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EWE Associates Ltd Windy Ridge Barn

Thealby Lane Winterton

Scunthorpe North Lincolnshire

DN15 9TG t: 0845 8377783

M: 07875 972270 e: [email protected]

This document has been prepared solely as a Flood Risk Assessment for Conder Developments Ltd. EWE Associates Ltd accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared.

Conder Developments Ltd

Proposed Development High Street Rocester

Staffordshire

Flood Risk Assessment

Prepared by EWE Associates Ltd Final Report RevE February 2014

Conder Developments Ltd Proposed Development, High Street, Rocester – Flood Risk Assessment Final Report RevE February 2014

EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire DN15 9TG. t: 01724 733349 : i

REVISION HISTORY

Revision Ref./

Date Issued

Amendments Issued to

Draft Report Rev A: 8th November 2008 Paul Shanley (Conder Developments Ltd) - 1No. Copy

Draft Report Rev B: 24th November 2008 Amendments following Client’s comments

Paul Shanley (Conder Developments Ltd) - 1No. Copy

Environment Agency - 1No. Copy

Final Report Rev C: 13th February 2012 Amendments to layout drawing Paul Shanley (Conder Developments Ltd) - 1No. Copy

Pegasus Planning - 1No. Copy

Final Report Rev D: 30th August 2012 Updated with latest EA flood data and subsequent revision of flood envelopes within site.

New EA reservoir maps added and commented.

Further local historical information added.



Final Report Rev E: 13th February 2014 Updated with latest EA flood data and subsequent revision of flood envelopes within site.

Updated drainage strategy



CONTRACT

This report describes work commissioned by Conder Developments Ltd following instruction by their representative on 4th August 2008. Conder Development Ltd representative for the contract was Mr Paul Shanley. Donna Metcalf and Lea Favill of EWE Associates Ltd carried out the work.

Date: 13th February 2014

Prepared by: ………………………………… ………………… Lea Favill Director


EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire DN15 9TG. t: 01724 733349 : ii

EXECUTIVE SUMMARY

Executive Summary

The site presently consists of farmland divided into a number of fields separated by established hedgerow and fence lines, attached to Churnet House Farm, located off High Street in the Staffordshire village of Rocester. Churnet House Farm and associated outbuildings occupy the area of the site to the north, with the remaining undeveloped area within the site, previously utilised as pasture for grazing cattle and horses. The parcels of land are linked by an unsurfaced track leading south from the farm buildings off High Street. The site is located 15 miles to the south east of Stoke on Trent.

The site covers a total area of 4.0 Hectares. It is considered that the existing site is 90% permeable due to the relatively small roof and paved areas within the site. During the day of the site visit there was evidence of a positive surface water drainage system within farm building complex via rainwater pipes and yard gullies. The remaining farmland area is not positively drained.

The site lies at levels between 84.80mAOD along the south boundary of the development area, and 86.63mAOD within the farm building complex. There are existing residential dwellings to the north along Churnet Row, and also to east of the site within Rocester village. JCB’s World Headquarters site is located 300m to the north west of the proposed development, and JCB Finance is located at The Mill, which is situated immediately adjacent to the west boundary of the site.

The proposed development is to comprise mixed development comprising residential units and an employment area covering a total of 2.14 Hectares. It is anticipated that the impermeable area of the site will be increased to 53% following completion of the development.

The development site is shown to lie partially within Zone 3 of the Environment Agency Flood Map (version 2.8.2), being the zone with risk of 1 in 100 year (1% AEP) or greater for river flooding. The remaining area of the site lies within Zone 2 of the Environment Agency Flood Map (version 2.8.2), being between a 1 in 100 and 1 in 1,000 annual probability (chance) of river flooding (1% -0.1%). It is understood that the flood zone plan provided by the Environment Agency has been mapped using LIDAR, and indicates that the primary flood risk to the site comes from the River Churnet. The watercourse is situated approximately 40m to the west of the proposed development. The development site is larger than 1 hectare in size.

There are three sources of flood risk – the River Churnet; the River Dove and surface water runoff – it is necessary to determine flood water levels at the site for the desired return periods emanating from these sources.

The River Churnet is in line with the development and is considered to represent the greatest flood risk to the site. The River Churnet estimated 1 in 100 year flood levels are 84.62mOD, 84.90mOD and 85.81mOD at Node References 02682, 02888u and 02945u respectively. The estimated 1 in 100 year plus climate change flood levels at these nodes are 84.77mOD, 85.07mOD and 85.96mOD. From the topographic survey of the development, existing ground levels are shown to be elevated above these modelled flood levels.

Following consultation with the Environment Agency it was suggested that the internal ground floor levels for the proposed residential development are set at a minimum of 600mm above the 1 in 100 year plus climate change flood levels in line with the site. It is also advised that the internal ground floor level for the commercial development within the employment area is set at a minimum of 300mm above the 1 in 100 year plus climate change flood level, incorporating flood resilience measures up to 300mm above the ground floor level of the proposed buildings. For the purposes of recommending finished ground floor levels within the area of the site set aside for residential development, the area has been divided into three sections corresponding to the locations of node references 02682, 02888u and 02945u.

In compliance with the Environment Agency’s requirements it is recommended that internal ground floor levels in line with Node Reference 02945u are set at a minimum of 86.560mOD. The lowest existing ground level within this area is shown to be 86.050mAOD and as such, some land/floor raising will be required, in order to provide adequate freeboard above the 1 in 100 year plus climate change flood level in line with the development.

The internal ground floor level of the proposed residential units in line with Node Reference 02888u should be set to a minimum of 85.430mOD. The lowest existing ground level within this section of the site is 85.430mAOD and as such, no land/floor raising will be required.


EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire DN15 9TG. t: 01724 733349 : iii

Additionally, it is recommended that the internal ground floor levels of the proposed dwellings in line with Node Reference 02682 are set to a minimum level of 85.110mAOD. The lowest existing ground level within this southern section of the proposed development has been surveyed as 84.800mAOD. Therefore some land/floor raising will also be required within this part of the development in order to provide adequate freeboard above the 1 in 100 year plus climate change flood level.

In order to mitigate against localised flooding which may be caused by heavy rainfall, it is also advocated that the internal ground floor levels of the proposed residential units are elevated at least 150mm above the adjacent external finished ground levels.

The area of the site set aside for commercial development is in line with Node Reference 02945u and therefore the 1 in 100 year plus climate change flood level utilised for this part of the development is estimated to be 85.96mOD. In compliance with the Environment Agency’s requirements it is recommended that internal ground floor levels in line with Node Reference 02945u are set at a minimum of 86.260mOD. The lowest existing ground level within this area is shown to be 86.050mAOD and as such, some land/floor raising will be required, in order to provide adequate freeboard above the 1 in 100 year plus climate change flood level in line with the development.

For the low lying areas within the commercial site, where ground levels are less than 600mm above the 1 in 100 year plus climate change flood level, it is recommended that flood proofing measures are incorporated into the commercial building design up to a level of 600mm above the 1 in 100 year plus climate change flood level, at 86.56mOD.

In order to mitigate against localised flooding which may be caused by heavy rainfall, it is also advocated that the internal ground floor levels of the proposed commercial units are elevated at least 150mm above the adjacent external finished ground levels.

It is concluded that there is a low risk of flooding due to overtopping of the Rivers Dove and Churnet during the 1 in 100 year and 1 in 100 year plus climate change flood events within Rocester. An additional flood risk is from increased surface water runoff following development. The implementation of mitigation measures as described above will ensure that any risk of flooding at the proposed development may be reduced to an acceptable level.


EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire DN15 9TG. t: 01724 733349 : iv

CONTENTS

Page REVISION HISTORY CONTRACT EXECUTIVE SUMMARY CONTENTS LIST OF FIGURES LIST OF TABLES

i i ii iv v v

1 INTRODUCTION--------------------------------------------------------------------------------------------------------------------------------- 1

1.1 Terms of Reference........................................................................................................................................................1 1.2 Consultation with the Environment Agency ....................................................................................................................1 1.3 Approach to the Assessment..........................................................................................................................................1 1.4 Application of Sequential & Exceptions Test ..................................................................................................................2

2 DETAILS OF THE SITE ----------------------------------------------------------------------------------------------------------------------- 3

2.1 Site Details......................................................................................................................................................................3 2.2 Site Description...............................................................................................................................................................4

3 INITIAL ASSESSMENT------------------------------------------------------------------------------------------------------------------------ 5

3.1 Past Flooding History......................................................................................................................................................5 3.2 Local Flooding History ....................................................................................................................................................5 3.3 Environment Agency Flooding History............................................................................................................................5 3.4 Environment Agency Flood Map.....................................................................................................................................6 3.5 Environment Agency Reservoir Flood Map ....................................................................................................................6 3.6 Possible Flooding Mechanisms ......................................................................................................................................7

4 QUANTATIVE FLOOD RISK ASSESSMENT-------------------------------------------------------------------------------------------- 9

4.1 Requirements of the Environment Agency .....................................................................................................................9 4.2 River Dove......................................................................................................................................................................9 4.3 River Churnet................................................................................................................................................................10 4.4 Existing and Proposed Foul Drainage ..........................................................................................................................13 4.5 Increase Run-off due to the Development ....................................................................................................................13

5 MITIGATION MEASURES ------------------------------------------------------------------------------------------------------------------ 16

5.1 Raising Floor Levels/Land Raising ...............................................................................................................................16 5.2 Emergency Access and Egress....................................................................................................................................17 5.3 Flood Compensation.....................................................................................................................................................17 5.4 Control of Runoff...........................................................................................................................................................17 5.5 Proposed Outfall to River Churnet................................................................................................................................18 5.6 SUDS............................................................................................................................................................................18 5.7 Flood Resilience Measures – Proposed Commercial Development.............................................................................19 5.8 Conclusion....................................................................................................................................................................24

APPENDICES:

APPENDIX A: - EXISTING GROUND LEVELS

APPENDIX B: - PROPOSED DEVELOPMENT LAYOUT PLAN

APPENDIX C: - ENVIRONMENT AGENCY FLOOD DATA


EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire DN15 9TG. t: 01724 733349 : v

APPENDIX D: - WINDES MODEL RESULTS

APPENDIX E: - PROPOSED DRAINAGE STRATEGY

APPENDIX F: - FLOOD ENVELOPES

LIST OF FIGURES

Figure 2.1: Existing Site Viewed South East. ..........................................................................................................................................4

Figure 3.1: Environment Agency Flood Zones ........................................................................................................................................6

Figure 3.2: Environment Agency Reservoir Flood Map...........................................................................................................................6

Figure 4.1: The River Dove at Rocester Bridge.......................................................................................................................................9

Figure 4.2: The River Churnet Viewed Downstream from Churnet Bridge............................................................................................11

Figure 5.1: Design Strategy Flowchart – Resistance/Resilience...........................................................................................................20

Figure 5.2: Water Ingress into building through the ground – ground bearing concrete floor slab........................................................21

Figure 5.3: Ground supported concrete floor.........................................................................................................................................22

Figure 5.4: Cavity external wall – part filled cavity.................................................................................................................................23

LIST OF TABLES

Table 1.1: Flood Risk Vulnerability and Flood Zone ‘Compatibility’.........................................................................................................2

Table 2.1: Development Location...........................................................................................................................................................3

Table 3.1: Possible Flooding Mechanisms .............................................................................................................................................7

Table 4.1: Summary: Comparison of Modelled Flood Levels with Existing Ground Levels within the Site ...........................................12

Table 5.1: Summary: Recommended Floor Levels (Residential) ..........................................................................................................16

Table 5.3: SUDS Techniques and Suitability of Use .............................................................................................................................19

Table 5.4: Flood Resilience Characteristics of Walls ............................................................................................................................22


EWE Associates Ltd Windy Ridge Barn, Thealby Lane, Winterton, Scunthorpe, North Lincolnshire DN15 9TG. t: 01724 733349 1

1 INTRODUCTION

1.1 Terms of Reference

This report was commissioned by Conder Developments Ltd to support a planning application for a mixed residential and commercial development located off High Street, Rocester. The site is within the village of Rocester in Staffordshire and can presently be accessed directly from High Street. The location of the site is shown on Table 2.1.

The development site is shown to lie partially within Zone 3 of the Environment Agency Flood Map (version 2.8.2), being the zone with risk of 1 in 100 year (1% AEP) or greater for river flooding and 1 in 200 year (0.5% AEP) or greater for tidal/coastal flooding. The remaining area of the site lies within Zone 2 of the Environment Agency Flood Map (version 2.8.2), being between a 1 in 100 and 1 in 1000 annual probability (chance) of river flooding (1% -0.1%); or between a 1 in 200 and 1 in 1000 annual probability (chance) of sea flooding (0.5%-0.1%). It is understood that the flood zone plan provided by the Environment Agency has been mapped using LIDAR, and indicates that the primary flood risk to the site comes from the River Churnet. The watercourse is situated approximately 40m to the west of the proposed development. The development site (4Ha) is larger than 1 hectare in size. The High Street directly adjacent to the northern boundary of the site lies within flood zone 1, low risk.

It is usual for the Agency to raise an objection to development applications within the floodplain or Zone 2 or 3 of the flood map until the question of flood risk has been properly evaluated. The Agency will also object to developments where the total site area is in excess of 1 hectare until suitable consideration has been given to surface water runoff.

1.2 Consultation with the Environment Agency

Following the planning submission dated March 2012 the Environment Agency provided comment in their letter dated 24th April 2012 (UT/2012/110278/01-L01). The requested that the latest flood data be obtained for the River Churnet as the data used has been superseded. Subsequently the River Churnet flood data was requested from the Environment Agency flood mapping team during May 2012.

Further to this the Environment Agency has been consulted with regards to additional information on local historical flooding and the risk from reservoir flooding.

During 2013 the Environment Agency commented that they could not support the application due to the current Churnet model being unfit for use. The Environment Agency confirmed that the remodelling works had been commissioned and the developer would be provided with a copy of the flood data when available. The FRA would need to be updated with this information.

Following the completion of the remodelling of the River Churnett a meeting was held on the 11th September 2013 with the Environment Agency to discuss the development and the modelling results.

Following the meeting it was confirmed that the primary flood risk was from the River Churnet and the River Dove to the east would not flood the site.

1.3 Approach to the Assessment

As there are three sources of flood risk – the River Churnet; the River Dove and surface water runoff – it is necessary to determine flood water levels at the site for the desired return periods emanating from these sources. Consideration has also been given to the site flooding from either overland flow or ponding of localised rainfall within the site.

The River Churnet is a tributary of the River Dove and is located approximately 40m to the west of the proposed development site. The confluence of the rivers is situated approximately 1.5km to the south of Rocester and the site. The Environment Agency does have modelled flood data which may assist in predicting the design flood level for the river adjacent to the proposed development site.

The River Dove is a tributary of the River Trent and is located approximately 750m to the east of the proposed development site. The Environment Agency does have modelled flood data which may assist in predicting the design flood level for the river in line with the proposed development site.



It is considered that the proposed development will significantly increase the paved and roofed area within the site resulting in an increase in surface water runoff from the development. As such the existing method of draining the site will be appraised.

A walk over survey of the site was conducted by Mr Lea Favill, a Principal River Engineer and Miss Donna Metcalf, Senior Engineer on 23rd September 2008; during the visit a photograph survey of the site was undertaken. A topographic survey of the development area was undertaken in September 2008 by the Greenhatch Group, drawing reference 8788_OGL. The survey has been calibrated to OS GB 36, using GPS and the resulting levels have been used within this report.

