appendix 4.1 surface water drainage strategy

Veolia ES Staffordshire Limited

Project W2R: Energy Recovery Facility

Environmental Statement August 2010 Appendix 4.1: Surface Water Drainage Strategy

Appendix 4.1 Surface Water Drainage Strategy




1. Introduction

1.1.1 This document describes the proposed Surface Water Drainage Strategy for the ERF proposed

by Veolia Environmental Services.

1.1.2 It draws upon the Sustainable Drainage Strategy Statement prepared by Enviros Consulting in

2008 for the consented EfW facility. The salient conclusions of this report were:

• Proposed surface water drainage system design in compliance with Planning Policy

Statement 25: Development and Flood Risk (PPS25), i.e. up to and including the 1%

annual probability event, inclusive of climate change (20% added to rainfall intensities).

• Due to likely low infiltration rates, attenuation storage was proposed, rather than infiltration.

• Attenuation would be provided by the living roof, swales adjacent to access roads and two

ponds.

• Discharge rates from the attenuation system would be equal to greenfield runoff rates and

would discharge into the small ditch that flows to the east and south of the site.

1.1.3 Given that the proposed site layout and design of the ERF now proposed differs from the

consented EfW facility, and that a new, stand-alone planning application is required, this Surface

Water Drainage Strategy has been prepared.

1.1.4 The principal design alterations relevant to the drainage strategy are as follows:

• Alteration in the site layout resulting in the following adjustments to site areas:

• Roof area of approximately 10,280 m2

(1.3 ha), including 7,860m2 (0.8 ha) of living

roof. The consented EfW facility had a roof of 1.3 ha, 1.1 ha of which was living roof.

• Road, access and parking areas of approximately 1.39 ha. The consented EfW

facility had an area of 1.2 ha.

• All developed areas of the site are to drain via traditional gravity-fed pipes into the ponds.

1.1.5 The principal attenuation features will remain as the living roof and two ponds, which will be

located in the same locations as for the consented EfW facility, namely one in the southeast

corner of the site (the ‘southern pond’) and one in the northern extent of the site (the ‘northern

pond’). For the purposes of this Surface Water Drainage Strategy, it is assumed these ponds will

be permanently wet with fluctuating water levels according to rainfall and associated runoff levels.

In addition to the ponds, some runoff attenuation and pollution control will be provided by grassed

swales/filter strips. The proposed living roof and attenuation ponds are deemed acceptable to

provide sufficient biodiversity benefits to the site.

1.1.6 Therefore the strategy is considered to comply with Planning Policy Statement 25 (PPS 25) and

Policy D2, in particular item (g), of the Adopted Staffordshire and Stoke-on-Trent Structure Plan.




2. Baseline Conditions

2.1 Hydrological Receptors

2.1.1 The principal receptor for surface water runoff discharging from the proposed development will

be the small ditch that flows south along the eastern boundary of the site, before turning west

along the southern boundary. This ditch joins the Saredon Brook to the south west of the site.

2.2 Drainage Areas

2.2.1 For the purposes of this assessment, the site has been separated into two drainage areas: one

area draining into the northern pond and one draining into the southern pond.

2.2.2 It has been assumed that the area draining to the northern pond is 0.86 ha and consists of the

access areas in the northern portion of the site along with the roof area not designated as living

roof.

2.2.3 The drainage area for the southern pond is 1.35 ha and consists of the access areas in the

southern extent of the site, along with the condensers and the portion of the roof area dedicated

as living roof.

2.2.4 As part of this assessment and to ensure a conservative approach to the proposed attenuation

provided by the principal attenuation features (ponds and living roof), the proposed grassed

swales/filter strips have not been considered within the runoff calculations.

2.2.5 The proposed drainage arrangement considered by this assessment is shown in Annex A.

However, as this assessment is primarily concerned with providing an estimated surface area

and storage volume for the attenuation ponds, the drainage areas are subject to alteration via

more detailed designs.

