maldon central - maldon district 1.pdf · snc lavalin | new concept for maldon fsa system...

Maldon Central Technical note - outline design update

Essex County Council

23 November 2018

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Contains sensitive information 5154071-DG-70-005 | 1.0 | 23 November 2018 SNC Lavalin | new concept for maldon fsa system 23-11-2018 of 18

Notice

This document and its contents have been prepared and are intended solely as information for Essex County Council and use in relation to the updated outline design for Maldon Central.

SNC Lavalin assumes no responsibility to any other party in respect of or arising out of or in connection with this document and/or its contents.

This document has 18 pages including the cover.

Document history

Revision Purpose description Origin-ated Checked Reviewed

Author-ised Date

Rev 1.0 Client comment AL, SW, RR

CA GAC GAC 23/11/18

Client signoff

Client Essex County Council

Project Maldon Central

Job number 5154071

Client signature / date

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Contents

Chapter Page

1. Introduction 4

2. Outline Design 5

3. Hydraulic Modelling 6

4. Economics 9

5. Key risks 12

6. Conclusion and next steps 13

APPENDIX A 14

Key risks 14

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1. Introduction The aim of this technical note is to explain the updated concept design for the Maldon Flood Storage Area system, further to comments from the Essex County Council highways team and Maldon District Council. The new objectives of the scheme are as follows:

1. All highways surface water from Marlowe Close to drain in to the new storage area.

2. Storage area not to be wet all the time.

In line with these objectives, the scope of works was updated to include the following items:

Prepare an updated concept drawing;

Investigate whether any clashes between the proposed design and existing services will occur;

Investigate the feasibility of SUDs;

Develop the hydraulic model to represent the updated design and run it;

Re-run the economics to determine the benefits of the updated scheme;

Revise the cost estimates to include the changes to the design;

Estimate the frequency of use of the flood storage area;

Prepare document and drawing to summarise findings.

This technical note summarises the design changes that have been implemented, and highlights key considerations, since the issue of the CDA02 Maldon Central Feasibility Report, dated 11th January 2018, in order to meet the additional objectives outlined above. This technical note supplements the Feasibility Report.

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2. Outline Design The components of the updated proposed scheme are as shown on drawing 5154071-ATK-MC-DR-C-002. The elements that have changed from the previous arrangement have been noted below:

The existing surface water network pipe(s) running along Marlowe Close will be disconnected and the road gullies will be removed.

New grated road drainage channels will be installed along both sides of Marlowe Close, within the road. Piped connections will be used to pass the flow from the grated road drainage channels to the flood storage area inlet.

The use of SUDs was investigated but deemed unfeasible due to the limited depth available above existing services.

The new grated road drainage channels will be able to capture the road drainage previously captured by the existing road gullies connecting into the surface water network pipe, as well as capturing additional overland flow. Therefore, the channels will replace the gully pots and their associated drainage pipe as well as providing additional benefits. The addition of extra gullies into the surface water network was investigated but not found to provide sufficient benefits.

In normal conditions, the grated road drainage channels will direct all the flow (road drainage that would have previously entered the gullies) towards the Flood Storage Area (FSA) inlet structure, from where the water will flow into a pipe beneath the FSA. The downstream end of the pipe will connect to the FSA outlet structure on the south side of the FSA, from which it will be piped directly back into the surface water network adjacent to the south east corner of the allotment plots.

In high flow conditions (storm events), the flow entering the FSA will exceed the capacity of the low flow pipe (beneath the FSA) and water will fill up the intake structure and then spill into the FSA and fill up the basin. As the storm subsides, water will drain out of the basin through the outlet structure and back into the surface water network via a new piped connection. The sizing of the pipes will be confirmed during detailed design, in order to obtain the optimal flow through the pipe under the FSA and acceptable flows returning into the surface water network downstream of the FSA.

The basin falls towards a depression located between the inlet and the outlet and this depression falls towards the outlet. This arrangement ensures that at the beginning/end of filling flows are concentrated between the two structures and that the overall basin drains to the outlet – minimising flooding of the basin floor as far as possible.

