010-158-42395 Flood Risk Assessment

7/30/2019 010-158-42395 Flood Risk Assessment

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GEOENVIRONMENTAL

CONSULTING ENGINEERS clarkebond

Completed by: Andrew Crewdson Page 1of 8 Date: 01 October 2007

FLOOD RISK ASSESSMENT

SITE: H17 Newtown Road, Highbridge Project No: EE00431NGR: 331400, 147350 Revision: C

CLIENT: Gaia Partnership Date of Revision: 01/10/07

AREA LIABLE TO FLOODING

The site lies within the Environment Agencys indicative flood risk areas for Zone 2 and 3.

The proposed site is part of the H17 residential allocation included in Sedgemoor DistrictCouncils development framework. The site lays approximately 100m to the north of the tidalestuary of the River Brue. The Brue reaches the sea at Highbridge, 1500m to the west. Thecourse of the Brue has been altered and straightened over the years upstream of the site.The current tidal limit is immediately to the south of the site at the sluices at New ClyceBridge.

To the north and east of the site are residential properties. To the south is a boat buildingyard and housing, to the west is a wastewater treatment works with the Apex Leisure Parkcontaining several lakes and a caravan park on land further to the west. The Brue catchmenthas an area of approximately 259km2 and is predominantly wetland within the SomersetLevels.

The existing site varies in level from 5.6 to 6.8mAOD. The existing topographical survey hasbeen tied into Ordnance Datum from the benchmark on New Clyce Bridge. The total sitearea is approximately 27120m2. The site falls away from the River Brue towards theresidential dwellings alongside the northern boundary.

An earth bank tidal defence, which runs along the southern and western boundaries, defendsparts of the area, such as the Sewage works to a level varying between 8.45m and 8.60mAOD. On the southern boundary of the site there is a raised parcel of land that reaches8.00mAOD.

To the east of the site on Huntspill Moor the River Brue is linked to the manmade HuntspillRiver, dug in the 1940s, and to the south by an artificial channel known as Cripps River. TheHuntspill and Cripps rivers were built to improve drainage of the area.

The attached drawing EE00431/Fig01 shows the site location.

The attached drawing EE00431/Fig02 shows the existing site topographical survey, andpredicted flood extents.

The drawings are included in Appendix A.

A copy of the EA flood map is included in Appendix B.


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GEOENVIRONMENTAL






PROBABILITY OF FLOODING AND POTENTIAL CHANGE

The impact of urban development and land use change on the River Brue is likely to beminimal, particularly in view of recent policies such as PPG and PPS25 aimed at reducingsuch impacts.

The Environment Agency have stated that there is a maximum fluvial 1 in 100 year level of5.0mAOD for the River Brue because of the interlinked watercourses and wetland areasupstream of Highbridge. Due to the Highbridge Sluice there is no tidal influence on the Brueupstream of Clyce Bridge. The River Brue is therefore highly unlikely to have any majorflood influence on the site but will need to be considered if any surface water drainageoutfalls are considered to the Brue.

EXTENT AND STANDARD OF FLOOD DEFENCES

An earth bank currently defends western parts of the site, which is part of the EnvironmentAgencys tidal defence scheme for Highbridge. The bank varies in level between 8.60 and8.54mAOD over a distance of approximately 290m.

An additional bank running along the southern boundary provides some further defence at alevel of 8.14m AOD.

The lowest point in the area exposed to tidal influence is at 7.81m AOD, or 50mm short ofthe 1 in 200 year predicted flood level.

This defence level of 7.81m equates to an approximate return period of 1 in 150 years fortidal flooding.

LIKELY DEPTH OF FLOODING

1 in 200 Year EventThe 1 in 200 year still water level in the tidal Brue for Highbridge has been agreed with theEnvironment Agency as 7.86mAOD.

Behind the existing defence at 7.81m the ground drops away to a low-dished area with aninvert of approximately 7.20m. The perimeter of the dished area has a level of 7.59m and aplan area of 2347m2.

Any flows overtopping the 7.81m level would flow east into this dished area and impound upto a level of 7.59m, before then flowing north, east and south into existing areas ofdevelopment. This dished area has an approximate storage volume of 445m3.


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GEOENVIRONMENTAL






Using the design flood level of 7.86m and the existing defence level of 7.81m a tide wouldspill over the low spot for just under 14 minutes based on tide curves derived from HinkleyPoint gauging station. The width of the spill is between 4.5 and 5.0m with cross sectionalflow area varying between 0.0m2 and 0.14m2. Assuming the tide is travelling at a rate of0.5m/s calculates out as a total volume of 28.3m3 travelling through the lowest point.

From this it can be seen that no flood water would enter this site, part of the H17 allocation,

during a 1 in 200 year tide event. The full tidal overtopping calculations are included inAppendix C.