2.3 Greenfield Runoff Rates

2.3.1 PPS 25 requires that the runoff rates leaving a proposed site be equal to the pre-development

rates of runoff. The existing site is undeveloped and consists of reclaimed land. Therefore, in

order to ensure a conservative estimate, the existing site is considered to be ‘greenfield’ in

drainage terms. As a result, the reference discharge rate from the proposed development should

be equal to greenfield runoff rates. Greenfield runoff rates have been estimated using the

Microdrainage WinDes software suite. As the site has been considered as two drainage areas,

reference greenfield runoff rates from each area have been calculated. Table A4.1.1 below

provides greenfield runoff rates for each drainage area.

Table A4.1.1: Greenfield Runoff Rates

Drainage Area Surface Area (ha) Greenfield Runoff Rate (l/s)

Northern area 0.86 2.8

Southern area 1.35 4.4




3. Attenuation Requirements

3.1 Methodology

3.1.1 In order to estimate the required attenuation for the proposed development, the Source Control

module of Microdrainage WinDes Version 11.4 software has been utilised. This is based on the

Modified Rational Method.

3.1.2 Green/living roofs typically act as a temporary holding facility, slowing down the rate of runoff

from the roof rather than as a permanent storage facility. This is particularly prevalent during

longer duration storms or following prolonged rainfall when the green/living roof may become

saturated. The Source Control module in WinDes incorporates a function to include a

green/living roof within the area contributing runoff into the attenuation feature. This provides a

realistic and conservative estimate of required attenuation as it considers the green/living roof to

be contributing runoff, albeit at a reduced rate. As stated in Section 1.1.4 above, the area of

living roof is approximately 0.8 ha.

3.1.3 In order to control the discharge from the ponds, it is proposed that a Hydro-brake device or

similar is used. The reference flow rate for the southern pond Hydro-brake is 4.4 l/s (equal to

greenfield runoff rates).

3.1.4 The rate of flow through a Hydro-brake device is linked to its diameter, i.e. the larger the diameter

of a Hydro-brake, the greater the flow is passed through it. In order to limit flow to 4.4l/s, a Hydro-

brake of approximately 83mm diameter is required. Hydro-brake manufacturers’ guidelines

suggest that the diameter of a Hydro-brake should be greater than 75mm (for maintenance

reasons). In order to ensure the Hydro-brake that is controlling outflow from the northern pond is

greater than 75mm, the reference runoff rate has been increased from 2.8 l/s to 3.3 l/s. This

should be revised and agreed during detailed designs in accordance with chosen manufacturer

recommendations and the Environment Agency.

3.1.5 An addition of 20% has been included to account for climate change, commensurate for the

lifetime of the proposed development, in accordance with PPS 25.

3.2 Results

3.2.1 Table A4.1.2 below provides a summary of results with additional results provided in Annex B.

Table A4.1.2. Required Attenuation for Each Site Area During the 1% Annual Probability Storm Event, Inclusive of Most Storm Durations and Climate Change

Minimum Pond Dimensions

Drainage Area Reference Runoff Rate (l/s) Required Attenuation (m3)

Area (m2) Depth (m)

Northern area 3.3 752 835 0.9

Southern area 4.4 1,358 1,030 1.4




3.2.2 The northern pond will require a surface area of at least 835 m2. The southern pond will require

a surface area of at least 1,030 m2. These areas are consistent with those shown on the site

layout drawing, Figure 4.1.

3.2.3 It is therefore considered that the proposed site is able to meet the requirements of PPS 25 by

providing suitable and sustainable methods of attenuating surface water runoff generated from

the site.




4. Assumptions and Recommendations

4.1.1 A number of assumptions have been made:

• Because finished floor levels were not available at the time this strategy was prepared, the

pipe network has not yet been modelled. This would be undertaken during detailed design.

• The site has been divided into two large drainage areas. During detailed designs prior to

construction, the site may be sub-divided into smaller drainage areas.

• It is assumed that given the raised topography of the site in relation to the ditch, the

drainage system can feed by gravity into the existing ditch.

• The grassed swales/filter strips have not been included in the attenuation calculations, in

order to provide a conservative approach.

4.1.2 In order to ensure the proposed drainage system functions correctly, it is recommended that the

following be confirmed during detailed design:

• The proposed drainage pipe network will be able to drain by gravity into the ponds and

outflow receptor.

• Roof downpipes and road gully drains are sufficient to accommodate the 1% annual

probability event, inclusive of climate change.

• In order to avoid water pollution, pipes serving access and parking areas should be fitted

with suitable oil and silt interceptors.

