rps report - eaststaffsbc.gov.uk application/627000/62… · c 2012 rps group notes 1. this drawing...

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Highfield House, 5 RidgewayQuinton Business Park, Birmingham B32 1AF

T: +44(0)121 213 5500 E: [email protected] F: +44(0)121 213 5502

Client

Title

Status

Date CreatedScale @ A0

PM/Checked byDrawn By

Drawing Number Rev

Project

c 2012 RPS Group

Notes1. This drawing has been prepared in accordance with the scope of RPS’s

appointment with its client and is subject to the terms and conditions of thatappointment. RPS accepts no liability for any use of this document other thanby its client and only for the purposes for which it was prepared and provided.

2. If received electronically it is the recipients responsibility to print to correctscale. Only written dimensions should be used.

Job Ref

rpsgroup.com

David Wilson Homes Mercia

Havelwall FarmUttoxeter

East SiteEngineering Concept

Status

AAC5085

RAA

1:200 Dec 14

SK108 -

Rev Description Date Initial Checked

For guidance only. Do not scale off this drawing

Due to the steep nature of the site therewill be considerable soil earthworksmovement on this site. This drawinghighlights the many retaining walls whichwill be required to develop the site. Theretaining walls are shown in simplifiedform, and will become move intricate atdetailed design stage, particularly nearbuildings and in back gardens where stepsetc will need to formed. The majority ofproperties will require exposed brickwork,upto 1.5m in places.

In determining the slab levels gardengradients of 1 in 10, and some drivegradients of 1 in 8 have been used.

The smaller red level within the housesrepresents the existing ground level at thecentre of the house.

The larger magenta level is the anticipatedslab level of the house.

A max gradient of 1 in 12 has been usedfor the highway design, with a maxgradient of 1 in 20 directly from junctions.

sarah.tittertonText BoxP/2015/01497Received 22 Oct 2015

45 rpsgroup.com

Planning & Development

APPENDIX M

MicroDrainage Attenuation Calculations 1 in 100 year + 30% return periods storage calculations

RPS Group Plc Page 1Highfield House Hazelwall Farm5 Ridgeway Quinton Business Park Site ABirmingham B32 1AF Indicative StorageDate 20.01.2015 Designed by O PocockFile Site A 4700 cubed storage 7.5... Checked by A.GrangerMicro Drainage Source Control 2014.1.1

Summary of Results for 100 year Return Period (+30%)

©1982-2014 XP Solutions

StormEvent

MaxLevel(m)

MaxDepth(m)

MaxControl(l/s)

MaxVolume(m³)

Status

15 min Summer 100.346 0.346 48.0 1625.7 O K30 min Summer 100.456 0.456 48.1 2144.1 O K60 min Summer 100.570 0.570 48.1 2678.9 O K120 min Summer 100.680 0.680 48.1 3195.3 O K180 min Summer 100.737 0.737 48.1 3462.6 Flood Risk240 min Summer 100.769 0.769 48.1 3616.6 Flood Risk360 min Summer 100.801 0.801 48.1 3766.7 Flood Risk480 min Summer 100.814 0.814 48.1 3825.5 Flood Risk600 min Summer 100.814 0.814 48.1 3827.6 Flood Risk720 min Summer 100.810 0.810 48.1 3805.0 Flood Risk960 min Summer 100.798 0.798 48.1 3748.8 Flood Risk

1440 min Summer 100.765 0.765 48.1 3595.8 Flood Risk2160 min Summer 100.703 0.703 48.1 3302.1 Flood Risk2880 min Summer 100.637 0.637 48.1 2993.3 O K4320 min Summer 100.518 0.518 48.1 2432.6 O K5760 min Summer 100.419 0.419 48.1 1970.7 O K7200 min Summer 100.344 0.344 48.0 1615.1 O K8640 min Summer 100.288 0.288 47.4 1351.7 O K10080 min Summer 100.247 0.247 46.7 1162.1 O K

15 min Winter 100.388 0.388 48.1 1824.9 O K30 min Winter 100.513 0.513 48.1 2409.5 O K60 min Winter 100.641 0.641 48.1 3014.1 O K120 min Winter 100.767 0.767 48.1 3607.1 Flood Risk180 min Winter 100.832 0.832 48.1 3912.2 Flood Risk240 min Winter 100.870 0.870 48.1 4090.3 Flood Risk

StormEvent

Rain(mm/hr)

FloodedVolume(m³)

DischargeVolume(m³)

Time-Peak(mins)

15 min Summer 118.781 0.0 1628.6 2630 min Summer 78.851 0.0 2131.6 4160 min Summer 49.937 0.0 2806.7 70

120 min Summer 30.571 0.0 3436.8 128180 min Summer 22.634 0.0 3817.0 188240 min Summer 18.173 0.0 4086.7 246360 min Summer 13.263 0.0 4473.9 364480 min Summer 10.612 0.0 4765.6 482600 min Summer 8.919 0.0 4987.0 600720 min Summer 7.734 0.0 5170.2 654960 min Summer 6.172 0.0 5458.4 7701440 min Summer 4.483 0.0 5834.1 10302160 min Summer 3.250 0.0 6579.0 14322880 min Summer 2.584 0.0 6974.8 18244320 min Summer 1.868 0.0 7561.8 25965760 min Summer 1.482 0.0 7998.3 33367200 min Summer 1.237 0.0 8349.6 40328640 min Summer 1.068 0.0 8645.2 4672

10080 min Summer 0.942 0.0 8900.3 534415 min Winter 118.781 0.0 1814.8 2630 min Winter 78.851 0.0 2371.6 4060 min Winter 49.937 0.0 3143.7 68

120 min Winter 30.571 0.0 3849.8 126180 min Winter 22.634 0.0 4275.5 184240 min Winter 18.173 0.0 4577.4 242




StormEvent

MaxLevel(m)

MaxDepth(m)

MaxControl(l/s)

MaxVolume(m³)

Status

360 min Winter 100.909 0.909 48.1 4273.7 Flood Risk480 min Winter 100.927 0.927 48.1 4357.4 Flood Risk600 min Winter 100.932 0.932 48.1 4379.5 Flood Risk720 min Winter 100.928 0.928 48.1 4363.8 Flood Risk960 min Winter 100.908 0.908 48.1 4269.2 Flood Risk

1440 min Winter 100.865 0.865 48.1 4066.7 Flood Risk2160 min Winter 100.782 0.782 48.1 3675.7 Flood Risk2880 min Winter 100.682 0.682 48.1 3207.4 O K4320 min Winter 100.498 0.498 48.1 2338.5 O K5760 min Winter 100.358 0.358 48.0 1683.7 O K7200 min Winter 100.266 0.266 47.1 1248.0 O K8640 min Winter 100.220 0.220 44.4 1031.8 O K10080 min Winter 100.196 0.196 39.9 919.4 O K

StormEvent

Rain(mm/hr)

FloodedVolume(m³)


Time-Peak(mins)

360 min Winter 13.263 0.0 4997.9 356480 min Winter 10.612 0.0 5306.5 470600 min Winter 8.919 0.0 5550.3 580720 min Winter 7.734 0.0 5750.5 688960 min Winter 6.172 0.0 6061.0 8761440 min Winter 4.483 0.0 6435.4 11002160 min Winter 3.250 0.0 7369.1 15642880 min Winter 2.584 0.0 7812.0 19964320 min Winter 1.868 0.0 8469.4 27685760 min Winter 1.482 0.0 8961.9 34567200 min Winter 1.237 0.0 9351.9 40408640 min Winter 1.068 0.0 9682.3 4664

