temporary flood defence measures : technical...

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M A Y O R A L F L O O D T A S K F O R C E

T e m p o r a r y F l o o d D e f e n c e M e a s u r e s : T e c h n i c a l R e p o r t

A P P E N D I X A : F I N A L D R A F T

M a y 2 0 1 4

May 2014. Temporary Flood Defence, Measures: Technical Report, APPENDIX A: FINAL DRAFT

This report contains technical information prepared by the Christchurch City Council Mayoral Taskforce on Flooding.

As of 19 May 2014 the Christchurch City Council has yet to make any decisions on the report and the matters contained in the report. The report was prepared on the basis of the best available information at the time. That information may be subject to change (in whole or in part) and the Council does not represent or warrant the completeness or accuracy of the information within the report.

Christchurch City Council has no control over and is not responsible or in any way liable, to any person or entity that chooses to rely upon the information, or for any errors, omissions, or inaccuracies, whether arising from negligence or otherwise or for any consequences arising there from.

Any person or entity wishing to rely on the information is advised to seek independent advice as may be necessary.

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Appendix A Detailed area reports Table of Contents A1 Lower Avon: River stopbanks A2 Lower Avon: Avondale area A3 Lower Avon: Residential Red Zone A4 Lower Avon: Brittans Drain area A5 Lower Avon: Knights Drain area A6 Dudley Creek A7 Upper Heathcote A8 Lower Heathcote: Riverside properties A9 Lower Heathcote: Bells Creek catchment A10 Lower Heathcote: Tennyson Area A11 Heathcote Valley A12 Little River A13 Lyttelton A14 Lyttelton: Slips A15 Lyttelton: Stormwater drainage network A16 Southshore A17 Sumner: Sumner Village A18 Sumner: Moncks Bay

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A1 Lower Avon: River stopbanks A1.1. Background Stopbanks were first constructed along some lengths of the Avon in the early 20th Century. Following overtopping during major storm events, the stopbanks have been progressively extended and raised.

The earthquakes resulted in significant slumping along the banks of the Avon as a result of liquefaction and lateral spreading and the major subsequent earthquakes have caused further damage and settlement. The stopbanks were subsequently reconstructed along the river’s edge as a

temporary measure. The stopbanks continue to suffer from bank slumping and fill consolidation which results in areas of the stopbanks being lower than the river flood level.

New alignments for the permanent stopbanks are currently being considered.

Location The stopbanks are present along the entire length of the Lower Avon.

Existing studies The following existing studies have been identified:

Emergency Management Plan was created in 2012 following identification of the priority areas for stopbank monitoring in the event of high tide, high rainfall and aftershocks.

The full stopbank length was inspected in March 2014 prior to Cyclone Lusi and low-lying sections were topped up as required.

The council has requested Davie Lovell Smith to undertake a new survey of the stopbanks. This work was to start in the first week of May 2014.

Flooding history Stopbanks were first constructed along some lengths of the Avon in the early 20th Century. During major storm events the river has over topped the stopbanks resulting in a progressive extension and raising of the stopbanks. A notable significant historic event occurred following a snow storm in 1992. The river also over topped its banks during the March 14 storm events in some locations, e.g. New Brighton Road at Pump Station 205.

A1.2. Key drainage infrastructure The stopbanks incorporate stormwater outfalls, some of which have some backflow prevention installed some of which are in need of repair.

A1.3. SCIRT projects Stopbanks are not within SCIRT scope.

A1.4. Existing flood protection infrastructure The Avon stopbanks are the primary flood protection for the Lower Avon area. In order to prevent backflow and flooding each stormwater outfall through the stopbanks is fitted with a flap valve for backflow prevention.

A1.5. March 2014 flood extent The stopbanks are a barrier to prevent tidal or river flooding of urban land along the river. However with river flood levels peaking above ground level the land drainage is inhibited, this resulted in flooding on the inside of the stopbanks. Flooding has also been caused by back flow through the drainage network where flaps valve were not functional.

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Figure 1 Lower Avon River location plan

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Cause The flooding was caused by a lack of channel capacity, the overtopping of banks and the poor condition of the flap valves.

Local effects The affected properties can be grouped into three locations;

Brittans Drain, around Kerrs Road

Avondale, generally between Avondale & Wainoni Rd

Knights Drain, around Pages Rd ANZAC Drive

A1.6. Community response and drivers The stopbanks are essential to prevent river and tidal flooding. However they also cause other adverse effects such as ponding within the protected area. Improved maintenance and operational activities are required to minimise this flooding.

A1.7. Field studies The Emergency Response Plan identified the following as critical areas:

Kibblewhite Street

Evans Avenue

Owles Terrace

New Brighton Road

Locksley Avenue

Dallington Terrace

River Road

Avonside Drive

Hulverstone Drive

A full survey of the stopbanks has been completed by Davie Lovell Smith on behalf of Council.

Investigation completed in March 2014 identified that topping up of the stopbanks is required in some locations, particularly Hulverstone Drive and the Owles Terrace boat ramp. It was also identified that multiple flat gates in the lower Avon are either missing or blocked open.

A1.8. Earthquake effects The effects of the 2011 Canterbury earthquake sequence on the area are described in this section and have been derived from information provided by the Canterbury Geotechnical Database (CGD), SCIRT Geodatabase (SGD), CERA and field observations.

Land level changes The CGD vertical and horizontal ground movement database indicates land changes between -0.1 m to -0.5 m have been observed along the Avon with some localised movement of -0.5 m to -1.0 m.

Lateral spread Moderate to severe lateral spread has been exhibited at along the length of the Avon and within the Red Zone to Owles Terrace.

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The Avon River banks suffered significant slumping as part of liquefaction and lateral spreading. Temporary stopbanks were reconstructed along the river’s edge. Each major earthquake event

caused further damage and settlement to the stopbanks. These stopbanks continue to suffer from bank slumping and fill consolidation. This results in areas of the stopbanks being lower than the river flood level.

Liquefaction Moderate to significant liquefaction has occurred along the Lower Avon. CERA has therefore indicated the area is situated within the Red Zone and also the Green Zone. The red zone was created due to the significant effects of liquefaction and lateral spreading. Land in the CERA Green Zone has been divided into three technical categories (TC). These categories describe how the land is expected to perform in future earthquakes. Outside the Red Zone the land is classified as TC2 and TC3 indicating minor to moderate and moderate to significant land damage from liquefaction is possible in future significant earthquakes, respectively.

Ground cracking The CGD ground cracking database indicates cracks of 10 to 50 mm along the length of the Avon with some 50mm to 200 mm cracks identified with some in excess of 200mm in the Red Zone.

Waterway observed changes Bed heave will have occurred with the Lower Avon

Infrastructure damage The locate drainage network will have suffered a reduction in capacity during peak river levels.

The emergency response placement of the stopbanks has inadvertently covered some manholes and sumps causing local drainage issues. Ongoing maintenance difficulties are being experienced.

The local drainage network will have suffered a reduction in capacity during peak river levels due to ground settlement.

A1.9. Key photos Flooding at the stopbanks is presented below; a further photo presented below provides an example of a lot point in the stopbanks is at the Owles Tce boat ramp.

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Figure 2 Lower Avon flooding behind stopbanks

Figure 3 Lower Avon stopbanks at Owles Terrace

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A1.10. Options New alignments for the permanent stopbanks are currently being considered. The stopbanks will stay in their current location until the River & Tidal Flood Study is completed. Ongoing monitoring will continue, including survey and walk overs.

Constraints The stopbanks cannot be removed until an alternative alignment for the permanent stopbanks has been determined.

Opportunities The relocation of the stopbanks away from the river bank will reduce the risk of ground settlement and lateral spread.

Potential temporary protection measures Ongoing monitoring and maintenance of the emergency stopbanks is required. They need to be regularly surveyed and topped up as identified by the survey. In particular, the Owles Terrace boat ramp is a gap in the network which requires filling. This could be temporarily sand bagged before an event or bunded at a gentle grade to allow use of the ramps by vehicles to be maintained.

A1.11. Recommendations

Timeframe A survey of the stopbanks is underway and topping up will subsequently proceed as required.

Consenting requirements Consent to maintain the stopbanks is not required.

Emergency response plan An Emergency Response Plan has been developed and used since 2012 during periods of high tides and/or rainfall. Emergency response is also triggered in the event of significant aftershocks. It is recommended that this Plan is continued and updated as required, for example to account for the abandonment of Residential Red Zone land

A1.12. Recommendations for future studies The Davie Lovell Smith survey is part of a detailed assessment of the stopbanks, and management of the temporary stopbanks is included in the Land Drainage Recovery Programme. It is recommended that these projects proceed with haste.

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A2 Lower Avon: Avondale area A2.1. Background The Avondale area suffered area wide ground settlement as a result of the Canterbury Earthquake sequence and as a result, the ground level is now typically RL10.5 – RL11m. This change in level has resulted in a reduced hydraulic head for the operation of the local stormwater network, thus reducing the networks efficiency. Some parts of the area are now below annual high tide level and the river flood level for major events.

The area is protected by the Avon River stopbanks and back flow prevention devices on the stormwater network outfalls.

Location The Avondale area is located between Breezes Road in the south to the River Avon in the north and west and to Hulverstone Drive and Waratah Street to the east.

Existing studies

Investigation of the operation of Pump Station 220 by SCIRT

Survey of the river stopbanks

Flooding history One significant historical storm event was the 1992 snow storm when the river over topped the stopbanks along Hulverstone Drive. During the March 2014 storm event the river also overtopped the stopbanks in some locations, e.g. New Brighton Rd at Pump Station 205.

A2.2. Customer service reports Customer service reports indicate that seven properties have experienced flooding up to the building foundations and two properties suffered garage flooding. While no house floors were flooded during the March 2014 event, feedback from local residents confirms that house were at risk. The properties are clustered in two groups.

A2.3. Key drainage infrastructure The area is served by stormwater network piping which discharges directly to the river. The main pipeline is Wainoni Drain which discharges at Hulverstone Drive, this pipeline includes Pump Station 220 which pumps stormwater into the river during peak tides and flood events.

There are numerous stormwater outfalls (ranging in size from DN 225-825 mm) which also discharge to the river.

A2.4. SCIRT projects The following SCIRT Projects are applicable to the area:

11086 Avondale Rd rebuild in Residential Red Zone,

10802 Pump Station 54 Stage 1 roading renewal (includes stormwater network repairs)

11071 NE4-Aranui North, repairs to roading and stormwater assets.

SCIRT has completed an investigation of the operation of Pump Station 220 which found the following:

The existing outlet is damaged and held open allowing back flow of water from the river

The outlet scour protection is gone

The pump size is considered adequate

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The other local flap gate have been removed or are damaged allowing back flow into the network

A2.5. Existing flood protection infrastructure The wider area is protected by the Avon stopbanks and the council stormwater network which discharges through the stopbanks. Some of the piped outfalls have flap value back flow prevention and it has been identified that some are in need of repair.

A2.6. March 2014 flood extent

Depth Areas within the Residential Red Zone experienced above floor level flooding and the Residential Green Zone areas experienced property and garage flooding.

Cause Flooding along the urban side of the stopbanks was caused by both fluvial and pluvial sources. River back flowed through the flap gate inundating the streets and surface water flooded behind stopbanks because it was unable to drain to the river. A resident stated that, “the water flows back up Prestwick Street at high tide, then onto Avondale Rd”. These comments seem to suggest the local flap gates may not be sealing correctly as the flooding occurs at ‘all’ high tides. The Avondale Rd properties have also been flooded 3 times since the earthquakes.

The main outfall for the area, Pump Station 220 is reported to have a damaged flap gate, possibly allowing back flow into the stormwater network.

The high river flood level reduced the capacity of the stormwater network. No stopbank overtopping was reported at this location.

General description of damage The GIS mapping indicated that continuous flooding was experienced inside the stopbank from Porrit Park to the Avondale Rd bridge along Avonside Drive and on the adjacent Red Zone properties. Between Avondale Rd and Wainoni Rd Street flooding was noted with property surface flooding.

Number of affected floors House floor level flooding has not been identified, however 4 garages were flooded. It can be concluded at as the garages were flooded to a depth of 100mm, water would have also have been present under the floor of the houses. Some degree of flooding has been observed at 20-30 properties across seven streets and further flooding occurred in the Residential Red Zone.

Local effects The affected properties were clustered along Avondale and Breezes Road.

A2.7. Other 2014 floods It was reported by one resident that their property had been flooded three times since the Canterbury earthquake sequence.

A2.8. Community response and drivers

Social impact A report specific to this area has not been received however street flooding would cause access difficulties for the wider community. The flooded properties, especially those who have had repeated garage flooding would experience increased levels of anxiety, possible loss of property and increased insurance costs.

Community feedback The community has reported flooding as previously recorded.

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A2.9. Field studies Site inspections were undertaken on 3rd to 4th May 2014 to inspect the area to identify area and individual property flooding.

A2.10. Earthquake effects The effects of the 2011 Canterbury earthquake sequence on the area are described in this section and have been derived from information provided by the Canterbury Geotechnical Database (CGD), SCIRT Geo database (SGD), CERA and field observations.



The Avon River banks suffered significant slumping as part of liquefaction and lateral spreading. Temporary stopbanks were reconstructed along the river’s edge. Each major earthquake event caused further damage and settlement to the stopbanks. These stopbanks continue to suffer from bank slumping and fill consolidation. This results in areas of the stopbanks being lower than the river flood level. Ongoing maintenance and ‘top up’ is required to maintain the bank levels and protection. Maintenance issues with the flap gate protection is also reported.

Liquefaction Moderate to significant liquefaction has occurred along the Lower Avon and CERA have designated areas along the reach both Residential Red Zone and Residential Green Zone. The Residential Red Zone was created due to the significant effects of liquefaction and lateral spreading. Land in the CERA Residential Green Zone has been divided into three technical categories (TC). These categories describe how the land is expected to perform in future earthquakes. Outside the Residential Red Zone the land is classified as TC2 and TC3 indicating minor to moderate and moderate to significant land damage from liquefaction is possible in future significant earthquakes, respectively.

Ground cracking The CGD ground cracking database indicates cracks of 10 to 50 mm along the length of the Avon with some 50mm to 200 mm cracks identified with some in excess of 200mm in the RRZ.

Waterway observed changes Bed heave will have occurred with the Lower Avon.

Infrastructure damage No damage to the infrastructure has been identified but a lower level of service may not be afforded due to settlement in the catchment.

Bed heave/hydraulic capacity loss The locate drainage network will have suffered a reduction in capacity during peak river levels.

A2.11. Key photos The photograph included below indicates the level of flooding at Avondale.

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Figure 4 Lower Avon Avondale example of flooding up to house foundations and in garage

A2.12. Options

Key assumptions and gap analysis Local drainage and flood protection could be improved by repairs or replacement of damaged back flow protection devices in the area.

Constraints The following constraints have been identified:

The stopbanks cannot be removed until an alternative alignment for the permanent stopbanks has been determined.

Access to maintain the existing flap gate is difficult due to high tides and soft soils within the river channel.

Opportunities Shallow stormwater holding basins could be formed in the Residential Red Zone away from the residential Green Zone properties. Runoff could be directed to these basins. During major events, stormwater could be pumped direct from these basins into the river.

A2.13. Potential temporary protection measures

Single residence measures It is considered that local area solutions will be more effective than individual property measures.

Local catchment measures If repairs can be made to the existing local drainage network, improved drainage can be provided to the wider area and hence a reduction in risk to individual properties.

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Maintenance measures The following maintenance measures have been identified:

The existing flap gate at Pump Station 220 should be replaced.

All flap gates long Avonside Drive and Hulverstone Drive should be checked, repaired or replaced as required.

Temporary pumps could be installed during major events to keep ponded runoff from reaching low lying property. Suggested sites are, DN 375 outfall at Avondale bridge, DN 825 outfall at Orrick Street. Swales and stormwater ponding basins could also be installed to aggregate outfalls together.

Social impacts of measures No adverse effects envisaged from proposed mitigation.

A2.14. Recommendations The following actions are recommended and can occur immediately:

Restore operation of Pump Station 220 by replacing flap gate

Check all other flap gates.

Extra pump capacity maybe required to prevent street and thereby house flooding.

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A3 Lower Avon: Residential Red Zone A3.1. Background The Residential Red Zone (RRZ) area suffered area wide ground settlement as a result of the Canterbury Earthquake sequence. As a result the ground level is now typically RL10.0 – RL11m and the hydraulic head necessary for the local stormwater network has been reduced, thus reducing the stormwater network’s efficiency. Some parts of the RRZ area are now below annual high tide level

and the river flood level for moderate to major storm events.

Hence the area is protected by the Avon River stopbanks and back flow protection devices on the stormwater network outfalls.

The March 2014 flood maps indicate that six areas within the RRZ experienced property and house flooding, these areas are;

Avonside Dr, Retreat Rd river loop, including land adjacent to the Dudley Creek confluence with the Avon,

Dallington Tce, Locksley Ave Gayhurst Rd river loop,

Avonside Dr, Porrit Park river loop,

Avonside Dr and Hulverstone Dr, Avondale,

New Brighton Rd at Horseshoe Lake outfall,

Knights Drain Outfall, Pages Rd & ANZAC Drive

Location The RRZ is situated along the River Avon predominantly in Avonside and includes the areas outlined above.

Existing studies The CCC has requested Davie Lovell Smith to undertake a resurvey of the stopbanks. The field survey component of this work is completed.

Flooding history Stopbanks were first constructed along some lengths of the Avon River in the early 20th Century. During major storm events the river has over topped its banks leading to progressively higher and longer lengths of stopbanks. Stopbank over topping during the 1992 snow storm resulted in a major extension of the stopbank system. The river also over topped the stopbanks during the March 2014 storm events in some locations, e.g. New Brighton Rd at PS205.

A3.2. Customer service reports As the area is classified as RRZ there are no customer service reports available for recent flooding events.

A3.3. Key drainage infrastructure There are numerous piped and open drain outfalls to the river, including discharges from Pump Stations 205, 204 and 220.

A3.4. SCIRT projects SCIRT are currently undertaking a large number of projects that interact with the RRZ, however due to the size of the area covered and number of projects involved they have not been listed.

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A3.5. Existing flood protection infrastructure The RRZ is located along the banks of the Avon River. The river stopbanks and the local council stormwater network provide flood protection and drainage. The piped outfalls have flap gate back flow prevention.


Depth Property level flooding was not investigated for this study because this study area is classified as RRZ.

Type Flooding along the urban side of the stopbanks was caused by both the river flood waters back flowing through the flap valves and rainfall within the catchment, i.e. behind the stopbanks, being unable to drain out due to high river levels.

Cause Bank overtopping has occurred as a result of stopbank slumping and settlement and the flooding in these areas has been exacerbated by post-quake maintenance practices of blocking off some piped stormwater outfalls. In some areas this has been undertaken as the location of the recovery stopbank over manholes prevents access for maintenance of the flap gates. In other areas this has been carried out to prevent backflow from the river but has resulted in surface ponding.

The capacity of the stormwater network has been reduced due to land settlement.

General description of damage The GIS mapping indicated that 396 properties experienced some flooding, however the majority of these are in the RRZ.

Number of affected floors 128 houses have been reported as flooded, however all of these are in the RRZ and only garages (8) were confirmed as flooded outside the RRZ area. A number of properties also reported flooding up to the house foundations. It is likely that the properties with garage flooding also had flooding under the house floor.

Local effects The affected properties can be grouped into six areas as noted in the Background section above.

A3.7. Other 2014 floods Some areas within the RRZ have been noted as flooding at least three times since 2011.

A3.8. Community response and drivers The land has been placed in the RRZ and is unoccupied. Community feedback has not been investigated for this report.

A3.9. Field studies As the area is RRZ it has not been investigated for this report.

A3.10. Earthquake effects The effects of the 2011 Canterbury earthquake sequence on the area are described in this section and have been derived from information provided by the Canterbury Geotechnical Database (CGD), SCIRT Geo Database (SGD), CERA and field observations.

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Lateral spread Moderate to severe lateral spread has been exhibited along the length of the Avon and within the Red Zone to Owles Terrace.

The Avon River banks suffered significant slumping as part of liquefaction and lateral spreading. Temporary stopbanks were reconstructed along the river’s edge. Each major earthquake event has

caused further damage and settlement to the stopbanks. These stopbanks continue to suffer from bank slumping and fill consolidation. This results in areas of the stopbanks being lower than the river flood level.

Liquefaction Moderate to significant liquefaction has occurred along the Lower Avon. CERA has therefore indicated the area is situated within the Red Zone and also the Green Zone. The Red Zone was created due to the significant effects of liquefaction and lateral spreading. Land in the CERA Green Zone has been divided into three Technical Categories (TC). These categories describe how the land is expected to perform in future earthquakes. Outside the Red Zone the land is classified as TC2 and TC3 indicating minor to moderate and moderate to significant land damage from liquefaction is possible in future significant earthquakes, respectively.

Ground cracking The CGD ground cracking database indicates cracks of 10 mm to 50 mm along the length of the Avon with some 50 mm to 200 mm cracks identified in the Red Zone.


Infrastructure damage The local drainage network will have suffered a reduction in capacity during peak river levels due to ground settlement.

The emergency response placement of the stopbanks has inadvertently covered some manholes and sumps causing local drainage issues. Ongoing maintenance difficulties are being experienced.

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A3.11. Key photos

Figure 5 Lower Avon RRZ – flooding within stopbank

A3.12. Options

Key assumptions and gap analysis No flood protection or drainage works will be undertaken for the purposes of protecting the RRZ

Opportunities The large area of the RRZ may provide opportunities for the construction of flood protection measures as part of other studies or works. However these works will not be part of this Flood Task Force activity which is only considering house protection.