4.1.3 In addition to the above, the attenuation requirements (see Table A4.1.2) are provided as

minimum storage volumes. Therefore, a permanently wet pond would require a certain volume to

remain wet, and to be lined with an impermeable membrane. The required storage would be

provided as an additional volume of the pond (see sketch overleaf). Without the permanently wet

aspect, the pond would be a dry basin.

4.1.4 In order to comply with PPS 25 and its Practice Guide, the ponds and drainage system should

include a design for exceedance. This would reduce the chance of flood water inundating the

buildings in the unlikely event that a storm of greater magnitude than the 1% annual probability

event (inclusive of climate change) was to occur. It would also offer some protection against a

blockage of the drainage system. In order to design for exceedance, it is recommended that

detailed designs should consider constructing the ponds with excess storage capacity. The pond

banks could also be aligned to discourage any overspill flowing towards the buildings by lowering

the furthest banks from the developed area. The graphic overleaf provides an indicative sketch

to further explain.




Indicative sketch of a typical pond cross-section

Constant water level to ensure pond

remains wet

Water level during the 1% annual

probability storm Outflow pipe

with Hydro-brake

Development

Designing for Exceedance level

The lower bank on the non-development side encourages

any overflow away from the development

To connection with drainage

ditch Impermeable lining




Annex A Drainage Plan




Annex B WinDes Outputs

©1982-2009 Micro Drainage

Scott Wilson Ltd Page 1The Crescent Centre Veolia StaffsTemple Back Surface Water DrainageBristol BS1 6EZ North PondDate 100316 Designed By PDGFile Checked ByMicro Drainage Source Control W.11.4ICP SUDS Mean Annual FloodInputReturn Period (years) 100 SAAR (mm) 680.000 Urban 0.000Area (Ha) 0.856 Soil 0.400 Region Number 4ResultsResultsResultsResults l/sl/sl/sl/s QBAR Rural 2.8QBAR Urban 2.8Q 100 years 7.2Q 1 year 2.3Q 30 years 5.5Q 100 years 7.2


Scott Wilson Ltd Page 1The Crescent Centre Veolia StaffordshireTemple Back Surface Water DrainageBristol BS1 6EZ North PondDate 100316 Designed By PDGFile North pond_835m2_0.9mD_3.3ls.src Checked ByMicro Drainage Source Control W.11.4

Summary of Results for 100 year Return Period (+20%)

StormDuration(mins)

MaximumControl(l/s)

MaximumOutflow(l/s)

MaximumWater Level

(m OD)

MaximumDepth(m)

MaximumVolume(m³)

Status

15 Summer 2.4 2.4 100.3298 0.3297 275.4 O K30 Summer 2.4 2.4 100.3802 0.3802 317.4 O K60 Summer 2.4 2.4 100.4367 0.4367 364.6 O K120 Summer 2.4 2.4 100.4982 0.4982 416.1 O K180 Summer 2.5 2.5 100.5357 0.5357 447.5 O K240 Summer 2.5 2.5 100.5622 0.5622 469.6 O K360 Summer 2.6 2.6 100.5983 0.5983 499.4 O K480 Summer 2.7 2.7 100.6208 0.6208 518.5 O K600 Summer 2.7 2.7 100.6363 0.6363 531.4 O K720 Summer 2.7 2.7 100.6468 0.6468 540.1 O K960 Summer 2.7 2.7 100.6453 0.6453 538.9 O K

1440 Summer 2.7 2.7 100.6268 0.6268 523.5 O K2160 Summer 2.6 2.6 100.5963 0.5963 497.8 O K2880 Summer 2.6 2.6 100.5688 0.5688 475.0 O K4320 Summer 2.5 2.5 100.5192 0.5192 433.5 O K5760 Summer 2.4 2.4 100.4737 0.4737 395.5 O K7200 Summer 2.4 2.4 100.4317 0.4317 360.7 O K8640 Summer 2.4 2.4 100.3927 0.3927 328.0 O K10080 Summer 2.4 2.4 100.3557 0.3557 297.1 O K