10080 min Winter 0.942 0.0 9968.8 5352


Model Details


Storage is Online Cover Level (m) 101.000

Tank or Pond Structure

Invert Level (m) 100.000

Depth (m) Area (m²) Depth (m) Area (m²) Depth (m) Area (m²) Depth (m) Area (m²) Depth (m) Area (m²)0.000 4700.0 0.600 4700.0 1.200 0.0 1.800 0.0 2.400 0.00.100 4700.0 0.700 4700.0 1.300 0.0 1.900 0.0 2.500 0.00.200 4700.0 0.800 4700.0 1.400 0.0 2.000 0.00.300 4700.0 0.900 4700.0 1.500 0.0 2.100 0.00.400 4700.0 1.000 4700.0 1.600 0.0 2.200 0.00.500 4700.0 1.100 0.0 1.700 0.0 2.300 0.0

Hydro-Brake Optimum® Outflow Control

Unit Reference MD-SHE-0289-4830-1000-4830Design Head (m) 1.000

Design Flow (l/s) 48.3Flush-Flo™ CalculatedObjective Minimise upstream storage

Diameter (mm) 289Invert Level (m) 99.950

Minimum Outlet Pipe Diameter (mm) 375Suggested Manhole Diameter (mm) 1800

Control Points Head (m) Flow (l/s) Control Points Head (m) Flow (l/s)Design Point (Calculated) 1.000 48.1 Kick-Flo® 0.780 42.7

Flush-Flo™ 0.439 48.1 Mean Flow over Head Range - 38.7

The hydrological calculations have been based on the Head/Discharge relationship for the Hydro-BrakeOptimum® as specified. Should another type of control device other than a Hydro-Brake Optimum® beutilised then these storage routing calculations will be invalidated

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)0.100 9.0 0.800 43.2 2.000 67.2 4.000 94.1 7.000 123.70.200 29.8 1.000 48.1 2.200 70.4 4.500 99.7 7.500 127.90.300 46.7 1.200 52.5 2.400 73.4 5.000 105.0 8.000 132.00.400 48.0 1.400 56.6 2.600 76.3 5.500 109.9 8.500 136.00.500 47.9 1.600 60.3 3.000 81.8 6.000 114.7 9.000 139.90.600 47.0 1.800 63.9 3.500 88.2 6.500 119.3 9.500 143.6

RPS Group Plc Page 1Highfield House Hazelwall Farm5 Ridgeway Quinton Business Park Site BBirmingham B32 1AF Indicative StorageDate 20.01.2015 Designed by O PocockFile Site B 950 cubed storage 1.47... Checked by A.GrangerMicro Drainage Source Control 2014.1.1



StormEvent

MaxLevel(m)

MaxDepth(m)

MaxControl(l/s)

MaxVolume(m³)

Status

15 min Summer 100.335 0.335 9.5 317.8 O K30 min Summer 100.441 0.441 9.5 419.3 O K60 min Summer 100.551 0.551 9.5 523.0 O K120 min Summer 100.655 0.655 9.5 622.0 O K180 min Summer 100.706 0.706 9.5 670.6 Flood Risk240 min Summer 100.734 0.734 9.5 697.0 Flood Risk360 min Summer 100.758 0.758 9.5 720.0 Flood Risk480 min Summer 100.764 0.764 9.5 726.1 Flood Risk600 min Summer 100.760 0.760 9.5 721.9 Flood Risk720 min Summer 100.750 0.750 9.5 712.6 Flood Risk960 min Summer 100.730 0.730 9.5 693.2 Flood Risk

1440 min Summer 100.686 0.686 9.5 652.1 O K2160 min Summer 100.617 0.617 9.5 586.1 O K2880 min Summer 100.545 0.545 9.5 517.6 O K4320 min Summer 100.422 0.422 9.5 400.8 O K5760 min Summer 100.321 0.321 9.5 304.8 O K7200 min Summer 100.243 0.243 9.5 231.0 O K8640 min Summer 100.187 0.187 9.4 177.3 O K10080 min Summer 100.146 0.146 9.2 138.9 O K

15 min Winter 100.376 0.376 9.5 357.2 O K30 min Winter 100.497 0.497 9.5 471.8 O K60 min Winter 100.621 0.621 9.5 589.8 O K120 min Winter 100.740 0.740 9.5 702.9 Flood Risk180 min Winter 100.800 0.800 9.5 759.6 Flood Risk240 min Winter 100.833 0.833 9.5 791.7 Flood Risk

StormEvent

Rain(mm/hr)

FloodedVolume(m³)


Time-Peak(mins)

15 min Summer 118.781 0.0 328.3 2630 min Summer 78.851 0.0 435.9 4160 min Summer 49.937 0.0 552.3 70

120 min Summer 30.571 0.0 676.3 128180 min Summer 22.634 0.0 751.3 188240 min Summer 18.173 0.0 804.1 246360 min Summer 13.263 0.0 880.4 364480 min Summer 10.612 0.0 939.3 482600 min Summer 8.919 0.0 986.8 600720 min Summer 7.734 0.0 1027.0 670960 min Summer 6.172 0.0 1092.7 7841440 min Summer 4.483 0.0 1190.5 10422160 min Summer 3.250 0.0 1294.9 14562880 min Summer 2.584 0.0 1372.7 18404320 min Summer 1.868 0.0 1488.2 26005760 min Summer 1.482 0.0 1574.2 33367200 min Summer 1.237 0.0 1643.3 40328640 min Summer 1.068 0.0 1701.3 4672

10080 min Summer 0.942 0.0 1751.5 534415 min Winter 118.781 0.0 367.7 2630 min Winter 78.851 0.0 488.3 4060 min Winter 49.937 0.0 618.7 70

120 min Winter 30.571 0.0 757.6 126180 min Winter 22.634 0.0 841.4 184240 min Winter 18.173 0.0 900.7 242




StormEvent

MaxLevel(m)

MaxDepth(m)

MaxControl(l/s)

MaxVolume(m³)

Status

360 min Winter 100.866 0.866 9.5 822.5 Flood Risk480 min Winter 100.878 0.878 9.5 834.2 Flood Risk600 min Winter 100.878 0.878 9.5 834.3 Flood Risk720 min Winter 100.871 0.871 9.5 827.4 Flood Risk960 min Winter 100.844 0.844 9.5 802.0 Flood Risk

1440 min Winter 100.789 0.789 9.5 749.6 Flood Risk2160 min Winter 100.698 0.698 9.5 662.9 O K2880 min Winter 100.591 0.591 9.5 561.8 O K4320 min Winter 100.398 0.398 9.5 377.8 O K5760 min Winter 100.254 0.254 9.5 241.6 O K7200 min Winter 100.161 0.161 9.3 153.4 O K8640 min Winter 100.110 0.110 8.9 104.7 O K10080 min Winter 100.092 0.092 8.0 87.2 O K

StormEvent

Rain(mm/hr)

FloodedVolume(m³)


Time-Peak(mins)

360 min Winter 13.263 0.0 986.1 358480 min Winter 10.612 0.0 1052.0 470600 min Winter 8.919 0.0 1105.2 582720 min Winter 7.734 0.0 1150.3 690960 min Winter 6.172 0.0 1223.9 8881440 min Winter 4.483 0.0 1324.8 11102160 min Winter 3.250 0.0 1450.2 15802880 min Winter 2.584 0.0 1537.5 20204320 min Winter 1.868 0.0 1666.9 27685760 min Winter 1.482 0.0 1763.5 34567200 min Winter 1.237 0.0 1840.4 40408640 min Winter 1.068 0.0 1905.5 4584