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A4 Lower Avon: Brittans Drain area A4.1. Background Brittans Drain serves a residential area south of the Avon River Porritt Park loop. The wider area is protected by stopbanks on the southern bank of the Avon River, although the Avon itself does not typically threaten Green Zone houses. Properties immediately adjacent to the Avon River have been red zoned by CERA.

Several complaints in recent years have been lodged to address maintenance issues causing localised flooding. The March 2014 event caused section flooding on multiple properties as well as flooding at or near foundation level for properties directly adjacent to Brittans Drain.

Location Brittans Drain is located between Pages Road and the River Avon and includes Wainoni Road south of Porritt Park.

Existing studies Brittans Drain was modelled by SCIRT as part of a catchment investigation. This identified some capacity issues given the large catchment area feeding the drain. In particular, modelling indicated surface ponding potential around the commercial properties of Eastgate Mall at the head of the catchment.

The drain outfall at Porritt Park was also investigated by SCIRT. This identified a lack of a flap gate at the outfall.

Flooding history Residents in Avonside Drive report that Brittans Drain regularly floods; however, this area is now part of the Residential Red Zone. The Green Zone experiences localised flooding which is typically confined to the road carriageway.

A4.2. Customer service reports Multiple reports have been lodged both pre- and post-earthquakes for the area. These relate to maintenance issues such as blocked sumps or constriction in Brittans Drain. Instances of reports have increased since the earthquakes with the infrastructure rebuild contributing to sump blockages.

A4.3. Key drainage infrastructure The area is served by Armstrongs and Brittans Drain which feed into the Porritt Park loop of the Avon River. Brittans Drain is an open drain which flows along the back of residential properties before entering 900 mm brick barrel along Wainoni Road. It transitions into 1200 mm concrete pipe after being joined by Pannell Drain. The catchment drainage network is complex and included six manholes where flow can occur in multiple directions.

A4.4. SCIRT projects SCIRT project 10876 modelled the Brittans Drain stormwater network and project 10809 investigated the outfall on the Avon loop. Piped repairs and renewals are recommended within the catchment.

Roading repairs have been made in isolated areas, and project 10876 recommends further repairs with significant carriageway work for two sections of road. This has yet to be undertaken.

A4.5. Existing flood protection infrastructure Stopbanks on the south side of the Avon River protect the area from large scale fluvial (river) flooding.

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Depth The worst affected house in March experienced flooding to a depth of 50 mm up the foundations. Brittans Drain was reported as 2 m deep at Kerrs and Wainoni Roads intersection.

Type Several properties experienced pluvial (rainfall) flooding of their driveways and front yards with water overtopping the kerb and channel. More significant fluvial flooding was experienced by properties backing on to Brittans Drain.

Cause Green Zone properties on the corner of Kerrs Road and Dunarnan Street experienced flooding from Brittans Drain. These properties in particular were flooded due to a large tree creating a significant constriction in the drain at this location.

Other nuisance flooding is a result of the March event exceeding the capacity of local kerb and channel. This may have been exacerbated by the earthquake as localised changes to the grade are apparent. This flooding is not however a risk to floor levels and is therefore a lower priority.

General description of damage No damage to property, garages or houses has been identified in the Residential Green Zone. Impacts were limited to inundation of properties.

Number of affected floors No properties experienced floor level flooding in March 2014. Two properties adjacent to Brittans Dain experienced foundation level flooding.

Local effects The worst affected properties were clustered around the intersection of Dunarnan Street and Kerrs Road. This coincides with the Brittans Drain constriction created by a large tree.

A4.7. Other 2014 floods The residential area is not known to have experienced any other significant flooding in 2014.Avonside Drive along the river in this area is frequently inundated however this does not threaten Green Zone properties or their direct access.

A4.8. Community response and drivers March flooding in this area is believed to be at a nuisance level. No roads in the area have been closed due to flooding. However disruption to property access is a potential issue for the two properties which have experienced inundation across their section to their foundation. Water is not believed to have extended beneath foundations for any Green Zone houses in the area.

A4.9. Field studies The area was inspected in the days following the March 2014 flood event. Flood extents were derived by identifying and surveying remaining debris marks. Residents gave additional information such as the frequency and depth of flooding. This information was used to produce the flood extent maps.

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Residential land in the Brittans Drain area has settled by approximately 300 mm due to the earthquakes. This settlement is greater along areas immediately adjacent to Brittans Drain and in the Residential Red Zone with settlement in these locations of up to 1 m. The fall of the land however is relatively unchanged.


The Avon River banks suffered significant slumping as part of liquefaction and lateral spreading. Temporary stopbanks were reconstructed along the river’s edge. Each major earthquake event caused further damage and settlement to the stopbanks. These stopbanks continue to suffer from bank slumping and fill consolidation. This results in areas of the stopbanks being lower than the river flood level.

Liquefaction Moderate to significant liquefaction has occurred along the Lower Avon. CERA has therefore indicated the area is situated within the Red Zone and also the Green Zone. The Red Zone was created due to the significant effects of liquefaction and lateral spreading. Land in the CERA Green Zone has been divided into three Technical Categories (TC). These categories describe how the land is expected to perform in future earthquakes. Outside the Red Zone the land is classified as TC2 and TC3 indicating minor to moderate and moderate to significant land damage from liquefaction is possible in future significant earthquakes, respectively.

Ground cracking The CGD ground cracking database indicates cracks of 10 to 50 mm along the length of the Avon with some 50mm to 200 mm cracks identified in the Red Zone.


Infrastructure Damage The emergency response placement of the stopbanks has inadvertently covered some manholes and sumps causing local drainage issues. Ongoing maintenance difficulties are being experienced.

The local drainage network will have suffered a reduction in capacity during peak river levels due to ground settlement. SCIRT modelling investigations suggest that any post-earthquake reduction in service is predominantly due to damage to piped infrastructure rather than land settlement. Concept design of the stormwater system recommends repair of 8% of the pipes in the catchment and renewal of 32%.

Damage has also been identified to roads in the area with significant deformation and kerb and channel damage identified for Dunarnan Street and parts of Woodham Road. Isolated damage has also been observed for Kerrs Road and other minor roads in the area

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A4.11. Key photos

Figure 6 Lower Avon Brittans Drain - Debris mark showing water level on foundation of Kerrs Road

property after March 5th event.

Figure 7 Lower Avon Brittans Drain - Sump two days after March 5th event.

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Figure 8 Lower Avon Brittans Drain - Large constriction in Brittans Drain

A4.12. Options

Key assumptions and gap analysis Temporary flood mitigation options are proposed on the basis that there was no flooding of residential floor levels in the area and therefore it is of lower priority for additional protection. Suggested options are only those which can be quickly and easily implemented. Flooding of floor levels in the Residential Red Zone has been excluded from analysis.

Constraints Brittans Drain flows between residential properties with fences backing onto the drain. This restricts options to significantly alter the drain profile.

Opportunities There are opportunities to provide a ‘quick fix’ for the most severe flooding by immediate removal of constrictions in Brittans Drain.


Single residence measures Localised protection using sandbags could be considered for properties along Brittans Drain, although this is not a priority due to the lack of floor level flooding in the area. It is anticipated that local catchment scale measures will be the most effective option for relieving flooding.

Local catchment measures Immediate removal of a large tree blocking Brittans Drain would relieve the foundation level flooding experienced.

Maintenance measures Clearance of all constrictions in Brittans Drain is recommended immediately and on a regular basis. Street and sump clearing is also a common customer request and would mitigate nuisance localised flooding. Current maintenance activities may therefore need to increase in frequency.

Social impacts of measures No adverse effects are envisaged from proposed mitigation.

A4.14. Recommendations It is recommended that the large constriction identified in Brittans Drain be removed immediately. No consent is required as this forms part of regular maintenance activity.

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Emergency response plan It is recommended that maintenance crews review this area prior to a forecast event to remove any drain or sump blockages. Crews also need to be available during the event to address any blockages which may arise.

Sand bagging of houses backing onto Brittans Drain could provide additional emergency protection in a future event if required. This needs to be assessed against the size of the event as floors are not expected to be at risk in a March size event once effective maintenance is undertaken.

A4.15. Recommendations for future studies A more in depth investigation of Brittans Drain is included in the current Land Drainage Recovery Programme. A project to review the maintenance contract post-earthquake also has particular relevance to this area given the sump and drain blockages reported. It is recommended that these projects proceed with haste.

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A5 Lower Avon: Knights Drain area

A5.1. Background

The ANAZC Drive area suffered area widespread ground settlement as a result of the Canterbury

Earthquake sequence. As a result of the ground level dropping (now typically RL 9.5 m – RL 10.5 m)

and the hydraulic head for the operation of the local stormwater network has been reduced, and thus

the network’s efficiency has been reduced. Some parts of the area are now below annual high tide

level and the river flood level for major events.

The area is protected by the Avon River stopbanks. A storm water pump station (PS204) on Knights

Drain provides the necessary lift to provide drainage for this low lying area.

Location

The study area is located between Pages Road and Anzac Drive

Existing studies

SCIRT investigation of the operation of the pump station (PS204).

Davie Lovell Smith survey of the river stopbanks.

Flooding history

One significant historical storm event was the 1992 snow storm when the river over topped the stopbanks along Hulverstone Drive. During the March 2014 storm event the river also overtopped the stopbanks in some locations, eg New Brighton Rd at Pump Station 205.

A5.2. Customer service reports

Seven properties were indicated as experiencing flooding up to the building foundations and four properties with garage flooding. While no house floors were reported as flooded during the March 2014 event, the level of the ground would indicate that some flooding under the floor of houses would also have occurred.

A5.3. Key drainage infrastructure

The area is served by stormwater network piping which discharges to Knights Drain and then Pump Station 204 which pumps directly to the river. Other numerous local pipelines also discharge to the river.

A5.4. SCIRT projects

SCIRT has undertaken an investigation into the operation of Pump Station 204. A low flow bypass existed on the pump station pre-quake. However this has been removed and the pump station now pumps all of the storm water discharged from the local area.

A5.5. Existing flood protection infrastructure

The wider area is protected by the Avon stopbanks and the council stormwater network which discharges through the stopbanks. The piped outfalls have flap valves to reduce back flow from the river.


Type

Flooding along the urban side of the stopbanks was caused by a combination of the river flood flows back flowing through the flap values, inundation of the streets, and rainfall within the residential area being unable to drain out due to high river levels. Cause

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The flooding was primarily caused by the low lying nature of the ground and the relatively high flood level of the Avon during the March 14 event. The properties have only been reported as experiencing flooding once during the March 14 event. However the land is very low lying and at Pages Rd the carriageway level is being raised, leaving some properties below the road level. This will increase their vulnerability to future flooding. No stopbank overtopping has been reported at this location. However the river flood levels of RL 11 to 11.14 m recorded in the local area are above the level of identified low points in the stopbanks. It is likely that some overtopping of the stop bank has occurred. Knights drain is poorly graded with a high point in the midpoint of the drain. This high point maybe the primary cause of the flooding.

General description of damage The GIS mapping indicated that sheet flooding was experienced inside the stop bank for the area bounded by the river, ANZAC Dr and Pages Rd. Flooding on the east of Pages Rd also occurred.

Number of affected floors No house floors have been reported as flooded. However four garages were flooded. In addition 20 to 30 properties experienced some degree of flooding across seven streets. Flooding in the Residential Red Zone has not been included. The wide spread surface flooding would have resulted in ponded water under the house floors.

Local effects The affected properties were clustered along Pages Rd and between Knights Drain and Pages Rd. Up to 30 properties were affected by the surface flooding.

A5.7. Other 2014 floods No information about previous flooding is known.

A5.8. Community response and drivers No specific report received. However, street flooding would cause access difficulties for the wider community. The flooded properties, especially those who have had repeated garage flooding, would experience increased levels of anxiety, possible loss of property value and increased insurance costs.

A5.9. Field studies The area was visited in May 2014 to inform this study.

A5.10. Earthquake Effects The effects of the 2011 Canterbury earthquake sequence on the area are described in this section and have been derived from information provided by the Canterbury Geotechnical Database (CGD), SCIRT Geo database (SGD), CERA and field observations.

Land Level Changes The CGD vertical and horizontal ground movement database indicates land changes between -0.1 m to -0.5 m have been observed along the Avon with some localised movement of -0.5 m to -1.0 m.

Lateral Spread Moderate to severe lateral spread has been exhibited at along the length of the Avon and within the Red Zone to Owles Terrace.

The Avon River banks suffered significant slumping as part of liquefaction and lateral spreading. Temporary stopbanks were reconstructed along the river’s edge. Each major earthquake event caused further damage and settlement to the stopbanks. These stopbanks continue to suffer from bank slumping and fill consolidation. This results in areas of the stopbanks being lower than the river flood level.

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Liquefaction Moderate to significant liquefaction has occurred along the Lower Avon. CERA has therefore indicated part of the area as being within the Red Zone and part within the Green Zone. The Red Zone was created due to the significant effects of liquefaction and lateral spreading. Land in the CERA Green Zone has been divided into three technical categories (TC). These categories describe how the land is expected to perform in future earthquakes. Outside the Red Zone the land is classified as TC2 and TC3 indicating minor to moderate and moderate to significant land damage from liquefaction is possible in future significant earthquakes, respectively.

Ground cracking The CGD ground cracking database indicates cracks of 10 to 50 mm along the length of the Avon with some 50mm to 200 mm cracks identified with some in excess of 200mm in the Red Zone.

Waterway observed changes Bed heave is expected to have occurred within the Lower Avon River.

Infrastructure Damage The placement of the stopbanks during the initial emergency response has inadvertently covered some manholes and sumps causing local drainage issues. Ongoing maintenance difficulties are being experienced.

The local drainage network will have suffered a reduction in capacity during peak river levels due to ground settlement.

A5.11. Key photos

Figure 9 Ponding remaining around property three days after the March event

A5.12. Options

Constraints The stopbanks cannot be removed until an alternative alignment for the permanent stopbanks has been determined. Access to the existing flap valves for maintenance is difficult due to high tide levels and soft soils within the river channel.

Opportunities Knights Drain between ANZAC Drive and Pump Station 204 is piped. This pipeline could be day-lighted potentially increasing its capacity. In addition shallow stormwater holding basins could be formed in the Residential Red Zone away from the Green Zone properties. Runoff could be directed to these basins away from the Green Zone properties.

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Single residence measures It is considered that local solutions will be more effective than individual property measures.

Local catchment measures If repairs can be made to the existing local drainage network, improved drainage can be provided to the wider area and hence a reduction in risk to individual properties. These measures would include:

Re-grading of Knights Drain invert

Repairs to the timber lined length of the drain

Removal of trees and vegetation within the waterway, especially immediately upstream of ANZAC Drive

Repairs or replacement of damaged back flow protection devices in the area

Maintenance measures Undertake waterway cleaning as above:

Regrading of Knights Drain invert,

Repairs to the timber lined length of the drain,

Removal of trees and vegetation within the waterway, especially immediately upstream of ANZAC Drive.

A5.14. Optioneering The proposed works are all maintenance activities. Further works could be undertaken but these will all required detailed investigation and a long term implementation. The Pages Road rebuild design should be reviewed to ensure drainage is being provided to the adjacent low lying properties. There was one house identified as having flooding under the floor level putting it in vulnerability category 2.


Preferred options Undertake recommended maintenance activities:

Regarding of Knights Drain invert

Repairs to the timber lined length of the drain

Removal of trees and vegetation within the waterway

These works could begin immediately. No consent is required.

Emergency response plan The area should be inspected during storm events to ensure Pump Station 204 and Knights Drain are operating correctly and that the low lying properties below Pages Rd are not being flooded.

A5.16. Recommendations for future studies Investigate:

Day-lighting the piped length of Knights Drain

Providing detention storage upstream of Pump Station 204 in the Residential Red Zone

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A6 Dudley Creek

A6.1. Background

Existing studies

The Dudley Creek catchment including the Flockton area has been extensively studied by Jacobs SKM under the Land Drainage Recovery Programme: Dudley Creek project. This has been taken from six values assessment, through options identification to feasibility assessment of the long-term preferred solutions for managing flood risk in the area.

The majority of the houses within the Dudley Creek area have been surveyed for floor level and ground level, and this information has been electronically recorded (on GIS). Detailed hydraulic modelling has been completed to assess pre earthquake, post-earthquake scenarios and taken forward for remediation solutions.

The following reports have been produced by Jacobs SKM on behalf of Christchurch City Council:

Dudley Creek Catchment: Issues and Options Report, Revision F, 13 November 2013

Post-earthquake operations of the upper and lower diversion, Revision 0, 30 September 2013

Effect of Stormwater Flooding in Dudley Creek Catchment on the Wastewater Network, Draft C, 13 August 2013

Land Drainage Recovery Programme: Assessment of the effects of filling building platforms, case study report, 12 April 2013

Jacobs SKM has also been working on a Dudley Creek area rainfall response plan. For this plan investigations were completed and preliminary solutions were identified prior to commencement of this Task Force.

Flooding history

The Dudley Creek catchment has a history of flooding. Floods have been documented across the area dating back to the early 1900’s. The area was then occasionally flooded up to the 1970’s, until the Dudley Creek Diversion was constructed. It is likely for this reason that no flooding to the area has been reported (until the earthquakes) by residents who have lived in the area for 30 years. After the earthquakes, there have been several floods in the area:

August 2013

June 2013

March 2014

April 18th and 30

th 2014

A6.2. Customer service reports

Numerous Customer Service Reports have been raised across the assessment area. The following is a general summary of the issues raised since the earthquakes:

Flooding of the street

Drain blockages

Requests for sandbags

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Figure 10 Dudley Creek location plan


Dudley Creek and its tributaries are demonstrably under capacity in rainfall events both within the open channels and at culverts. One of the most vulnerable areas with the Dudley creek catchment is around Flockton Street which has the topography of a basin and during flood events often suffers from significant ponding. Underground infrastructure helps to drain this ponding; however, many of the water levels within the structure rely on hydraulic pressure. The Flockton Invert, located on Harrison Street, is a submerged pipe and water levels equalise with the Dudley Creek. An issue with this is that when equilibrium is achieved, the resultant water level in Flockton area is higher than the ground level. This is due to the height of the Dudley Creek banks at the invert’s outlet.


SCIRT studies across the assessment area are listed below:

11069 St Albans and Mairehau (Roading, Stormwater, Water Supply)

11068 St Albans and Mairehau (Wastewater)

10811 Richmond and Shirley (Roading, Stormwater, Water Supply)

10581 PS7 Catchment Wastewater

10915 Shirley NW2 (Stormwater Water Supply & Roading)

11044 Edgeware Road

Stormwater works across the Lower Dudley Creek area are proposed under the following projects:

11131 Richmond (Roading, Stormwater, Water Supply)

11132 Shirley South (Roading, Stormwater, Water Supply):

11047 Edgware & Richmond Bridges

The SCIRT catchment study for the Upper Dudley Creek area, including the Flockton Street area, is currently at concept design stage and the works are yet to be finalised.

The SCIRT works currently identified in the lower Dudley Creek area through Richmond primarily relate to repairs to existing drainage infrastructure and culverts.

It should be noted that the SCIRT pluvial assessment has been led by the same team completing the fluvial assessment and details of flooding identified have been captured for this assessment.

A6.5. Existing flood protection infrastructure

Two engineering solutions currently exist:

Dudley Creek Diversion – The Flockton area has historically had high flood risk. In the 1970s a diversion structure was constructed to divert flood waters from the upper catchment to Horseshoe Lake. This diversion has been investigated by Jacobs SKM in 2013 and was found to be operating as designed. There were some changes in the structures level; however, it was not suggested that this would impact on its operation.

Cranford Basin – Cranford Basin is located at the market gardens between Cranford Street and Philpotts Road. It is used to provide storage during high rainfall events. This allows PS219 to prevent increased flow from being conveyed to lower Dudley Creek. The basin is currently to be used as part of the NZTA northern arterial scheme and further storage enhancement may be possible at such time.


Depth

In the March event pluvial flood waters were recorded as being over 600mm in depth through the centre of Carrick Street.

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Cause Flooding in the Dudley Creek catchment is a result of under capacity waterways which restrict flows and back up in heavy rainfall. The catchment has suffered from significant settlement and the waterways specifically have suffered from lateral spreading and bed heave.

The inability of the waterways to convey flow has a direct impact on the ability of drainage infrastructure to function. Fluvial water backflows the associated piping and spills into the Flockton basin exacerbating pluvial flooding issues. The Aylesford Street Drain (aka Mairehau Drain) is an example of this type of situation. The significance of Aylesford Street Drain spilling is that it will equalise in water level with that contained within the Flockton basin. The resulting depth of the water is known to flood many houses above floor level.

General description of damage During flooding to the Flockton area, many houses were flooded above floor level. For the wider area, the majority of properties experienced flooding across the section or under foundations.

Flood maps produced by Jacobs SKM highlight the wide spread house and property damage from the March flooding. Vehicles were also flooded and suffered damage.

Number of affected floors 97 properties are estimated to have experienced above floor levels based on GIS data and the surveyed flood extent.

Local effects Many houses flooded above floor level. A majority flooded to foundations and others across the property.

Local optional evacuations were required due to the risk of flooding to residents.

A local welfare center was opened; however, this was largely un-utilised.

A6.7. Other 2014 floods Flockton area has experienced flooding on three occasions in 2014. These include:

March 4th

April 18th

April 30th


Social impact Flooding in the Dudley Creek catchment has caused a significant stress to the residents. Many residents have moved out of their properties for this reason.