The requirements for flood risk assessments are generally as set out in NPPF and technical guidance. The detail and complexity of the study required should be appropriate to the scale and potential impact of the development. For the purposes of this study, the following have been considered:-

Available information on historical flooding in the area.

Site level information.

Details of structures, which may influence hydraulics of the watercourse and consideration of the effect of blockage of structures.

Estimates of design levels, equivalent to a 200-year (coastal/tidal) and a 100-year (fluvial) return period flood event.

Allowances for increased flows resulting from the effects of climate change.

Allowances for sea level rise resulting from the effects of climate change.

Assess the existing runoff characteristics and the potential impact the proposed development will have on the runoff.

Further guidance is also provided in the CIRIA Research Project 624 “Development and Flood Risk: Guidance for the Construction Industry”.

1.4 Application of Sequential & Exceptions Test

The development site is shown to lie partially within Zone 3 of the Environment Agency Flood Map (version 2.8.2) being the zone with risk of 1 in 100 year (1% AEP) or greater for river flooding and 1 in 200 year (0.5% AEP) or greater for tidal/coastal flooding. However, the findings of this flood risk assessment indicate that the proposed buildings are situated largely within Zone 1 of the Environment Agency Flood Map (version 2.8.2), being the zone with a risk of less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%).

The proposed development is considered to be a mixture of residential (more vulnerable) and commercial development (less vulnerable). As such, Table 1.1 below indicates that the proposed development is appropriate within the existing site.

Table 1.1: Flood Risk Vulnerability and Flood Zone ‘Compatibility’

Flood Risk Vulnerability classification

Essential Infrastructure

Water compatible

Highly Vulnerable

More Vulnerable

Less Vulnerable

Zone 1

Zone 2

Exception Test required

Zone 3a



Flood Zone

Zone 3b


Development is appropriate Development should not be permitted



2 DETAILS OF THE SITE

2.1 Site Details

Table 2.1: Development Location

Site Name: High Street, Rocester

Purpose of Development: Mixed residential and commercial development

Existing Land Use: Undeveloped agricultural land and farm buildings

OS NGR: SK 107 392

Country: England

County: Staffordshire

Local Planning Authority: East Staffordshire Borough Council

Internal Drainage Board: Not Applicable

Other Authority (e.g. British Waterways/ Harbour Authority)

Not Applicable

Location Plan:



2.2 Site Description

The site presently consists of farmland divided into a number of fields separated by established hedgerow and fence lines, attached to Churnet House Farm, located off High Street in the Staffordshire village of Rocester. Churnet House Farm and associated outbuildings occupy the area of the site to the north, with the remaining undeveloped area within the site, previously utilised as pasture for grazing cattle and horses. The parcels of land are linked by an unsurfaced track leading south from the farm buildings off High Street. The site is shown below in Figure 2.1. The extent of the proposed development site is shown at Appendix A of this report. The site is located 15 miles to the south east of Stoke on Trent.

The site covers a total area of 4.0 Hectares. It is considered that the existing site is 90% permeable due to the relatively small roof and paved areas within the site. During the day of the site visit there was evidence of a positive surface water drainage system within farm building complex via rainwater pipes and yard gullies. The remaining farmland area is not positively drained.

The site lies at levels between 84.80mAOD along the south boundary of the development area, and 86.63mAOD within the farm building complex. The ground levels within the site are illustrated at Appendix A of this report. There are existing residential dwellings to the north along Churnet Row, and also to east of the site within Rocester village. JCB’s World Headquarters site is located 300m to the north west of the proposed development, and JCB Finance is located at The Mill, which is situated immediately adjacent to the west boundary of the site.

The proposed development is to comprise mixed development comprising residential units and an employment area covering a total of 2.14 Hectares. Detailed layout and floor plans for the proposed development are not yet available; however a concept masterplan indicating the proposals for the site is included within at Appendix B of this report. It is anticipated that the impermeable area of the site will be increased to 53% following completion of the development.

Figure 2.1: Existing Site Viewed South East.



3 INITIAL ASSESSMENT

3.1 Past Flooding History

A search on the British Hydrological Society Chronology of British Hydrological Events website1 found two records of past flooding within the Churnet Valley. The first recorded event occurred in August 1880 when heavy rainfall caused flooding of the Churnet Valley. Heavy rainfall during February 1881 caused houses to become flooded, bridges washed away, and railway traffic stopped on the Churnet Valley line; part of railway line was washed away at Consall, and considerable damage done between Oakamoor and Alton, where the water was 5 ft. deep on the railway line.

Undertaking an internet based search for flooding in the area provided no further information.

3.2 Local Flooding History

During the consultation period historical flood information was provided from a local source. The historical evidence was in the form of photographs taken during the 1947, 1956 and 1998 floods. The photographs are taken from the west side of the B5030 where the large JCB factory is located. There are no photos of the proposed development site flooding with the exception of two photos which show the southern part of the site being inundated which is predicted by the Environment Agency modelling data. As such we consider that the photographs provided simply confirm the relevance and accuracy of the current EA flood maps and modelling. The Environment Agency confirmed they agreed with the above, however, confirmed that site investigation would be required to be able to comment in detail.

3.3 Environment Agency Flooding History

The Environment Agency was approached during May 2012 for any historical flood information. The Environment Agency provided a copy of the 1964 historical flood map which is provided at Appendix C of this report. The flood map shows that the site was not flooded during this event. The Environment Agency confirmed that they hold no other historical flood data for the area.

1 http://www.dundee.ac.uk/geography/cbhe/



3.4 Environment Agency Flood Map

3.5 Environment Agency Reservoir Flood Map

Figure 3.1: Environment Agency Flood Zones

Source: Environment Agency Website (www.environment-agency.gov.uk)

Figure 3.2: Environment Agency Reservoir Flood Map

Source: Environment Agency Website (www.environment-agency.gov.uk)



3.6 Possible Flooding Mechanisms

Table 3.1: Possible Flooding Mechanisms

Source/Pathway Significant? Comment/Reason Fluvial Yes River Churnet, River Dove

Tidal/Coastal No

Pluvial (urban drainage) Yes On site run off

Groundwater No

Overland flow No No higher ground adjacent to site which could promote overland flow.

Blockage No No significant culverts or bridges close to site which could cause a blockage.

Infrastructure failure No

Rainfall Ponding No No depressed areas within the site which could encourage ponding.

As there are three sources of flood risk – The River Churnet, the River Dove and surface water runoff – it is necessary to determine flood water levels at the site for the desired return periods emanating from these sources.

The first is from the River Churnet which is a tributary of the River Dove and is located approximately 40m to the west of the development. The watercourse is classified as Main River and is the responsibility of the Environment Agency and modelled flood data is available to assist in predicting the design flood level for the river adjacent to the proposed development site. The area is not defended from flooding by earth embankments or hard defences extending above normal bank level, therefore only overtopping of the natural river bank will require further consideration.

The second is from the River Dove, which is a tributary of the River Trent and is located approximately 750m to the east of the proposed development site. A mill race, branches off the main channel of the River Dove to the east of Dove Lane, and flows southwards, rejoining the River Dove to the south of the village. The River Dove is classified as Main River and is the responsibility of the Environment Agency. Modelled flood data for both the main channel and the mill race has been provided which may assist in predicting the design flood level for the river in line with the proposed development site. The watercourse at Rocester is undefended and as such, only overtopping of the natural river bank will require consideration.

It is considered that the proposed development will significantly increase the paved and roofed area within the site resulting in an increase in surface water runoff from the development. As such the existing method of draining the site will be appraised. Therefore further consideration will need to be given to the existing drainage route and the drainage characteristics in order to evaluate the impact surface water runoff from the site will have on the site and elsewhere.

Due to the local topography the site itself is relatively flat with a gentle gradient in a south westerly direction towards the River Churnet. There are localised depressions within the site which could encourage ponding, however it is anticipated that these areas will be levelled during the construction process and as such ponding has not been considered further.