10080 min Winter 0.942 0.0 1961.8 5248


Model Details


Storage is Online Cover Level (m) 101.000

Tank or Pond Structure

Invert Level (m) 100.000

Depth (m) Area (m²) Depth (m) Area (m²) Depth (m) Area (m²) Depth (m) Area (m²) Depth (m) Area (m²)0.000 950.0 0.600 950.0 1.200 0.0 1.800 0.0 2.400 0.00.100 950.0 0.700 950.0 1.300 0.0 1.900 0.0 2.500 0.00.200 950.0 0.800 950.0 1.400 0.0 2.000 0.00.300 950.0 0.900 950.0 1.500 0.0 2.100 0.00.400 950.0 1.000 950.0 1.600 0.0 2.200 0.00.500 950.0 1.100 0.0 1.700 0.0 2.300 0.0

Hydro-Brake Optimum® Outflow Control

Unit Reference MD-SHE-0142-9500-1000-9500Design Head (m) 1.000

Design Flow (l/s) 9.5Flush-Flo™ CalculatedObjective Minimise upstream storage

Diameter (mm) 142Invert Level (m) 99.950

Minimum Outlet Pipe Diameter (mm) 225Suggested Manhole Diameter (mm) 1200

Control Points Head (m) Flow (l/s) Control Points Head (m) Flow (l/s)Design Point (Calculated) 1.000 9.5 Kick-Flo® 0.675 7.9

Flush-Flo™ 0.305 9.5 Mean Flow over Head Range - 8.2

The hydrological calculations have been based on the Head/Discharge relationship for the Hydro-BrakeOptimum® as specified. Should another type of control device other than a Hydro-Brake Optimum® beutilised then these storage routing calculations will be invalidated

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)0.100 5.1 0.800 8.5 2.000 13.1 4.000 18.3 7.000 23.90.200 9.2 1.000 9.5 2.200 13.8 4.500 19.4 7.500 24.70.300 9.5 1.200 10.3 2.400 14.3 5.000 20.4 8.000 25.50.400 9.4 1.400 11.1 2.600 14.9 5.500 21.3 8.500 26.30.500 9.1 1.600 11.8 3.000 15.9 6.000 22.2 9.000 27.00.600 8.7 1.800 12.5 3.500 17.2 6.500 23.1 9.500 27.7

46 rpsgroup.com


APPENDIX N

Severn Trent Water Sewer Capacity Assessment Report No. DE1310-256 and Subsequent Correspondence

Sewer Capacity Assessment

Hazelwalls Farm, Uttoxeter

DE1310-256

Version 2

Date: December 2013

AMEC Partnership House Regent Farm Road Gosforth Newcastle upon Tyne NE3 3AF

Hazelwalls Farm, Uttoxeter Sewer Capacity Assessment Severn Trent Water

AMEC November 2013

Sewer Capacity Assessment Summary

Sewer Capacity

Assessment prepared for

RPS Planning and Development

Highfield House, 5 Ridgeway, Quinton Business Park, Birmingham, B32 1AF

Development location and

existing use

The development is located on greenfield land at Hazelwalls Farm, in Uttoxeter.

The centre of the site is located at GR 408366, 332345.

Development proposals

Approximately 365 Residential Dwellings and one school area. The foul flows from the new developments will be drained to the existing system (total foul predicted flow into the system 1.8/s).

Surface water disposal to local watercourse

Study aim The aim of the study is to identify the potential impact of foul flows from the proposed development on the sewerage system.

Impact of proposed

development on public

sewer network

Sewer flooding Low

Combined Sewer Overflows Medium

Sewage Pumping Stations Low

Requirement for Capacity

Improvements

Capacity/storage improvements are required to accommodate flows from the whole of the proposed development. However, a maximum of 275 properties could be built without the need for capacity improvements.

Sewage Treatment Works

capacity

The site drains to Uttoxeter sewage treatment work. There is sufficient capacity at the STW to accommodate flows from this development.

Important Information:

This Sewer Capacity Assessment has been prepared by Amec on behalf of Severn Trent Water Ltd for RPS Planning and Development. This report is based on the best available information at the time of undertaking, including Severn Trent Water hydraulic models and development proposals submitted by RPS Planning and Development. If there are any changes to the development proposals after the date of submission that may affect waste water, Severn Trent Water must be informed as there may be a requirement to revisit the assessment. If there is a delay in submitting the planning application or commencing construction on site from the anticipated dates provided, the information in this report may have become out of date and Severn Trent Water must be informed as there may be a requirement to revisit the assessment based on new information.


AMEC November 2013

Table of Contents

1 Introduction ________________________________________________________________________ 1

2 Sewer Capacity Assessment ___________________________________________________________ 3

3 Conclusions and Recommendations _____________________________________________________ 7


AMEC 1 November 2013

1 Introduction

1.1 Site Location The site is located at Hazelwalls Farm, in Uttoxeter. The site is located near postcode area ST148UP and the OS Grid reference is 408366, 332345.

The site is described as greenfield.

The site location is shown in Figure A-1, Appendix A.

1.2 Local Sewerage Network The sewerage system in the vicinity of the site is a gravity drained separate system.

The site is located approximately 2.2km south-west of the Uttoxeter STW. The site adjoins some existing residential areas that are drained on separate systems. There is one critical asset located between the site and the works. This is Dovefields pumping station and overflow. There is also one critical asset that is located off line from system downstream of the development, but may be influenced by the increased flows. This is the Dove Bank Road overflow.

There is one reported external flooding issue in the near vicinity of the new development area.

The local sewerage network and the location of critical sewer assets are shown in Figure A-2.1 and Figure A-2.2, Appendix A.

1.3 Proposed Development The new development comprises approximately 365 residential dwellings and one potential new school. With no information to the contrary, the development is assumed to be built in a single phase. The site does not have planning permission.

The foul flows will drain to the existing system. The preferred manhole connections have been provided by the developer and these are SK08327501 and SK08326602. The surface water is assumed to drain to a local unnamed watercourse, which passes through the centre of the site. The foul flows from the new developments will eventually drain to Uttoxeter STW.

The proposed development is summarised in Table 1-1.

Table 1-1: Summary of proposed development

Development Type Units

Housing 365 dwellings

School 150 pupils

(assumed value)



1.4 Study Aims and Objectives The aim of the study is to identify the potential impact of flows from the proposed development on the public sewer network. This will be achieved through undertaking hydraulic computer modelling of the proposed development and assessing the impact at key points on the sewer network. Where capacity improvements are likely to be required to accommodate flows from the development, the preferred notional solution is provided.



2 Sewer Capacity Assessment

2.1 Methodology Hydraulic modelling has been used to assess the impacts of the proposed development to the existing network. The methodology is summarised below:

The best available model for the area was used as the ‘baseline model’. A review of the model was undertaken to ensure that it is suitable to inform the assessment.

MICAS has not been undertaken

The new development scheme involves the construction of approximately 365 dwellings and one new school area at Hazelwalls Farm, in Uttoxeter. The new foul flows will drain by gravity into the existing system. The preferred foul connection manhole locations are: SK08327501 and SK08326602. These connection points were provided by the developer. 65 houses and a school are to be connected to MH SK08327501 and 300 properties to manhole SK08326602.

Details of proposed development flows used in the assessment are included in Section 2.2.

The ‘baseline model’ and ‘proposed model’ were run for dry weather flow analysis and the 20 and 40 year return period events for a suite of storm durations (15,30,60,90,120,240). The results for the critical storm duration are reported throughout this report.

The model results were analysed to determine the impact of the additional flows on network performance and identify whether capacity improvements are required.