In addition to the drainage amendments, ground reprofiling is proposed between the end of Marlowe Close and the FSA, including removal of the existing speed hump. The purpose of this is to redirect overland flow into the storage area and to reduce flooding of Brickhouse Farm Community Centre once the capacity of the grated road drainage channels is exceeded. Any ground reprofiling proposed along the vehicle access route at the end of Marlowe Close and the cycle path will be subject to appropriate design codes with regards to the gradient and surface finish.

Erosion protection is likely to be required to protect the ground where overland flow enters the storage area. The type will be decided during the detailed design phase but it is likely to be reinforced grass or other suitable solution for the expected flows, which will be in keeping with the surrounding landscape.

In the event of the storage area capacity being exceeded (either during larger flood events or due to blockages in the outlet system), an emergency spillway along the existing access track to the north of the allotments area is proposed. This will direct flow towards and into Poulton Close, which is the area that would have been flooded before the construction of the FSA. A raised kerb between the cycle path and the allotments will reduce the risk of water flowing into the allotments and a ramp over the kerb will allow access to the allotments.

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It is proposed to relocate the playground just to the north of the storage area, as shown on the drawing. Play equipment will be chosen during the detailed design stage. It may not be appropriate to have a raised area as per the existing due to the proximity of the relocated playground to the fence line and the potential to overlook gardens and houses.

3. Hydraulic Modelling The hydraulic modelling has been updated to reflect the changes to the design, as described in Section 2 above. A range of Annual Exceedance Probability (AEP) events have been modelled, as listed below:

- 50% (1 in 2)

- 20% (1 in 5)

- 10% (1 in 10)

- 5% (1 in 20)

- 2% (1 in 50)

- 1% (1 in 100)

- 0.5% (1 in 200)

Key modelled elements are summarised below.

Grated road drainage features:

The grated road drainage feature is key to the viability of the scheme. Given this importance, two grating types were modelled to identify the sensitivity of the option to the efficiency of the grating used. Type P and Type S gratings were identified, using the Design Manual for Roads and Bridges1 (DMRB), as low and high efficiency grating types, respectively.

Figure 1 - Type P grating Figure 2 - Type S grating

The performance of the grating types was determined using design tables from the DMRB and the drowned orifice equation. The design tables in the DMRB only provide information, from which flow rates through the grating can be calculated, for water depths of less than 4 cm (up to 0.0375 m). The drowned orifice equation was used to create an additional head-discharge curve. The two curves were used to produce a composite head-discharge curve, which uses the lowest value to avoid over-estimating flow through the grating.

1 Highways England, 2018. Design Manual for Roads and Bridges: Volume 4. CD526 Spacing of Road Gullies. Available at: http://www.standardsforhighways.co.uk/ha/standards/dmrb/vol4/section2/CD526.pdf [Accessed on: 21/09/2018]

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The gratings have been modelled without any blockage, as blockages would impact a much smaller percentage of the available grating area and are less likely to occur. The continuous grating means water can flow around, and potentially dislodge debris which might block point drainage features (e.g. gullies) where water has nowhere else to go. Blockages are therefore not expected to have a significant impact on the performance of the grating, although there is a small risk that blockage of the gratings could reduce the effectiveness of the works. Additionally, the testing of a low-efficiency (Type P) grate provides insight into the sensitivity of the system to grating performance.

There is some uncertainty over the way that the model treats the flow passing through the gratings when the drainage system is surcharged. Assessments made suggest that the outlet pipe acts as the hydraulic control on the system, however there is some risk that this issue could affect the modelling results and scheme benefits presented. Queries have been raised with the model provider and Atkins will confirm whether this has a material effect on the findings of the study when responses are received.

Overland Flow route:

The flow routes for the 5% AEP (1 in 20) event Do Minimum and Option scenarios are shown in Figure 3 and Figure 4. The Do Minimum option assumes the existing system is in place and 50% of the gullies will be blocked to represent the average state between maintenance clearing. The Option is modelled to include the features as described in Section 2. It is seen to divert water from Marlowe Close into the storage area, via the grated road drainage features and overland flow, reducing flood depths in the flow path downstream of Marlowe Close. The grated road drainage redirects a volume of 1,500 m3 towards the storage area.