The Environment Agency reservoir risk map shows that the site and Rocester are located in an area which could be affected by a reservoir failure. To the north west is the Tittesworth and Rudyard Reservoirs which may also include the Caldon Canal which is 23km from the site. It is conceivable that the River Churnet would be a flood route if this reservoirs/canal failed. The second route is from Carsington Reservoir to the north east which is 18km away. This appears to produce the greatest volume of the two routes. Carsington Reservoir is 36331 million litres volume and Tittesworth is 6460 million litre volume. The breach flow from the Tittesworth and Rudyard Reservoirs appears to be confined to the River Churnet main channel and does not impact on the site. The Environment Agency agreed with this assumption and confirmed that the flood route from the Carsington Reservoir to Rocester was via Henmore Brook valley. As such, the Tittesworth and Rudyard Reservoirs have not been considered further in this assessment. The reservoir flood map shows the consequences of a full breach of the dam structure with the reservoir at full capacity, as such a worse case scenario. However, the flood maps do not consider the probability of such a failure. Following discussions with Severn Trent Water who are responsible for Carsington Reservoir it was commented that the probability of failure is low as the reservoir is inspected and maintained in accordance with the Reservoirs Act. As such the dam is regularly inspected for defects and any necessary works undertaken prior to a failure. As such the probability of a failure is extremely low.



There are no areas of higher ground close to the development which could encourage overland flow towards the site. As such this flood mechanism has not been considered further.



4 QUANTATIVE FLOOD RISK ASSESSMENT

4.1 Requirements of the Environment Agency

The Environment Agency, as part of its development control procedures, generally require finished floor levels within residential development to be set at a minimum of 600mm above the 1% AEP plus 20% for climate change flood water level at the site. Finished floor levels within commercial development are to be set at a minimum of 300mm above the 1% AEP plus 20% for climate change flood water level at the site, incorporating flood resilience measures up to 300mm above the ground floor level of the proposed buildings.

Part of the development is residential in nature and as such, it is considered that access and egress from the site will be essential during times of extreme floods.

4.2 River Dove

The River Dove is the principal river of the south-western Peak District in the East Midlands, and is approximately 40 miles in length. The river rises on Axe Edge Moor near Buxton and flows generally south to its confluence with the River Trent at Newton Solney.

The River Dove is situated approximately 750m to the east of the development site and is predominantly open channel, and as it flows in a southerly direction through Rocester is undefended by raised earth embankments or hard flood defences. A mill race, branches off the main channel of the River Dove to the east of Dove Lane, and flows southwards, rejoining the River Dove to the south of the village. Therefore consideration need only be given to overtopping of the natural river bank, in order to ascertain the flood risk to the proposed development site.

Figure 4.1: The River Dove at Rocester Bridge.

The Environment Agency commissioned the River Dove Flood Mapping Report in 2006. From this data, the Environment Agency was able to provide modelled flood levels for specific sections of the River Dove and the existing mill race, near to the proposed development.

From the data provided by the Environment Agency it was determined that the flood level data for Node References RD100 (main channel) and RD49G (mill race) would provide the best points of reference due to their location relative to the proposed development site. Therefore an in channel flood level of 87.09mAOD has been adopted as the estimated 1 in 100 year flood level for the River Dove in line with the proposed development site. Additionally, the 1 in 100 year flood level within the mill race is estimated as 87.79mAOD.

The lowest level within the existing site 84.80mAOD and as such, the estimated 1 in 100 year flood levels for the River Dove and the mill race are over 2m and 3m respectively above this ground level. Topography within the village however,



indicates that ground levels rise in a westerly direction along Mill Street to a high point in excess of 88.00mAOD at the junction of Mill Street with High Street and Ashbourne Road. As such, natural ground between the River Dove and the site is elevated sufficiently to prevent flood water from flowing westwards towards the development.

Consideration will also need to be given to the effects of climate change over the design life of the development and overtopping during an extreme flood event (1 in 1000 year).

Increase in estimated flood level due to Climate Change

NPPF states that ‘…Flood risk assessment should be carried out to the appropriate degree at all levels of the planning process, to assess the risks of all forms of flooding to and from development taking climate change into account. The future users of the development must not be placed in danger from flood hazards and should remain safe throughout the lifetime of the plan or proposed development and land use.’

As the proposed development includes a mix of residential and commercial land use, consideration has therefore been given to take into account the potential effects of climate change over the next 100 years in accordance with NPPF and technical guidance. Adopting the recommended contingency allowance for fluvial rivers requires an increase of 20% for climate change.

The flood level estimates provided by the Environment Agency are for a range covering the 1 in 2 year to 1 in 200 year return periods, and unfortunately corresponding flow estimates have not been provided. The 1 in 100 year flood level adjacent to the site has been estimated as 87.09mAOD, application of a 20% increase to the 1 in 100 year flow provides an estimate of the 1 in 100 year plus climate change flood level. We have assumed an increase of 200mm for climate change over the next 100 years. Therefore the predicted 1 in 100 year plus climate change flood level is 87.290mAOD. Experience would suggest this increase is conservative.

It is also considered that due to the relatively small flows within the mill race, compared to the River Dove, the effect of climate change is likely to cause a minimal increase in flood levels within the watercourse.

Although ground levels within the existing site are lower than the 1 in 100 year plus climate change flood level, an area of high ground is situated between the River Dove and the development. As such, it is considered that out of bank flooding of the River Dove in line with the development would be contained to the east side of the high ground and will not reach the development site.

Consideration has also been given to the extreme 1 in 1000 year flood event. Unfortunately the Environment Agency was unable to provide flow estimates for this return period flood level. Therefore it is assumed that the 1 in 1000 year flood would increase flood levels within the River Dove by an additional 0.5m above the estimated 1 in 100 year plus climate change flood level, hence, 87.790mAOD. Existing ground levels to the west side of the River Dove are known to be elevated and therefore it is anticipated that the site will remain dry during this flood event within the River Dove.

In conclusion, the River Dove is considered to present a low flood risk to the proposed development. This corresponds with the flood map provided by the Environment Agency, which is illustrated within Appendix C of the report.

4.3 River Churnet

The River Churnet rises in the Staffordshire moorlands near the Roaches. It flows through Tittesworth reservoir, and down through Leek, where it once supplied the town's textile industry with water. South of Cheddleton, the Caldon Canal merges with the river at Oak Meadow Ford Lock, for about a mile, as far as Consall Forge. At this point the canal diverges from the river. The River Churnet joins the River Dove to the south of Rocester.

The watercourse is situated approximately 40m to the west of the development site and is predominantly open channel, and as it flows in a southerly direction through Rocester is undefended by raised earth embankments or hard flood defences. Therefore consideration need only be given to overtopping of the natural river bank, in order to ascertain the flood risk to the proposed development site.



Figure 4.2: The River Churnet Viewed Downstream from Churnet Bridge.

The Environment Agency commissioned the River Churnet Section 105 Flood Mapping in 2007 which was then superseded by the Dove Tributary Study 2011. The River Churnet was then reemodlled during September 2013. From this data modelled flood levels for specific sections of the River Churnet, adjacent to the proposed development have been provided.

From the data provided by the Environment Agency it was determined that the flood level data for Node References 02682, 02888u and 02945u would provide the best points of reference due to their location relative to the proposed development site. Therefore 1 in 100 year flood levels of 84.62mOD, 84.90mOD and 85.81mOD have been adopted as the estimated 1 in 100 year flood levels for the River Churnet in line with the proposed development site.

Node Reference 02682 is in line with the south boundary of the development site. The lowest ground level within this area of the existing site is 84.80mAOD and as such, the estimated 1 in 100 year flood level for the River Churnet at this location is 0.18m below this ground level.

Node Reference 02888u is located on the upstream side of the road bridge which provides access to the JCB site from the B5030 and High Street. The ground level along the east boundary of the development in line with this location is 85.76mAOD and as such, the estimated 1 in 100 year flood level for the River Churnet at this point is 0.86m below this ground level.