2.2 Proposed Development Flows A development comprising of 365 dwellings and one school area have been included in the model. The foul flows have been added to the existing baseline model. The foul flows arising from the proposed development have been derived using Severn Trent Water standard guidance. An allowance of 2.8 persons / property has been allocated with a consumption rate of 140 litres / head / day. For the new school area, an approximate number of 150 pupils with a consumption rate of 80 litres /day/pupil were used (as no further details were provided by the developer). Making no allowance for infiltration, the overall average dry weather flow will be 1.8 l/s. The foul flows from the new developments will drain to the existing system at the locations identified by the developer. These locations are: SK08327501 and SK08326602 for foul flows. The surface water is assumed to drain to a local unnamed watercourse, which passes through the centre of the development area. Figure A-3.2 in Appendix A shows the developer’s proposed plan of how the site is split and assigned to each connection point.

2.3 Impact of Proposed Development on Sewer Capacity The impact of the proposed development on sewer flooding is summarised in

Table 2-1. The impact at each location is assigned an ‘Impact Risk Level’, which considers whether a change in performance as a result of the development is acceptable based on the risk of sewer flooding.



Table 2-1: Predicted impact on sewer flooding for modelled scenarios (baseline and post-development)

Location Baseline performance Post-development impact Impact

Risk Level Road Manhole

reference DWF 20 year event 40 year event DWF 20 year event 40 year event

Foxglove Avenue

SK08327704 (foul)

No surcharge 0 m3 16.2 m3 No surcharge 0 m3

36.8 m3

(Increase in flood volume >20m3,

external flooding)

Low

Foxglove Avenue

SK08327705 (foul)

No surcharge 0 m3 64.6 m3 No surcharge 0 m3

70.7 m3

(Minor increase in flood

volume



Table 2-2: Predicted Combined Sewer Overflow performance for modelled scenarios (baseline and post-development)

Combined Sewer Overflow

Receiving watercourse Baseline spill volume (m3)

Post-development spill volume

(m3) Impact

Risk Level Name

Special

designation?*

1year 60 minute

event

5 year 90 minute

event

1year 60

minute event

5 year 90

minute event

Dovefields SPS CSO Local ditch No 91 m3 404 m3 103 m3

(Increase 13%)

423 m3

(Increase 4.6%)

Medium

Dove Bank Road CSO Local ditch No 152 m3 356 m3 153 m3

(Minor increase 0.6%)

358 m3

(Minor increase 0.3%)

Low

* ‘Special designation’ refers to environmental designations such as Special Areas of Conservation (SAC) and Sites of Special Scientific Interest (SSSI). The special designation may be at the location of the overflow or in downstream reaches of the receiving watercourse.

Table 2-3: Predicted Sewage Pumping Station performance for modelled scenarios (baseline and post-development)

Sewage Pumping Station

Emergency overflow Baseline performance (m3) Post-development performance (m

3)

Impact

Risk Level Receiving

watercourse

Special

designation? DWF

20 year

event

40 year

event DWF

20 year

event

40 year

event

Dovefields SPS CSO Local ditch No 0 m3 1534 m3 3257 m3 0 m3

1603 m3

(Minor increase,

4.5%)

3311 m3

(Minor increase,

1.7%)

Low



2.4 Capacity Improvement Requirements The predicted flooding volumes at a number of manhole locations (as presented in Table 2-2) are not considered to be of significant risk. However, it should be noted that the minor flooding volume increase at the manhole SK08338101 is in an area that is already prone to flooding.

There is a small impact on the spill volumes at Dove Bank Road. However at Dovefields CSO/PS, the spill volume increase is such, that storage improvements will be required to restore performance to the March 2101 baseline.

Overall, the impact of the new development to the existing system is considered to be of low risk. However, in strict accordance with STW specification, capacity/storage improvements will be required to accommodate flows from the development.

Model testing has demonstrated that by restricting development to 275 properties capacity improvements would not be required. The spill volume at the Dovefields PS CSO would increase to 100m3 from 91m3 which is a net increase of 9.9%. This level of development is below the STW trigger level for capacity increases.



3 Conclusions and Recommendations

3.1 Conclusions The impacts of foul flows arising from the proposed development at Hazelwalls Farm, Uttoxeter on the sewer network have been assessed using hydraulic modelling. The following are the conclusions of this assessment.

The proposed development is predicted to have the following impacts according to ST Capacity Assessment Specification:

o Sewer Flooding: Low

o Combined Sewer Overflows: Medium

o Sewage Pumping Stations: Low

Capacity / storage improvements will be required to accommodate foul flows from the whole of the proposed development.

By restricting development to 275 properties, downstream capacity improvements would not be required.

3.2 Recommendations Modelling has shown that the new developments (365 residential dwellings and one school area)

trigger medium risk, Following discussions with our operations regarding Dovefield CSO, it has been decided to monitor the spills from this CSO.

It is recommended that downstream improvements are investigated in order to reduce the system performance back to the March 2010 baseline.

It is recommended that if the monitoring above worsens then further modelling work is carried out to investigate reducing the impact of the development on the Dovefields PS CSO. This work would involve testing the following solutions.

Provide storage upstream of the Dovefields pumping station Increase the pump capacity at Dovefields pumping station Increase the spill level at Dovefields pumping station.


AMEC November 2013

Appendix A: Site and Development Information

Figure A-1: Site location plan


AMEC November 2013

Figure A-2.1: Local sewerage network


AMEC November 2013

Figure A-2.2: Critical assets – Overview Plan


AMEC November 2013

Fig A-3.1: Location plan from developer / consultant


AMEC November 2013

Fig A-3.2: Preferred connection locations by the developer


AMEC November 2013

Appendix B: Model Review Proforma

This appendix is for internal use only and must be removed prior to external distribution

Information contained within this appendix must not be referred to elsewhere within this report


AMEC November 2013

Appendix D: Supplementary Information

This appendix is for internal use only and must be removed prior to external distribution

Information contained within this appendix must not be referred to elsewhere within this report

1

Oli Pocock

From: Arkesden, James Sent: 12 December 2014 11:38To: Oli Pocock; Net Dev WestSubject: RE: AAC5085- Hazelwall Farms, Uttoxeter - DE-1310-256

Hi Oli Thanks for the new proposals, I have forwarded them to our catchment planner, and I will respond shortly with a quote for this modelling work. With regards to improvement works, these will be funded internally by STW and normally take 18-24 months to complete depending on complexity. This is a gated process undertaken by Severn Trent water and we will only commence this process once we have confirmation of planning permission (provide by yourselves) Many Thanks James Arkesden Modelling Specialist Planning & Performance Severn trent Water Please note I am only seconded to Severn Trent Water Wed - Fri From: Oli Pocock [mailto:[email protected]] Sent: 12 December 2014 10:55 To: Arkesden, James; Net Dev West Subject: RE: AAC5085- Hazelwall Farms, Uttoxeter - DE-1310-256 Hi, Following on from the receipt of the modelling report, attached, we have updated site proposals, please see the attached. New proposals are for approximately 470 dwellings, and no longer incorporate proposals for a school. Our calculations indicate that, the removal of the school more than offsets the proposed additional development. Can you please advise of the process to update the modelling report to reflect the revised proposals. I have attached the revised site layout for information. We also note that there is capacity for approximately 275 dwellings at present. Can you please advise of the process and path to achieve the necessary improvement works prior to completion of the 275th unit, and the timeframes involved for doing so. Please let me know if you require any further information, Kind Regards, Oli Oli Pocock Assistant Engineering Hydrologist - RPS Planning & DevelopmentHighfield House, 5 Ridgeway, Quinton Business Park, Birmingham, B32 1AF. United Kingdom Tel: +44 (0) 121 213 5500 Fax: +44 (0) 121 213 5502