Figure 3 - Overland flow route, 5% AEP (1 in 20) event, Do Minimum Scenario

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Figure 4 - Overland flow route, 5% AEP (1 in 20) event, Option Scenario

In a 50% AEP (1 in 2) event, the grated road drainage feature becomes drowned, allowing water to flow over the grating and overtop the eastern kerb of Marlowe Close. The overtopping occurs due to a combination of the large volume of flow and the low kerb heights along Marlowe Close. Despite overtopping in such a frequent event, the option reduces flood depths downstream of the grated road drainage for all modelled events.

The impact of the option on the flow route is most notable downstream of Marlowe Close where the flow through Brickhouse Farm Community Centre and the flow heading south-east through Poulton Close and Chichester Way are both reduced. The reduction in flow provides benefits as follows:

by reducing flood risk for some properties. A reduction in flood risk is defined, in this instance, as first flooding in a higher return period event than the baseline (“Do Nothing”) scenario.

by reducing the flood depth, and therefore the damage, for properties which flooded during the same return period event (as the baseline scenario).

Further information on option benefits is provided below in Section 4 - Economics.

Exceedance of low flow pipe capacity: In all modelled events; 50% AEP (1 in 2) to 0.5% AEP (1 in 200), the capacity of the low flow pipe is exceeded and water enters the storage area via an overflow from the inlet structure. The flood storage area is designed to receive water from the improved road drainage on Marlowe Close during large rainfall events. It will likely become “wet” due to the exceedance of the low flow pipe capacity on a number of occasions each year during larger rainfall events.

The modelled events indicate that pond will take approximately 2 - 3 hours to fill the storage area and water will be within the storage area for 3 – 9 hours in the 50% AEP (1 in 2) and 0.5% AEP (1 in 200) events respectively.

If required, by adding this to the scope of the detailed design phase, further work could be carried out to optimise the design of the low flow pipe and determine the size of rainfall event at which water begins entering the storage area via an overflow from the inlet structure.

Spill Frequency:

When the capacity of the storage area is exceeded, water will spill out at the lowest point, located in the north-east corner of the FSA, and be directed along the spillway (the cycle path towards Poulton Close). The spillway will direct water back to where it currently flows in the existing Do Minimum scenario. The proposed level of the spillway is approximately 6.0mAOD and will be confirmed during detailed design. The spillway is therefore likely to be in use in events greater than the 10% AEP (1 in 10) event as can be seen on Table 1 which shows a summary of peak water levels in the

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storage area for each event. The lowest level of the pond is 4.9mAOD. Velocities and depths along the spillway will be determined during detailed design.

Annual Exceedance Probability Event (% / 1 in X)

Maximum Water Level (mAOD)

Maximum Water Depth in Basin (m)

50% (1 in 2) 5.4 0.5

20% (1 in 5) 5.8 0.9

10% (1 in 10) 6.0 1.1

5% (1 in 20) 6.2 1.3

3.33 % (1 in 30) 6.3 1.4

2% (1 in 50) 6.3 1.4

1.33% (1 in 75) 6.3 1.4

1% (1 in 100) 6.4 1.5

0.5% (1 in 200) 6.4 1.5

Table 1 - Maximum Water Levels in the Storage Area

4. Economics Scheme cost:

The estimated cost of the updated scheme has been calculated, taking into account the following elements:

- Storage area excavation and landscaping

- Storage area inlet and outlet

- Grated road drainage channels (excavation and purchase of units)

- Pipework to the storage area and from the storage area to the existing surface water network

- New manhole

- New playground (area of raised ground to be created to locate playground)

- Removal of hazardous waste (contaminated land) amounting to 5% of the total volume of removed material for the storage area

The unit cost of each element was sourced from SPONS 2015, aside from the cost of the grated road drainage channels, which was obtained from a supplier website. Costs were adjusted to reflect today’s costs.