Node Reference 02945u is located on the upstream side of an existing weir, adjacent to the refurbished mill buildings occupied by JCB Finance. The lowest ground level along the east boundary of the development in line with this location is 86.05mAOD and as such, the estimated 1 in 100 year flood level for the River Churnet at this point is 0.24m below this ground level.

Consideration will also need to be given to the effects of climate change over the design life of the development and overtopping during an extreme flood event (1 in 1,000 year).

Increase in estimated flood level due to Climate Change

NPPF states that ‘…Flood risk assessment should be carried out to the appropriate degree at all levels of the planning process, to assess the risks of all forms of flooding to and from development taking climate change into account. The future users of the development must not be placed in danger from flood hazards and should remain safe throughout the lifetime of the plan or proposed development and land use.’

As the proposed development includes a mix of residential and commercial land use, consideration has therefore been given to take into account the potential effects of climate change over the next 100 years in accordance with NPPF and technical guidance. Adopting the recommended contingency allowance for fluvial rivers requires an increase of 20% for climate change.

The flood level estimates provided by the Environment Agency range from the 1 in 5 year up to the 1 in 1000 year return period which also included the 1 in 100 year plus climate change flood event.



Therefore 1 in 100 year flood plus climate change levels of 84.77mOD, 85.07mOD and 85.96mOD have been estimated for the River Churnet in line with the proposed development site.

Existing ground levels within the site and in particular along the east boundary of the development indicate that the proposed development is also elevated above the estimated 1 in 100 year plus climate change flood levels.

Consideration has also been given to the extreme 1 in 1,000 year flood event. Modelled flood levels at node references 02682, 02888u and 02945u have been estimated as 85mOD, 85.36mOD and 86.16mOD respectively.

Table 4.1: Summary: Comparison of Modelled Flood Levels with Existing Ground Levels within the Site

Node Existing Ground Level

1 in 100 year Flood Level

1 in 100 year + 20% Flood Level

1 in 1000 year Flood Level

02682 84.80 84.62 84.77 85.00

02888u 85.76 84.90 85.07 85.36

02945u 86.05 85.81 85.96 86.16

In conclusion, the River Churnet overall is considered to present a low flood risk to the proposed development the flood envelopes have been annotated onto the topographical survey and is provided at Appendix F.



4.4 Existing and Proposed Foul Drainage

Existing

From the sewer record plans supplied by Severn Trent Water Ltd, foul drainage within the vicinity of the development is shown to be conveyed via a system of foul and combined sewers to a pumping station located immediately adjacent to the north west boundary of the site.

A 225mm diameter foul sewer traverses the existing site in a north westerly direction, from the rear of residential properties located at Riverfield Drive to join the pumping station. Combined foul and surface water flow from the village is then transferred via pumping main in a south westerly direction over a distance of approximately 5 kilometres to the sewage treatment works located at Uttoxeter. The existing pumping main traverses west part of the proposed development.

Proposed

It is proposed that the foul drainage from the proposed development will be directed towards the pumping station, subject to capacity of the pumping station and agreement with Severn Trent Water Ltd.

Should capacity of the existing pumping station prove to be inadequate, it may be necessary to upgrade the pumping station in order to support additional foul flows from the proposed development.

It is also noted that Severn Trent Water require an easement of 3 metres either side of an existing public sewer which is up to 300mm diameter; increasing to 5 metres for larger sewers. As the size of the pumping main is yet to be determined, it is recommended that the wider easement is adopted for this development.

Flood Risk

In the event of a failure of the pumping station, due to its location, it is anticipated that foul flows would pump out into the adjacent JCB Finance site.

4.5 Increase Run-off due to the Development

The total area within the site boundary equates to 4 Hectares. A drainage strategy for the proposed development is currently being undertaken by EWE Associates Limited. The strategy involves draining the site via gravity to a retention pond, located in the south east part of the development site. It is anticipated that surface water will be freely discharged into the pond, where it will be attenuated. Outflow from the retention pond will be limited using a flow control device such as a Hydrobrake, to the equivalent greenfield runoff rate. Surface water will subsequently be directed eastwards towards the River Churnet. Consultation with the Environment Agency has established that a greenfield runoff rate of 4l/s/ha is acceptable and this rate has subsequently been utilised in order to determine attenuation volumes for the proposed development.

Storage volumes required to attenuate surface water flow from the development to accommodate the required 1 in 100 year plus 30% climate change event, have therefore been calculated, using the proposed drainage strategy.

The pond will be designed to incorporate a permanent wetland area. To enhance the environmental benefits, each pond base will be contoured, creating a range of water depths to encourage differing vegetation, such as meadow grass, marshland and open water areas. The balancing pond will have the added benefit of improving water quality prior to discharge into the River Churnet.

Current development proposals indicate that the development is to be gravity drained via a positive drainage system to a retention pond, which is to be located adjacent to the south boundary of the site.

It is proposed that the discharge rate from the balancing pond will be restricted to a nominal greenfield runoff rate of 4l/s/ha. Therefore the limited discharge rate into the River Churnet from the pond is calculated to be 16l/s.

The Environment Agency will request that the proposed 1 in 2 year surface water runoff can be maintained and also insist that the 1 in 30 year event is not allowed to flood the surface; hence surface water must remain within the pipes,



manholes, and storage systems. It is usual that the 1 in 100 year plus 30% climate change event will be allowed to flood the surface but flood water will not be permitted to enter any of the buildings within the site. Additionally, the 1 in 100 year plus 30% climate change flood must also be limited to the development boundary and must not be allowed to migrate to adjacent properties.

It is proposed that the new pond will attenuate flows up to and including the 1 in 100 year plus 30% climate change event, prior to discharge into the River Churnet.

Proposed Drainage Strategy

The proposed site is a mixed development comprising residential units and an employment area. Detailed layout and floor plans for the proposed development are not yet available; however a concept masterplan indicating the proposals is included within at Appendix B of this report.

It is proposed to ultimately discharge any surface water flows generated by the development of the site which cannot drain via infiltration to the River Churnet.

The proposed impermeable area for the development site has been calculated to be approximately 2.14 ha which is 54% of the total site area, with the remainder of the site proposed as gardens, landscaping and open space.

The drainage strategy utilises an appropriately sized hydro brake to restrict the flow rate to 16 l/s. This allows the discharge through the hydro brake to vary as the head increases due to the increase in upstream flows. As such, the flow will vary for each of the design storms shown above and it is expected that during the more extreme return periods there will be a considerable betterment as the hydro brake is likely to restrict flows to a lesser rate than estimated at present.

Based upon the assumption that the EA will agree to the maximum discharge rates of 16 l/s, a preliminary surface water network has been developed and attenuation has been sized using MicroDrainage software.

The model data for the proposed surface water drainage network has been obtained from the proposed development layout drawing and the drainage strategy drawing is provided at Appendix E of this report. A model has been developed to represent the main drainage runs within the proposed drainage network and contributing drainage areas within the development.

Overall, the hydraulic models include the following;

22 pipes to represent the proposed system

1 hydro-brake

1 pond

1 outfall into the River Churnet with no surcharging

Impermeable area contributions have been based on those supplied on the proposed layout drawing, considered to be 100% impermeable, comprising of roofed and paved areas.

The models have been set up as a fixed runoff model assuming 100% runoff coefficient for roofed and paved areas. The rainfall characteristics for Rocester have been utilised with a value for M5-60 given as 20mm (the depth of rain in a once in five years one hour duration event); and r given as 0.40 (the ratio of the M5-60 rainfall to the M5-2day rainfall). For durations over 60 minutes the FEH runoff data for Rocester has been used.

Hydraulic Modelling Results

The proposed MicroDrainage models have been simulated with the 1 in 100 year plus climate change (30%) return period design storm events with durations of 15, 30, 60, 120, 240, 300, 360, 480, 600, 720, 900 and 1440 minutes. At the request of the Environment Agency seven day 10080 minute duration was also undertaken. The durations were run in both Winter and Summer profiles. It was found that the Winter profile was critical.