1 in 200 Year + 30% Climate Change EventTo allow for potential rises in tide levels initially an additional 0.40m was added to the 1 in200 year tidal level and a further 0.30m for freeboard giving a 1 in 200 year + climate tidelevel of 8.56m AOD for any earth flood defences.

Further work to revise the climate change allowances following publication of PPS25 hasnow been undertaken. We have calculated a revised tidal design level of 8.87mAOD for2110 to cover a development life span of 100 years.

We have carried out an assessment of existing ground levels within the Highbridge /Burnham area to create some contouring of the surrounding area. These contours areshown on EE00431/Fig5 and Fig6. We have then assumed that we will construct the Phase1 development including our flood alleviation proposals and that the land to the south ofPhase 1 (Boatyard site) will be raised to 8.90m. Any overtopping floodwater passing alongthe south side of the site, will still run through Smith Way, Sparks Way and out into NewtownRoad before heading north toward Newtown Lake.

To assess the maximum volume of flood water that the tide could generate we have drawnup a series of spill profiles to match the predicted tide curve level and calculated the possibleflow area available through each cross sectional area of the spill.

If tide levels were to reach the level of 8.87m, spilling through the area with the lowestground levels between 8.11 and 7.81mAOD there would be approximately 150,000m3 ofwater entering into the Highbridge area over the spill length of 70m. This tidal volume isbased on a single tide cycle in just over 1 hours of flow between 0.76 and 1.06m deep.

We have considered 3 tide cycles passing over this spillway and a total volume of floodwaterof 450,000m3 dispersed in the Highbridge/Burnham area.

The lowest areas immediately to the north of the site are at 5.2m, rising to 5.5m. At 5.5mfloodwater would start to enter the northern side of the site, as the low-lying areas below5.5m will have filled. Therefore using the 5.5m contour area as a storage pond level (less a20% allowance for buildings) we have calculated the maximum impounding level of 3 tide


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GEOENVIRONMENTAL






cycles by dividing the total flood volume in 3 tide cycles (450,000m3) by the potential storagearea (221.04ha). This equates to a total water depth of 200mm, or a level of 5.70mAODwithout any allowances for draw down by the highway and surface water drainage network orthe volume of storage available below the 5.5m contour.

At the entrance to the site off Newtown Road the existing ground level is at 6.40m, wepropose to raise the access road up to a level of 7.65m as soon as highway designstandards permit (approx 15m distance). This will be 1.25m above the existing ground level

to the south of the site where flood flow depths would be up to 1.1m deep based on our flowpath/depth analysis. Then the access road or site level would rise up further to a level of8.90m along the southern site frontage to provide an engineered flood defence at point 3 onour development option layout. Alternatively if the access road follows a more northerlyroute along the old railway line then the site frontage levels will be raised to a similar profile.This raised defence area would be constructed, as a minimum width of 5m to ensuresurcharging by floodwater did not cause a breach failure.

The width of flow available through the housing area varies between 40 and 70m and weestimate that the flow depth through this area would vary between 1.0 and 1.3m because ofthe housing layout. By adopting the proposed levels on the southern boundary shown ondrawing EE00431/Fig4C we would ensure that the proposed site frontage is always at least1.4m above the existing ground levels along the boundary with Smith Way / Sparks Wayhousing area.

On this basis if the remainder of the Phase 1 site area to the rear of the proposeddevelopment option defences were raised to a minimum of 6.5m, floodwater would not enterthe Phase 1 development area.

MHWS tide is approximately 5.80m AOD.

The site is generally not at risk from waves or storm surges due to its distance inland, butshould it occur there is 300mm freeboard above the 1 in 200 year + climate change tidallevel.

EFFECTS OF CLIMATE CHANGE

The impacts of climate change on the hydrological regime of the River Brue are not fullyknown. In general climate change in the west of England is expected to result in an increasein rainfall intensity and higher rainfall totals during the winter period. This might be expectedto lead to an increase in heavy rainfall events such as occurred in October and December2000 and July 2005.

An additional 30% flow above the 1 in 100 year flow should be considered for climatechange, however the main flood risk in this location is tidal.


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GEOENVIRONMENTAL






Tide levels are predicted to increase due to climate change. Generally it is believed that tidelevels in the southwest will increase by 4mm a year. When combined with the rate thesouthwest peninsula is sinking a value of 5mm is used.

Tide 1 in 200 year design tide level has been increased in line with Annex B of PPS 25 tocover a life span of 100 years.

In a higher flood level tidal event flows would travel through the site north, east and south

towards the existing development areas once the low ponding area was full.