The details associated with some smaller elements of the design are not known at this stage. These elements are: ground raising and raised kerbs, erosion protection, reinstatement of the road surface and cycle path, repairs to the car park that will be used as the construction compound and disconnection of the existing pipes. An allowance of £25k has been made for these items.

The capital scheme cost has been developed based on the following assumptions:

- Optimism bias of 60%

- Contractor’s on-costs (mobilisation etc.) are 30%

- Contractor’s profit is 15%

- Design fees are 15%

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Based on the methodology and assumptions summarised above, the implementation cost of the scheme has been estimated at £700k. There is some risk that costs may escalate due to contractor’s costs being higher than those calculated using SPONS; if the geotechnical investigation identifies significant quantities of contaminated land; if service diversions are required or and if vehicle movement timings have to be restricted. These risks are assumed to be included in the 60% optimism bias allowance at this stage.

The whole life cost of the scheme has been calculated based on the following additional assumptions:

- Routine maintenance will be required annually (or throughout each year), including cleaning of the grated channels, grass cutting and inspection of the inlet and outlet structures.

- Further maintenance will be required on a 10-yearly basis. This may include items such as minor maintenance of earthworks, replacement of grates on the road channels, full jetting of channels and pipes and removal of trees growing on the slopes of the storage area.

- The scheme has a 30-year design life. After 30 years, capital investment will be required to extend the life of the scheme. This may include works such as repairs to the slopes of the storage area, replacement of inlet and outlet screens, undertaking of pipe repairs and any works not completed in the yearly and 10 yearly maintenance.

Based on the assumptions summarised above, the whole life present value cost of the scheme has been estimated to be £722k.

Annual maintenance cost of £10,000 has been assumed to account for the upkeep of the existing network. The existing network includes all of the storm water network south of the River Chelmer (Anglian Water pipes and highways gully connections in Maldon Central area).

Scheme benefits:

The economic flood assessment included calculation of the following:

• Residential and non-residential property damages, using the MCM methodology and depth damage curves (as updated in 2018).

• Evacuation costs for residential properties experiencing above floor level flooding using the MCM data

• Cost of emergency services, estimated as 5.6% of the total property damages

• Vehicle damages, using the average value of a UK motor vehicle of £3,100, and a percentage of vehicles per residential property of 115%.

• Human Intangible benefits of the option

For the purpose of the economic assessment, Average Annual Damages (AADs) are discounted over a period of 100 years using a discount factor to generate a Present Value Damage (PVd).

It is standard for economic appraisal to assess the benefits of options against both Do Nothing and Do Minimum scenarios. The baseline used in this economic appraisal was the Do Nothing scenario, against which the other options can be compared.

The options appraised were:

• Do Nothing – this is the economic baseline, representative of the current flood risk situation without doing any kind of maintenance.

• Do Minimum – this represents the current flood risk situation, including the current maintenance of the existing system.

• Improve 1 – this option consists of Type S grated road drainage on Marlowe Close connected to the flood storage, including the normal maintenance of the existing system.

• Improve 2 – this option consists of Type P grated road drainage on Marlowe Close connected to the flood storage, including the normal maintenance of the existing system.

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It is possible that a combination of the different types of grated road drainage will be used. This will be finalised during the detailed design stage.

Table 2 - Summary of the Option Costs and Benefits

Option Do

Nothing Do Minimum Improve 1 Improve 2

PV Capital Costs (£k) £0 £0 £413 £413

PV Existing Maintenance Costs (£k)

£0 £298 £298 £298

PV Option Maintenance Costs (£k)