Reports in the media have also cited that some residents are unable to sleep when it rains.

Financial stress is also affecting the community who reported to have been subject to increases in insurance excesses. Additional financial pressure exists for those which have left their homes and are now paying mortgages, rates and rents at more than one property.

Community feedback Dudley Creek Information Evenings were held in March 2014 to discuss the two long term options presented in the Jacobs SKM report. Feedback from the community was that they welcomed the flood remediation but were concerned of the timeframe for this to be in place (approximately 2-3 years). A public focus group was held by Council to discuss the temporary options which are being explored. It became apparent that residents were open to large scale pumping if it relieves flooding. A question

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was posed to the group that they would be willing to put up with the noise if a pump was utilised during flood events. This was received positively.

The public focus group indicated they did not see sand bags as an option, but were open to house tanking. The idea of temporarily raising the houses drew concern in relation to future earthquakes and house being shaken whilst on temporary structures.

A6.9. Field studies After the June 2013 and March 2014 flood events, the area was surveyed. This included house surveys, flood level surveys and extent mapping. There were also a series of engineering walkovers as part of the Dudley Creek long term option studies.

As part of the task force, a site walk over was conducted and areas for increased maintenance and possible solutions were identified and evaluated. Properties which were known to be affected were visited by staff who recorded the information through the task force software application.

A6.10. Classification of earthquake effects

Bed heave/hydraulic capacity loss Bed heave has occurred extensively along Dudley Creek and its tributaries and a subsequent loss in hydraulic capacity has occurred.

Settlement A large proportion of the catchment has subsided between 200 and 500 mm.

Flood defences There are no stopbanks or flood defences in the Dudley Creek catchment.

Infrastructure damage Damage to culverts and failure of timber retaining structures has been observed across the lower catchment. Culvert repairs and repairs and replacements are proposed by SCIRT and Council have been undertaking remedial works such as silt clearance since the earthquakes.

Further damage to the drainage infrastructure is being addressed through the SCIRT programme.

A6.11. Key photos

Figure 11 Flockton area flooding in March 2014 event

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Figure 12 Required flow (arrows) and waterway capacity (red) during a 50 year ARI (Sourced from Dudley Creek information session presentation)

Figure 13 LiDAR image of the Dudley Creek catchment

A6.12. Options

Assumptions

Long term options are expected to be implemented within 3 years.

The underground stormwater network has been maintained and is operational.

The SCIRT programme is not affecting drainage during works.

Prior modelling is accurate as evidenced through validation of real world events.

House floor levels within the GIS data are accurate.

Dudley Creek Diversion has capacity to receive 2 m3/s.

Rainfall runoff is will cause water levels in Flockton area to rise slower than in the waterways.

House assessments are accurate.

Property owners will comply with proposed option implementation.

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Gaps

Available capacity for the Dudley Creek Diversion to receive pumped flood waters.

House specific detail for properties not flooded above floor level (i.e. over section or under foundations).

Detailed topographical survey of the entire Flockton area.

Receiving capacity of waterways in rainfall events.

Hydraulic certainty of option benefits.

Constraints

Narrow corridors

Accessibility of watercourse/property

Private property

Planning and building consents

Property construction type

Capacity of receiving waterbodies / infrastructure

Costs of implementation

Time to implement

Equipment or product availability

Opportunities

Utilisation of the Dudley Creek Diversion

New technologies to protect houses

Possibilities to integrate temporary implantations into permanent solutions

A6.13. Potential temporary protection measures Flood defence measures

Overpumping (street pooling and waterways)

Street bunding

Local drain improvements

House raising

House tanking

Lower catchment capacity enhancement works

Maintenance

Temporary buy-outs

Risk reduction measures

Traffic management plans

Wastewater overflow reduction

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Maintenance measures

Mairehau Drain – During site visits, the drain was observed to have some vegetation growth, so at a minimum this should be cleared. It is also not known what the design depth of the drain is, so it is suggested that the drain is dug to determine this and assess whether it should be further excavated.

Lower Dudley Creek – A site walk over identified opportunities to enhance the capacity of the network through minor works and maintenance activities. Council has initiated this.

Blocking Mairehau Drain intake from Dudley Creek - This could be blocked either through plugging or adding backflow control.

Social impacts of measures

Reduced risk of flooding to property

Reduced property damage from flood events

Return of residents to houses which have been empty due to flood related stress

Reduced migration of residents from the area


Catchment level Solutions

Lower Dudley Creek improvements – conveyance through the waterways can be improved through targeted works on constraints in the network. These works include the removal of Chancellor Street culvert and the Guild Street footbridge, the replacement of 2 private bridges on Stapletons Road is also recommended. These works will be complemented by Council’s maintenance works that are currently underway. Agreement would be required as to whether culvert or footbridge replacement is necessary, it is assumed that it is not.

Backflow prevention – The Flockton Drain, also known as the Flockton Invert, discharges to Dudley Creek upstream of Hills Road. As Dudley Creek rises, backflow occurs up the Flockton Drain and prematurely prevents functionality of the Flockton Drain. By preventing this backflow and providing overpumping from the Flockton Drain to Dudley Creek, the Flockton drain and the local drainage network feeding it are afforded service.

An option to pumping flood flows from the catchment to the Lower Dudley Creek Diversion could remove one cubic metre per second from the system and will ease the pressure on the network. The pressure main would span 650 m from Kensington Avenue to the Lower Dudley Creek Diversion at Philpotts Road.

Culvert booster pumping is suggested at the North Parade Culvert to augment capacity in the lower reaches of Dudley Creek. This will increase capacity by increasing the velocity of the stream at this location.

Permanent Option Solutions Component Acceleration – The acceleration of either the Warden Street bypass (long term Option 2) or the lower Dudley Creek channel works (long term Option 1) would offer significant enhancement. It is acknowledged that the permanent solutions are considered to take up to 2 years to implement but components can be progressed in isolation and delivered in a shorter timeframe.

Street Level Solutions

Bunding of selected properties. This would be implemented on Stapletons Road at 2 properties. Drainage through the bunding fitted with flap valves would be required.

Street overpumping – This proposal would be applicable up to a 5 year event and is predicated on the basis that ponded water within the Flockton cluster can be pumped to the Mairehau drain on Aylesford Street. As the Mairehau drain overtops beyond a 5 year ARI event the pumps would need to be deactivated at this time.

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Backflow protection on local drainage network – local drains at Westminster Park backflow from the main stormwater pipe when levels in the downstream channel (Mairehau Drain) rise and the outlet becomes submerged.

Property Level Solutions

House raising of selected properties with flood risk not addressed through catchment solutions. It should be noted that there may be opportunities to align this solution with the permanent options.

House tanking of selected properties with flood risk not addressed through catchment solutions.

Consenting requirements Resource Consents will be required for earthworks, diversion of water, to use land for a pumpstation, works in waterways, damming and diversion, and discharge of dewatering water.

Emergency response plan A monitoring point with notification alarms currently exists in Dudley Creek downstream of the Aylesford Street culvert.

The lowest house in the Flockton area is 300mm above ground level in a high flood risk area of the catchment.

Drain levels are known to rise in rainfall intensities exceed 2 mm/hr. An intensity of 4 mm/hr will cause a rapid rise in levels and the drain will likely spill after 3-4 hours of rainfall this intensity.

Wet antecedent conditions will increase the risk of flooding.

When Aylesford Drain spills the area will flood rapidly and residents welfare should become primary concern.

Any pumping to Aylesford Drain must cease when the drain begins to spill.

Pilot study recommendations

House wrap at two Carrick Street properties

Pumping from Mairehau Drain to the Dudley Creek Diversion

A6.15. Recommendations for future studies The waterways of Dudley Creek are under capacity and the required flow to remediate the area permanently is higher than that being suggested in this scheme. The present scheme aims to behead the catchment with 1 cumec of conveyance to the Dudley Diversion. Prior studies by Jacobs SKM indicate this to be successful in reducing the extent and depth of flooding events up to a 10 year ARI. It is also suggested that adding a pumpstation will reduce the duration of flooding by allowing a faster drain of the area. Further modelling is required to evaluate the pumping requirements to reduce flood levels.

The lower Dudley Creek hydraulic capacity enhancement works will provide increased capacity than is presently available. Whilst the extent of these works are not at the scale required in the long-term options it should help to reduce flood risk. Further installation of a culvert booster pump will augment capacity of the channel by increasing the velocity of the water and subsequently the flow rate. Further modelling is suggested to evaluate the success of this form of pumping.

As Dudley Creek Diversion is the proposed receiving drainage system, modelling is required to evaluate its current utilisation and available capacity. If further capacity is required, Cranford Basin and the optimisation of PS219 is a likely solution to provide this.

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A7 Upper Heathcote A7.1. Background

Location The Upper Heathcote study area looks at flooding on both sides of the Heathcote River from Halswell Road to Cashmere View Street.

Existing studies Council’s 50 year Average Return Interval (ARI) flood maps predict flooding across approximately 12 sections.

Flooding history Residents report a history of flooding that predates the earthquakes. However there is a perceived increase in frequency following the earthquakes, with flooding across sections and into Weir Court on approximately six occasions.

Land damage mapping EQC land damage maps do not extend east to this area. Comparison of July 2003 LiDAR with July 2011 LiDAR shows very little land damage in this area. The subsidence observed is generally less than 100 mm.

A7.2. Customer service reports Customer service reports are consistent with the observations noted in the Flooding History section above.

A7.3. Key drainage infrastructure Weir Place drainage network consists of two single sumps that outfall to the Heathcote River via a 300 mm internal diameter pipe fitted with an inline check valve.

Greenpark Street drainage consists of two 225 mm and one 300 mm internal diameter outfalls.

A7.4. SCIRT projects The existing 225 mm internal diameter pipe was replaced with a 300 mm pipe outfall under a SCIRT work package to the Heathcote River.

A7.5. Existing flood protection infrastructure There is no existing flood protection infrastructure in this area.


Depth The depth of flooding increases as the sections slope towards the river. The depth near the river would have been in excess of 1 m and up to 500 mm around some houses and over the carriageway.

Cause Flooding is a combination of fluvial and pluvial. The river overtopped the banks and flowed overland through some properties on Weir Place into the road and also in and around adjoining properties.

28 properties along Greenpark Street had water observed within the section either from water overflowing or backflowing from the river or the street catchment.

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Figure 14 Upper Heathcote - Location Plan

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General description of damage Flooding caused damage to houses, garages and gardens. Several houses were surrounded by floodwaters making access/escape dangerous or impossible. Weir Place was completely covered in floodwater and not safely passable.

Number of affected floors Two properties were known to have flooded above floor level and two or three other properties came close to being inundated.

Local effects The primary area of impact was a cluster of three houses adjacent to the river and one Weir Place property, along with one garage in Smartlea Street. A further nine garages were reported to have had garages flooded above floor level.

A7.7. Other 2014 floods During the other 2014 flood events the Heathcote River has risen onto sections and carriageway, limiting access to Weir Place but has not severely limiting property access along Greenpark Street.


Social impact The community has been impacted by property damage, temporary access difficulties and ongoing concerns about the potential impact of future flood events.

Community feedback Individual residents were consulted in the week following March 5 and again during taskforce investigations were conducted to determine the extent and nature of flooding in this area.

A7.9. Field studies Individual residents were consulted following the March flooding event and site visits and further investigations were undertaken during the taskforce work.


Land level changes The CGD vertical and horizontal ground movement database indicates the area has generally settled -0.1m to -0.3 m during earthquake events.

Lateral spread Site observations and conversations with property owners indicate that some areas have experience lateral spread. It was also commented by residents that the river bed is now lower.

Liquefaction Minor to moderate liquefaction occurred. CERA has therefore indicated the area is situated within the Green Zone.

Land in the CERA Green Zone has been divided into three technical categories (TC). These categories describe how the land is expected to perform in future earthquakes. The area has both TC2 and TC3 land which indicating minor to moderate and moderate to significant land damage from liquefaction is possible in future significant earthquakes, respectively.

Ground cracking The CGD ground cracking database indicates that there was no obvious ground cracking.

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Waterway observed changes Localised bed heave is likely to have occurred. Comments for residents suggest in some areas the bed of the river is now lower.

Infrastructure damage No infrastructure damage was observed.

A7.11. Options

Constraints The proposed area solutions are based on general flood conveyance principles and have not been modelled. Any estimated reductions in flood levels are based on engineering judgement only.

Opportunities Further analysis of this reach through collection of detailed survey information and creation of a hydraulic model would improve confidence in the flood level reduction estimate. It may also allow for the benefit of individual aspects of the area wide solution to be assessed and the most cost-effective options selectively implemented.


Flood defence measures Flood defence measures such as a bund would need to be implemented across three properties in Weir Place adjacent to the river on the upper bank level.

Mobilisation of a pump to the head of Weir Place to pump water out to the river.

Local area schemes The general level of flood risk along this reach of the Heathcote River waterway could be reduced by increasing the capacity of the waterway. This could be achieved by removing some of the constraints within the channel, on the banks and across the floodplain.

There are several sites along the river where retaining walls that have partially failed and banks have slumped which could be rectified to increase channel capacity.

There are a large number of fences that extend to the stream bank. Removal or reconfiguration of these fences would be a very cost effective way to improve flood capacity. Where the fences cannot be relocated it may be possible to replace them with other more open fence types that allow for some flood conveyance, or types that can be laid flat during a flood.

In some cases the vegetation that has been planted along the banks or within the river’s floodplain is not appropriate. Selective replacement of some vegetation would improve the stream capacity without compromising environmental objectives.

The sumps and outfalls from Greenpark Street should be inspected and consideration be given to the installation of inline check valves to the three outfalls to prevent backflow from the river.

Social impacts of measures Implementation of the proposed bank remediation works and setback enforcement will impact on private yards and landscaping that has been done within the floodplain area.


Preferred options and cost Implementation of the risk reduction and maintenance measures is likely to result in a wider community benefit at a lesser cost. In summary the preferred option includes:

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Installation of bunding across critical properties in Weir Place and with further investigation individual properties along Greenpark Street.

Reconfiguration of fences within the floodplain; and replacement of some vegetation on the channel banks within the floodplain.

Timeframe These works could be implemented within one month.

Consenting requirements No consent is likely to be required to complete these works.

Pilot study recommendations This area is quite compact and not suited a pilot study.

A7.14. Recommendations for future studies This section of the Heathcote River would benefit hydraulically (i.e. reduced flood levels) and environmentally from the proposed works. The recommendation to complete the area wide works is therefore valid without further study.

However, if flooding persists following completion of the temporary works, a study may be required to identify further opportunities to increase capacity through increasing culvert sizes, modifying stream cross-sections or removing floodplain obstructions

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A8 Lower Heathcote: Riverside properties A8.1. Background

Location plan The Lower Heathcote catchment has been defined as the catchment area contributing to the length of river between Colombo Street and Radley Street. Refer to Figure 4 for a location plan.

Existing studies

‘Heathcote River Catchment Investigation Stage 2’ Christchurch Drainage Board, 1985

‘Heathcote River Floodplain Management Strategy’, CCC, ECAN, 1998

‘Storm Report – 11-13 October 2000’, Ken Couling, CCC, 2000

‘Mid Heathcote River Masterplan’,

Additional Council studies on the Woolston Cut and Heathcote modelling by CCC, NIWA and ECAN. Royds, T.B. Oliver, 2009

Flooding history The Heathcote River Flood Plain Management Strategy (1998) highlights the historic flooding of the Heathcote River. Summarised below:

Recorded flooding of 73 houses before 1998 (Above floor level); Floodwaters have reached heights above floor level on at least four different occasions (1968,1975,1977 and 1980) up to 1998; The most affected areas have been noted as Waimea Terrace, Richardson Terrace, Aynsley Terrace, Claredon Terrace, Riverlaw Terrace and Eastern Terrace.

Physical works previously undertaken include:

Widening and deepening of the river channel and dragline operations (from 1920s); Construction of the Woolston Cut (1986) to provide a more efficient flow of water and the installation Woolston Tidal Barrage (1994); Construction of the Wigram East Retention Basin; Raising of 19 house from 1983 to 1998.

Although flooding had previously been experienced in the catchment, a number of residents in the area indicated that there had been no previous property flooding on their properties prior to the earthquakes. This potentially indicates a change in vulnerability or increase in the risk of flooding of the area due to the earthquake sequence. It is fairly common for riverside properties in this catchment to experience shallow flooding or have restricted access during large rainstorms.

There have been three severe rain events from March 2014 to April 2014 which have resulted in the recent floor level flooding in the area, along with widespread road, property and ancillary buildings flooding.

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Figure 15 Lower Heathcote riverside properties - location plan

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Land damage mapping LiDAR mapping from CERA Project Orbit indicates that there has been both land uplift and settlement over the Lower Heathcote Area.

The Lower Section of the Heathcote River is tidally influenced and is reported to have risen by up to 400mm, while the upstream portion of the river has settled by 100-300mm. This has resulted in a flatter hydraulic grade reducing the hydraulic capacity of the River and leading to increased bed levels. The River is beginning to re-establish lower inverts in the downstream section.

River stopbanks in the region of Radley to Garlands are lower in places than the 10.9m level constructed in 1986. In isolated areas, e.g. in vicinity of railway, there is an area that is now at 10.40m. This area was historically lower than 10.9 because of difficulties installing a river barrier in the confined space but unlikely as low as 10.40m

Recent rock movement, slope cracking and slumping in the Port Hills has resulted in additional silt loading on the River and consequentially increased bed levels. The area also had extensive liquefaction and pipe damage (wastewater and stormwater) which has resulted in additional silt and sand loading.

Residential properties on the banks of the Heathcote River have suffered some lateral spreading and settling within the property boundary, this settlement increases the susceptibility to flooding.

A8.2. Customer service reports Flood reports from the 3 March 2014 event indicate a range of concerns in the catchment.

Priority concerns indicate that siltation and detritus in the sumps are a constant issue. Requests to clean the sumps prior to rain events are a common occurrence. Residents highlight access to properties as a major concern. Residents have requested sandbags to protect property. Concerns over wastewater overflows have also been raised due to the flooding.

A8.3. Key drainage infrastructure The Lower Heathcote Catchment consists of approximately 100 km2 of urban and semi urban drainage. The River is the terminal outfall for major streams and drains in the South of the city. The river is tidal within much of the urban catchment. Since the construction of the Woolston Cut and the tidal control (RL10.8m) stopbanks the Heathcote River tidal and flood flows were mostly contained within the river banks.

The local stormwater drainage network within the areas adjacent to the river consists of both road drainage and large catchment drainage such as Tennysons Drain. SCIRT assessment of the drainage infrastructure indicated that there was some damage to outfall pipes. This damage is currently being assessed and some of the outfalls are highlighted for repair.

Drainage outfalls diameters range from DN150 to above DN2100. Some outfalls in the lower reaches have flapgates to prevent river and tidal backflow (many SW outlets between Radley and Opawa have a double flapgate system). These flapgates are in need of maintenance and residents indicate that backflow is the primary cause of flooding in some areas of the downstream section of the River, indicating that the valves are not operating as designed.

A8.4. SCIRT projects Due to the size of the Lower Heathcote catchment area there are numerous SCIRT project areas covering the catchment. Condition assessment reports indicate more damage to the stormwater network closer to the River and on older drainage systems. Some wastewater repairs have been undertaken. However stormwater drainage repair is still in construction or planned within the next three years, these are mostly repairs and relays of existing pipe capacities.

A8.5. Existing flood protection infrastructure Lower Heathcote properties were historically protected by local sumps and gravity piping to the Heathcote River with double flapgates installed in some areas to prevent river and tidal backflow. The Woolston Cut was installed to help the drainage capacity of the River in the downstream reaches. There are no large scale stopbanks installed on the Heathcote River, only some lower ‘tidal’ banks

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protecting Richardson and Claredon Terrace during more frequent high tides. Survey undertaken between Garlands Road and Radley Street in May 2014 indicates that there has been some localised reduction in tide bank heights; the design level for this bank (1986) was set at 10.9 mRL, however the survey indicates that approximately 20% of the banks have lower levels than designed, with some areas now having a level as low as 10.4 m RL.


Depth Flooding depths varied along the length of the River; depths were anecdotally recorded from 300 -1000mm on properties. The flood depth on the road corridor was too deep for vehicle passage and resulted in isolation of residents.

Type River flooding, with some due to local rainfall intensity (Mackenzie Street). Tidal influence contributes to increased flooding. Some localised stormwater flooding due to inability of stormwater to enter the river.

Cause The Heathcote River has sustained earthquake damage with lateral spreading, siltation and bed heave reducing the capacity of the river system. This has contributed to an increase in property level flooding within the catchment. Overtopping of the river occurs at RL10.9m (10.8m MH lids causing lower flooding points) with overtopping of the tidal banks especially where land settlement has occurred. Some properties may have settled due to earthquakes and could be prone to more frequent flooding.

Residents reported that flooding was initially coming from the street sumps indicating that the flap gates are no longer operating correctly. Visual inspection of the flap gates indicates that they no longer close fully and allow river backflow into the drainage system.

Flooding was indicated in the Talbot/Judge Street area. There are no reported above ground floor occurrences in this area. However the flooding extents highlight that this area is likely to have had on property flooding with some access issues for elderly residents. The flooding in this area has not been assessed further due to the absence of above floor flooding data.

Flooding at the Colombo/Somerfield intersection will be covered off as a separate catchment report.

Bells Creek Catchment will be covered off as a separate report. The report indicates that maintenance, capacity and hydraulic loss of the system are a major cause of flooding in combination with the rainfall intensities.