2

Email: [email protected] www: www.rpsgroup.com

From: Arkesden, James [mailto:[email protected]] Sent: 03 December 2014 16:27 To: Oli Pocock Cc: Net Dev West Subject: RE: AAC5085- Hazelwall Farms, Uttoxeter - DE-1310-256 Oli Please find attached your SCA for Hazelwalls, If you have any more queries regarding your connection please contact our [email protected] team regards James Arkesden Modelling Specialist Planning & Performance Severn trent Water Please note I am only seconded to Severn Trent Water Wed - Fri From: Oli Pocock [mailto:[email protected]] Sent: 28 November 2014 10:49 To: Arkesden, James Cc: Sewer Capacity Assessment Subject: RE: AAC5085- Hazelwall Farms, Uttoxeter - DE-1310-256 James, Just following this up, have yet to hear back, Kind Regards, Oli Oli Pocock Assistant Engineering Hydrologist - RPS Planning & DevelopmentHighfield House, 5 Ridgeway, Quinton Business Park, Birmingham, B32 1AF. United Kingdom Tel: +44 (0) 121 213 5500 Fax: +44 (0) 121 213 5502 Email: [email protected] www: www.rpsgroup.com

From: Oli Pocock Sent: 27 November 2014 09:52 To: 'Arkesden, James' Cc: 'Sewer Capacity Assessment' Subject: RE: AAC5085- Hazelwall Farms, Uttoxeter - DE-1310-256 Hi James, Haven’t heard back about this at all- can you send over the report please, Oli

3

Oli Pocock Assistant Engineering Hydrologist - RPS Planning & DevelopmentHighfield House, 5 Ridgeway, Quinton Business Park, Birmingham, B32 1AF. United Kingdom Tel: +44 (0) 121 213 5500 Fax: +44 (0) 121 213 5502 Email: [email protected] www: www.rpsgroup.com

From: Oli Pocock Sent: 19 November 2014 10:51 To: 'Arkesden, James' Cc: 'Sewer Capacity Assessment' Subject: AAC5085- Hazelwall Farms, Uttoxeter - DE-1310-256 Importance: High Hi James, Can you please advise when the SCA report for Hazelwall Farms will be available, the instruction to proceed was sent in November last year, and the VAT receipt was received 27/11. This has been outstanding for a significant period, and we are well beyond the 6/8 weeks you advise in the quote as the turnaround period from commissioning. Please advise when the report will be available. Can you please also provide contact details for the department I would need to address a letter, to take this further. Many thanks for your earliest response, Kind Regards, Oli Oli Pocock Assistant Engineering Hydrologist - RPS Planning & DevelopmentHighfield House, 5 Ridgeway, Quinton Business Park, Birmingham, B32 1AF. United Kingdom Tel: +44 (0) 121 213 5500 Fax: +44 (0) 121 213 5502 Email: [email protected] www: www.rpsgroup.com

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RPS Planning and Development Limited, company number: 02947164 (England). Registered office: 20 Western Avenue Milton Park Abingdon Oxfordshire OX14 4SH. RPS Group Plc web link: http://www.rpsgroup.com

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47 rpsgroup.com


APPENDIX O

Hydraulic Modelling Report Report No. RCEF270069-002 R Rev 2

RCEF27069-002 R Rev 2 – Hazelwalls Farm March 2015

March 2015 Our Ref: RCEF27069-002 R Rev 2 RPS Health, Safety & Environment Unit 12 Watersedge Business Park Modwen Rd, Salford Quays M5 3EZ

HAZELWALLS FARM, UTTOXETER

MODELLING REPORT

FOR

MOSAIC ESTATES AND DAVID WILSON HOMES MERCIA


Report Status: DRAFT REV 2

Project Reference: RCEF27069 (Related ref: RCEF30548)

Name: Signature:

Report Authors:

Rory Clements

Environmental Scientist

Laura Howe

Environmental Consultant

Technical Reviewer: Neil Bagley

Technical Director

Date: 27/02/2015

This report has been prepared in the RPS Group Quality Management System to British Standard EN ISO 9001:2008

RPS Health, Safety & Environment is part of the RPS Group Plc with around 5,000 staff based at over 85 offices located throughout the UK, Ireland and the Netherlands and in the USA, Canada, the Russian Federation, Australia, Malaysia,

Singapore and Abu Dhabi. RPS offers an unparalleled range of commercially focused services relating to property and land due-diligence, site development and geo-environmental investigations (including liability reviews, planning feasibility, EIAs and

flood risk, energy & sustainability assessments).


RPS HEALTH, SAFETY & ENVIRONMENT

General Notes

1. The following notes should be read in conjunction with the report:

2. This report contains only that available factual data for the site, which was obtained from the sources, described in the text. These data were related to the site on the basis of the location information made available to RPS by the client.

3. The assessment of the site is based on information supplied by the client. Relevant information was also obtained from other sources.

4. The report reflects both the information provided to RPS in documents made available for review and the results of observations and consultations by RPS staff.

5. Where data have been supplied by the client or other sources, including that from previous site audits or investigations, it has been assumed that the information is correct but no warranty is given to that effect. While reasonable care and skill has been applied in review of this data no responsibility can be accepted by RPS for inaccuracies in the data supplied.

6. This report is prepared and written in the context of the proposals stated in the introduction to this report and its contents should not be used out of context. Furthermore new information, changed practices and changes in legislation may necessitate revised interpretation of the report after its original submission.

7. The copyright in the written materials shall remain the property of the RPS Company but with a royalty-free perpetual licence to the client deemed to be granted on payment in full to the RPS Company by the client of the outstanding amounts.


CONTENTS

1.1 Introduction

1.2 Background to the modelling

1.3 Modelling approach

1.4 Topographic data

1.5 Existing and Proposed Scenarios

1.6 Model boundaries

1.7 Structures

1.8 Roughness

1.9 Model limitations

1.10 Model runs

1.11 Results

1.12 Sensitivity test

1.13 Offsite Flooding

1.14 Conclusions


TABLES

1 PROPOSED CULVERT DIMENSIONS

2 PROPOSED SCENARIO INFLOWS

3 ONLINE ATTENUATION POND INITIAL CONDITIONS

4 MODELLED POND MAXIMUM VOLUMES & WATER LEVELS

FIGURES

1 LOCATION PLAN

2A MODEL SCHEMATIC - EXISTING

2B MODEL SCHEMATIC - PROPOSED

3A EXISTING SCENARIO - 1 IN 20

3B EXISTING SCENARIO - 1 IN 100

3C EXISTING SCENARIO - 1 IN 100 + CLIMATE CHANGE

3D EXISTING SCENARIO - 1 IN 1000

4A PROPOSED SCENARIO - 1 IN 100 + CLIMATE CHANGE

4B PROPOSED SCENARIO – 1 IN 1000

5A SENSITIVITY TEST - 90% BLOCKAGE AT TIMBER LANE CULVERT IN 1 IN 100 + CLIMATE CHANGE

5B SENSITVITY TEST – 90% BLOCKAGE AT WESTERN ATTENUATION POND OUTLET IN 1 IN 100 + CLIMATE CHANGE

5C SENSITVITY TEST – JULY 2012 BREACH EVENT COMPARISON - 1 IN 20

6 1 IN 100 + CLIMATE CHANGE EXISTING & PROPOSED EXTENT COMPARISON

APPENDIX

A TOPOGRAPHIC DATA

B HYDROLOGY


1.1 Introduction

RPS was commissioned by Mosaic Estates and David Wilson Homes Mercia to undertake a hydraulic

modelling exercise to support a Flood Risk Assessment (FRA) for a proposed development at

Hazelwalls Farm in Uttoxeter, East Staffordshire. The location of the site is shown in Figure 1.

This modelling report (Rev 2) supersedes modelling report Rev 1 (doc ref: RCEF27069-002 R Draft

Rev 1). Since completion of Rev 1 report where mitigation measures were recommended, the

proposed plans have progressed and mitigation measures have been determined. This modelling

report describes the modelling approach and presents the results obtained. The report should be read

in conjunction with the FRA report.