The table overleaf shows a summary of the 1 in 100 year plus climate change model runs and the impact on the drainage system in terms of peak depth within the pond and flow through the hydro-brake.

The 720 minute duration produced the largest flow through the hydro-brake (15.6 l/s) which is less than the existing peak 1 in 2 year runoff rate (16l/s). The modelled result for the 720 minute Winter model run is provided at Appendix D. There was only minor flooding (1.155m³) during the 15 minute storm.



Return Period Profile Duration

(min) Peak depth

in pond Peak flow into River Churnet

Status

100 year+CC Winter FSR 15min 84.403 12.6 Flooding 1.155m3 100 year+CC Winter FSR 30min 84.481 13.3 surcharge 100 year+CC Winter FEH 60min 84.600 13.9 surcharge 100 year+CC Winter FEH 120min 84.759 14.7 surcharge 100 year+CC Winter FEH 240min 84.861 15.2 surcharge 100 year+CC Winter FEH 300min 84.896 15.3 surcharge 100 year+CC Winter FEH 360min 84.909 15.4 surcharge 100 year+CC Winter FEH 480min 84.934 15.5 surcharge 100 year+CC Winter FEH 540min 84.941 15.6 surcharge 100 year+CC Winter FEH 600min 84.945 15.6 surcharge 100 year+CC Winter FEH 720min 84.949 15.6 surcharge 100 year+CC Winter FEH 900min 84.923 15.5 surcharge 100 year+CC Winter FEH 1440min 84.844 15.1 surcharge 100 year+CC Winter FEH 10080min 84.089 11.0 ok



5 MITIGATION MEASURES

5.1 Raising Floor Levels/Land Raising

The River Churnet is in line with the development and is considered to represent the greatest flood risk to the site. The River Churnet estimated 1 in 100 year flood levels are 84.62mOD, 84.90mOD and 85.81mOD at Node References 02682, 02888u and 02945u respectively. The estimated 1 in 100 year plus climate change flood levels at these nodes are 84.77mOD, 85.07mOD and 85.96mOD. From the topographic survey of the development, existing ground levels are shown to be elevated above these modelled flood levels.

Following consultation with the Environment Agency it was suggested that the internal ground floor levels for the proposed residential development are set at a minimum of 600mm above the 1 in 100 year plus climate change flood levels in line with the site. It is also advised that the internal ground floor level for the commercial development within the employment area is set at a minimum of 300mm above the 1 in 100 year plus climate change flood level, incorporating flood resilience measures up to 300mm above the ground floor level of the proposed buildings.

Residential

For the purposes of recommending finished ground floor levels within the area of the site set aside for residential development, the area has been divided into three sections corresponding to the locations of node references 02682, 02888u and 02945u.

In compliance with the Environment Agency’s requirements it is recommended that internal ground floor levels in line with Node Reference 02945u are set at a minimum of 86.560mOD. The lowest existing ground level within this area is shown to be 86.050mAOD and as such, some land/floor raising will be required, in order to provide adequate freeboard above the 1 in 100 year plus climate change flood level in line with the development.

The internal ground floor level of the proposed residential units in line with Node Reference 02888u should be set to a minimum of 85.430mOD. The lowest existing ground level within this section of the site is 85.430mAOD and as such, no land/floor raising will be required.

Additionally, it is recommended that the internal ground floor levels of the proposed dwellings in line with Node Reference 02682 are set to a minimum level of 85.110mAOD. The lowest existing ground level within this southern section of the proposed development has been surveyed as 84.800mAOD. Therefore some land/floor raising will also be required within this part of the development in order to provide adequate freeboard above the 1 in 100 year plus climate change flood level. A summary table is provided below:

Table 5.1: Summary: Recommended Floor Levels (Residential)

Node Ref. 1 in 100 year plus climate change

flood level (mAOD)

Lowest existing ground level

(mAOD)

Recommended internal ground

floor level (mAOD)

Land/floor raising required (/)

02945u 85.960 86.050 86.560

02888u 84.830 85.430 85.430

02682 84.510 84.800 85.110

In order to mitigate against localised flooding which may be caused by heavy rainfall, it is also advocated that the internal ground floor levels of the proposed residential units are elevated at least 150mm above the adjacent external finished ground levels.



Commercial

The area of the site set aside for commercial development is in line with Node Reference 02945u and therefore the 1 in 100 year plus climate change flood level utilised for this part of the development is estimated to be 85.96mOD. In compliance with the Environment Agency’s requirements it is recommended that internal ground floor levels in line with Node Reference 02945u are set at a minimum of 86.260mOD. The lowest existing ground level within this area is shown to be 86.050mAOD and as such, some land/floor raising will be required, in order to provide adequate freeboard above the 1 in 100 year plus climate change flood level in line with the development.

For the low lying areas within the commercial site, where ground levels are less than 600mm above the 1 in 100 year plus climate change flood level, it is recommended that flood proofing measures are incorporated into the commercial building design up to a level of 600mm above the 1 in 100 year plus climate change flood level, at 86.56mOD.

In order to mitigate against localised flooding which may be caused by heavy rainfall, it is also advocated that the internal ground floor levels of the proposed commercial units are elevated at least 150mm above the adjacent external finished ground levels.

5.2 Emergency Access and Egress

Proposals for the site comprise a mixture of residential and commercial development and as such, it is considered that dry access and egress from the development site will be essential during extreme flood events.

From the flood risk analysis carried out within Section 4 of this report, the development is considered to lie predominantly within Flood Zone 1 and therefore it is considered that dry access and egress from the development site will be available at all times.

5.3 Flood Compensation

The development site is shown to be elevated above the 1 in 100 year plus climate change flood levels at the site and as such, it is considered that flood storage compensation measures are not required.

5.4 Control of Runoff

The current drainage strategy being undertaken by EWE Associates Ltd, indicates that the development is to be drained via gravity to a retention pond, where flow will be attenuated prior to discharge ultimately into the River Churnet.

It is proposed that surface water discharge from the development is limited to the nominal greenfield runoff rate of 4l/s/ha. Therefore the restricted discharge from the balancing pond is calculated to be 16l/s.

It is proposed that the new retention pond will attenuate flows up to and including the 1 in 100 year plus 30% climate change event, prior to discharge into the River Churnet.

Consideration has been given to the hierarchy for surface water disposal which recommends the SUDs approach which includes infiltration as the first tier. Further investigation is required to confirm that infiltration drainage will be a practical solution for the site.

However, other SUDs techniques can be used within the site and they have been considered. The second tier is to discharge to a watercourse. The existing site is considered to be predominantly permeable with only a small paved area. Following the proposed development the impermeable area will be significantly increased to approximately 53% of the total site area. It is considered that the site currently discharges runoff via a combination of infiltration, evaporation and overland flow to the River Churnet.

Using software developed by Microdrainage the required attenuation has been calculated for the 1 in 100 year plus climate change (30%) event. The site will discharge into the River Churnet to the south west of the site at a peak discharge rate equal to or less than the current peak runoff rate for the site. The primary attenuation will be provided within a balancing pond which is 1m deep with a bed area of 1087m² and a bank top area of 1566m².

The balancing pond will be used to accommodate the storage during 1 in 1 year, 30 year, 100 year and 100 year +CC storms (worse case scenario).



The proposal is to provide a hydro-brake to restrict flows from the site. The hydro-brake will reduce the runoff from the development site during higher return periods, hence, there will be a significant reduction in runoff and as such the development will provide significant betterment in terms of runoff being passed forward from the site into the local watercourse.

It is recommended that during the detailed phase of the development the following items are considered.

The proposed surface water drainage system should be modelled using Micro Drainage WinDes or similar. The model should be used to analyse the possibility that the design for surface water may fail or becomes blocked and as such should design a backup plan. Overland floodwater should be routed away from vulnerable areas. Acceptable depths and rates of flow are contained in EA and Defra document FD2320/TR2 “Flood Risk Assessment Guidance for New Development Phase 2”.