General description of damage Flooding ‘Above floor level’ has resulted in damage to houses and contents. On property damage includes flooding of cars and garages. Restricted access to property increases the feeling of vulnerability among residents.

Number of affected floors Total known identified ‘Above Floor Level’ flooding for the Lower Heathcote Catchment Study is 19 properties. LiDAR contour maps predict that unrecorded flooding exists in other streets in these areas.

Local effects River flooding was seen along the lower reaches of the Heathcote River and the streets adjacent to the river banks. Localised rainfall intensity flooding was seen in the intersection of Colombo and Somerfield Streets.

A8.7. Other 2014 floods The streets adjacent to the Heathcote River have flooded three times in 2014 to 30 April. The Colombo/Somerfield Streets area has flooded only once in March 2014.

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Social impact River flooding has a severe social impact with the restriction of access to and from houses on the riverbanks. Residents highlighted the main concern was emergency, food and water access.

Public health issues arise where wastewater has been seen coming out of gully traps and manholes. This was highlighted in the Claredon Terrace area where residents comments perceived this is more prevalent post SCIRT wastewater repairs. It should be noted that most floodwaters have some contaminants.

Community feedback Residents indicated access to and from properties as a main concern. Flooding and velocity of river flows concerned some residents. Residents highlighted lack of maintenance and visibility of Council contractors during and post flooding events as a key issue. Access to food and water when property access was limited was a concern. A Beckenham neighbourhood group is collecting flood data to present to the local community board. To date, this highlights frequent on-property flooding in the Waimea Terrace area.

A8.9. Field studies Investigations were conducted following the March 2014 event and again to inform this May report.


Land level changes Between Barrington Street and Opawa Road land changes post earthquake show a change on average of -0.1 to -0.2 m in ground level. Some localised areas have experienced changes of -0.4 m and -0.5 m.

From Opawa Road to the estuary land levels have generally changed by +0.2 to +0.3 m.

The area at the end of Tennyson Street and the rear of Somerfield Street has experience settlement of -0.2 to -0.30m. In addition local information indicates a spring appeared in a garage on Somerfield Street.

SCIRT GIS Land drainage to road maps indicate that some properties along the banks of the Heathcote have slumped and therefore are more susceptible to flooding in peak events. There are also isolated areas of differential settlement in the Tennyson area.

Lateral spread Moderate to major lateral spread has been exhibited in around Hanson Park, Clarendon Terrace and downstream of Radley Street to Woolston Cut. An area downstream of Woolston Cut on the northern bank to the estuary has also suffered lateral spread.

Liquefaction Liquefaction occurred at locations of the Lower Heathcote area which has contributed to significant volumes of material entering the Heathcote River.

CERA has included the area is within the Green Zone. Land in the CERA Green Zone has been divided into three technical categories (TC). These categories describe how the land is expected to perform in future earthquakes. Within the Lower Heathcote land has been classified as TC2 and TC3 indicating minor to moderate and moderate to significant land damage from liquefaction is possible in future significant earthquakes, respectively. This will increase possible risk to future siltation with the Heathcote River.

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Ground cracking The CGD ground cracking database indicates minor cracking around the Lower Heathcote.

Waterway observed changes Settlement has resulted in reduced hydraulic capacity of drainage network around the Tennyson Street flooding area. The lower reaches of the Heathcote suffered raised bed level in the region of 400 mm and therefore has reduced the hydraulic gradient of the river resulting in a loss of capacity.

Bed heave and increase sediment deposition has occurred between Aynsley Terrace and the Heathcote Tow Path. It has been estimated this change in sediment is 130,000m3 of deposited silt post earthquake. Localised stop bank damage was observed at Clarendon Terrace.

There are some informal ‘tidebanks’ on the Lower Heathcote which may have settled between 100-300mm. The initial tidebanks were designed to generally contain a 50 year event of 10.8 -10.9m. Settlement may mean that tidebanks are now lower than design. A May 14 survey between Radley and Garlands identifies about 20% of the tide banks are up to ~200mm below 10.90m. SCIRT works and other contractors have caused isolated low points in the tidebanks due to construction activities. Flapgates may have rotated at the outlets and no longer close. Woolston Cut has risen up to 400mm and therefore has reduced its hydraulic capacity.

Infrastructure damage SCIRT damage assessment maps indicate that there was considerable damage to pipes adjacent to the river. This includes damage to outfalls and flap gates. SCIRT works to be undertaken include repair of some damaged stormwater pipes in the area. Residents highlighted issues with sumps blocked with silt and other detritus post-earthquake on a regular basis.

A8.11. Key photos

Figure 16 Lower Heathcote riverside properties - floor level flooding at Clarendon Terrace

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Figure 17 Lower Heathcote riverside properties - flooding on Riverlaw Terrace

Figure 18 Lower Heathcote riverside properties - extensive floor level flooding on Riverlaw Terrace.

A8.12. Options

Key assumptions and gap analysis Options presented fall into three categories; individual property solutions (for properties with the highest flood risk – approximately 13 properties), ‘interim option’ to reduce the short term flood risk for up to 168 properties in the area(not priced) and ‘area’ solutions (for groups of properties at risk – approximately 780 properties).

Key assumptions are that the drainage network will operate efficiently except where operation and maintenance are identified as an issue.

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The options recommended will be acceptable to the local community and stakeholder engagement will be held to mitigate any concerns on the options.

Local drainage and flood protection for smaller storm events could be improved by repairs or replacement of damaged back flow protection devices in the area with more modern and reliable inline checkvalves.

Further flood data will be received as a result of local surveys and media coverage.

The area options presented have been made on a high level basis. Further assessment and/or modeling, along with survey, is required for area schemes identified to ensure schemes do not exacerbate flooding elsewhere in the catchment.

Constraints Placement of stopbanks may require placement around trees, some landscaping on the river bank may be removed.

On street parking and/or two way traffic may be temporarily restricted along the river bank.

Drivable bunding/speed humps may cause some noise or vibration,

Bunding may be required on private property.

Opportunities There are opportunities to uses dredging bags as temporary or permanent stopbanks on the Heathcote river. See pilot recommendations for further opportunities.


Flood defence measures Area solutions, in the form of temporary bunding along the river bank could be used to reduce the risk of flooding to groups of the most vulnerable houses from river and local stormwater flooding. This needs to be combined with stormwater and wastewater backflow prevention and pumping to remove stormwater which will collect behind the bunds.

Temporary ‘interim’ solutions using relocatable bunding (1m3 ‘unit’ bags) and pumps may reduce the

risk in the short term for some~168 properties affected by underfloor flooding/low ground levels adjacent to high tide areas.

Local area schemes Residents have reported fast moving vehicles on Opawa Road sending water down Clarendon Terrace and into houses. Traffic management would assist in mitigating this particular cause of flooding. Bow waves can raise the flood risk to floors by 300-500 mm.

Removal of earthquake derived sediments from the river bed via dredging has been proposed as a flood reduction measure. This will help address the reduction in hydraulic capacity of the Heathcote River. However this cannot be quantified at this stage and is being considered as a pilot study.

Maintenance measures For all options flap gates must be regularly inspected to ensure these are not missing or malfunctioning and contributing to flooding. Switching these gates for inline check valves would reduce this potential issue. It has been identified that a number of outfalls in the city are not included as part of the current stormwater asset maintenance programme; this will be addressed by CCC and CityCare as a priority.

Social impacts of measures Raising houses will provide certainty to residents around flow mitigation however it will not resolve issues of disrupted property access and water beneath homes. A combination of bunding and

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pumping during an event should mitigate road and property flooding but the house floor protection outcome is less certain and more maintenance dependant than simply raising the house.

A8.14. Optioneering Flooding in the Lower Heathcote catchment has been widespread – the river has a large floodplain with many houses and roads in the potentially impacted area. Water depths of up to 1m in places have been reported. Area solutions assessed involve localised stopbanking / bunding and pumping, however these solutions will not prevent flooding at all locations in the catchment during a major tidal/river flood event, and analysis would be required to ensure these solutions do not exacerbate flooding elsewhere in the catchment.

Individual house protection solutions for the most vulnerable houses would be the only option that could provide certainty in terms of protection from major flood events. Currently, it is estimated that this would affect 13 homeowners. Notwithstanding house protection, it would be expected that ancillary structures, properties and roads would still be inundated.

The construction of either permanent or temporary stopbanks within the floodplain may have adverse effects. Any reduction in the cross section area of the floodplain will most likely raise flood levels either upstream, if the capacity of the channel is reduced, or downstream if peak river flood flows coincide with a peak tide.

A8.15. Recommendations-

Preferred options and cost

Individual property options

Individual property options that were considered for the Lower Heathcote Area include: House Purchasing, House Raising; House Wrapping and House bunding. This is recommended if protection of habitable floors is prioritised above access and property flood issues.

Interim options

It is proposed that a temporary solution for high-frequency under floor and property access flooding could involve temporary bunding (using large and small sandbags) and re-locateable pumps. This could reduce the flood risk for up to 168 properties in the area while longer term solutions are investigated. Discussions with CityCare indicate that temporary pumps and sandbag / bunding equipment could be available within a two-week period. (refer to implementation matrix), and emergency response plan below.

Raising the tidebanks upto 1986 design levels of at least 10.9m in the region of Radley St upstream to Garlands and further upstream subject to survey would improve a level of service to adjacent residents and carriageways. Special river containment design may be needed under railway and motorway to confine10.9m high river flows

Local area options

Due to the nature of the flooding in the catchment it is recommended that the existing tidebank heights are increased beyond the current design level to incorporate flood level protection, sea level rise and climate change considerations (potentially by up to 500 mm). This would affect the area between Radley Street and Garlands Road. Upstream of Garlands needs further survey work to further assess issues/options.

A maintenance issue has been highlighted during this study that maintenance contracts may not yet be aligned with the increased numbers of outfalls found with recent survey work, and recommendations include the inspection and, regular maintenance of flapgate and if appropriate installation of more effective less maintenance intensive inline check valves, particularly in low lying tidal areas such as Clarendon Tce and Richardson Tce. An undetermined number of valves may need work

Risk Reduction Strategies that are recommended for the wider area involve stopbank protection for groups of houses, with event-based pumping solutions. These sub-catchment options could be

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cheaper than individually protecting the most vulnerable houses and will also protect additional adjacent properties in the cluster which experience below floor level flooding. However quantification of flood level reduction has not been undertaken, and impacts on other parts of the catchment would need to be assessed to reduce community risk exposure.

The stopbanks could ‘wrap around’ groups of properties and in some cases into private property to create fully bunded areas with provision for appropriately sized pumps installed.

These options will generally also include:

Bunding across roads (e.g. creating asphalt speed humps height dependant on location, maximum 0.5m)

Temporary pumping of water ponded behind bunds

Assessment should be made to ensure the areas outside the bunding are able to drain efficiently. The direction of flow and capacity in this area needs to be investigated to ensure any impacts are mitigated.

Consideration should be given to the use of river dredging bags to both lower the river bed and provide temporary stopbanks using Ecobags, however more investigation would be required prior to implementing this solution.

Recommended Actions

A: Offer individual dwelling protection ( ~13 vulnerability 1 properties)

B: Offer support &/or technical dwelling specific design for measures to speed up recovery from

underfloor flooding or/& design & install measures to reduce impact of underfloor flooding (~158

Vulnerability 2 properties)

C: Pilot Study/Investigate/Design/Model activity

Also recommend undertaking the following concurrent actions:

(Note this may not significantly immediately reduce community flood risk)

1) Dredge:

Pilot study dredging in region of Woolston Cut/ Radley St will gain more accurate estimate of potential cost of ~$40 mill if decide extensive length of

river dredging has significant benefit

will gain better knowledge of sediment migration to help determine if a regularly emptied sediment trap 'hole' in river offers long term estuary /river health & flood benefits

will improve local expertise in regards to construction dredging techniques and equipment needed

will help determine if practical /cost effective to use dredgings to build temp stopbanks out of eco bags or 1x1m re-locatable bags

2) Reinstate 10.9 tidebank upstream of Radley

-with temporary sand bags

(Generally will only reduce nuisance property/street flooding. Higher permanent stopbank works may

be warranted depending on modelling outcome. See Modelling section 6A below)

3) Back flow prevention improvement: (Generally will only reduce nuisance property/street flooding)

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Locate, document, check/ rationalise, repair upgrade faulty flapgates,

Ensure maintenance process & contract aligned with back flow preventors actually in the field

Determine most effective inspection method. Boat inspection at low tide?

Identify post-quake low lying land and critical valves (document location & marker post? pipe marker?)

4) Design For Vulnerability 2 where in a cluster, Fast track design option for tidy long term improved

protection incorporating local ' CCC Strategy vision' . Refer to Point 5.

For the Area schemes adjacent to the river, fast track Design Option for a tidy long term improved protection incorporating local ' CCC Strategy vision' .

Don't proceed to construction unless modelling and proposed extent of works confirms no adverse affects on the community elsewhere within the catchment.

5) Confirm concise long term drainage & multi value community ' CCC Strategy vision'

Confirm/Anticipate this option will reduce road to one lane, no street parking, have a combined stopbank/ walkway/cycleway/landscaping with a reduced number of point sw discharges into river, some outfalls discharging to a swale on upstream side, standby pumping support for more extreme events in vulnerable areas)

Sketch typical solution cross-section through river, road and property for low properties.

6A) Modelling & Survey. ( Significant risk of stopbanks shifting problem)

Undertake Computor Modelling of the proposed mitigation. Variable factors to include: Obtain extend survey information for various options to model,

Sea level rise allowance

Various AEPs etc

10.9 tide banks only,

11.2 isolated stopbanks only around-158 Vulnerability 2 props,(Interim Option)

11.2 isolated stopbanks for ~780 property ~nine 'area wide' schemes.

Variety of dredge extents

Wider river profile at restrictions

Heathcote master-plan aspirations

Water quality improvement options incorporating IWI Management Plan Mahaanui IMP aspirations

6B) Explore if any merit in modelling 12.4m banks

Bridge jet pumps to speed water or temp barrier & over pump at several bridges few hrs before/during storm to increase river velocity capacity

Over pumping river at Woolston Cut several hours before rain event

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Barrier gates/pumping at Ferrymead bridge

7) Modify SCIRT mandate requirement to identify SW infrastructure design capacity improvement

options. Give Land Drainage Asset Rep/ SWTech advisor some delegated authority to include obvious

cost effective drainage improvements where new SW works adjacent to Heathcote river proposed.

Timeframe It is estimated that a lead time of approximately two weeks would be required for maintenance tasks (check valves) and pumping / bunding preparedness for interim options. Implementation time following this would be dependent on availability of resources and priorities.

Consenting requirements Consenting would normally be required for anyworks within the riverbed from a District Plan and Regional Plan. Stakeholder engagement should be considered with residents and Tangata Whenua in the area. All consent issues would need to be assessed. Some emergency works do not require consents.

Emergency response plan Pump(s) must be available during an event to remove stormwater ponding behind the bunds. Sumps should also be checked for blockages prior to an event as part of maintenance activities. Residents have reported vehicles on Opawa road sending water down Clarendon Terrace, therefore emergency traffic management also need to be considered. Bow wave impact reduction measures need to be trialled e.g. castellated sand bag layout by busy traffic route.

A8.16. Pilot study recommendations All options require further investigation for required length and profile prior to construction.

The following pilot study is currently being investigated for implementation:

Pilot study dredging just upstream of Woolston cut stockpipe/drain then to waste.

Pilot study dredging just upstream of Woolston cut to fill Eco bags which could be used as a temporary stopbank. The ecobags can then be landscaped at a later date to blend into the environment.

The following pilot studies could be undertaken in addition to the above.

Dredge ~5km of River from Aynsley to Woolston Cut.

Pilot study dredging at Woolston Cut

Pilot study closing gates at Woolston Cut (&loop outlet), trapping high tide water upstream of gates and releasing rapidly 2 hours before low tide to shift sediment near gates downstream towards estuary.

Pilot Study River bank energy adding pumps/lazy river principle perhaps first trialling with several jet boats tied together in strategic spot identified by modelling to shift sediment downstream and to increase flow velocity.

Pilot Study, increase local river velocity by choking through a floatable/sinkable ‘V’ shaped flow focus structure spanning the river.

Dredge by daisy chain in river pumping between pumps on caissons floated/sunk in river bed to an outlet downstream retreating/abandoning lifting caissons as move downstream

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A8.17. Recommendations for future studies It is recommended that a gap analysis is undertaken for assessing flood data over the past decade for trends in street and property flooding.

It is recommend that a survey of and hydraulic modelling of the Heathcote River bed and banks is undertaken to quantify the reduction in hydraulic capacity as a result of the earthquakes.

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A9 Lower Heathcote: Bells Creek catchment A9.1. Background The Bells Creek sub catchment of the Heathcoate River may have suffered differential land movement and localised ground settlement (through liquefaction) as a result of the Canterbury Earthquake sequence. This may have reduced grades across the sub-catchments drainage network increasing flood risk.

Location This area is located to the north east of the Heathcote River and is bound by Randolph Street to the east, Ferry Road to the south, Bordesley Street to the west and Harrow street to the north.

Existing studies Project soon to be initiated as part of the Land Drainage Recovery Plan.

Flooding history Feedback from council staff is that the Bells Creek has not been a problem area in the past, though flood modelling has highlighted the area is at risk. This may be due to its low lying nature relative to the surrounding land, being similar is level as the banks of the Heathcote (at least post-quake).

Land damage mapping EQC have determined this catchment is subject to increased vulnerability. Pre and post change in catchment level indicates settlement may have been differential resulting in a reduction in catchment slope. EQC identified catchment as having increased vulnerability.

A9.2. Customer service reports No properties were reported being flooded above floor level, with 21 reported as having flooding to their foundation and 113 with flooding on their sections. Property inspections during 4/5 May 2014 reported one resident who claimed they experienced above floor level flooding on three occurrences. In addition the resident identified an additional property that may have experienced above floor level flooding, although no evidence of floor level flooding was found. This is an area of low confidence around habitable floor flooding and frequency.

A9.3. Key drainage infrastructure The area is drained via a mix of shallow drains and pipework, all of which discharges to the Bells Creek, which in turn flows into the Lower Heathcote River.

A9.4. SCIRT projects Projects 10945, 11160 & 11158 covering SW, WW, WS, RD. Also works underway at PS11 (Bass Street).

A9.5. Existing flood protection infrastructure The area relies on the existing Council drainage infrastructure for flood protection.


Depth No properties reported being flooded above floor level, with 21 reported as having flooding to their foundation and 113 with flooding on their sections. A resident of Smith street identified two properties which may have experienced above floor level flooding. The area experienced widespread ponding, photographs taken during flooding shows depths above kerb and across footpaths and into adjacent sections indicating flood depths of 100 – 300mm depth.

This is an area of low confidence around habitable floor flooding and frequency.

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Type Intensity and rainfall depth flooding is due to limited drainage, so higher depth events result in larger flood volumes and greater depths.

Cause The primary cause of flooding in Wildberry Street, Bordesly Street and Ryan Street appears to be due to street sumps being completely blocked with sediment.

Flooding in Randolph Street is in a low area along the Bells Creek and the sump there was blocked on inspection.

Flooding in Smith Street appears to have been caused by inadequate drain capacity (Tamai No.2 Drain and Staffa Invert) due to sediment build-up. The one property of Smith Street that reported repeated habitable floor flooding appears to be flooding due to a surcharging grate that is flush with the floor level of their property. The drainage infrastructure in the property is unclear. It is likely to be private drainage and the internal flooding is due to the property floor levels being flush with the drain grate. If a flow path was provided the issue would probably be resolved.

Water from the drain also floods out into the back of the gardens. The upper drain was reported as being silted up whilst the lower section was ok. This may indicate that the drain has limited grade.

General description of damage Flooding in Ryan Street, Bordesley Street and Wildberry Street appears to have been limited to ponding of flood waters in the road corridor and sections but without entering foundations or exceeding habitable floor level.

Randolph Street was similar in nature, but did reach the foundations of one residential property and may have flooded it internally. This property is at a low point.

Smith Street appears to have been the worst affected area, with one resident reporting being flooded internally three times. Inspection of adjacent properties did not suggest they had been flooded internally; adjacent properties may have had flooding to foundations. A photograph of flooding at this location confirmed at least one other property had below floor level flooding. The one property that reported internal flooding is suspected as being due to a private drainage issue combined with no flow path away from the building and no building freeboard.

Number of affected floors 1 house has reported as being flooded above their floor level. 21 properties are estimated as being flooded to their foundations whilst an estimated 113 properties had their sections flooded.

Local effects All the flooding appears to be due to localised issues that may be resolved through maintenance of blocked sumps and clearing of silt from drains. The single property that meets V1 appears to be due to a private drain, no flow path and no building freeboard.

A9.7. Other 2014 floods One resident on Smith Street reporting having been flooded three times since the earthquakes, but it is unclear if they all occurred in 2014.

A9.8. Community response and drivers Social impact appears to be limited to reduced access (ponding in the road corridor) and section flooding. Only one possibly two properties are known to have flooded above their floor level, whilst a number (estimated 21 properties) may have had water under their floor level. Only one property is known to have flooded several times.

A9.9. Field studies Field visits were conducted on 3 and 4 May 2014.

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Land level changes Between Barrington Street and Opawa Road land changes post-earthquake show a change on average of -0.1 to -0.2 m in ground level. Some localised areas have experienced changes of -0.4 m and -0.5 m.

From Opawa Road to the estuary land levels have generally changed by +0.2 to +0.3 m.