15 Winter 2.4 2.4 100.3697 0.3697 308.7 O K30 Winter 2.4 2.4 100.4262 0.4262 355.8 O K60 Winter 2.4 2.4 100.4897 0.4897 408.9 O K120 Winter 2.5 2.5 100.5597 0.5597 467.2 O K180 Winter 2.6 2.6 100.6022 0.6023 502.9 O K240 Winter 2.7 2.7 100.6328 0.6328 528.2 O K360 Winter 2.8 2.8 100.6743 0.6743 562.9 O K480 Winter 2.8 2.8 100.7013 0.7013 585.6 O K600 Winter 2.8 2.8 100.7203 0.7203 601.3 O K720 Winter 2.9 2.9 100.7333 0.7333 612.3 O K960 Winter 2.9 2.9 100.7348 0.7348 613.6 O K

1440 Winter 2.8 2.8 100.7208 0.7208 602.0 O K2160 Winter 2.8 2.8 100.6833 0.6833 570.4 O K2880 Winter 2.7 2.7 100.6503 0.6503 542.8 O K4320 Winter 2.6 2.6 100.5853 0.5853 488.5 O K5760 Winter 2.5 2.5 100.5222 0.5222 436.2 O K

StormDuration(mins)

Rain(mm/hr)

Time-Peak(mins)

15 Summer 172.92 2330 Summer 99.99 3860 Summer 57.82 68120 Summer 33.44 126180 Summer 24.27 186240 Summer 19.33 246360 Summer 14.03 366480 Summer 11.18 484600 Summer 9.37 604720 Summer 8.12 724960 Summer 6.35 962

1440 Summer 4.50 14042160 Summer 3.19 17282880 Summer 2.49 21044320 Summer 1.77 29045760 Summer 1.38 37447200 Summer 1.14 45448640 Summer 0.98 5360

10080 Summer 0.86 615215 Winter 172.92 2330 Winter 99.99 3760 Winter 57.82 66120 Winter 33.44 126180 Winter 24.27 184240 Winter 19.33 242360 Winter 14.03 360480 Winter 11.18 476600 Winter 9.37 594720 Winter 8.12 708960 Winter 6.35 936

1440 Winter 4.50 13842160 Winter 3.19 19682880 Winter 2.49 22244320 Winter 1.77 31565760 Winter 1.38 4040




StormDuration(mins)

MaximumControl(l/s)

MaximumOutflow(l/s)

MaximumWater Level

(m OD)

MaximumDepth(m)

MaximumVolume(m³)

Status

7200 Winter 2.4 2.4 100.4627 0.4627 386.5 O K8640 Winter 2.4 2.4 100.4067 0.4067 339.6 O K10080 Winter 2.4 2.4 100.3522 0.3522 294.0 O K

StormDuration(mins)

Rain(mm/hr)

Time-Peak(mins)

7200 Winter 1.14 49048640 Winter 0.98 5792

10080 Winter 0.86 6560



Rainfall Details

Region FEH Rainfall Model F (1km) 2.403Return Period (years) 100 Cv (Summer) 0.750Site Location GB 392150 308250 SJ 92150 08250 Cv (Winter) 0.840C (1km) -0.032 Shortest Storm (mins) 15D1 (1km) 0.357 Longest Storm (mins) 10080D2 (1km) 0.296 Summer Storms YesD3 (1km) 0.296 Winter Storms YesE (1km) 0.320 Climate Change % +20

Time / Area Diagram

Total Area (ha) = 0.856

Timefrom:

(mins)to:

Area(ha)

Timefrom:

(mins)to:

Area(ha)

0 4 0.428 4 8 0.428



Tank/Pond Details

Invert Level (m) 100.000 Ground Level (m) 101.000

Depth(m)

Area(m²)

Depth(m)

Area(m²)

Depth(m)

Area(m²)

Depth(m)

Area(m²)

Depth(m)

Area(m²)

Depth(m)

Area(m²)

0.00 835.0 0.20 835.0 0.40 835.0 0.60 835.0 0.80 835.00.10 835.0 0.30 835.0 0.50 835.0 0.70 835.0 0.90 835.0

Hydro-Brake Outflow Control

Design Head (m) 1.000 Hydro-Brake Type MD6 Invert Level (m) 100.000Design Flow (l/s) 3.3 Diameter (mm) 77

Depth(m)

Flow(l/s)

Depth(m)