£0 £0 £61 £61

PV Optimism Bias Costs (£k) £0 £0 £248 £248

Total PV Costs (£k) – excluding existing maintenance

£0 £0 £722 £722

Total PV Costs (£k) – including existing maintenance

£0 £298 £1,020 £1,020

PV Residential Property damages £35,200 £32,494 £27,587 £27,788

PV Non-Residential Property damages

£8,749 £8,574 £8,267 £8,281

PV Evacuation monetised flood losses

£4,844 £5,408 £4,612 £4,609

PV Emergency Services monetised flood losses

£2,082 £1,971 £1,685 £1,697

PV Vehicles flood damages £665 £651 £447 £447

PV Risk to Life monetised flood damages

£0 £0 £0 £0

Total PV Damages (£k) £51,540 £49,099 £42,598 £42,822

PV Human Intangibles flood benefits

£92 £171 £163

Total PV Benefits (£k) £2,534 £9,113 £8,882

Net Present Value NPV £2,235 £8,093 £7,862

Average benefit/cost ratio BCR 8.5 8.9 8.7

Incremental benefit/cost ratio IBCR

9.1 -

The properties benefitting from a reduction in flood risk are shown in Figure 5. A reduction in flood risk is defined, in this instance, as first flooding in a larger event than the baseline (“Do Nothing”) scenario.

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Figure 5 - Properties benefitting from a reduction in flood risk

Figure 6 - Combined Benefits Map

5. Key risks The key risks have been identified and a table of modelling risks and a table of design risks is presented in Appendix A. Risks identified as being high risks are as follows:

Grating performance (Risk 2 in Modelling Risks Table A-1)

Pipe clashes (Risk 8 in Design Risks Table A-2)

Anglian Water (Risk 9 in Design Risks Table A-2)

Unexpected ground conditions (Risk 12 in Design Risks Table A-2)

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6. Conclusion and next steps The modelling, economics and outline design have been revised in line with updated scope of work, as detailed in Section 1. The scheme has been revised in line with the following objectives:

1. All highways surface water from Marlowe Close to drain in to the new storage area.

2. Storage area not to be wet all the time.

Based on current modelling results the economics show that the scheme provides a cost benefit ratio of between 1:8.9 and 1:8.7, depending on the type of grating used on Marlowe Close. Due to modelling uncertainties there is some risk that this level of benefit will not be fully achieved.

It should be noted that the modelling shows the flood storage area is likely to fill at least partially during most rainfall events but will drain once the storm event has subsided. The length of time it takes to drain will depend on the size of the event, but the model shows that even for a 0.5% AEP (1 in 200 year) event the storage area will have drained within 9 hours.

The next step is to gain approval for the scheme with key stakeholders. At the meeting on 6th November 2018 attended by Dave Chapman and Marc Inman (Essex County Council) it was agreed to proceed with the ground investigation and detailed design in parallel with the consultation process. The design submitted with this technical note will form the basis of the detailed design.

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APPENDIX A

Key risks The key risks identified during the design process are summarised below:

Table A-1 Modelling Risks

Risk

Risk rating

Mitigation

Con

se

que

nce

Lik

elih

oo

d

Ratin

g

Model uncertainties: The new model was validated against the original Capita model which was assumed to be correct. Key assumptions are as follows:

1 The model shows large volumes of overland flow some of which the grated road drainage channel is able to capture. If the flow path is further north, the grated road drainage channel will not capture this flow and the expected benefits would not be realised.

M L M Ground survey data was obtained for the original study area including the southern part of Marlowe Close. Ground survey data correlated well with LiDAR data within the original study area and hence the probability that actual ground levels on Marlowe Close differ from the levels in the model is low. To increase confidence further, topographic survey information could be obtained for the northern section of Marlow Close and “upstream” of Marlowe Close.

2 Grating performance

The grated road drainage feature may not perform as expected as modelled under drowned conditions.

H M H The performance of the grating types was determined using design tables from the DMRB and drowned orifice equation. Modelling high and low efficiency grating types (under normal conditions) suggested that the viability of the scheme is not determined by the performance of the grating. However, it is not clear if the model is able to fully represent the hydraulic interaction between overland and drain flow.

Model behaviour is currently being investigated with supplier. Further sensitivity testing could be undertaken if required.

3 Overland flow around the buildings in the study area may not behave as per the model and there is a risk that this could have an effect on the damages (over or under estimation) and hence it could have an effect on the benefits (over or under estimation).