The area at the end of Tennyson Street and the rear of Somerfield Street has experience settlement of -0.2 to -0.30m. In addition local information indicates a spring appeared in the garage at the property at 134 Somerfield Street.

Lateral spread Moderate to major lateral spread has been exhibited in around Hanson Park, Clarendon Terrace and downstream of Radley Street to Woolston Cut. An area downstream of Woolston Cut on the northern bank to the estuary has also suffered lateral spread.

Liquefaction Liquefaction occurred at locations of the Lower Heathcote area which has contributed to significant volumes of material entering the Heathcote River.

CERA has therefore indicated the area is within the Green Zone. Land in the CERA Green Zone has been divided into three technical categories (TC). These categories describe how the land is expected to perform in future earthquakes. Within the Lower Heathcote land has been classified as TC2 and TC3 indicating minor to moderate and moderate to significant land damage from liquefaction is possible in future significant earthquakes, respectively. This will increase possible risk to future siltation with the Heathcote River.


Waterway observed changes Settlement has resulted in reduced hydraulic capacity of drainage network around the Tennyson Street flooding area. Bed heave and increase sediment deposition has occurred between Aynsley Terrace and the Heathcote Tow Path. It has been estimated this change in sediment is 130,000m3 of deposited silt post-earthquake. Localised stop bank damage was observed at Clarendon Terrace

Infrastructure damage SCIRT condition assessment maps indicate non critical damage to the drainage network in the area.

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A9.11. Key photos

Figure 19 Photo of private grate that surcharges and enters nearby doorway due to lack of flow path

away from building and freeboard above the adjacent paving.

Figure 20 Upper Bells Creek – scope to increase channel capacity

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Figure 21 Drain off of Okeover Street – silt in invert of drain

Figure 22 Blocked sump at PS11 – blockage possibly due to recent construction activity

A9.12. Options Local drainage and flood protection could be improved by maintenance of existing sumps, cleaning of stormwater pipes and cleaning of drains. This is constrained by limited access to the drains due to property boundaries and encroachment.

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Single residence measures The property that reported as having flooded several times (Smith Street) may be experiencing flooding due to issues with their private drainage. Council should investigate and notify the resident if it is a private issue. The solution would be to either sandbag the doorway that floods (low confidence solution due to flat grades) and pump water out of the existing drainage grate, or create a flow path away from the house by re-levelling an existing area of concrete paving adjacent to the house (more expensive solution but requires no flood response in future).

Local area schemes Sand bagging and over-pumping if required, following maintenance recommended below. Bells Creek will be looked at in detail soon as part of the Land Drainage Recovery Plan which will identify any non-maintenance related issues around hydraulic capacity.


Undertake inspections and maintenance of sumps and pipework in the area.

Clear silt from drains to try and restore capacity.

Monitor area post maintenance during rainfall and mobilise sand bags and pumps if needed.

Social impacts of measures Minor nuisance due to cleaning of sumps, pipework and open channels.


Clean out sumps

Check pipework is clear of silt

Clear drains of silt and any other obstructions

These options could be implemented immediately with no resource consent required.

Emergency response plan Sandbag worst affected properties and over-pump to protect habitable floor if required.

A9.15. Recommendations for future studies Future studies should assess: How land movement has affected the capacity of existing drainage network and if any open drains need to be re-graded or widened.

Backwater effect of Heathcote River on the catchment.

The catchment will be addressed in more detail as part of the LDRP.

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A10 Lower Heathcote: Tennyson Area A10.1. Background The Tennyson area is located to the north of the Heathcote River. The area is bound by Colombo Street to the east, Somerfield Street in the north, Selwyn Street to the West and Aylmer street to the south

The sub catchment bounded by Colombo Street and Somerfield Street may have suffered localised ground settlement as a result of the Canterbury Earthquake sequence. As a result, there is a visual low point to the rear of a Somerfield Street property which falls towards the Heathcote River. This appears to have been the significant factor in above floor level flooding of up to 16 properties.

Existing studies

SCIRT #10949 Spreydon/Somerfield RD, SW, WS

SCIRT #10958 Beckenham/Waltham/Opawa Rd, SW, WS

Flooding history There is no known record of flooding in this area.

Customer service reports Up to 16 properties were indicated as experiencing above floor level flooding with additional properties experiencing garage flooding. The properties are clustered within the bounded area.

Key drainage infrastructure The area is served by Tennyson Drain which flows between Somerfield Street and Aylmer Street. It enters a 900mm brick barrel stormwater trunk main which flows through to Colombo Street and down Tennyson Street to discharge to the Heathcote River. The inlet to the 900 mm brick barrel may have blocked in the March 2014 event, contributing to the flooding. The 900mm brick barrel is known to have capacity issues pre-earthquake, although it did not result in floor level flooding. The impact of the limited capacity appears to have been exacerbated by the Canterbruy earthquake sequence and land level change.

A10.2. SCIRT projects The following SCIRT projects were undertaken prior to the flooding and covered the associated areas, although no work was proposed on the drainage in the area. SCIRT 10949 Spreydon/Somerfield RD, SW, WS

SCIRT 10958 Beckenham/Waltham/Opawa Rd, SW, WS

A10.3. Existing flood protection infrastructure The area relies on the existing Council drainage infrastructure for flood protection.


Depth Up to 16 properties experienced above floor level flooding from surface water. LiDAR shows localised settlement of 200-300 mm which coincides with indications from resident of areas which experienced up to 500 mm deeper flooding than the rest of the catchment.

Cause The flooding in this area was attributed to four main causes: Localised settlement of the properties near Somerfield Street

Capacity of existing Council drainage infrastructure (Tennysons Drain and the DN900 brick barrel)

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Blockages of sumps and the point where Tennysons Drain enters theDN900 brick barrel

Surface flooding from Somerfield Street and Colombo Street

General description of damage There was considerable flooding at some properties in the vicinity of a Colombo Street residence that experienced above floor level flooding of up to 100mm and additional properties which experienced flooding above 100 mm. There were also numerous properties that experienced garage and car flooding.

Number of affected floors The floors of 16 houses have been reported as flooded and an additional 8 garages were reported as flooded. It is suspected that further properties experienced on property or garage flooding.

Local effects Road flooding at the intersections of Colombo Street and Tennyson Street, Selwyn Street and Somerfield Street, Aylmers Street and properties bounded within the Somerfield and Colombo Streets.

A10.5. Other 2014 floods A resident of Somerfield Street indicated his garden has flooded to considerable depth twice since the March flood.

A10.6. Community response and drivers No specific report has been received. Considerable social impact of above floor level flooding and residential property flooding is expected.

A10.7. Field studies Field inspections were undertaken on 3 and 4 May 2014 to inform this report.


Land level changes The area at the end of Tennyson Street and the rear of Somerfield Street has experience settlement of -0.2 to -0.3 m. In addition local information indicates a spring appeared in a garage on Somerfield Street.

Liquefaction CERA has designated the area within the Green Zone. Land in the CERA Green Zone has been divided into three technical categories (TC). These categories describe how the land is expected to perform in future earthquakes. Within the Lower Heathcote land has been classified as TC2 and TC3 indicating minor to moderate and moderate to significant land damage from liquefaction is possible in future significant earthquakes, respectively. This will increase possible risk to future siltation with the Heathcote River.


Hydraulic capacity loss Settlement has resulted in reduced hydraulic capacity of the drainage network around the Tennyson Street flooding area.

Infrastructure damage SCIRT projects indicate non-critical damage to the drainage network in the area.

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A10.9. Options Local drainage and flood protection could be improved by repairs and maintenance of existing sumps and drains. This is constrained by access to Tennysons Drain and at the rear of residential properties limits the ease of construction of temporary solutions.

Opportunities Existing SCIRT investigations could be utilised. General improvements to the local drainage network could be undertaken at the time of temporary measures.


House defence measures Due to the complexity of the issue and the requirement for capacity upgrades in Tennysons Drain DN900 brick barrel, property mitigation measures such as local bunding and sand bags should be considered.

Local area schemes The local catchment receives flood water from Selwyn Street, Somerfield Street and Colombo street. Recommendations are: Local bunding of driveways to reduce flooding from streets

Increase the drainage in problem areas in Colombo Street to reduce flooding at the back of properties.

Install check valve in a new manhole to prevent backflow flooding from the brick barrel

Provide temporary pump installed in the manhole to pump over the bunding

Maintenance measures The inlet screen on Tennyson Drain to the Brick Barrel needs regular maintenance prior to major storms. Local road sumps need to be cleaned regularly.

Social impacts of measures Property access is required. Water may impede those with reduced mobility.

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Preferred options

Local bunding of driveways to reduce flooding from streets

Increase the drainage in problem areas on Colombo Street to reduce flooding at the back of properties.

Install check valve in a new manhole to prevent backflow flooding from the brick barrel

Provide temporary pump installed in the manhole to pump over the bunding.

These options could be implemented immediately with no resource consent required

Emergency response plan Extra pump capacity maybe required to prevent street and thereby house flooding.

A10.12. Recommendations for future studies Prior to any permanent bunding, piping etc it is necessary to better understand highest risk of flow direction/volume and survey levels of land. Inappropriate placement of bunds may aggravate ponding depth by dwellings.

Other options to consider include beheading street catchment with additional upstream sumps, raised downpipe wells on dwellings discharging to a separate piped system beyond bund ie reduce catchment to confined local surface drainage near dwellings.

Investigations should be undertaken to pipe Tennysons Drain from Somerfield Street to the rear of Aylmer Street.

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A11 Heathcote Valley

A11.1. Background

Existing studies

CCC Heathcote Valley Report

Flooding history During significant rainfall events flooding has occurred around the lower lying properties of Pawaho Place and Stedley Place from two sources.

Firstly when water flowing along Bridal Path timbered drain adjacent to the properties breaches the timbering and back flows along the outfall pipes serving the area.

Secondly when there have been blockages in the upstream network at critical inlet structures sites. Water has spilt over and flowed along the roading network to Martindales Road and into Pawaho Place.

A11.2. Customer service reports Reports of flooding on Martindales Road, blocked drains, serve silt deposits in Pawaho Place up to 0.5m depth and on Martindales Road and request to close road due to flooding.

A11.3. Key drainage infrastructure The key drainage networks in the Heathcote area are the Bridle Path Waterway, Heathcote Valley Drain and Matuku Waterways These systems are a combination of piped and open channel sections. There is a piped system in Martindales Road conveying water from Tunnel road.

Currently the Bridle Path Waterway route follows the west side of the railway embankment then passes under the embankment via a DN1200 brick barrel approximately 300m downstream of Martindales Road to connect with the Truscotts Stream Branch timbered drain that discharges to the river.

Note: A contract to divert the flow from the west side of the embankment drain to the east side drain is currently being let.

The water from Heathcote Valley Drain now receives additional water from the Matuku Waterway that was constructed as part of the Morgan’s Valley subdivision development. The current outfall is a combination of sections of DN750 and DN900 pipes and open channel system that runs from Bridle Path Road to Martindales Road to an outfall at Truscotts Road. This network was probably sized for the flows from the original farmland draining to Heathcote Valley Drain and does not have the capacity for the additional flows from the Matuku Waterway.

The current outfall is a temporary arrangement and to provide the long term outfall with the capacity to receive storm flows, land was purchased to extend the Matuku Waterway approximately 400m north adjacent to a proposed carriageway through a proposed subdivision. (Much is now Red Zoned land).

The lower section of Matuku Waterway section was constructed along with the subdivision at Cooks Lane. An outlet structure and a section of DN1600 pipe was laid out into Bridle Path Road for the future extension of approximately 65m to connect with the extended waterway from the Morgan’s Valley waterway development.

Flooding in was a combination of water breaching the timbered drains and blockages of inlet structures at upstream locations in the network. These blockages forced the water out on to the carriageways and overland flow paths towards the lowest point of the catchment at Pawaho Place and Stedley Place. The breaching of the drains allowed water to flow over the adjoining properties in Pawaho Place and Stedley Place and over Truscotts Road into Deavoll Place

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A11.4. SCIRT projects The preliminary design for the Port Hills Area 3 project number 11065 show there are proposed renewals to pipes in Marsden Road, Martindales Road and Port Hills Road. Repairs are to be carried

Figure 23 Heathcote Valley location plan

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out on part of the network discharging from Tunnel Road. None of this repair work directly affects the pipe network in the area of flooding.

A11.5. Existing flood protection infrastructure Tide gates and stopbanks


Depth and depth Up to 500mm of pluvial (rainfall) flooding occurred.

Cause The rain event on the 5th March resulted in approximately 60m3 of material deposited into the inlet structure of the DN1200 pipe at the south upper end of the former Malt works site. This piped section is part of the Bridle Path Waterway system.

This resulting blockage caused the water to bypass over the sides of the inlet structure and flow across the site and out on to Martindales Road where it proceeded west towards the low point of Martindales Road and then down Pawaho Place on the north side. The properties at the end of Pawaho Place are on some of the lowest land in this area.

The inlet grill to the Bridle Path Waterway culvert on Port Hills Road blocked and sent water over flowing along Port Hills Road to Martindales Road and into Pawaho Place.

The inlet grill that receives the flow from Heathcote Valley Drain on Bridle Path Road blocked and water overflowed along the east side of Bridle Path Road towards Cooks Lane. Some also flowed across Bridle Path Road and down Marsden Road, Station Road and then into Martindales and lastly Pawaho Place. Some water flowed across some properties on the north side of Marsden Road at the Bridle Path Road end.

Overflow from Heathcote Valley Drain that headed north along east side of Bridle Path Road crossed to the west side at the intersection of Cooks Lane and flowed past the existing triple sump down the driveway and across a Bridle Path Road property.Approximately 0.5m (confirm with survey data requested)Maintenance, lack of capacity, overtopping of banks

General description of damage Damage to carpet, wall linings, possessions and deposition of silt over private property and carriageway.

Number of affected floors

10 properties reported flooding above floor level

2 two properties reported flooding above garage level

43 properties reported flooding up to foundations

Local effects The clusters of houses in Pawaho Place, Stedley Place, Hamlet Lane and Deavoll Place.

A11.7. Other 2014 floods

18th April 2014 The rain event on the 18th April resulted in approximately 30m3 of material from the true left side of the inlet to the Bridle Path Waterway culvert being scoured out and entering the culvert and consequently being deposited into the inlet structure of the DN1200 pipe at the upper end of the former Malt works site.

This resulting blockage caused the water to bypass over the sides of the inlet structure and flow across the site. This water flowed across the site and out on to Martindales Road where it proceeded

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west towards the low point of Martindales Road and then down Pawaho Place on the north side. The properties at the end of Pawaho Place are on some of the lowest land in this area.

The inlet grill at the time of the rain event was not able to be cleared and water continued to overflow. The contractor who is carrying out the site development cut a channel from the inlet structure to a section of broken DN1200 pipe to allow the water back into the piped section of the system. This resulted in a reduction of flow across the site towards Martindales Road. A small cut off channel was made at the north end of the site to divert the remaining water flowing across the site back into the open section of the Bridle Path Waterway just upstream of Martindales Road.

Silt and water from the piped network collecting Tunnel Road storm silt laden water flowed out of existing sumps connected to the DN675 pipe along Martindales Road. This system should discharge into the timbered drain on the west side of the railway embankment. This additional silt laden water which bubbled out of the sumps then flowed down Pawaho Place contributing to the flooding around and through a couple of these the lower houses.

29th April 2014 Prior to the rain event of the 29th April the inlet grill to the piped section of Bridle Path Waterway at the south end of the former Malt Works site, the inlet to this system on Port Hills Road was and the inlet grill that receives the flow from Heathcote Valley Drain on Bridle Path Road were cleared.

The developer of the former Malt works site was instructed to excavate another cut off channel at the north end to divert flows from the site to the open section of waterway.

During the rain event water from the DN675 pipe system from Tunnel Road again poured water from the sumps as a result of the DN675 being partially blocked at the outlet and from silt within the pipe itself.

A pump and sand bags were used to contain and then disperse the water to the west timbered drain at the railway embankment.

Approximately 90m of the Truscotts Stream timbered Drain has dropped in level due to earthquake events. The lower board level enabled the water to breach the timbering at this low point adjacent to Deavoll Place and flow across Truscotts Road and into Deavoll Place with the flood levels threatening houses in this area. The depth of water increased until it was able to cross the carriageway further along Deavoll Place and enter the swale system on the north side and drain to the Matuku Waterway branch adjacent to Truscotts Road.

A11.8. Community response and drivers A meeting was held on the 20th March 2014 with representatives of the Community and Council Staff to visit specific sites along the stormwater network and to discuss what was observed during the 5th March event.

A11.9. Field studies Areas visited were:

Cooks Lane entrance at Bridle Path Road.

Heathcote Valley Drain inlet structure at Bridle Path Road opposite Marsden Road.

Heathvale Place

Port Hills Road inlet structure of Bridle Path Drain.

Inlet structure to DN1200 piped section of Bridle Path Waterway at the South end of the former Malt works site.

Stedley Place

Pawaho Place

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Martindales Road.

Truscotts Road.

Modelling of upper Bridle Path Waterway is currently being carried out to determine storm flows and existing network capacity.


Land level changes The only area of significant land change is along Truscotts Stream Branch timbered Drain which has been shown to settle by -0.8 m to -0.9 m. The lower board level enabled the water to breach the timbering at this low point adjacent to Deavoll Place and flow across Truscotts Road and into Deavoll Place with the flood levels threatening houses in this area.

Lateral spread No lateral spread has been mapping in the Heathcote Valley.

Liquefaction CERA has therefore indicated the area is situated within the Green Zone. Land in the CERA Green Zone has been divided into three technical categories (TC). These categories describe how the land is expected to perform in future earthquakes. The area is TC2 indicating minor to moderate risk from liquefaction and non residential land. In addition area to the south is classified under the Port Hills and Banks Peninsular area.

Ground cracking The CGD ground cracking database indicates minor cracking at the eastern extend of Martindales Road and in the northern extent of Avoca Valley.

Waterway observed changes The invert of the timbered drain along Truscotts Road has heaved and has reduced the depth from the top board to invert from 1-1.2m to 0.6m over approximately 0.4m and to a lesser amount further downstream.

Infrastructure damage Sinking of Truscotts Stream Branch was observed.

A11.11. Key photos

Figure 24 Heathcote valley - Truscotts Road showing subsidence of timbered drain

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Figure 25 Heathcote valley- Truscotts road north of Deavoll Place

Figure 26 Heathcote Valley - Truscotts Road north of Deavoll Place

Figure 27 Heathcote Valley - Trsucotts Road at outlet of DN1200 brick barrel

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Figure 28 Heathcote Valley - Truscotts Road outlet of DN1200 brick barrell after trimming sections of berm

Figure 29 Heathcote Valley - Truscotts Road - berm cut down to allow flow into Matuku Waterway

Figure 30 Heathcote Valley - DN1200 brick barrel

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Figure 31 Heathcote Valley - Bridle Path drain west of rail embankment property side timbering to the raised

Figure 32 Heathcote Valley - drain inlet structure at 78 Bridle Path Road

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Figure 33 Heathcote Valley - Bridle Path valley waterway - inlet at Port Hills Road

Figure 34 Heathcote Valley -Bridle Path valley waterway - Inlet structure at south end of former malt works site

A11.12. Options

Constraints Flood levels in the area are impacted by the capacity of the existing timbered drain sections below Martindales Road and capacity of the DN1200 brick barrel culvert passing under the rail embankment.

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Opportunities The diversion of the Bridle Path Waterway at the completion of a contract currently being let will reduce the risk of flood waters breaching timbered drains.



Bridle Path Road Raise threshold and adjoining footpaths each side and along fence on Bridle Path Road at Marsden Road intersection to create a bund to divert overflows north along Bridle Path Road.

Tunnel Road Place individual sandbags and safely cone approximately every 15m along Tunnel Road kerb to trap more hillside silt while maintaining safe traffic flows. NZTA

Truscotts Road Install approximately 90m of 300 x 50 timber board to existing timbering along the low section of drain and bund over the adjacent berm to a higher level to contain flows to a point downstream of Deavoll Lane.

Construct two temporary high level bypasses across Truscotts Road by lowering the carriageway and berm approximately 200-300mm over 20 and 26 metres. The first location shall be downstream of the existing kiosk and the second opposite the existing brick barrel outlet to divert storm flows into the Matuku system where a First Flush and Detention Basins have been constructed.

Trim the grass berm along both sides of Truscotts Road between the two high level bypass locations, with the west side being trimmed to top board level to edge of seal. On the opposite side trim berm to edge of seal level to allow additional high level flows to pass over Truscotts Road to Matuku Branch Waterway.

Remove approximately 0.6m depth of invert material from Truscotts Stream Branch timbered drain from approximately 80m downstream of Martindales Road to bend and to a lesser depth downstream of bend as required. Pre earthquake depth from top board to invert was 1–1.2m

Pawaho and Stedley Place Increase height of timbering along property side of Bridle Path timbered drain behind the properties of Pawaho and Stedley Place from Martindales Road to Romar Lane.

Reprofile east side excavated material to create a swale adjacent to timbered drain.

Install flap gate on the DN225 pipe from Stedley Place.

Pump mobilised to head of Pawaho Place and Stedley Place to over pump to Bridle Path drain.

Maintenance measures Inspect and maintain clear entry to inlet structures and outlets at critical sites within the network.

Inspect DN675 piping along Martindales Road to ensure it is free of silt deposits pre and post storm events.

A11.14. Optioneering

Bridle Path Road - Cooks Lane

Modify existing double sump on west side of Bridle Path Road adjacent to No 111 to increase inlet capacity.