Flow(l/s)

Depth(m)

Flow(l/s)

Depth(m)

Flow(l/s)

Depth(m)

Flow(l/s)

Depth(m)

Flow(l/s)

0.10 2.0 0.60 2.6 1.60 4.2 2.60 5.4 5.00 7.5 7.50 9.20.20 2.4 0.80 3.0 1.80 4.5 3.00 5.8 5.50 7.8 8.00 9.50.30 2.3 1.00 3.3 2.00 4.7 3.50 6.3 6.00 8.2 8.50 9.70.40 2.3 1.20 3.7 2.20 5.0 4.00 6.7 6.50 8.5 9.00 10.00.50 2.4 1.40 4.0 2.40 5.2 4.50 7.1 7.00 8.8 9.50 10.3


Scott Wilson Ltd Page 1The Crescent Centre Veolia StaffsTemple Back Surface Water DrainageBristol BS1 6EZ South PondDate 100316 Designed By PDGFile Checked ByMicro Drainage Source Control W.11.4ICP SUDS Mean Annual FloodInputReturn Period (years) 100 SAAR (mm) 680.000 Urban 0.000Area (Ha) 1.347 Soil 0.400 Region Number 4ResultsResultsResultsResults l/sl/sl/sl/s QBAR Rural 4.4QBAR Urban 4.4Q 100 years 11.4Q 1 year 3.7Q 30 years 8.7Q 100 years 11.4


Scott Wilson Ltd Page 1The Crescent Centre Veolia StaffordshireTemple Back Surface Water DrainageBristol BS1 6EZ South PondDate 100512 Designed By PDGFile 100512 SOUTH POND_1030M2_1.4MD_4.4LS.SRC Checked ByMicro Drainage Source Control W.11.4


StormDuration(mins)

MaximumControl(l/s)

MaximumOutflow(l/s)

MaximumWater Level

(m OD)

MaximumDepth(m)

MaximumVolume(m³)

Status

15 Summer 2.8 2.8 100.5218 0.5217 537.6 O K30 Summer 3.0 3.0 100.6108 0.6108 629.1 O K60 Summer 3.2 3.2 100.7128 0.7128 734.0 O K



15 Winter 2.9 2.9 100.5903 0.5903 608.1 O K30 Winter 3.1 3.1 100.6898 0.6898 710.5 O K60 Winter 3.4 3.4 100.8038 0.8038 828.0 O K

120 Winter 3.6 3.6 100.9328 0.9328 960.8 O K180 Winter 3.8 3.8 101.0138 1.0138 1044.4 O K240 Winter 3.9 3.9 101.0728 1.0728 1105.2 O K360 Winter 4.0 4.0 101.1563 1.1563 1191.1 O K480 Winter 4.1 4.1 101.2143 1.2143 1250.7 FLOOD RISK600 Winter 4.2 4.2 101.2573 1.2573 1294.7 FLOOD RISK720 Winter 4.3 4.3 101.2897 1.2898 1328.4 FLOOD RISK960 Winter 4.3 4.3 101.3102 1.3102 1349.6 FLOOD RISK

1440 Winter 4.3 4.3 101.3183 1.3183 1358.0 FLOOD RISK2160 Winter 4.3 4.3 101.2927 1.2928 1331.6 FLOOD RISK2880 Winter 4.2 4.2 101.2488 1.2488 1286.1 FLOOD RISK4320 Winter 4.1 4.1 101.1708 1.1708 1205.9 O K

StormDuration(mins)

Rain(mm/hr)

Time-Peak(mins)

15 Summer 172.92 9230 Summer 99.99 10260 Summer 57.82 122

120 Summer 33.44 162180 Summer 24.27 210240 Summer 19.33 262360 Summer 14.03 374480 Summer 11.18 488600 Summer 9.37 608720 Summer 8.12 726960 Summer 6.35 966

1440 Summer 4.50 14422160 Summer 3.19 20522880 Summer 2.49 23644320 Summer 1.77 31125760 Summer 1.38 39207200 Summer 1.14 47608640 Summer 0.98 5552

10080 Summer 0.86 637615 Winter 172.92 9430 Winter 99.99 10460 Winter 57.82 122

120 Winter 33.44 164180 Winter 24.27 210240 Winter 19.33 262360 Winter 14.03 370480 Winter 11.18 484600 Winter 9.37 600720 Winter 8.12 718960 Winter 6.35 950