1.2 Background to the modelling

The proposed development site is split into 2 regions; Area 1 (adjacent to the B5013), and a smaller

area east of this, Area 2. There are three ordinary watercourses which cross Area 1 and Area 2.

Model schematics for the existing site and proposed development are included in Figures 2A and 2B.

Whilst the Environment Agency indicative flood map identifies that the site is within Flood Zone 1, the

flood risk associated with the three watercourses has not been established through Environment

Agency floodplain modelling and mapping. The Environment Agency have therefore requested that a

hydraulic study of the three watercourses should be undertaken within the FRA to determine any

localised areas that could be at risk of flooding from the watercourses.

Area 1 and Area 2 are split by two watercourses (Eastern Channel and Middle Channel), with a third

watercourse (Northern Channel) passing through Area 1. Eastern Channel flows north alongside the

western edge of Area 2; this drains the largest catchment of the three watercourses. Middle Channel

flows north alongside the eastern edge of Area 1. Northern Channel flows east through the centre of

Area 1.

Consultation with the Environment Agency and East Staffordshire County Council (ESCC) has

advised there are known flooding issues downstream of the site affecting an existing residential area

and community park. It has been requested this flood risk is also assessed, and any potential flood

mitigation to reduce the existing flood risk included in the final Masterplan. This is discussed further in

the FRA.

A site visit was undertaken on 17th October 2012. Further consultation has been undertaken with the

Environment Agency in order to inform the approach taken within the modelling exercise. The

following sections of the report describe the model produced for the purposes of this study and the

results of the site-specific modelling exercise.


1.3 Modelling approach

MIKE FLOOD by Danish Hydraulic Institute (v2011 Service Pack 7) modelling software has been used

to model the watercourses and catchment.

The in-bank channel is almost entirely modelled as 1D in the existing model, with the upstream section

of the Northern Channel, modelled as 2D for the existing model. The 1D channel data has been

derived from the topographic channel cross section survey. The floodplain and upstream Northern

Channel is modelled in 2D using a Digital Terrain Model (DTM) derived from topographic data. The

upstream section of the Northern Channel is a shallow ditch 2-3 metres across from top of bank; the

DTM has been edited to match the topographic survey. The upstream section of the Northern Channel

is modelled as 1D in the proposed model; alterations have been made to the location of the channel,

further details on this are below. The floodplain is represented in 2D using a DTM for both the existing

and proposed models.

A short 21m stretch of the Middle Channel at Hazelwalls Farm was not surveyed; the channel and

culvert dimensions here were estimated following discussions with Mr Michael Jones (owner of

Hazelwalls Farm) and a review of aerial imaging.

1.4 Topographic data

The in-bank 1D element of the model is based on a topographic channel cross section survey

undertaken by Tower Surveys in October 2013. The topographical data for the 2D floodplain element

of the model is derived from a topographic survey of the ground levels across the site undertaken by

Tower Surveys in January 2013, and 2m resolution LiDAR (Light Detecting and Ranging) data

(+/-0.15m accuracy) purchased in September 2013. Available Lidar data does not cover the south-

west corner of the site. Lidar data is included as Appendix B. The LiDAR data at Timber Lane has

been raised slightly so that ground levels accurately represent those shown on the topographical

survey.

1.5 Existing and Proposed Scenarios

Existing Scenario

The existing site is grassland with a small area covered by Hazelwalls Farm. All three watercourse 1D

river networks begin in steep agricultural land. The Northern Channel flows east through the centre of

Area 1. Both the Middle Channel and the Eastern Channel flow north between Area 1 and Area 2. The

Northern Channel and Middle Channel currently enter separate culverts beneath the existing

Hazelwalls Farm. The two culverts connect into one culvert which passes beneath Timber Lane. On

the east side of Timber Lane, the Eastern Channel enters the culvert connecting all three

watercourses together. From here, the Eastern Channel turns east and flows through the Hazelwalls


Community Park (passing through the Foxglove Avenue Culvert) for a short distance, where it enters a

long culvert (Hazelwalls Park Culvert) draining in to Picknall Brook. The model schematic is included in

Appendix C.

Proposed Scenario

The development proposals are to demolish Hazelwalls Farm in Area 1 and remove the existing

culverts diverting flow in the Middle and Northern Channels, leaving only the Timber Lane Culvert as

the controlling structure. The proposed diameter of the Timber Lane Culvert is 0.86m, this is smaller

than the existing 0.9m culvert. In the place of the farm an attenuation pond will be constructed to

attenuate flows from both the watercourses and surface water from the proposed site. Another online

additional attenuation pond will be constructed along the Northern Channel in Area 1, to attenuate

flows from both the watercourses and surface water from the proposed site. The outfall pipe from this

culvert will have a diameter of 0.3m. The dimensions of the culverts are shown in Table 1.

Table 1: Proposed Structure Dimensions

Existing

Diameter

(m)

Proposed

Diameter

(m)

Length

(m)

Upstream

Invert Level

(mOD)

Downstream

Invert Level

(mOD)

Timber Lane Culvert 0.9 0.86 17 88.22 88.00

Western Attenuation

Pond Outfall Pipe

- 0.30 20 96.65 95

Bed

Level

(mOD)

Notch

Invert

Level

(mOD)

Notch

Width

(m)

Crest Invert

Level

(mOD)

Crest Width

(m)

Eastern Weir 93.00 93.50 1.0 94.06 5.0

For the Existing Scenario the Northern Channel is not defined in the upper reach of Area 1 and so is

only represented in the 2D domain. In the Proposed Scenario a 1D channel was developed and the

bed was lowered by 0.5m to create a defined channel. Due to existing established hedgerows along

the Northern Channel, the proposals are to move the watercourse north by a short distance to ensure

the hedgerows aren’t disturbed.

In Area 2, removal of the two 0.32m diameter pipe culverts (see Appendix A) along the boundary of

Area 2 has been simulated to investigate any effect on local flood extents.

It is proposed that a 100m reach of the Eastern Channel adjacent to Area 2 will be lowered by 0.5m,

and a notch weir constructed to attenuate flows and remove flood risk from Area 2.


1.6 Model boundaries

The downstream boundary of the 1D element of the model is controlled by the Hazelwalls Park Culvert

inlet. The inlet is located at Hazelwalls Park, and the outlet (approximately 330m downstream)

discharges into Picknall Brook. Limited information is available for the outlet and culvert route, as such

estimations were made using Lidar data and aerial mapping as to the invert level and channel shape

at the culvert outlet. Dimensions of Picknall Brook were estimated using Lidar and aerial mapping. The

Brook has been included in the model to allow the culvert to discharge into the Brook. A normal depth

boundary was used for Picknall Brook as the downstream boundary of the 1D model.

The three upstream boundaries are time-flow hydrographs generated using the Revitalised Flood

Hydrograph (ReFH) method. Catchment descriptors were generated using FEH CD-ROM 3; this

represents the three watercourses as one whole catchment with a total area of 1.4km2. Catchment

descriptors for the area were extracted and a hydrograph for the whole FEH catchment (1.4km2) was

generated using the Revitalised Flood Hydrograph (ReFH) spreadsheet. OS mapping has been used

to determine the watershed of each of the three individual catchments. The ReFH hydrograph was

then scaled to the area of each of the three catchments creating three time-flow hydrographs for the

existing scenario. Details of the hydrology calculations are in Appendix D.