Install double sump long capacity inlet on east side opposite existing double sump and connect the two sumps with a DN450 pipe.

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Inspect inlet structure of DN600 pipe at boundary and clean drain upstream as required.

Increase length of inlet to existing triple sump across driveway to Bridle Path Road. (only required if above additional sump and modifications do not capture storm flows)

Bridle Path Road – Marsden Road (Heathcote Valley Drain/Matuku Waterway outfall)

Remove existing top grate, remove existing grill and clean out invert. Build up existing walls of inlet structure and create a controlled overflow at end of the structure to direct flow into adjacent sump. Install new longer and flatter grill.

Increase the inlet capacity of existing double sump adjacent to inlet structure.

Install double sump with long capacity inlet on west side of Bridle Path Road and make direct connection to existing DN750 pipe or build over DN750 pipe.

Martindales Road

Excavate cut off channel on former Malt Works site to intercept overland flow from site and direct into existing drainage channel up stream of DN1200 pipe across Martindales Road. (By Developer)

Increase inlet capacity of single sump

Install double sump with large capacity inlet outside and connect to existing DN675 pipe system.

Install double sump with large capacity inlet or modify existing sump across entrance to Pawaho Place.

Remove sediment from timbered drain on west side of railway embankment from headwall at the existing DN675 and DN1200 pipes to downstream brick barrel.

Remove sediment from pipes.

Tunnel Road – Inspect silt traps and clean out, enlarge/modify trap inlets. NZTA

Port Hills Road – Bridle Path Waterway

Investigate where water and material has gone from area adjacent to inlet. Material may have passed through hole in floor of culvert. To be confirmed. Backfill scour hole adjacent to culvert inlet and repair invert as required.

Increase the grill area at inlet to existing culvert

Investigate possible installation of debris trap upstream of inlet.

Bubble-up cross connection from west to east at approximately to divert secondary flows.

A channel has been scoured out behind the kerb and channel from the pedestrian cut down opposite No 37. Install further cut down sections along kerb and channel and stabilised ground behind kerb and create a swale through the reserve towards open waterway to divert secondary flows. (This recently laid new kerb may have contributed to spillage being deflected down Port Hills Road rather than through the reserve as probably historically happened)

Increase inlet capacity of two single sumps on east and west sides to capture secondary flows.

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Truscotts Road

Reconfigure weir boards in First Flush Basin outlet structure to correct position over Dn1350 outlet.

Install bubble up connections from Truscotts Stream Branch timbered drain to the Matuku Branch Waterway.

Install two additional DN750 pipes across Ferrymead Park Drive to provide additional outfall capacity from the Detention Basin to Saltmarsh pond.

Install permanent tide gate at the reconstructed outlet of Saltmarsh Pond.

A11.15. Recommendations Implementation of maintenance and flood defence measures are likely to reduce the risk of water leaving the stormwater network and flowing to the lowest part of the catchment.

These measures could be implemented within one month. No consent is likely to be required to complete these works

Emergency response plan Check critical inlet structures and pipeline in the catchment to ensure they are clear of debris and silt. Regular checks and removal of any debris during storm event

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A12 Little River A12.1. Background

Location plan The Little River study area looks at the little river township along Christchurch Akaroa Road from Morrisons Road to Fleet Street. This extent is shown in Figure 35.

Existing studies Flood extent mapping following the 5 March 2014 event by Jacobs SKM March 2014 has been used to inform this study.

Flooding history The Little River area has a long history of flooding.

No evidence was found showing any difference between flooding pre and post earthquakes. There is little visible land damage and no evidence of land rising or falling in the catchment.

Land stability issues in the upper catchment that have caused slips and waterway blockages may be earthquake related but these could also be attributable to the recent high intensity rainfalls.

The recent series of flooding seems to be related to the high rainfall intensity during recent events rather than earthquake effects.

A12.2. Customer Service Reports (CSR) A review of CSRs showed very few reports of residential flooding above the floor level. The records showed one property including a house and sleep-out that had been flooded and a second property where a sleep-out had flooded.

There are a number of CSRs for commercial properties in the area.

A12.3. Key drainage infrastructure Generally flooding in the Little River area is due to flowing rather than ponding water. The area consists of a steep upper catchment that funnels into a flatter lower valley where the township is situated. From here water drains via the Okana River system through Te Roto Wairewa / Lake Forsyth to the sea. The outlet of the lake is artificially opened by Council to manage the lake level.

There is a small amount of drainage infrastructure around the main township area and in other areas related to conveying water under the roads.

The top end of Barclays Rd including the southern end of the Heritage Park is a relatively low area that is drained via a pipe that runs from open channel drains on the north side of 10 Barclays Rd to an open channel on the east side of the Christchurch Akaroa Highway. The area is often flooded as a result of high water level at the pipe outlet restricting flow out of the basin area. On occasion water has been reported to be flowing back up through the pipe.

This pipe is also scheduled for replacement and will be upsized following capacity concerns however the area will still be flooded until the water level at the outlet can be reduced.

This is a pipe / culvert located on east side of main road past 4230 to 4240 Christchurch Akaroa Rd (includes Library / Service Centre). This pipe carries water between the open channels above and below it on the side of the main road.

A larger pipe may ease some of the flooding around the area of the shops by reducing the amount of water flowing over the road.

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Figure 35 Little River study area extents

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Wairewa Pa Road Bridge (Kinloch Road end) In large events the bridge does not pass the entire flow. Overflows bypass on either side of the bridge causing flooding on the Highway, private property and local roads (Wairewa Pa Rd and Kinloch Rd). Local observations have been made of a ‘wave’ or back surge of water travelling back upstream and through the overland flow into the township once the water level reaches the beams under the bridge.

A12.4. SCIRT projects There are no SCIRT projects in this area.

A12.5. Existing flood protection infrastructure Currently the lake outlet is artificially opened. Lake openings are initiated by the Christchurch City Council for flood prevention reasons. The operation of this outlet has not been affected by earthquake damage.

In some areas the remains of what look like bunds along river banks are visible. However, it is unclear whether these are merely spoil piles or bunds placed purposefully to mitigate flooding issues.


Depth and type Flooding is predominantly as a result of fluvial flowing water rather than ponding, depth therefore varies based on topography and flow paths, but has been anecdotally identified to be as deep as 0.5-1 m in places.

Cause Flooding in Little River has a number of causes. Field investigation has identified that the predominant cause of flooding is likely to be inadequate channel capacity and the constraining impact of the Wairewa Pa Road Bridge.

The minimal maintenance within the Okana River corridor and its tributaries has led to the establishment of extensive vegetation, particularly willow trees, along and within the river corridors. This effect is catchment wide.

Flooding of the Little River township is most likely caused by water overflowing from the Okana River Branch No3 (Police Stream) which is severely choked. On the downstream side of the main road conveyance is heavily constrained by vegetation. Once flood water spills into the township the local drainage infrastructure becomes submerged and unable to function causing more widespread ponding from other sources e.g. at Barclays Rd / Heritage Park area.

The Wairewa Pa Road Bridge (Kinloch Road end) appears to form a significant restriction to channel flow and also forms a bund impeding flow away from the township. However, secondary flow paths do form and are essential to allow flow to escape the township area.

The Opuahou Stream and its tributaries in Cooptown are potentially under capacity and the banks have overtopped in places directing water into properties. However, further investigation of this is required for increased certainty.

General description of damage Property flooding in the area was reasonably widespread, but typically limited to below floor levels. Houses in Little River have been constructed to varying standards but many have been constructed with high floor levels. This has resulted in significantly less damage than would otherwise have been expected. One house has been designated as a rebuild following flood damage and there are others that have now been vacated to allow for repairs works to be completed. A number of garages and other ancillary buildings that have been constructed with a lower floor level were flooded. The risk of flood waters being contaminated by sewage is far less than urban areas.

Number of affected floors

6 residential buildings

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4 commercial buildings

Local effects All houses identified as having been flooded (with the exception of a sleep out in Barclays Rd) are located in the lower Cooptown / Church Rd area.

Flooding of the road and township has the effect of isolating the community and disconnecting other areas of Banks Peninsula such as Akaroa.

A12.7. Other 2014 floods There have been three flood events that have resulted in on property flooding in 2014:

March 4th

April 18th (Good Friday)

April 30th

Anecdotally there have been seven on property flood events since the earthquakes.


Social impact While flooding in the Little River area only enters a few houses above floor level (six identified) it does have a large impact on the community. A large number of properties have been inundated multiple times including close to floor level under houses and also entering into garages and ancillary buildings.

A number of commercial buildings have been flooded impacting on the owner and the associated businesses.

Road closures on the Christchurch Akaroa Highway impact the community in a number of ways including isolation of Peninsula. Preventing travel and limiting tourist activity affects both individuals and businesses.

Deep flowing water on the main road, often at night, creates a hazard to the local community and visitors.

Community feedback There have been very few complaints logged with the Council regarding residential flooding. In order to address the uncertainty, the investigations have involved discussions with property owners in the area to identify vulnerable homes.

Key community members were contacted and their local knowledge used to identify which buildings to visit and also to recommend other people to contact. While in the field conversations with the residents also help to gather more information and identify further sites to investigate.

A12.9. Field studies During the field studies known problem areas along the rivers and streams from upper Cooptown down to the lake outlet were visited.

Prior to and during our visit enquires were made with members of the community, business owners, members of the Community Board, and Council staff to identify known and suspected flood prone houses. These were all visited and inspected.


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Land level changes Land stability issues in the upper catchment that have caused slips and waterway blockages may be earthquake related but these could also be attributable to the recent high intensity rainfalls.

No settlement has been identified in this area.

Lateral spread, liquefaction and infrastructure damage Lateral spread and liquefaction has not been identified in Little River. There is no known damage to infrastructure.

Waterway observed changes The earthquakes are possibly linked to slips and increased sediment run off from catchment leading to deposition in the rivers and lake. There is no evidence of bed heave, lateral spreading or other direct land damage impacts. No known earthquake damage to flood defences.

A12.11. Key photos

Figure 36 Little River – An indication of the level of vegetation in Okana River Branch 3

Figure 37 Little River – Wairewa Pa Road Bridge; a key constraint on the network.

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A12.12. Options

Key assumptions and gap analysis There is limited data to assess level changes across the catchment, post-earthquake LiDAR for the area is not available. Further investigation of ground levels may yield confirmation of whether there have been any significant earthquake related changes, but this is unlikely to add significant value.

All assessments are at qualitative level and no hydraulic modelling of the catchment has been completed. In order to quantify the benefits and changes through implementing options a detailed study would be required.

Constraints In the past, under the Wairewa Borough Council, river maintenance was funded (at least partly) by national funding from the government. This funding ended around the time the Banks Peninsula District Council was founded and a ratings district was proposed. Following community meetings and consultation in 1993 a resolution was passed by community vote that rather than a rating being introduced land owners would look after the maintenance of the waterways. This approach was not successful as while some properties were well maintained others were neglected. Later, circa 2008 the Regional Council sought to introduce a ratings district and the vote was not passed.

Currently the maintenance of waterways, including the control of trees and removal of debris is the responsibility of the property owner. Management and maintenance of crown land around waterways is unclear and will require further work to identify who this responsibility lies with.

As many of the waterways are located on private land, any works that could lead to increased flooding or land requirements will require land owner agreement. This is particularly important for any works required for the construction of flood flow channels across the flood plain downstream of the Wairewa Pa Road Bridge.

Vegetation clearance and in channel works would lead to increased flows downstream, the downstream affects would need to be assessed. Further removal of trees could also lead to increased bank erosion and stability issues; this is a key consideration when assessing what works are required.

Plans for vegetation clearance will need to balance the benefits of removing channel constrictions with the increased risk of erosion, loss of streamside habitat and loss of stream shading.

Opportunities Channel clearing work could be combined with Environment Canterbury’s proposed stream enhancement project which includes willow removal work.


House defence Low built addition to a house on Church Rd could be protected by sandbagging the extension to the level of the original house foundation.

Local area schemes

Clearing of river and stream channels throughout area.

Creation of secondary flow channels to contain spill from main channel and move it more efficiently to the lake.

Maximizing capacity of Wairewa Pa Rd Bridge and creating bypass channel to ease backup of water in lower part of township and contain spillage from flooding highway.

Ensuring drainage off the Christchurch Akaroa Highway through Cooptown is effectively drained to the river.

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Social impacts of measures Reducing flooding in the lower village area will help reduce the time the highway link to the Peninsula is closed reducing the impact on businesses, tourism, supply of goods etc.


Lower Cooptown / Church Road area

Excavation of a terraced bank to create secondary capacity for the river in the area of 4411 and 4421 Christchurch Akaroa Rd.

Placement of excavated material to deflect overflows away from houses and back into stream channel.

Improvements to drainage from the Christchurch Akaroa Road to the stream.

New open drain near boundary between 4411 and 4421 Christchurch Akaroa Rd.

Improvements to roadside drainage along highway past 4411 and along Church Road to the Opuahou Stream including upsizing culvert pipes at vehicle crossings.

Selective willow removal to improve flow in the Opuahou Stream.

Further investigation

Possible options for re-routing drainage from the Christchurch Akaroa Highway and ensuring water is not being diverted into the area by overflows etc. from higher up the highway.

Potential to create a high level spill way or realign river to reduce the volume of water in the river loop behind the Wairewa Motors site and improve drainage from the upstream area.

Little River Lower Township Area

Improve conveyance of water through lower flats to the lake by removing bunds / overbank deposits from the downhill sides of banks from select locations to allow spilling from river into river flats.

Increase capacity of Wairewa Pa Road Bridge.

Reshaping of inlet and outlet areas of bridge.

Possible removal of bank material under bridge (dependant on further investigation regarding foundations etc.).

Creation of spillway beside the bridge by either lowering the road or installing culverts between the highway and bridge.

Willow clearance from waterway channels to prevent overflow into township. Immediate benefits could be achieved through a quick but rough initial clearing. This would need to be followed up at a later date by more permanent work.

Okana River Branch No3 (Police Stream) – approx. 300 m of dense willows plus several individual or small stands of trees.

Okana River Highway Bridge to Wairewa Pa Road Bridge – removal of main willows issues and debris from channel.

Further recommended work

Removal or maintenance of willows on Department of Conservation land along the Christchurch Akaroa Rd and on the banks of the Takiritawai River

Increase capacity of Okana River Highway Bridge by excavating accumulated material from under north side

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Timeframe The implementation of the works is likely to require months of vegetation clearance, this could be significantly reduced if general clearance was implemented and the spoil could be deposited on locally on site.

Additionally the clearing could be staged with an initial rough sweep clearance to increase capacity quickly. This would then need to be followed by tidy up work to further improve the benefit and increase the longevity of the work, also reducing ongoing maintenance costs.

Consenting requirements Waterway channel works for willow removal and reshaping earthworks will require resource consent.

Emergency response plan The fast flowing fluvial nature of the flooding means that the safety risks in some areas are significant. Road closures in areas of deep and/or fast flowing water should be continued. It is recommended that vulnerable property owners are identified in the response plan and prior to forecasted flooding, flood warnings are given and support offered to prevent isolation during the floods. A text message based system could be highly beneficial.

A12.15. Recommendations for future studies

Effect of Lake Forsyth water level on the flooding of Little River and the Christchurch Akaroa Highway

Drainage from Christchurch Akaroa Highway to river

It is recommended that an integrated solution for highway flooding is pursued with the New Zealand Transport Agency (NZTA) who are also investigating flooding in the area.

Environment Canterbury (ECan) have also completed studies in the area and should be engaged, as a potential source of knowledge, in the development of solutions for Little River

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A12 Lyttelton Lyttelton is located on the southern side of Christchurch’s Port Hills. Much of the area falls steeply down to the Lyttelton Harbour, see Figure 38. Due to the topography the flooding risks in Lyttleton are very different to those across the flatter areas of the city. Therefore the vulnerability measures and the resultant options selection criteria cannot be directly applied in this area. Instead a greater focus has been put on the risks of injury and property damage that flooding can cause through fast flowing water, scour or reducing slope stability. This report it has been divided into two different sections, one focused geo-technical slope stability and risks associated with long duration rainfall events that saturate the catchments soils, and a second study focusing on the surface flooding that occurred during the March 5th event and the other significant rainfall events.

Figure 38 Lyttleton Study Area

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A13 Lyttelton: Slips A13.1. Background Lyttelton does not appear to have been covered by any post-earthquakes area-wide land stability studies. Therefore this investigation has relied on what can be gathered from the field in the given timeframes and from recent customer service requests from the public.

Site walkover produced a map of observed slips as at 4/5 May 2014. There were 19 significantly sized slips observed, mostly onto roads. There were also two calls from the public reporting the collapse or retaining walls.

SCIRT projects There is a major project underway by SCIRT in relation to retaining walls, which is somewhat related to the slip problem. However there is not a direct SCIRT project addressing land instability in Lyttelton.

Existing flood protection infrastructure The CCC stormwater infrastructure in this area is made up of those typical of drainage networks.

Number of affected floors An informal estimate by David Bell (senior lecturer, Geological and Mineral Sciences, University of Canterbury) is that 50 – 100 houses in Lyttelton are potentially threatened by landslip.

Local effects These include slips blocking roads, breaking utilities and diverting stormwater runoff. This can cause further problems elsewhere.

Community response and drivers Two houses are known to be abandoned, one house is most likely abandoned (from visual observation), and the true number of houses affected is unknown.

Events have been too recent for the CCC to initiate consultation.

A13.2. Field studies The field study for this catchment included a site walkover/driveover to locate and photograph slips, identify common features and causes. The causes of slips included:

1. Many observed slips in the area appear to have been influenced to varying degrees by a flow of water

2. Many slips on private property appear to be initiated by a private stormwater pipes discharging onto the hillside.

3. Surface cracks caused by the earthquakes are a major cause of hillside instability. The cracks are extensive, in many cases hard to detect, and hard to protect against water ingress.

4. Broken water pipes many in private ownership could be leaking into loess where water will be stored for years contributing to instability.


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Land level changes The loess mantle covering basement rock on the Port Hills had a different dynamic response to the underlying rock, which led to cracking (short features), fissuring (extended linear features), and formation of disconnected blocks prone to toppling. Most of these features were accompanied by surface cracking, which in many places, including Lyttelton, is difficult to detect. Surface cracking on residential properties and hillsides tends to be hidden by vegetation and buildings. Surface cracks direct rain water and surface water into the ground where it destabilizes the hillside by adding weight, reducing ground strength (making the ground slippery) and carrying away dissolved loess, leaving parts of the hillside unsupported. This results in landslips.

A common earthquake effect has been settlement of downslope road edges. These areas now direct stormwater over the road edge and onto susceptible hillsides (Area Maintenance Manager, CCC)

No liquefaction occurred in the Lyttelton area. CERA has therefore indicated that the technical categories of land damage are not applicable in much of this area.

There is unquantified damage to water supply pipes on private land. This is significant because leaking water will saturate underlying loess and destabilize the area. Loess can take years to decades to dry out; the water leak will initiate an instability that will persist for many years, ready to be activated by extended periods of wet weather or heavy rainfall.

A13.4. Key photos

Figure 39 Lyttelton - private stormwater pipe (blue, upper right) likely initiator of this slip

Figure 40 Core sample of wet loess that has lost all strength. The cause is infiltration from a cracked hill slope. Sourced from Reserve Terrace at 3-4 m depth.

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A13.5. Key assumptions and gap analysis Surface cracks are a major cause of hillside instability. The cracks are extensive, in many cases hard to detect, and hard to protect against water ingress. Most areas of cracking are on private property and tend to be concealed by vegetation.

It is important that all stormwater that falls on hard surfaces be directed into the stormwater network, retained in the network, and discharged safely to the sea. This does not occur at present due to grate blockages, some shortfall in catch-pit clearing, runoff from the carriageway onto private property, disconnected private downpipes, and misdirected private stormwater pipes.

Uncontrolled stormwater runoff from roads onto hillsides is a cause of hillside erosion. It is a less extensive phenomenon than infiltration, but can cause large local effects.

Constraints The limited capacity of the current stormwater drainage network means that during heavy rain stormwater flows overland on roads or down hillsides. While the trunk mains, large brick barrel pipes, appear to have sufficient capacity the wider network can struggle in extreme rainfall.


Stormwater defence measures

Street sumps should be regularly cleaned and where possible protected from blockage from gravel and floating debris

The stormwater drainage network on hillsides susceptible to slipping should be increased in capacity so that:

o The primary flow is carried in a pipe

o Where necessary there is provision for a lined or protected secondary flow path

Rainfall runoff from the road should be directed to a stormwater pipe or channel

Private roof runoff should be checked in at-risk areas to make sure that down-pipes discharge to the road or into the drainage network.

Local area schemes

Educate and work with all occupiers in susceptible areas to identify and redirect sources of water,

Educate and work with all occupiers in susceptible areas to identify and seal ground cracks

As much as possible, locate and redirect under-runners into pipes

Locate and repair water supply leaks


Preferred options and cost All of the above would assist in controlling hillside instability

Timeframe

Catchpit upgrades – 2 months

Road edge bunds – 2 months

New kerb – 4 months

Site visits – should be completed before winter

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Emergency response plan

Keep all grates, catchpits and inlets clear at all times.

Update the CCC response plan

o initial response based on expected precipitation

o follow-up response based on actual precipitation

Responders to be on site during storms until the peak has passed. Adequate resources on site during that period.

Pilot study recommendations Hillside immediately below Ticehurst Terrace

A13.8. Recommendations for future studies This report was prepared by a drainage engineer, with information, advice and encouragement from Dr David Bell (engineering geologist, Univ. Cant) and Joyce Searle (geologist, GHD/SCIRT). The information and recommendations should be reviewed and verified by geotechnical engineers conversant with the issues.