1440 Winter 4.50 14062160 Winter 3.19 20682880 Winter 2.49 26604320 Winter 1.77 3304




StormDuration(mins)

MaximumControl(l/s)

MaximumOutflow(l/s)

MaximumWater Level

(m OD)

MaximumDepth(m)

MaximumVolume(m³)

Status

5760 Winter 3.9 3.9 101.0958 1.0958 1128.5 O K7200 Winter 3.8 3.8 101.0223 1.0223 1053.0 O K8640 Winter 3.7 3.7 100.9528 0.9528 981.5 O K10080 Winter 3.5 3.5 100.8878 0.8878 914.2 O K

StormDuration(mins)

Rain(mm/hr)

Time-Peak(mins)

5760 Winter 1.38 42167200 Winter 1.14 51208640 Winter 0.98 6048

10080 Winter 0.86 6888



Rainfall Details

Region FEH Rainfall Model F (1km) 2.403Return Period (years) 100 Cv (Summer) 0.750Site Location GB 392150 308250 SJ 92150 08250 Cv (Winter) 0.840C (1km) -0.032 Shortest Storm (mins) 15D1 (1km) 0.357 Longest Storm (mins) 10080D2 (1km) 0.296 Summer Storms YesD3 (1km) 0.296 Winter Storms YesE (1km) 0.320 Climate Change % +20

Green Roof

Area (m²) 7860 Evaporation (mm/day) 3Depression Storage (mm) 5 Decay Coefficient 0.050

Time / Area Diagram

Timefrom:

(mins)to:

Area(ha)

Timefrom:

(mins)to:

Area(ha)

Timefrom:

(mins)to:

Area(ha)

Timefrom:

(mins)to:

Area(ha)

0 4 0.667832 32 36 0.028837 64 68 0.005822 96 100 0.0011754 8 0.641941 36 40 0.023610 68 72 0.004767 100 104 0.0009628 12 0.095743 40 44 0.019330 72 76 0.003903 104 108 0.00078812 16 0.078388 44 48 0.015826 76 80 0.003195 108 112 0.00064516 20 0.064178 48 52 0.012957 80 84 0.002616 112 116 0.00052820 24 0.052545 52 56 0.010609 84 88 0.002142 116 120 0.00043224 28 0.043020 56 60 0.008686 88 92 0.00175428 32 0.035222 60 64 0.007111 92 96 0.001436



Tank/Pond Details

Invert Level (m) 100.000 Ground Level (m) 101.400

Depth(m)

Area(m²)

Depth(m)

Area(m²)

Depth(m)

Area(m²)

Depth(m)

Area(m²)

Depth(m)

Area(m²)

Depth(m)

Area(m²)

0.00 1030.0 0.30 1030.0 0.60 1030.0 0.90 1030.0 1.20 1030.00.10 1030.0 0.40 1030.0 0.70 1030.0 1.00 1030.0 1.30 1030.00.20 1030.0 0.50 1030.0 0.80 1030.0 1.10 1030.0 1.40 1030.0

Hydro-Brake Outflow Control

Design Head (m) 1.400 Hydro-Brake Type MD6 Invert Level (m) 100.000Design Flow (l/s) 4.4 Diameter (mm) 81

Depth(m)

Flow(l/s)

Depth(m)

Flow(l/s)

Depth(m)

Flow(l/s)

Depth(m)

Flow(l/s)

Depth(m)

Flow(l/s)

Depth(m)

Flow(l/s)

0.10 2.2 0.60 3.0 1.60 4.8 2.60 6.1 5.00 8.4 7.50 10.30.20 2.8 0.80 3.4 1.80 5.1 3.00 6.5 5.50 8.8 8.00 10.60.30 2.6 1.00 3.8 2.00 5.3 3.50 7.0 6.00 9.2 8.50 11.00.40 2.6 1.20 4.1 2.20 5.6 4.00 7.5 6.50 9.6 9.00 11.30.50 2.8 1.40 4.5 2.40 5.8 4.50 8.0 7.00 10.0 9.50 11.6

appendix 4.1 surface water drainage strategy

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