L L L The buildings in the new model have been modelled as a 100mm plinth with a Mannings roughness of 1, in line with the methodology of the original Capita model.

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4 The catchment runoff percentages are high, approximately 60-80% for the 5% AEP (1 in 20) – 0.5% AEP (1 in 200) events. High runoff is expected in this area due to the characteristics of the land but the high values may be overestimating the runoff volumes. Over-estimation of runoff volumes may mean an over-estimation of damages and an under-estimation of the standard of protection provided by the option.

M M M The infiltration model and associated parameters match the original Capita model.

5 The Do Minimum scenario assumed 50% blockages in existing pipes as it was noted that the existing drainage is not routinely cleared. The Option scenario has also assumed 50% blockages in existing pipes but no blockages in the grated road drainage channel. There is a risk that if the grated road drainage channel outlet pipes become blocked over an extended length the capacity of the outlet pipes could be exceeded and the scheme will not perform as expected.

M L M Water in the grated road drainage features backs up and flows out onto the road, at the southern end of Marlowe Close, when the capacity of the outlet pipes is exceeded. A flow route from the southern end of Marlowe Close into the storage area will be incorporated into the design to accommodate for this.

6 The representation of the downstream boundary condition may result in an over-estimation of the damages and an under-estimation of the standard of protection provided by the option.

M M M The downstream boundary conditions were set as a constant water level of 3.0mAOD, to represent bank full level in the River Chelmer, as per the Capita model. The model was simulated for 9 hours to capture the peak flows and establish the length of time water is in the storage area. It is unlikely that stage in the Chelmer will be at bank full level for 9 hours given the tidal location.

7 The downstream outfall to the River Chelmer, by Northey Island, is modelled as being open. Water intrudes into the drainage network due to the constant water level condition of 3.0mAOD. The intrusion reduces the capacity of the drainage network, relative to an empty network with a tidal-locked flap valve and may over-estimate damages as water is forced overland. Consequently there may be an under-estimation of the standard of protection provided by the option.

M L M It is likely that there is a control (e.g. a flap valve) on this downstream outfall given the presence of flood embankments along the waterfront.

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Table A-2 Design Risks

Risk

Risk rating

Mitigation

Con

se

que

nce

Lik

elih

oo

d

Ratin

g

8 Pipe clashes between the proposed pipes and existing surface water and foul pipes.

Tolerances are low for the pipes upstream of the FSA so if any of the existing pipes are not at the elevations stated in the Anglian Water records, there could be potential clashes. Therefore, the risk associated with pipe clashes is high, due to the cost implications of a pipe clash occurring.

H M H Pipe locations and levels have been assessed based on the information available.

The likelihood is low as some of the information has been verified through a drainage survey, which correlated well with the Anglian Water records.

There is some scope to lower the pipes further if a greater distance between existing and new pipes is required.

Trial pits should be undertaken at key points at the same time as the ground investigation to verify the location and level of the pipes to eliminate the risk during construction.

9 Connection into the Anglian Water surface water network.

The storage area outlet is proposed to connect into the existing Anglian Water surface water network. If this is not acceptable to Anglian Water, it would pose a significant risk to the project.

H L H Design advice has been received from the drainage team who have previously worked with Anglian Water, during the outline design stage.

Anglian Water will be consulted on the proposed design, in order to remove this risk. Essex County Council to action.

10 Agreement from third parties and planning permission

The design involves works in the public highway and in the park, so the scheme needs to be acceptable to ECC Highways team, Maldon District Council and requires planning permission. Key stakeholders have had issues with previous iterations of the design. Possible issues include: removal of trees, playground equipment and disruption to the community.

M M M Discussions have taken place throughout the outline design stage and are ongoing. Essex County Council to action.

Ecology report noted that the trees do not have significant ecological value. Ecologists / landscape architects to propose replacement trees and landscaping features during detailed design phase.

Playground equipment to be proposed during detailed design phase.

Contractor to plan works such that disruption to community is minimised.