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A14 Lyttelton: Stormwater drainage network A14.1. Background Local stormwater run-off and flows from ephemeral watercourses above the township are directed to several brick barrel masonry pipes. These pipes flow down through the township of Lyttelton and discharge into the harbour. The brick barrels are key infrastructure for Lyttelton, and if the inlets to these pipes block or they surcharge it results in high velocity flows down through the streets of Lyttelton which can result in damage to properties due to flooding or the high energy flows. Fast flowing water can also be a risk to the people in the area.

Existing studies Considerable condition information exists for the stormwater network, both pre and post-earthquake. There is less information on capacity.

Earthquake Effects In addition to the effects of the 2011 Canterbury earthquake sequence described in the above section the earthquakes have settled the downslope road edges in many locations. According to the CCC Area Maintenance Manager, these areas now direct stormwater over the road edge and onto susceptible hillsides.

Flooding history A grate was fixed over the Canterbury Street brick barrel inlet subsequent to the death of a child in Waitakere City in 2009. It blocked in 2011 and March and April 2014 sending a large flow down Canterbury Street, causing a large amount of road and property damage.

170 mm of rain fell between midday 4 March 2014 and midday 5 March. This amount of rain in a 24 hour period is rare, although a return period is yet to be calculated from the Lyttelton Port Company record. Catchment debris brought down by the storm covered and blocked the grate. A lesser amount of rain on 16 April, falling on an already wet catchment again caused a blockage of the inlet grate and overtopping.

The grate has now been removed, which reduces the risk of the overflow hazard. Interim measures have been taken to warn residents of a potential, though small, hazard at the inlet, and design of a grate of adequate capacity has commenced.

Land damage mapping No earthquake land damage mapped above Lyttelton. Slips and rock slides will over time carry rocks to the brick barrel inlets, with potential for blockages.

A14.2. Key drainage infrastructure Lyttelton is served by 9 large brick stormwater mains, built c1900 within valleys, and a range of smaller pipes and overland flow paths that traverse roads and unprotected hillside routes. The brick barrels have adequate capacity, such that there is no record of overflow except as a result of inlet blockages. Anecdotal information suggests that inlet capacity may have been exceeded at one or more inlets on 5 March 2014.

Grates, when fixed over inlets to trap debris or, latterly, to exclude entrance by people are prone to blocking and require vigilance during sizeable storm events.

The outlying parts of the system include a mixture stormwater capture and delivery into pipes via sumps, and spillage of excess flows to the waterfront by typically by direct routes down the street.

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Figure 41 Lyttelton brick barrel (stormwater mains) locations

A14.3. SCIRT projects SCIRT repairs are generally minor repairs or like-for-like replacement of badly damaged pipes in a few cases. Some improvements have been achieved in Cunningham Terrace and Sumner Road but these are localised effects.

A14.4. March 2014 flood extent Typically the overland flows during this event were fast moving in some cases bypassing the primary drainage paths through private property or flowing across the road carriageways. These flows had a hazardous combination of depth and velocity particularly in the area of Canterbury Street and flowing on-road down to Exeter Street. Several hundred meters of road were damaged and many properties had gravel deposits. Several cars were damaged by moving gravel. Some pipes filled with gravel near the waterfront. This also contributed to spills of water onto the Port Company and Transrail property disrupting operations of both. A blockage of debris and gravel at the safety grate at the intake at the top end of Canterbury Street contributed to this flooding. The grate was installed to reduce the entry hazard to children.

A14.5. Other 2014 floods Some damage has occurred in a number of other heavy rainfall events since the earthquake including those in April 2014.

A14.6. Community response and impacts There is frustration and some anger amongst some in the community at the repeated events.

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A14.7. Field studies Visited each of 9 brick barrel inlets checking for potential to block, presence and adequacy of grate sizes, signs of debris in-stream, accessibility.

Figure 42 Canterbury brick barrel upper inlet at Somes Road


Flood defence measures

Investigate removal of grates, enlargement or secondary intakes for all major pipe entries.

Monitor and clear important pipe entries during storms.


Any new grates must be large, easily accessed, easily cleaned, low chance of blocking before a response can occur.

Install trapping upstream of all 9 inlets for floating debris and rocks.


Education of maintenance contractor and an operating protocol for Lyttelton inlets.

In the longer term ensure vehicle access to all inlets. This will involve creating vehicle access across private property, on a hillside, to the Cressy St inlet

Social impacts of measures The community are able to be reassured with a safety strategy arrived at by consensus.

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Preferred options The preferred solution for the immediate problem is a minimal cost option of removing a problem grate, advising locals, and monitoring. This has been done. A longer term solution is pending.

Debris trapping and waterway stabilization at the other 8 inlets is needed as an operational response to earthquake rubble and catchment debris generated by large rainfalls.

Timeframe

Canterbury Street grate removed.

Debris and rock trapping upstream of inlets – 2 to 4 months

Emergency response plan

CCC staff monitor weather forecasts

Sufficient crews in Lyttelton to monitor and clear 8 grates

Priority measures for the highest risk areas

Option Description

Catchpit upgrade Enlarge catchpit and grate, install sediment trapping/diversion

New catchpits Install new catchpit and grate, incl. sediment trapping/diversion

Contain water on roads Kerbing/bunding to contain water on roads

Upgrade or new SW pipes Increase the size of under-sized SW pipes or install new pipes

Install and secure secondary routes Install a flume or equivalent to carry secondary flow safely where hillsides are fragile

Ensure private SW connects to network. West of Hawkhurst Rd (danger area)

Visit all properties in susceptible areas to check SW connections, ensure compliance

Assist property owners with infiltration control

Visit all properties in susceptible areas and provide a toolkit for identifying and sealing surface cracking, capturing groundwater, locating under-runners

Locate and shut off water supply leaks

Visit all properties in susceptible areas, check water connections, repair leaks

Pick up under-runners Install new drainage to pick up under-runners

Improve maintenance actions Amend Road and Stormwater Maintenance Contracts to ensure SW collection systems remain continuously clear.

Response plan CCC to create a response plan in collaboration with its contractors

Priority measures for the medium risk areas

Option Description

Upgrade or new SW pipes Increase the size of under-sized SW pipes or install new pipes

Ensure private SW connects to network. Remainder of Lyttelton (less danger)

Visit all properties in susceptible areas to check SW connections, ensure compliance

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A15 Southshore A15.1. Background

Location The Southshore study area includes all areas on the Southshore Spit south of the Pages Road Bridge.

Existing studies SCIRT has undertaken several stormwater roading related projects throughout this study area and the recommendations can be found within the Concept and Detail Design reports. It should be noted a number of these projects have been or are nearing completion and early indications are that areas are no longer causing service related issues.

SCIRT projects within the study area:

11109 South Shore South of Beatty Street

11021 New Brighton -Owles Terrace

11070 Blake Street SW

10840 PS37-Catchment Study

10874 PS36 -Catchment Study

10429 Estuary Road

Addition investigation works can be found in catchment studies 11110 and 11111.

A15.2. Customer service reports Council records identify customer service requests indicating flooding relating to overflows over Bridge Street as well as poorly functioning flap valves exacerbate flooding issues. There is also historic flooding issues around this intersection of Owles Terrace and Beresford Street. Some of these known problems are being addressed as part of SCIRT projects.

A15.3. SCIRT projects SCIRT have observed that, on Ebbtide Street, numerous flap valves either block, are obscured or broken. City Care maintenance should be tasked as a high priority to rectify and clear all storm water outlets. A major concern in this area is the rapid erosion of the walkway on top of the old retaining wall. Emergency works will be required to prevent possible sea breaching along this section and putting the road at risk.

In Southshore, localised nuisance flooding at intersections has been observed as well as at low points on the carriageway, especially towards the southern end of Rocking Horse Road. City Care maintenance should be tasked as a high priority to rectify and clear all storm water outlets.

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Figure 43 Southshore location plan

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A15.4. Flood history

Post-earthquake Modelling by SCIRT demonstrates that South New Brighton and Southshore south of Estuary Road is vulnerable to flooding by tidal inundation. Throughout Rocking Horse Road, flooding depths of up to 0.4 m are shown to be possible during the scenario modelled - a 5 year storm and 1 year tide. Some private property floor levels were obtained during the SCIRT Southshore Project No. 11109. In a 5 year return period scenario there is some risk but not a certainty that private property floor levels would be at risk of flooding.

The depth and extent of flooding in a 50 year storm with a 5 year tide is somewhat worse than the 5 year storm scenario. The amount of on-road and on-property storage available buffers flood levels significantly. The 50 year storm model demonstrates depth of flooding to 0.5 m and significantly more properties are at risk of flooding. Those most susceptible to flooding are adjacent to Rocking Horse Road and Estuary Road. Some properties along the estuary edge are vulnerable to tidal flooding, and even 5 year high tide with strong south-westerly winds could breach the informal shoreline bund that exists.

To alleviate the flooding risk with a high level of certainty additional flood protection measures such as a stopbank and, potentially, pump stations are required. Such solutions were outside of SCIRT’s

scope and while some have been identified they have not been pursued by SCIRT. Further modelling is planned by CCC to determine the value of a shore-front stop bank.

SCIRT are currently planning or undertaking 3 stormwater pump stations and associated reticulation, as well as repairing or relaying a large proportion of existing reticulation. The estimate was $8.6M. Pre and post-earthquake stormwater modelling was carried out during detailed design to determine the change in level of service within the catchment. Modelling results showed that flooding levels after the earthquakes were approximately within 100 mm that of the pre earthquake condition. Given that accuracies in the input data and model have a tolerance of 100 mm, SCIRT moved forward on the basis that there has been no change to the level of service across the catchment. Approximately 40% of the network requires repair or renewal, like for like, to address earthquake damage. This damage impacts the level of service within the catchment. A further 40% requires no action. 20% of the network suffered minor damage; repairing this damage will not impact the level of service within the catchment, and hence SCIRT will not complete these repairs. 1200 m of stormwater reticulation requires relay and 650 m requires repair. The estimated SCIRT stormwater rebuild cost is $2.4 M inclusive of traffic management, P&G and contingency.


Depth No records could be located of flooding above floor levels however numerous properties were inundated and damage sustained to garage contents.

Type and cause of flooding The predominant flooding issues on South Shore are caused by tide flowing inwards through damaged storm water outlets, exacerbated by a number of additional factors. These include:

Earthquake damage to existing storm water infrastructure and carriageways disrupting flow paths to storm water outlets

Localised property damage (subsidence) giving rise to localised ponding

Flap valves need further maintenance; some are stuck open, some sealed shut. The consequence of this is there is no effective outlet for street drainage, or where flap gates are open high tides ‘hold back’ street run off until the tide recedes and sufficient head is generated to allow discharge

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Numerous street gully grates are blocked or restricted by vegetation. Cabbage Tree leaves are especially good at restricting flows at these outlet points

General description of damage There was no recorded flooding of house floors however numerous properties were inundated and damage was sustained to garage contents. Access in and out of certain properties was a cause for concern for some residents. These were generally alleviated once tidal waters in the Avon River and Estuary began to ebb.

A15.6. Community flood response and drivers

Social impact Discussions with the community illustrated that there were concerns with the tide levels, but a general understanding that once the SCIRT works are completed these would be resolved.

Community feedback There are wider community concerns in Southshore about sea level rise and the measures Council are considering to address this.

A15.7. Field studies Site surveys were undertaken as part of the Taskforce. This included checking all the flapgates into the estuary, both existing and recently constructed, and speaking to contractors working on existing SCIRT contracts on Kibblewhite Street to gain updates of their anticipated completion dates.

Where encountered local residents were surveyed, including residents who had raised concerns and had localised property flooding.

A subsequent site visit with SCIRT contractors was undertaken to all vulnerable locations throughout the study area and temporary work solutions and action plans in the event of another severe weather/ tide event were determined.


Land level changes Significant land changes have occurred post-earthquake. The eastern edge of Southshore has been shown to have changed by +0.4 to +1.0m. To the west of this and in the central section of Southshore a change of -0.1 to -0.5 m has occurred. Changes of -1.0 to -1.5 m have occurred at the south-western area of the spit and in the area south of Bridge Street and around Owles Terrace.

Lateral spread Moderate to major lateral spread has been exhibited on the western side of Southshore.

Liquefaction Minor to moderate liquefaction was observed at Southshore. CERA has indicated the area is situated within Red and Green Zones. Land in the CERA Green Zone has been divided into three technical categories (TC). These categories describe how the land is expected to perform in future earthquakes.

Southshore comprises of Red Zone on the western extent and a combination of TC2 and TC3 properties. This means the Red Zone land is deemed unsuitable for residential land use and TC2 and TC3 areas indicate minor to moderate and moderate to significant land damage from liquefaction is possible in future significant earthquakes, respectively.

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Ground cracking The CGD ground cracking database indicates cracking of 1- to 50 mm cracks on the western side of Southshore.

Infrastructure Damage There is outfall damage at a number of locations south of Owles Terrace. The construction of the temporary stopbanks and subsequent earthquakes has further damaged the connecting pipes and outfalls. Minor damage has occurred to pipes around Kibblewhite Street where the stopbanks have been raised resulting in poor flood management protection of this area. All the major outfalls sustained damage on the western side of Southshore.

A15.9. Options

Key assumptions and gap analysis The timing of SCIRT delivery is assumed to be as given. Sea level rise is excluded from this assessment

Opportunities Can SCIRT resources already available within the area can be utilised to undertake temporary measures. Current designs being implemented at Owles Terrace can be modified and utilised for area scheme one.


House defense No individual house protection measures are envisaged for this study area. SCIRT storm water and roading projects should address the damaged caused by the recent earthquake events. Increased maintenance and monitoring of storm water outfalls would decrease the number of street flooding occurrences. Projects completed or nearing completion throughout the study area appear to have resolved localized flooding issues. This can only improve as more SCIRT Projects are completed.

Local area schemes Due to the location of many of the storm water outfalls a more robust monitoring and maintenance regime should be developed. Poor maintenance and monitoring has been one of the main findings of this report and field observations as well as comments through the Councils Request For Service (RFS) system.


Preferred options and cost A number of SCIRT developed projects specifically address the problems arising through Earthquake damaged infrastructure. Many of these projects will be implemented and completed within the next 12 months, some are already complete. Funding for these projects is already accounted for.

Emergency localised pumping solutions can be developed with the SCIRT Delivery Teams. These can be included within the Temporary Works program for the individual SCIRT Projects and managed through the ECI (Early Contractor Involvement)-SCIRT process.

A maintenance contract is already in place between Christchurch City Council and City Care, a more robust process for repair, clearing and monitoring is required to ensure storm water outlets function properly.

Local area solutions to investigate and address the stop bank issues in and around Ebbtide Street should be undertaken. There is a current risk of a breach should it fail. This is the one of two main links providing emergency access from South Shore to South New Brighton in the event of a Tsunami or other natural disaster and is therefore an important evacuation route.

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Timeframe Construction timing of individual SCIRT projects all fall within 2013 -2014. Construction programs could effectively be modified to ensure improved security for residents is considered as a matter of priority.

Consenting requirements Works on the stopbank are likely to require a Resource Consent but this could be retrospectively obtained if undertaken utilising the emergency works provisions.

Emergency response plan CCC operational staff are to discuss with City Care a more robust monitoring and repairing regime for flap valves and gully grate clearing in vulnerable areas. This discussion should also be done in conjunction with SCIRT Delivery Teams currently undertaking Earthquake repair works within this entire study area.

Contact details of who does what should be drawn up between CCC City Care and the SCIRT Delivery Teams. The Communications/Call Center at CCC should also be informed of the main contacts and this list is to be kept current.

A15.12. Recommendations for future studies Recommend that a study is conducted on flap gate design to prevent siltation.

A full survey is required along the foreshore to highlight any potential ingress points in the high tide situation.

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A16 Sumner: Sumner Village A16.1. Background

Location The Sumner study covers two areas. One area extends from Augusta Drain to the eastern end of Moncks Bay and back to the base of the hills. The other area covers Sumner Village, from the intersection of Marriner Street and Nayland Street to the end of the Esplanade.

Existing studies

SCIRT Project Concept and Detailed design #11055

SCIRT Scope and Standards Paper 323

Walter. John.2009 “Sumner Flood Relief Pipe; Sediment and Ordour Issue Assessment” Capital Programme Group, Christchurch City Council

Couling, Ken. 2000 “Storm Report 11-13 October” City Solutions, Christchurch City Council

Flooding history Mariner Street and Esplanade have a history of flooding and were affected significantly in the October 2000 flood event. Sections on Clark Street and Arnold Street also have a history of flooding as a result of the low lying typography of this area and proximity to the Sumner Main Drain (SMD). Sections on Paisley Street near the Sumner Stream/SMD also have a history of flooding.

Land damage mapping On the Canterbury Land Information map released by CERA, the majority of the project area in Sumner is categorised as Technical Category 2 (TC2) in which “minor to moderate land damage from liquefaction in future large earthquakes” is possible. Only minor liquefaction was observed along Nayland Street immediately after the February and June 2011 earthquake events.

A small number of properties within the Sumner area have been Red Zoned due to associated risks with the surrounding cliffs in Sumnervale Drive, Ocean View Terrace, Heberden Avenue, Scarborough Hill, Searidge Lane and Wakefield Avenue.

Land maps illustrating the difference between 2003 and June 2011 LiDAR data indicates that the flat land of Sumner has lifted near the Port Hills and dropped slightly nearer to the coast.

A16.2. Customer service reports A number of formal complaints have been lodged to Council through the Council Call Centre both before and after the March 2014 flood event. The majority of complaints relating to the March event were associated with maintenance issues. These issues included blocked sumps and drains, insufficient capacity of the Sumner Stream along Paisley Street and the SMD located along the rear sections of Head Street as well as the existing beach outfalls being blocked, especially the Sumner Flood Relief Pipe (SFRP) and the Burgess Street outfalls at beach level. Also noted by residents was the increase in suspended silt in the flood waters in the March flood event compared to previous floods. One complaint referred specifically to significant silt deposits collecting at a Finnsarby Place property where flooding occurred to house foundation level.

During site inspections local Sumner residents suggested that the majority of surface flooding which occurs along Marriner Street and the Esplanade can be attributed to operation and maintenance issues associated with the stormwater outfalls at Cave Rock and Burgess Street. During the March flooding event the Cave Rock outfall was operational however the Burgess Street DN650 outfalls were potentially block which resulted in surcharging of the stormwater network upstream in Marriner Street. During previous flood events the Cave Rock outfall has been completely restricted by sand deposits which were removed in 2011 through an excavator in order to prevent further flooding on Mariner Street (Refer to Photo 1).

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Figure 44 Sumner location Plan

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Local residents on Paisley Street noted that very little maintenance had been completed on the Sumner Stream and SMD adjacent to Paisley Street and Wakefield Street. Residents also noted the overall depth of these open channels have gradually reduced due to accumulation of sediment.

Residents witnessed silty stormwater flowing across Wakefield Street into Paisley Street from under the shipping containers which border the Residential Red Zone on Wakefield Street. Also noted was that the majority of the flooding in Paisley Street was associated with the section of the Sumner Stream at the intersection of Paisley and Van Asch Streets.

The residents at a Wakefield Street property observed that the stormwater in the sump outside their property was not draining away during heavy rain periods even though the water level in the SMD was lower.

Residents at a Finnsarby Place property noted the stormwater runoff from the Red Zone on Sumnervale Drive contained a high concentration of suspended silt which then accumulated further downstream at Finnsarby Place and Sumnervale Drive in the sumps, kerb and channels.


Sumner Main Drain (SMD) This is a combination of open drains with timber sides and concrete sides/base, RCRR pipe and box culverts under roads. Recently a new box culvert has been added outside a property at Heberden Avenue.

SMD runs through numerous private properties (Refer to Photo 2). During the March flooding event high levels in the SMD contributed to flooding in adjacent properties in Arnold Street, Nayland Street, Wakefield Street and Paisley Street.

Sumner Flood Relief Pipe (SFRP) DN2100 RCRR pipe located through reserves and roads, discharging at the outfall located near Scarborough Park adjacent the SMD outfall. It was constructed to reduce the load on the SMD.

Ocean Outfalls:

DN300, Nayland Street

DN1200, Esplanade overflow for Richmond Hill Road drain and discharges to a swale at the corner of Mariner Street and Esplanade. This is secondary system to minimize flooding when the capacity of the Cave Rock / Burgess Street system is exceeded.

DN1350, Cave Rock, Esplanade. Flooding occurs on Mariner Street and Esplanade when the flap gate is restricted by sand and debris accumulation at the outfall and within the pipe (Refer to Photo 3 and 4).

2 x DN675 Burgess Street/Esplanade. A flap gate on the end of the Burgess Street DN900 pipe is located within a manhole structure immediately upstream of the twin outlet. This structure has been known to fill with sand, blocking the DN900. The DN675 outlets did not have backflow prevention during the March event but flap gates have been recently installed. Flooding occurs when the stormwater network surcharges back towards Mariner and Nayland Streets when the outfalls are not operating correctly or are restricted with sand (Refer to Photo 5 and 6).

DN225, Marriner Street/ Esplanade

DN300 Stoke Street/Esplanade

DN750 Hardwick Street

DN2100 SFRP Scarborough Park. Sluice water are nozzles attached to remove sand build up from the flap gate. Rocks/debris is often mobilized during nor-easterly storms and jam the flap gate open. Residents noted that the local fire service previously attempted unsuccessfully to open flap gate manually.