The proposed development is made up of a large area of hardstanding. To ensure the hydrology is

accurately represented in the proposed model, alterations have been made to the model inflows. The

two attenuation ponds in Site 1 will attenuate all surface water from the proposed development. As

such, the proposed development area of the catchments for the Middle and Northern channels has

been removed from the total area of each catchment. A pro-rata method has then been used to scale

the hydrographs of the Middle and Northern channels so that only flow from the non-developed area is

represented in the model inflows; see Table 2 below. Flow from the developed area will be

represented as a volume in each of the attenuation ponds; this is discussed below.

Table 2: Proposed Scenario Inflows

Catchment Area

(km2)

Proposed Dev

Area (km2)

1 in 100+CC Peak

Flow (m3/s)

1 in 1000 Peak Flow

(m3/s)

Northern Channel 0.32 0.116 0.50 0.79

Middle Channel 0.42 0.049 0.91 1.43

Preliminary development plans show that the attenuation volume required is 4400 m3; 3000 m3

storage in the eastern pond and 1400m3 in the western pond, both on Site 1. These volumes have

been represented in the 2D domain through an initial surface elevation level, set for each of the ponds,

see Table 3. These initial conditions are seen as a worst case scenario, as it is highly unlikely all


Table 2: Online Attenuation Pond Initial Conditions

Required surface

vol. from site

runoff (m3)

Modelled surface

water vol. from

site runoff (m3)

Modelled water level in pond

when all surface water

contained in pond (mOD)

Western Pond 1400 1446 97.29

Eastern Pond

(Timber Lane)

3000 3013 88.86

1.7 Structures

All hydraulically significant structures have been included within the model with dimensions informed

by the topographic channel cross section survey. Culverts have been represented using appropriate

MIKE 11 node types, representing the culvert orifice as a culvert structure, and the culvert deck as a

weir structure. Long culverts or connect culverts have been modelled as closed cross-sections, and

the resistance of the cross-sections increased by 10-20% to account for friction losses usually

calculated by the culvert structure.

The Northern Channel and Middle Channel enter culverts beneath Hazelwalls Farm. Discussions were

had with Mr Jones (Hazelwalls Farm owner) who advised on the route of these culverts beneath the

farm.

Outlet details for the Hazelwalls Park Culvert were unavailable and so estimations were made

following discussions with East Staffordshire Council and a review of available Lidar and aerial

mapping.

A proposed culvert will discharge flow from the Western Attenuation Pond and it is proposed to reduce

the diameter of the Timber Lane Culvert; details of this are in Table 1 above. In addition to this, a weir

is proposed adjacent to Site 2 in the Eastern Channel, details of this are also in Table 1.

1.8 Roughness

Mannings ‘n’ values have been assigned within the model based on the characteristics of the channel

and floodplain. Open channel sections are assigned a value of 0.030 - 0.04; culverts are assigned a

value of 0.015; floodplain areas are assigned values of 0.02 (roads), 0.035 (grass) & 0.5 (buildings).


The model has been developed for the specific purpose of identifying flood risk to the proposed

development site. The model is considered to be fit for this purpose. The model has not been



The model has been developed for the specific purpose of identifying flood risk to the proposed

development site. The model is considered to be fit for this purpose. The model has not been

developed to establish flood extents elsewhere on the catchment and therefore should not be used for

this purpose. The model has not been reviewed or signed off by the Environment Agency.

Topographical survey data for the bund at Hazelwalls Park is not available. The bund was modelled in

the DTM as 0.8m above the surrounding ground. This does not have an effect on flood extents at the

site.

Topographical survey data of the exact location of the outfall of Hazelwalls Park Culvert is not

available. Estimations were made following discussions with East Staffordshire Council.

1.10 Model runs

Existing Scenario

The model has been run for the following events for the existing scenario:

• 1 in 20 year

• 1 in 100 year

• 1 in 100 year plus climate change

• 1 in 1000 year

Climate change has been accounted for through a 20% increase in river flows, as advised within

NPPF Table 5: Recommended national precautionary sensitivity ranges for peak rainfall intensities,

peak river flows, offshore wind speeds and wave heights.

Proposed Scenario

The model has been run for the following events for the proposed scenario

• 1 in 100 year plus climate change

• 1 in 1000 year

The impact of a 90% blockage of the Timber Lane Culvert has been investigated through reducing the

diameter of the culvert within the model. The impact of 90% blockage of the western attenuation pond

outfall pipe has been simulated through reducing the diameter of the culvert within the model.


1.11 Results

The results of the modelling are presented as detailed below. Simulation results for Area 1 and Area 2

are discussed individually.

• Figure 3A – Existing Scenario – Maximum flood depths during 1 in 20 year event

• Figure 3B – Existing Scenario – Maximum flood depths during 1 in 100 year event

• Figure 3C – Existing Scenario – Maximum flood depths during 1 in 100 year plus climate

change event

• Figure 3D – Existing Scenario – Maximum flood depths during 1 in 1000 year event

• Figure 4A – Proposed Scenario – Maximum flood depths during 1 in 20 year event

• Figure 4B – Proposed Scenario – Maximum flood depths during 1 in 100 year plus climate

change event

• Figure 4C – Proposed Scenario – Maximum flood depths during 1 in 1000 year event

• Figure 5A – Sensitivity Test – 90% Blockage of Timber Lane Culvert

• Figure 5B – Sensitivity Test – 90% Blockage of Western Attenuation Pond Outfall Pipe

• Figure 5C – Sensitivity Test – Breach during 1 in 20 year event

Area 1

Existing Scenario

The existing scenario results show the channel capacity of the Northern Channel, passing through

Area 1, is not sufficient to accommodate the 1 in 20 year plus climate change flow; see Figures 3A-3D.

Floodwaters exceed channel capacity, flowing east across the uppermost region of Area 1, re-entering

the Northern Channel 350m downstream. Overland flooding in the upper reach of Area 1 is due to the

Northern Channel not having sufficient capacity for high flows.

Some minimal flooding occurs in Area 1 due to the Middle Channel exceeding capacity, during the

existing scenario 1 in 100 year plus climate change and 1 in 1000 year events. The flooding is limited

to the existing Hazelwalls Farm, due to lack of culvert capacity, and along the banks of the Middle

Channel. The maximum water level at Hazelwalls Farm is 90.2m AOD for the modelled 1 in 100 year

plus climate change event, and 90.4m AOD for the modelled the 1 in 1000 year event. The south-west

corner of Area 1 is not covered by the Lidar data. OS contour mapping shows this area is 10m+ above

bank levels and so would remain flood free from the Middle Channel during the 1 in 1000 year event.

Proposed Scenario

For the proposed layout the existing bed level was lowered by 0.5m to create a defined channel. With

this defined channel and the addition of the online western attenuation pond, Area 1 is now shown to


be flood free up to and including the 1 in 100 year plus climate change event; see Figure 4A.

Floodwater also no longer overtops Timber Lane during this event.

During the 1 in 1000 year event the pond reaches allowed capacity of 3000m3. A small overflow weir

has been introduced to the proposed western attenuation pond; with a crest level of 97.6mOD, so that

volumes in the pond will not exceed 3000m3. Volumes above 3000m3 overtop the east side of the

attenuation pond and flow easterly across the site to re-enter the Northern Channel. Depths where the

pond overtops during the 1 in 1000 year event do not exceed 0.34m; see Figure 4B.

Maximum modelled water levels and volumes in the western and eastern attenuation ponds is shown

in Table 4.

Table 4: Modelled Pond Maximum Volumes & Water Levels

1 in 100+CC Event 1 in 1000 Event

Max Water Level

(mOD)

Max Volume

(m3)

Max Water Level

(mOD)

Max Volume

(m3)

Western Pond 97.43 1884 97.83 3124

Eastern Pond 88.94 3400 89.49 6053

Area 2

Existing Scenario

The majority of Area 2 is flood free up to and including the 1 in 1000 year event. There is a small area

along the right bank of the Eastern Channel which is flood free during the 1 in 20 year event, but

inundated during more extreme events. The maximum water level here for the modelled 1 in 100 year

plus climate change and 1 in 1000 year events is 93.43m AOD and 93.29m AOD respectively.