11 Maintenance of low flow pipe.

As the low flow pipe under the storage area has a relatively small diameter and a shallow gradient, there is a risk that it may become blocked with silt.

There is a risk that blockage of this pipe could prevent the scheme performing as planned. If blockage

M M M The access chambers at the upstream and downstream ends of the pipe will allow for access to facilitate ease of clearing any debris or silt build up within this pipe.

Ongoing maintenance to ensure the pipe remains clear is essential. Essex County Council to discuss with Maldon District Council to ensure maintenance requirements are understood.

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of the low flow pipe occurred the storage area would fill more frequently and earlier in a storm event, potentially reducing its effectiveness.

12 Unexpected ground conditions e.g. groundwater, contaminated land, archaeology and buried services. Costs may increase due to the number of vehicle movements required and there may be restrictions regarding the vehicle movements.

H M H Ground investigation to be carried out at the start of the detailed design stage to reduce the risk of unexpected ground conditions during construction and to increase confidence in expected costs.

13 Extreme event greater than the design event leading to flooding of properties causing anger amongst stakeholders.

M M M Essex County Council to communicate the risk to key stakeholders.

14 Topography affecting flow routes e.g. southern section of Marlowe Close, spillway route.

M L M There is sufficient gradient for the proposed spill way along the cycle path and the flow route from Marlowe Close into the storage area, as confirmed by the model results.

Drawings to be checked during detailed design and ground profiling to be proposed to reduce the risk of ponding, if appropriate.

15 Flood Defence Grant in Aid funding

Lack of funds for scheme to go ahead.

M M M Economics approach has been agreed with ECC to date. Climate change to be included to obtain FDGiA funding.

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Alex Lorenz SNC Lavalin Woodcote Grove Ashley Road Epsom KT18 5BW

Direct: 01372 220664 [email protected]

© SNC Lavalin except where stated otherwise

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S

W

S

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S

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S

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S

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S

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S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

WS

W

F

S

F

S

F

S

F

S

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

F

S

F

S

F

S

S

W

S

W

S

W

S

W

S

W

F

S

F

S

F

S

F

S

S

W

F

S

F

S

F

S

F

S

S

W

S

W

S

W

S

W

S

W

S

W

F

S

F

S

F

S

S

W

S

W

S

W

S

W

F

S

F

S

F

S

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

S

W

NOT FOR CONSTRUCTION

PROPOSED

CONSTRUCTION

SITE ACCESS

PROPOSED 450Ø STORAGE

AREA OUTLET PIPE

KERB LEVELTO THE SOUTH

OF THE PATH TO BE RAISED

AS REQUIRED FOR THE

EMERGENCY SPILLWAY

PROPOSED CONNECTION TO

EXISTING STORM WATER NETWORK

WITH NEW MANHOLE AND TIDAL FLAP

EXISTING

FENCE LINE

EXISTING

MH 3859

PROPOSED EMERGENCY

SPILLWAY WITH

EROSION PROTECTION

PROPOSED OUTLET CHAMBER AND

CONCRETE CHANNEL WITH SAFETY

SCREEN.

INDICATIVE GRATED ROAD

DRAINAGE CHANNELS

500 WIDE TOP OF

PROPOSED

EMBANKMENT

PROPOSED CONSTRUCTION

SITE COMPOUND AREA

TREES WITHIN STORAGE

AREA TO BE REMOVED

INDICATIVE LOCATION OF

PROPOSED GRASSED

ACCESS RAMP.

INDICATIVE GROUND RAISING

(OR SIMILAR) TO PREVENT FLOW

THROUGH GATE OPENING.

EXISTING

CYCLE PATH

INDICATIVE LOCATION OF

PROPOSED GRASSED

ACCESS RAMP.