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SMD outfall and tide gate installed in 2004

A16.4. SCIRT projects SCIRT project #11055 Stormwater, Roading and Water infrastructure (Planned 11 August construction start, City Care)

SCIRT project #11054 Wastewater (Planned 4 August construction start, City Care)

A16.5. Existing flood protection infrastructure As a result of the October 2000 Sumner Flooding stormwater drainage improvements were completed on the Richmond Hill Road waterway. These included the Nayland Street culvert and high level weir which reduces the loading on the pipeline through Richmond Hill walkway by diverting flows through to Burgess Street. During the March flood the Cave Rock outfall was operational and flooding in this area was not as significant as previous events. However information from residents suggests the Burgess Street pipeline was surcharged which resulted in flooding in Marriner Street.

Tidal flood protection for the stormwater network in Sumner is limited to flap gates on approximately four of the ocean outfalls.


Cause The main cause of flooding in the March event was the depth of the rainfall event coinciding with the very high tide. Operation and maintenance deficiencies have likely contributed flooding in some cases. Operation and maintenance issues contributing to flooding include:

Main ocean outfalls at Cave Rock, Burgess Street and the SFRP not operating correctly due to debris/sand blockage

Reduced capacity of open drains due to silt build-up contributing to overflows from Sumner Stream and SMD around Van Asch and Paisley Streets

Surface flooding at existing low points on Clark Street, Arnold Street and Colenso Street possibly exacerbated silt build-up in SMD

Reduced performance and capacity of SFRP from sediment and marine deposits

General description of damage Above floor level flooding and flooding up to foundations was observed during the March flooding event in approximately 32-42 sections located in areas which are either adjacent or in close proximity to the SMD.

7 houses above floor levels

42 up to foundation level

This was confirmed by residents at public meeting on May 5th 2014.

Surface flooding was also noted by residents in Mariner Street, Wiggins Street and the Esplanade. Large quantities of silt and sediment were reported at Finnsarby Place downstream of the Red Zone.

Local effects The majority of the sections flooded in the March event were located in the Clark Street/Arnold Street/Colenso Street area. This area has a history of flooding and is within the modelled flood extents (Refer to Photo 7).

Wiggins Street also has a history of surface flooding which affects road access around the school. Mariner Street has a history of flooding which affects the retail sector of Sumner and major access route through to Sumner from Main Road.

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A16.7. Other 2014 floods 125 Wakefield Street has been flooded 4 times up to foundation level in 2014 and 2 times in 2013.


Social impact Households with floor level flooding experienced considerable impact. The wider community was impacted by residential property flooding and restricted access to amenities including Sumner School, the retail sector and the main access into Sumner through Mariner Street.

Community feedback During site inspections many residents noted that diligent operation and maintenance of the Cave Rock and Burgess Street outfalls is essential in flood prevention of Mariner Street and Nayland Street. Many residents actively maintain local kerb and channels by clearing silt and debris to prevent localized flooding such as Finnsarby Place.

A community meeting was held on 5th May where residents were able to confirm the extent of the March flood event and houses which were affected.

A16.9. Field studies Site visits were completed on the 3rd and 4th May.

Sand sediment was observed to be covering most of the Cave Rock outfall and SFRP outfall appears jammed partially open with rocks.

Large amounts of sediment were observed in invert of Sumer Stream and SMD along Wakefield and Paisley Streets. Water was ponding in the Wakefield Ave culvert crossing and in Sumner Stream.

Large amounts of silty sediment deposits were observed in the Red Zone on Finnsarby Place and in sumps and kerb and channels. Some deposits also from Red Zone were located downstream on Wakefield Avenue.


Land level changes The CGD vertical and horizontal ground movement database shows the difference between 2003 and June 2011 LiDAR data. This indicates that the flat land of Sumner has lifted near the Port Hills and dropped nearer to the coast.

Liquefaction Only minor liquefaction was observed along Nayland Street immediately after the February and June 2011 earthquake events.

Land in the CERA Green Zone has been divided into three technical categories (TC). These categories describe how the land is expected to perform in future earthquakes. CERA has therefore indicated the area is in the majority is categorised as TC2 indicating minor to moderate land damage from liquefaction is possible in future significant earthquakes.

Infrastructure damage Localized earthquake damage to pipe infrastructure to be completed under SCIRT project #11055. Damage also occurred to SMD however the drain has been propped and this has not affected the capacity.

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Bed heave/hydraulic capacity loss The invert of the Colenso Street SMD culvert has cracked and heaved along its length. The SMD bed has also heaved slightly in some locations.

Sediment deposits in Sumner Stream /SMD along Wakefield Avenue and Paisley Street are a possibly from unstable slopes within Red Zone and surrounding slopes in the rural zone.

A16.11. Key photos

Figure 45 Cave Rock sand removal

Figure 46 Sumner Main Drain with private access bridge

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Figure 47 Cave Rock outfall with sand restrictions

Figure 48 Cave Rock outfall during March event (flap gate partially open)

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Figure 49 Burgess Street outfalls with sand build-up

Figure 50 Burgess Street outfall with new flap gates

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Figure 51 March flooding up to foundations

A16.12. Options

Constraints Physical constraints include:

Removal and maintenance of sand/debris from outfalls during flooding event in conjunction with high tides.

Access to and location of the existing SMD through private properties.

Opportunities Redesign of outfalls could incorporate information from Variation 48 with regard impact of flooding from climate changes. Increased capacity could be achieved by increasing the single sump at the low point adjacent to a Wakefield Street property to a double sump and increasing pipe capacity to Sumner Stream/SMD.



Localised bunding to direct overland flow under shipping containers on Wakefield Street away from Paisley Street and into a stormwater sump.

Localised bunding around properties on intersection of Van Asch Street and Paisley Street to prevent overland flows from Sumner Stream. Provide temporary pump or use existing residences pump to pump over the bunding.


Sediment control of Red Zone properties and monitoring of slope stability by CERA including sediment control fences around properties and removal of sediment in downstream sumps, kerb and channels.

Removal of private access bridges/pipes to Red Zoned properties and approximately 38 m of pipe crossing the Sumner Stream/SMD on Wakefield Street.

Upgrading sumps and pipes to increase drainage capacity at the low point on Wakefield Ave and reduce flooding of the house on Wakefield Ave.

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Formalise the secondary flow path from DN1200 Cave Rock Drain outfall on Esplanade through to swale or ocean outfall (possibly as part of Surf Club redevelopment project).

Renew/rehabilitate SMD culvert on Colenso Street which has heaved in the invert. This defect does not however contribute significantly to capacity issues of the SMD therefore is not considered a critical fault which requires immediate attention.

Design review of Cave Rock and Burgess Street outfalls to minimize the effect of sand and debris so outfalls are both operational during storm events.


Removal of sediment and cleaning of Sumner Stream/SMD. Clean out single sumps and DN225 pipe outside the Wakefield Street property.

Removal of obstructing vegetation around the culvert on Campbell Street.

Removal of rocks from SFRP outfall at Scarborough Park and ensure that flap gate is operational. Redesign flap gate to allow manual operation in an emergency response situation.

Removal and cleaning of debris and marine deposits from SFRP.

Develop and implement emergency response plan to ensure that key outfalls are able to operate effectively prior to a forecasted storm event. This requires comprehensive documentation of key outfalls and inlets.

Social impacts of measures The implementation of maintenance and operation measures in conjunction with a design review of the Cave Rock and Burgess Street flap gates will potentially mitigate flooding in Mariner Street and Esplanade. This will impact significantly for residents and retail properties located in these areas and benefit the whole community by mitigating the impact of flooding through these main transport routes into Sumner.

Implementation of Flood Defense measures associated with a Wakefield Street property will significantly improve general living conditions for residents at this property.


Upgrading sumps and pipes to increase drainage capacity at the low point on Wakefield Ave and reduce flooding of house on Wakefield Ave

Removal of sediment from Sumner Stream/SMD and vegetation obstruction in Campbell Street culvert in conjunction with removal of private bridges to Red Zone properties crossing the Sumner Stream in Wakefield Street where feasible. Consultation with CERA is required regarding access bridges and proposed demolition timeframes to Red Zone properties on Wakefield Street.

Daylighting of piped section of Sumner Stream (approximately 38m)

Notification to CERA for sediment control of Red Zone properties and monitoring of slope stability including sediment control fences around properties and removal of sediment in downstream sumps, kerb and channels

Removal of sediment and debris from SFRP and SMD outfalls. Design review of Cave Rock, Burgess Street and SFRP outfalls in order to mitigate operational issues related to sand and marine deposits blocking these structures

Formalisation of the secondary flow path from DN1200 Cave Rock Drain outfall on Esplanade through to swale or ocean outfall

Removal of sediment and debris from SFRP

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Develop and implement emergency response plan to ensure that key outfalls are able to operate effectively prior to a forecasted storm event. This requires comprehensive documentation of key outfalls and inlets.

Timeframe Operation and Maintenance measures includes the flood defence of the house on Wakefield Street and notification of sediment control requirements to CERA can be implemented immediately. Design review of Cave Rock and Burgess Street outfalls is to be completed with 3 months in order to minimise the cost associated with water usage of current design and help mitigate flooding on Mariner Street and Esplanade. Daylighting of the pied section of Sumner Stream can occur within six months in conjunction with SCIRT project #11055. Removal of private bridges can be implemented with maintenance and cleaning of Sumner Stream/SMD. Confirmation with the Surf Club is required to formalise a secondary flow path in conjunction with future development of the Surf Club.

A16.15. Recommendations for future studies Further studies could investigate options for improving the capacity of SMD and options to mitigate sediment inflow from surrounding slopes.

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A17 Sumner: Moncks Bay A17.1. Background

Location The Sumner –Monks Bay area comprises Beachville Street to the end of Moncks Bay at Cliffs Road.

Existing studies

SCIRT Project Concept design #10924 Ferrymead to Sumner Catchment Study

SCIRT Project Detailed design #11107 Redcliffs McCormacks Bay

SCIRT Project Detailed design #11108 Redcliffs McCormacks Bay

SCIRT Project Detailed design #10667 Moncks Bay Seas Walls

Flooding history Cliff Street has a history of flooding due to area being the lower than surrounding streets. The properties that are adjacent to the Rifle Range Drain also have a history of section flooding in addition to the sections that are adjacent Barnett Park and the Rifle Range Drain overflow.

Main Road has a history of flooding as the Rifle Range Drain secondary flow path spills over the road, however only a few houses opposite Barnett Park and the Moa Community Kid Learning Centre reported flooding above floor level in the March flooding event.

A17.2. Customer service reports A number of formal flooding complaints have been lodged to Council through the Council Call Centre both before and after the March 2014 flood event. The majority of complaints relating to the March event were associated with flooding from the Rifle Range Drain which runs through private properties on Bay View Road.

Localised flooding was also reported in properties opposite Barnett Park which was caused by the secondary flow path from the Rifle Range Drain discharging across Main Road. The Rifle Range Drain was also reported to be close to bursting its banks near the sub-station at the top of Bay View Road. A large sink hole also appeared after the March flooding event where the joints of a DN1350 pipe have separated due to lateral spread approximately 8m from the ocean outfall (Refer Photo 1). Repair for this fault is scheduled to be completed under SCIRT project #11107.

The majority of historic and post-earthquake flooding complaints are related to maintenance of blocked sumps.

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Rifle Range Drain and overflow: A combination of open drain through Barnett Park, open drain with timber sides, concrete base with timber sides, RCRR pipe and box culverts under roads. A weir is located inside Barnett Park where high level flows are diverted through to an overflow drain and open grated inlet outside the Moa Community Kids Early Learning Centre on Main Road. This inlet is linked to the DN1200 stormwater pipe on Main Road which discharges at the Rifle Range Drain outfall located at the rear a property on Main Road.

Bay View Waterway: A timber lined drain which intersects the Rifle Range Drain inside private property on Bay View Road.

Ocean Outfalls:

DN1350 Main Road, repair and inline check valve to the completed under SCIRT project #11107

DN225 Main Road through sea wall



4x DN225 Main Road through sea wall

DN1800 Main Road through sea wall Rifle Range drain outfall




DN1300 Beachville Road

DN375 Beachville Road

2x DN225 Beachville Road

Approximately 70% of the ocean outfalls in Moncks Bay have flap gates. Under SCIRT project #11107 an inline check valve has been included in the stormwater design along with the removal of the existing concrete structure at the Rifle Range Drain outfall (Refer to Photo 2).

One of the DN300 outfalls opposite Main Road will be removed and the upstream pipe connected to the adjacent DN375 outfall as part of SCIRT project #11108.

Two DN225 outfalls will be removed and a short section of an existing DN375 will be relayed through the ocean wall as part of SCIRT project #11107. The smaller diameter pipes along Beachville Road are higher and will have tideflex/wastops installed in the pit upstream of the outfall however no flap gates or back flow prevention valves will be installed in the DN1300 as part of this project .


SCIRT Project #11107 Redcliffs McCormacks Bay (Planned 11 August 2014 construction start, Fulton Hogan. Construction date may vary due to proposed Coastal Pathway)

SCIRT Project Detailed design #11108 Redcliffs McCormacks Bay (Planned 9 June 2014 construction start, Fulton Hogan)

SCIRT Project Detailed design #10667 Moncks Bay Seas Walls (currently on hold due to mass movement area)

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A17.5. Existing flood protection infrastructure Flap gates are present on approximately 70% of outfalls in Moncks Bay to help protect against tidal flooding.

Weir and overflow drain located in Barnett Park to reduce load on Rifle Range Drain which drains into an open grated inlet structure in front of the pre-school (refer to photos).


Cause Land in Sumner was inundated due to fluvial and tidal flooding. This has various contributing causes:

Operation and Maintenance Potential operational issues and reduced capacity in the stormwater pipes upstream of the

Rifle Range Drain outfall and Beachville Road outfall due to sediment and marine deposits. CCTV investigations for condition assessments identified a number large diameter pipes in the Moncks Bay area were 30-50% full of marine silt.

Secondary path overland flow and existing low lying topography

Secondary path overflows from the Rifle Range Drain through Barnett Park, Moa Community Kids Early Learning Centre through to Main Road.

Secondary flow path from Main Road has no direct route to the estuary therefore contributes to flooding in Wakatu Avenue, Bayview Road and Cliff Street

Accumulation of surface flooding in Cliff Street due to the existing low point in the area

Tidal flooding in conjunction with rainfall events

General surface flooding during rainfall events which co-inside with high tide. As the road level is approximately at the high tide level, stormwater collects at low points in Cliff Street and Main Road and is unable to discharge through the ocean outfalls. Some ocean outfalls have no backflow prevention against tidal flooding

General description of damage Above floor level flooding and flooding up to foundations was observed during the March flooding event in properties located in low lying areas in Bayview Road, Cliff Street, Wakatu Avenue and properties opposite Barnett Park.

The Moa Community Kids Early Learning Centre also reported flooding above floor level and contamination from wastewater overflows. Pump Station 31 adjacent to the pre-school is also located in the Rifle Range Drain overflow secondary flow path and was flooded in the March event (Refer to Photo 4).

Affected properties included:

Approximately 61 up to foundation level (not confirmed by residents) Approximately 6 above floor level (not confirmed by residents) One vacant section was also flooded

Local effects The majority of sections that flooded in the March event were either adjacent or in close proximity to the Rifle Range Drain, the Rifle Range Drain overflow secondary flow path or within the low points in Cliff Street, Bay View Road and Wakatu Avenue where stormwater discharge to the ocean outfalls was restricted by incoming tidal flooding (Refer to Photo 5).

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A17.7. Other 2014 floods Residents from Main Road properties have confirmed flooding has occurred previously on Main Road opposite Barnett Park.


Social impact No specific social impact report has been received .There can be expected considerable social impact from above floor level flooding and residential property flooding. Surface flooding on Main Road can potentially restrict access to properties in Moncks Bay and other areas including Clifton, Sumner and Scarborough.

Community feedback During site inspections a resident on Main Road confirmed that Main Road was flooded opposite Barnett Park and several properties opposite and downstream of the park in the March event. The manager from the Moa Community Kids Early Learning Centre confirmed that the March event was the only time flooding has occurred above foundation level.

Reports and flooding complaints from residents suggest that when the Rifle Range Drain reaches high levels it often floods into the rear of sections on Bay View Road (refer to Photo 6 and 7).

A17.9. Field studies Site visits were completed on the 4th May.

The high flood level was visible along the Rifle Range Drain where vegetation had been flattened and debris was visible in the weir. The secondary overflows through the Moa Community Kids Early Learning Centre property was identifiable by silt deposits in the car park and access to the playground was still restricted. Immediately after the March event Chem-dry cleaners were onsite to decontaminate both the building and playground.

Silt deposits were also observed at along Bay View Road. The resident at a Bayview Road property confirmed that the garage and the property had been flooded in the March event by overflows from Rifle Range Drain located at the rear of her property.


Land level changes The CGD vertical and horizontal ground movement database shows the difference between 2003 and June 2011 LiDAR data. This indicates that the flat land of Sumner has lifted near the Port Hills and dropped nearer to the coast. Some areas of Moncks Bay have uplifted by 0.5m.

Liquefaction Only minor liquefaction was observed along Nayland Street immediately after the February and June 2011 earthquake events.

Land in the CERA Green Zone has been divided into three technical categories (TC). These categories describe how the land is expected to perform in future earthquakes. CERA has therefore indicated the area from Barnett Park along Main Road is categorised as TC2 in which “minor to

moderate land damage from liquefaction in future large earthquakes” is possible. The area from

Wakatu Avenue to Celia Street on the ocean side is categorised as TC3 where “moderate to

significant land damage from liquefaction in future large earthquakes” is possible.

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Infrastructure damage Localised earthquake damage to pipe infrastructure occurred and is to be repaired under SCIRT projects #11108 and #11107. SCIRT project #11107 includes road reshaping in front of the properties identified as the most vulnerable from flooding by the Rifle Range Drain secondary flow path on Main Road. A pulled joint at the Rifle Range Drain outfall will also be repaired as part of this project.

A17.11. Key photos

Figure 52: Tomo at Rifle Range Drain outfall

Figure 53:Existing Rifle Range Drain outfall at low tide

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Figure 54: Stormwater inlet structure for Rifle Range Drain overflow

Figure 55: Moa Community Kids Early Learning Centre day after March 2014 flooding event

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Figure 56: Surface flooding Bayview Road March 2014

Figure 57: Rifle Range Drain in Barnett Park. High flood level indicated by flattened vegetation and gravel deposits

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Figure 58: Top of Bayview Road with silt deposits day after March 2014 flooding event

A17.12. Options

Constraints Physical constraints include:

Location and access to Rifle Range Drain through private properties

Access to ocean outfalls located at the rear of private sections on Main Road

Tidal flooding in conjunction with high rainfall events and existing low lying properties

SCIRT projects on hold due to Coastal Path walkway and also mass movement area

Opportunities Conducting stormwater work in Sumer provides the potential opportunity to:

Install an inline check valve to stormwater outfall at Beachville Road to prevent sand deposits and debris blocking the upstream stormwater network

Review existing SCIRT project #11107 and integrate any further opportunities to protect driveways of vulnerable properties including raising the berm and reshaping the footpath

Modify the road design through Moncks Bay to create a new low point

Purchase land adjacent constructed low point (potentially on Main Road) and formalise a secondary flow path through to ocean outfall via purchased property

Progress SCIRT projects #11108 and #11107 to construction phase and implementation of new road levels, additional sumps and installation of inline check valves included the design for these projects

Formalise secondary flow paths on title or property records to prevent buildings or development blocking or impeding flow paths

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Localised bunding and reshaping of landscaping surrounding the Moa Community Kids Early Learning Centre to direct overland flow away from the pre-school building and playground. Reforming of secondary flow path through car park to Main Road from Rifle Range Drain overflow inlet structure

Localised bunding across driveways of properties vulnerable from flooding on Main Road

Floor raising of the most vulnerable properties to flooding from secondary flow path on Main Road


Bunding around the perimeter of Pump Station 31 to divert any potential wastewater overflows away from the Moa Community Kids Early Learning Centre building and playground


Removal of sediment and debris from stormwater pipes including the DN1200 and DN1350 on Main Road upstream of the Rifle Range Drain outfall and Beachville outfall where tidal inundation has caused silt deposits within these pipes

Maintenance of Rifle Range Drain weir. Reforming of open drain upstream of Rifle Range Drain weir to increase the cross sectional area and prevent high level flooding from the drain migrating down Bay View Road

Widening of the access point through the chain fence to allow vehicles/machines to enter into area to complete regular or emergency maintenance and cleaning of inlet structure

Social impacts of measures Localised bunding may cause visual impact and inhabit access to private properties. Reforming of the Moa Community Kids Early Learning Centre landscaping may cause interruption to operation of school during contract works

A17.14. Recommendations The preferred local area schemes include:

Bunding and reshaping of landscaping to divert the Rifle Range secondary flow away from Moa Community Kids Early Learning Centre .Reforming of secondary flow path through car park to Main Road from Rifle Range Drain overflow inlet structure

Localised bunding across driveways of properties vulnerable from flooding on Main Road

It is anticipated that the local area schemes and maintenance could be completed within one month of approval. An emergency response plan could also be developed to ensure critical outfalls are operational and not restricted by debris and sediment deposits

A17.15. Recommendations for future studies Further investigations could include:

Investigate impact of Variation 48 on existing low lying areas such as Cliff Street.

Investigate feasibility for increased retention upstream of Barnett Park.

Land purchase and formalizing secondary flow path from Rifle Range Drain

temporary flood defence measures : technical...

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