The maintenance and adoption regimes for all elements of the development should be considered for the lifetime of the development.

Consenting will be required from the Water Authority for any connections/outfalls into the River Churnet.

5.5 Proposed Outfall to River Churnet

Following discussions with the Environment Agency, it is recommended that the proposed outfall structure at the River Churnet is suitably clad with local materials, in order to reduce the visual impact of the structure within the natural landscape of Rocester village.

In addition, formal consent for the design and construction of the surface water outfall structure will need to be obtained from the Environment Agency.

5.6 SUDS

The impermeable area within the site will be increased following development. There will therefore be an increase in surface water runoff from the site when it is developed. The Environment Agency and Local Authority will request that the runoff from the proposed development is restricted to the nominal greenfield runoff rate of 4l/s/ha, which has been estimated as 16l/s. Additionally, the Environment Agency requires that adequate pollution control is incorporated into the drainage system in order to prevent deterioration of the quality of the water environment. However, this is only applicable for surface water originating from access roads and communal parking areas, which needs to be passed through a petrol/oil interceptor or equivalent system prior to discharge into the existing surface water sewer or infiltration system. It is noted however, that this will not apply to surface water originating from roof drainage.

To reduce the impact of surface water runoff from the development in accordance with the requirements of the Environment Agency and Local Authority, the employment of SUDS techniques to limit runoff volumes and rates from the site are recommended. SUDS techniques can also be used to provide an appropriate level of treatment to the runoff.

It is normal practice to ensure that the 1 in 30 year event is maintained within the drainage system and the 1 in 100 year is permitted to flood the surface as long as there is no flooding to buildings and the flood volume is contained within the site boundary in specific areas proposed for this purpose. However, it is proposed that the new balancing ponds will be of adequate size to accommodate flows up to and including the 1 in 100 year plus 30% climate change event.

The following section will provide some possible SUDS techniques which could be employed on the site to balance flows in excess of the 1 in 30 year event. SUDS techniques will also provide treatment to the runoff to remove a proportion of the pollution and protect the quality of the downstream watercourses. Following guidance from CIRIA Report C522 the following levels of treatment will be provided:

• Roofs – 1 level • Driveways – 1 level • Roads and communal parking areas – 2 levels.

The level of treatment indicates the number of SUDS techniques that will be used to treat pollution. For example if two levels are required the runoff may enter a filter drain that leads to a basin or pond before outfall. It is recommended that source control techniques are used. In practice there will be little outflow from these techniques for a 1 in 2 year storm as most of the rainfall will be held within the system and will disperse via evapotranspiration. Further detail of the potential to



use SUDS within this site it provided below within Table 5.3. The precise combination of methods used will be dependent upon the site constraints identified at the final design stage. With regards to the potential for infiltration methods of disposal of surface water from the development, it is known that the natural geology of the Rocester area consists predominantly of clay, shale and slate, which possess the characteristics of low permeability. This concurs with the catchment descriptors for the area containing the proposed development site which provides an SPRHOST (standard percentage runoff) of 32.7%, indicating that the ground potentially has low permeability.

In addition, the Flood Studies Report Soil Maps, which show Rocester to lie within WRAP Class 4 (Winter Rainfall Acceptance Potential), which is defined as ’clayey or loamey over clayey soils with an impermeable layer at shallow depth. Although a specific site investigation has not yet been carried out for the development; this indicates that infiltration methods are likely to be unsuitable for this development.

Table 5.2: SUDS Techniques and Suitability of Use

Method Description Potential for use at site Filter drains Drainage trench filled with gravel and

provided with a pipe Suitability to be determined following site investigation.

Swales Shallow grass ditch May be considered suitable for conveying surface water runoff along the west boundary of the site, preventing migration outside of the site boundary and onto the adjoining commercial land.

Permeable surfaces Pavement surfaces that allow water to pass through into underlying storage in sub base e.g. permeable concrete block paving or porous asphalt.

May be considered suitable for use within the access road and driveway areas, in order to reduce the volume for on-line attenuation.

Ponds and basins Open areas that are used to store and treat rainwater. Ponds are permanent bodies of water and basins are generally dry and occasionally store water.

A shallow pond containing a permanent wetland area is incorporated into the drainage strategy for this proposed mixed development.

Green roofs Roof system that is vegetated with plants (note sedum plants rather than grass so no mowing is required)

Suitability is dependent on design of buildings.

Infiltration devices Methods that allow rainwater to soak into the ground, e.g. soakaways.

Suitability is dependent on ground conditions following site investigation.

Storage tanks Underground tanks that temporarily store water in the drainage system.

These will be used as a last resort on this site if dictated by site constraints.

5.7 Flood Resilience Measures – Proposed Commercial Development

From the River Churnet Section 105 Flood Mapping in 2007, the 1 in 100 year plus climate change flood level at the proposed commercial development has been estimated as 84.74mAOD. Therefore to satisfy the requirements of NPPF, the internal ground level of the proposed commercial development should be set to a minimum of 300mm above this flood level, at 85.240mAOD. The Environment Agency have requested that flood resilience is incorporated into the building design to a minimum height of 300mm above the recommended finished floor level to mitigate against residual flooding.

The ‘water exclusion strategy’ as outlined within the DEFRA publication ‘Improving the flood performance of new buildings: Flood resilient construction’ has therefore been utilised to identify the appropriate measures which need to be incorporated into the building design. This strategy is preferred when low flood water depths have been identified (up to 300mm) and concentrates on minimising the entry of water into the building whilst maintaining structural integrity. The use of materials and construction techniques to facilitate drying and cleaning are also included within this strategy. In general building materials which are considered to be effective for a water exclusion strategy include:

Engineering bricks Cement based materials such as water retaining concrete Dense stone



Figure 5.1: Design Strategy Flowchart – Resistance/Resilience

Extract from DEFRA publication ‘Improving the flood performance of new buildings: Flood resilient construction’.

Foundations

Foundations are designed to suit the site conditions, which includes the local geotechnical characteristics and the building design. The foundation design will incorporate either strip or raft foundations, the selection of which will be determined by the Structural Engineer, following trial hole investigation of the ground conditions within the site, prior to submission of the Building Regulation application. In general, the selection of foundation type will be determined by ground conditions, rather than flood resilience considerations. However, improvements can be incorporated into the foundation design in order to increase flood resilience.

If a strip foundation is found to be the most suitable foundation type for the development, then insulation such as Kingspan / Celotex or similar should be utilised, with a minimum 65mm thick sand/cement screed. However, these details are to be finalized by the Structural Engineer.

The water exclusion strategy involves minimising the entry of water through permeable elements of the foundation. Concrete blocks placed below ground bearing concrete floor slabs provide a path for water ingress into wall cavities, as concrete blocks are substantially more permeable than concrete slabs. Where concrete ground floor slabs are to be utilised, the blockwork substructure is often the weakest point in terms of water penetration from the ground into a building. Whereas there is a general perception that water can ingress through the blockwork structure of the external face of a wall into the building, it is less apparent, but equally possible, that water will penetrate from the ground on the inside of the building.

Figure 5.3 below, illustrates the potential flow path of water from the ground adjacent to and beneath the building, through the porous substructure and into the wall cavity for ground bearing floor slabs.



Figure 5.2: Water Ingress into building through the ground – ground bearing concrete floor slab


Therefore it is recommended that concrete is used to encase these concrete blocks in order to prevent water movement from the ground to the wall construction. However, it must be highlighted that above ground, this measure may not fully prevent the ingress of water.

Floors

The behaviour of ground floors in floods is generally influenced by two different conditions:

Water ingress from the ground (potentially resulting in uplift pressures) Exposure to standing water

Water ingress from the ground is potentially more severe as it is more likely to affect the structural integrity of the floor. Structural calculations will therefore need to be carried out to ensure that the floor has the necessary strength to resist uplift forces without excessive deformation or cracking. Laboratory evidence on small slabs (0.5m x 0.5m) indicated that 150mm thick concrete slabs on supporting soil can withstand such forces without allowing water ingress. However, for larger slabs, uplift forces may cause deformation and induce cracking and lead to preferential paths for water ingress.