Proposed Scenario

Proposals are to construct an offline attenuation pond for Area 2 at the north-west corner of the site.

To remove this small area of flooding along the right bank of the Eastern Channel a 100m reach of the

river adjacent to Area 2 was lowered by 0.5m, and a notch weir proposed to attenuate flow. In addition

to this, proposed is the removal of the two 0.32m diameter pipe culverts located in the Eastern

Channel. Area 2 is now shown to be flood free up to and including the 1in 1000 year event; see Figure

4B. The proposed works have a negligible effect on discharge or water levels downstream.

1.12 Sensitivity test A variety of sensitivity testing has been carried out, assessing model parameters and a previous flood

event.


A 20% Manning’s ‘n’ increase and decrease in channel and floodplain resistance was simulated during

the Proposed Scenario 1 in 100 year plus climate change event. The increase/decrease had a minimal

increase in flood extent at the existing Hazelwalls Farm in Area 1 due to higher resistance causing

slightly higher levels in the Northern and Middle Channels (+/- 0.04m). The increase/decrease had a

negligible impact on in-channel water levels (+/- 0.15m) in Area 2.

A 90% blockage of the Timber Lane Culvert was simulated during the Proposed Scenario 1 in 100

year plus climate change event. This has a negligible effect on flood extents on site. Flooding occurs

at properties along Fennel Close and at Hazelwalls Community Park. This flooding is similar to the

existing 1 in 100+CC event modelled extents.

A 90% blockage of the proposed western attenuation pond outlet culvert was simulated during the

Proposed Scenario 1 in 100 year plus climate change event. The blockage causes the western

attenuation pond to fill to capacity, flows overtop the east side via the overflow weir and flow easterly

eventually re-entering the Northern Channel, there is no further affect to on site flood extents. The

blockage has no affect on flood extents downstream of the site.

Hazelwalls Community Park experienced flooding due to a breach in the Eastern Channel and a

partial blockage of the Hazelwalls Park Culvert in July 2012. Details of the flood event were provided

by East Staffordshire Council are discussed further in the FRA. The 1 in 20 year event was simulated

along with a breach (to adjacent ground level along the left bank) and 50% blockage of the Hazelwalls

Park Culvert. A similar flood outline was simulated. A comparison map is included in Figure 3B.

1.13 Offsite Flooding

There is a known flooding issue downstream of the site at Hazelwalls Community Park. The

Environment Agency and East Staffordshire Council requested any possible mitigation be included in

the proposed development to reduce this flood risk.

The existing flood risk to Hazelwalls Community Park and the surrounding area is due to the

insufficient capacity of the Eastern Channel at the park and the Foxglove Avenue Culvert. The area is

free from flooding during the modelled 1 in 20 year event. During the 1 in 100 year event flow is

constricted by the Foxglove Avenue Culvert, causing flow to exceed channel capacity. Flow overtops

the left bank, flowing down Fennel Road to inundate the Park and surrounding residential area.

A number of mitigation methods were investigated which could reduce flood risk to the Park area.

These were weirs, attenuation ponds, additional culvert and bund extension. It was concluded the

attenuation pond is the most viable option. This has been included in the proposed model.


The proposed attenuation ponds would reduce flood risk in the Hazelwalls Community Park area.

Flooding would still occur during the 1 in 100 year events and greater, but the flood extent would be

reduced. A difference map showing the difference in existing and proposed flood extents during the 1

in 100 year plus climate change event has been included in Figure 6A. In addition to this, floodwaters

would no longer overtop Timber Lane during the 1 in 100 plus climate change event.

It should also be noted that the banks of the Eastern Channel have breached in the past causing

localised flooding of Hazelwalls Park and the surrounding area. It is recommended these banks be

reinforced, and/or the channel bed lowered to prevent a future breach, and ensure the area remains

free from flooding up to and including the 1 in 20 year event.

1.14 Conclusions

A MIKE FLOOD 1D-2D model has been constructed for the purpose of identifying flood risk to the

proposed development site to support a Flood Risk Assessment.

The results show that Area 1 is currently at risk of flooding from the Northern Channel during the

modelled 1 in 20, 1 in 100, 1 in 100 plus climate change and 1 in 1000 year events. There is a limited

flood risk posed to Area 1 by the Middle Channel, with a small area of the site (Hazelwalls Farm)

flooded during the modelled 1 in 100 plus climate change and 1 in 1000 year events due to culvert

surcharging.

Area 2 remains mostly flood free during the 1 in 100, 1 in 100 plus climate change and 1 in 1000 year

events, with only the north-west boundary inundated.

Proposals for the site are to develop a defined channel in the upper reaches of Area 1, and construct

an attenuation pond in the place of Hazelwalls Farm, and along the Northern Channel, to attenuate

surface water flow from both the watercourses and the proposed development surface water runoff.

The outfall pipe from the eastern pond should be 0.3m diameter, and the Timber Lane Culvert should

be reduced to a 0.86m diameter. Modelled results show this removes flood risk to Area 1 during the

1 in 100+CC and 1 in 1000 year events. In addition to this, proposals are to lower the Eastern Channel

adjacent to the site and construct a v-notch weir; modelled results shown this removes flood risk to

Area 2.

The Hazelwalls Community Park is currently at risk of flooding during the 1 in 100 year event. The

Environment Agency requested investigation into various mitigation measures to reduce the existing

fluvial flood risk in the Park. A number of onsite options were investigated (weirs, additional culverts,

attenuation pond and bund extension). It was concluded attenuation ponds on site are the most viable

option. The storage offered by the proposed attenuation ponds in Area 1 reduces the modelled flood

extents/depths downstream of the site in the vicinity of Hazelwalls Community Park.

Figure 1 SITE LOCATION PLAN

rpsgroup.com

Unit 12, Watersedge Business Park, Modwen Road, Salford Quays, M5 3EZT +44 (0)161 874 3737 F +44 (0)161 877 3959 W rpsgroup.com

Client: Mosaic Estates & DWH

Project: Hazelwalls Farm Uttoxeter

Title: Site Location Plan

Date: March 2015

Figure: 1

Job Ref: RCEF30458

Scale: NTS

Rev: 02

Need to select borderless printing:

File > Print > Properties > Page Setup

Window size = x 193.6 mmy 207.4 mm

Site Location

Figure 2A EXISTING MODEL SCHEMATIC

rpsgroup.com

Client: Mosaic Estates & DWH

Status: DRAFT

Title: Existing Model Schematic

Scale: NTS Size: A3

Date: March 2015

Job Number: RCEF30458 Rev: 02

Unit 12, Watersedge Business ParkModwen Road, Salford QuaysT +44 (0)161 874 3737F +44 (0)161 877 3959W rpsgroup.com

Site: Hazelwalls Farm, Uttoxeter

Fig: 2a

The modelled outputs presented in this figure are derived from a model which has been developed for the specific purpose of identifying flood risk to the proposed development site. The model is considered to be fit for this purpose. The model has not been developed to establish flood extents elsewhere on the catchment and therefore should not be used for this purpose. The model schematic accompanying the modelling report demonstrates the model coverage from which the flood extents shown on the figure have been derived.

Site Boundary

Notes:

Cross sections(inc interpolated)

Culvert

1D river network

Figure 2B PROPOSED MODEL SCHEMATIC

rps report - eaststaffsbc.gov.uk application/627000/62… · c 2012 rps group notes 1. this drawing...

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