INDICATIVE ROUTE OF SHALLOW

BURIED DISCHARGE PIPES FROM

THE DRAINAGE TROUGH TO THE

STORAGE AREA

RAISED KERB TO BE ADDED

ADJACENT TO BUILDING

GROUND RAISING AS

REQUIRED TO DIRECT

SURFACE FLOW INTO BASIN

PROPOSED 450Ø

STORAGE

AREA INLET PIPE

M

A

R

I

N

E

R

S

W

A

Y

C

L

A

Y

T

O

N

W

A

Y

6.10mAOD

5.62mAOD

5.1mAOD

5.9mAOD

6.5mAOD

6.5mAOD

6.5mAOD

PROPOSED INLET CHAMBER

WITH TIDAL FLAPS, ENERGY

DISSIPATION AND SAFETY

SCREEN AND HANDRAIL.

F

A

L

L

5.9m AOD

INDICATIVE DEPRESSION - LOW

POINT FOR DRAINING OF STORAGE

AREA. BURIED LOW FLOW PIPE

BENEATH THE DEPRESSION

Date

DesignedScale

Drawing Title

Project Title

Drawing Status

Date Date Date

Drawn Checked Authorised

Millim

etre

s

10

01

00

Client

Copyright C Atkins Limited (2018)

www.atkinsglobal.com

Original Size

Suitability

Tel:

Fax:

Drawing Number Revision

Woodcote Grove

Ashley Road

Epsom

Surrey

KT18 5BW

+44 (0)1372 726140

+44 (0)1372 740055

Rev. DateDescription By

Chk'd

App'd

File

na

me

: P

:\IN

BL

C\W

&E

\C

ap

ita

l D

elive

ry S

olu

tio

ns\B

elfa

st S

tru

ctu

ra

l P

ro

je

cts\R

&C

\M

ald

on

C

en

tra

l\4

00

T

ech

nica

l In

fo

rm

atio

n\4

03

W

orkin

g D

ra

win

51

54

07

1-A

TK

-M

C-D

R-C

-0

02

_P

1.d

wg

US

ER

: B

SS

H5

52

9D

ate

: 1

7/9

/1

8 1

6:0

4:5

9

DO NOT SCALE

A1

SAFETY, HEALTH AND ENVIRONMENTAL

INFORMATION

In addition to the hazards/risks normally associated with the types of work

detailed on this drawing, note the following:

CONSTRUCTION

MAINTENANCE/CLEANING

DECOMMISSIONING/DEMOLITION

It is assumed that all works will be carried out by a competent contractor

working, where appropriate, to an approved method statement

TBC

TBC

TBC

DRAFT

P01 22/11/17 FOR COMMENT

RR/

DK

CJA GAC

P02 07/09/18 FOR COMMENT

RR/

DK

PROPOSED FLOOD DETENTION AREA

BOUNDARY

FOUL SEWER

SURFACE WATER DRAIN

DISCONNECTED SURFACE WATER DRAIN

LOW FLOW CHANNEL

EXISTING TREES

EXISTING FENCE

LEGENDS :

FS

SW

N

NOTES

1. ALL LEVELS ARE IN mAOD NEWLYN

2. ALL DIMENSIONS ARE IN mm UNLESS STATED

OTHERWISE

3. TREE SURVEY TO CONFIRM EXACT POSITION AND TYPE OF

TREES WITHIN AND IN THE VICINITY OF PROPOSED WORK

AREAS. TO BE CONFIRMED THAT TREE PRESERVATION

ORDER DOES NOT EXIST FOR TREES THAT

WILL BE MARKED FOR REMOVAL.

4. ALLOW FOR SERVICES PROTECTION IN THE VICINITY OF

EXISTING BURIED SERVICES.

5. SPILLWAY TO BE FORMED AS STANDARD TARMAC ROAD.

KEY PLAN

SCALE 1:5000

N

L

I

M

E

B

R

O

O

K

W

A

Y

W

O

R

D

S

W

O

R

T

H

A

V

E

N

U

E

M

U

N

D

O

N

R

O

A

D

PLAN

SCALE 1:500 Scale 1:500

10m 0m 10m 20m 30m

Scale 1:5000

100m 0m 100m 200m 300m

SW

APPENDIX 1

maldon central - maldon district 1.pdf · snc lavalin | new concept for maldon fsa system...

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