Ground supported floors are therefore recommended as the preferred option, and concrete slabs of at least 150mm thickness should be specified for non-reinforced construction. Hollow slabs are not suitable if the elements are not effectively sealed.

Further considerations regarding flood resilient design of the floor within the proposed structure are as follows:

For hardcore and blinding beneath concrete slabs, good compaction is necessary to reduce the risk of settlement and consequential cracking.

Damp Proof Membranes should be included within the design of the structure in order to minimise the passage of water through ground floors. Impermeable polythene membranes should be at least 1200 gauge to minimise ripping. Effective methods of joining membrane sections are overlaps of 300mm, and also taping (mastic tape with an overlap of 50mm minimum). Care should be taken not to stretch the membrane in order to retain a waterproof layer.

Water will lower the insulation properties of some insulation materials. Floor insulation should be of the closed-cell type to minimise the impact of flood water. The location of insulation materials, whether above or below the floor slab, is usually based on either achieving rapid heating of the building or aiming for more even temperature distribution with reduced risk of condensation. Insulation placed above the floor slab (and underneath the floor finish) rather than below would minimise the effect of flood water on the insulation properties and be more easily replaced, if needed. However, water entry may cause insulation to float (if associated with low mass cover) and lead to de-bonding of screeds. The placement of insulation below the floor slab may be adequate but it is important to recognise that the characteristics of the insulation may be affected by the uplift forces generated by the flood water.

Suitable floor finishes include ceramic or concrete-based floor tiles, stone, and sand/cement screeds. Concrete screeds above polystyrene or polyurethane insulation should be avoided as they hinder drying of the insulation material.

Under floor services using ferrous materials should be avoided



Recommended ground-supported floor arrangements are presented below in Figure 5.4.

Figure 5.3: Ground supported concrete floor


Walls

Table 5.3 overleaf classifies wall components as good, medium or poor with regard to water penetration, surface drying and structural integrity. The information contained within the table has been primarily based upon laboratory investigations and is supported by expert opinion and experience from the building industry.

Definitions of the characteristics used in Table 5.4 are:

Water penetration – the seepage (rate and volume) through the material. Drying ability – the capability to regain its original moisture condition (assessed by the average drying rate and

the time taken to reach the original value). Retention of pre-flood dimensions, integrity – the lack of deformation or change in form or appearance of the

material.

It is recommended that the wall construction consists of a partial fill construction as illustrated below in Figure 5.5 Option A, with engineering brick up to a level of 300mm above the internal finished floor level. It is also recommended that insulation is installed within the cavity space, with dense blockwork and internal render.

Table 5.3: Flood Resilience Characteristics of Walls

Resilience Characteristics* Material

Water Penetration Drying Ability Retention of Pre-flood Dimensions, Integrity

External Face Engineering Bricks (Classes A & B) Good Good Good

Facing bricks (pressed) Medium Medium Good Internal Face

Concrete blocks Poor Medium Good Aircrete Medium Poor Good

Cavity Insulation Mineral Fibre Poor Poor Poor

Blown-in expanded mica Poor Poor Poor Rigid PU foam Medium Medium Good

Renders/plaster Cement Render – External Good Good Good

Cement/lime Render – External Good Good Good Gypsum Plasterboard Poor Not assessed Poor Lime Plaster (young) Poor Not assessed Poor

*Resilience characteristics are related to laboratory testing and excludes aspects such as ability to withstand freeze/thaw cycles, cleanability and mould growth



Further considerations regarding flood resilient design of the walls within the proposed structure are as follows:

Should masonry walls be incorporated into the design, it must be ensured that mortar joints are thoroughly filled to reduce the risk of water penetration. If frogged bricks are used, they should be laid frog up so that filling becomes easier and coverage more certain. Bricks manufactured with perforations should not be used for flood resilient design. Where possible, use engineering bricks up to predicted flood level plus one course of bricks to provide freeboard (up to the required height of 0.3m depth above the finished ground floor level); this will increase resistance to water penetration. Blocks and dense facing bricks have much improved performance when covered with render.

There is evidence that thin layer mortar construction (or thin joint, as it is also commonly known) is a good flood resilience option.

Cavity insulation should preferably incorporate rigid closed cell materials as this retains integrity and has low moisture take-up. Other common types, such as mineral fibre batts, are not generally recommended as they can remain wet for several months after exposure to flood water, which slows down the wall drying process. Blown-in insulation can slump due to excessive moisture uptake, and some types can retain high levels of moisture for long periods of time (under natural drying conditions).

Internal cement renders (with good bond) are effective at reducing flood water leakage into a building and assist rapid drying of the internal surface of the wall. The extent to which render prevents drying of other parts of the wall is not currently clear. This may be important, particularly for solid wall construction. This applies also to external renders.

Standard gypsum plasterboard should be avoided as it tends to disintegrate when immersed in water. Splash proof boards do not necessarily offer protection against flood waters, which may remain for some time and exert pressure on the board.

Figure 5.4: Cavity external wall – part filled cavity


Doors and windows

Doors, windows and air vents are potential flow paths into buildings.

The recommended internal floor level of the building is 85.240mAOD, which is approximately 300mm above the 1 in 100 year plus climate change flood level estimated at the development and as such raises the threshold as high as possible, while complying with level access requirements.

It is recommended that the doors and windows should be powder coated aluminum framed, sealed double glazed units, with solid core fire doors or steel doors.



In flood prone areas, it is usual to use sealed PVC external framed doors, therefore, all effort should be made to ensure a good fit and seal to their frames.

Windows are vulnerable to flood water and similar measures to those used for doors should be taken. Special care should be taken to ensure adequate sealing of any window/door sills to the fabric of the building structure.

Fittings

The main principle is to use durable fittings that are not significantly affected by water and can be easily cleaned (e.g. use of plastic or stainless steel materials). Place fittings (e.g. electrical appliances) on plinths as high as practicable above floor so that they are out of reach of flood water.

Services

Where possible, all service entries should be sealed (e.g. with expanding foam or similar closed cell material).

Closed cell insulation should be used for pipes which are below the predicted flood level.

Non-return valves are recommended in the drainage system to prevent back-flow of diluted sewage in situations where there is an identified risk of the foul sewer surcharging. Maintenance of these valves is important to ensure their continued effectiveness.

Water, electricity and gas meters: should be located above predicted flood level.

Electrical sockets should be installed above flood level for ground floors to minimise damage to electrical services and allow speedy re-occupation

Heating systems: boiler units and ancillary devices should be installed above predicted flood level Conventional heating systems, e.g. hot water pipes are unlikely to be significantly affected by flood water unless it contains a large amount of salts. The less common, hot air duct heating would remain effective provided it is installed above the design flood level.

Communications wiring: wiring for telephone, TV, Internet and other services should be protected by suitable insulation in the distribution ducts to prevent damage. Any proposed design solution for flood insulation on all potentially vulnerable wiring should be discussed with the relevant service providers.

5.8 Conclusion

It is concluded that there is a low risk of flooding due to overtopping of the River Churnet during the 1 in 100 year and 1 in 100 year plus climate change flood events as existing ground levels within the site are generally elevated above the modelled flood levels. It is however recognised that increase in surface water runoff following development will present the greatest flood risk to the site. In order to minimise the residual flooding within the proposed development, the implementation of mitigation measures as described in Sections 5.1 to 5.7 should reduce the flood risk to an appropriate level.



APPENDICES



Appendix A: - Existing Ground Levels



Appendix B: - Proposed Development Layout Plan



Appendix C: - Environment

Agency Flood Data



Appendix D: - WinDes Model Results



Appendix E: - Proposed Drainage Strategy



Appendix F: - Flood Envelopes

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