NATURE AND LIFETIME OF DEVELOPMENTAND EXTENT OF FLOOD PROOFING

The site is currently defended to a 1 in 150 year return period tidal event, or 50mm short ofthe current 1 in 200 year design standard. It has been proven earlier that this part of the H17site would not flood in a 1 in 200 year tide event.

The site defence only becomes substandard when climate change and freeboard allowancesare added onto the predicted still water tide levels, i.e. development could proceed as longas the defences were improved within the timescale of the development of the whole of the

H17 allocation. Each landowner would need to deliver part of the defence scheme as part oftheir planning permission. To provide adequate flood protection to the whole of this sitewhen considering climate change the Environment Agency have agreed to the developmentoption as shown on Fig04. Refer to Fig04 for the reference locations:

Phased Development OptionDue to land ownership constraints the H17 site will proceed in two phases, phase 1 being thenorthern part, phase 2 being the boatyard site. Phase 1 will provide a raised flood defence to8.90m between the existing EA defences at point 1, through point 2 finishing at point 3,currently at 6.85m within the site. To the east of point 3 ground levels would be rampeddown to the site entrance at a level of 6.40m. Closing of the gap through the boat yard wouldoccur when the boatyard is developed within the next 5 years.

General Design Features

The raised land on the southern boundary between points 1 and 3 could be used as part of aresidential unit or garden area. A typical sketch showing how this could work is included onFigure 4 as section A - A.

Typically a minimum width of 5m of raised land is required as a defence before any buildingcan be located.

If no buildings are to be used to back onto the earth defence then a width of 8m earthbanking should be used.


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GEOENVIRONMENTAL






Finished floor levels on the properties fronting the tidal defences in the west and southshould be set at 9.05m AOD. Other property in the north and eastern parts of the site shouldhave minimum floor levels 300mm above the local ground level.

The proposed site access road from the east will have a landscape buffer on the south side,this will be shaped to form a dished channel to store and convey flood water to the eastalong the existing flood route in a climate change event. The road and landscape buffer

design can be detailed to mirror the existing flood regime where flows travel to the easttoward the existing development if Option 2 is promoted. Once the boatyard is developedthis potential climate change flood route would be closed off.

The attached drawing EE00431/Fig03 shows the proposed site layout.

The attached drawing EE00431/Fig04 shows the flood proposals for Phase 1 of thedevelopment.

Surface Water Run-off

Greenfield and Brownfield run-off for the existing site was calculated using the IH124 andWallingford methods. Run-off was calculated as the following:

Return Period Greenfield(l/s)

Brownfield(l/s)

1 in 2 year 1.9 195Mean Annual Flood 2.1 221.6

1 in 100 year 5.2 471.9

The maximum proposed site peak run-off based on the illustrative masterplan has beencalculated as follows:

Return Period Developed site(l/s)

1 in 2 year 196.9Mean Annual Flood 223.7

1 in 100 year 477.1Climate Change 620.2

Surface water run-off from the proposed development should be disposed of using SUDSTechniques if previous site use and ground levels permit. If a direct discharge to a


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GEOENVIRONMENTAL






than the existing run-off rate.

There is sufficient capacity to connect into the existing Wessex Water surface water sewer tothe west of the site to achieve a direct tidal outfall. The land needed to complete theconnection is under the control of Gaia Partnership. A copy of the Wessex Water sewerrecords is included in Appendix B.

If a direct discharge to the tidal estuary can be achieved then no reduction in the existingpeak flow or attenuation would be needed. Any outfalls through the existing / proposed tidal

defence wall should be fitted with a flap valve and adequate storage provided within the siteshould the outfall become hydraulically locked during a tidal event.

Pipe System

Due to the location of the proposed development there will be issues with the outfall to theestuary becoming tide locked. Using the tidal curves in Appendix D it has been calculatedthat during the climate change event the system would be tide locked for a total of up to 360minutes.

Using WinDes a maximum volume of 882.3m3 would be required to sufficiently store theflows from a 6 hour storm during a 100 year climate change event without flooding the site.

This can be delivered by using a 1.2m x 260m Pipe with a 225mm orifice plate or pipecontrolling the outflow.

The discharges from the proposed system are as follows:

Pipe SystemReturn period Allowable

run-off(l/s)

Proposed post developmentpeak site run-off (l/s) and

critical storm duration

Control Measures

2 year 196.9 65.3 (30min winter)30 year 374.6 82.3 (30min winter)

100 year 477.1 91.3 (60min winter)Climate change 620.2 101.8 (60min winter)

1.2m x 260m Pipewith 225mm Orifice

Plate or Pipe

We have therefore created a system with adequate storage in the event of a climate changetide locking event happening. Once the flows are able to discharge to the estuary, thesystem would be completely emptied within 150 minutes i.e. half the period of free flowingoutfall between tide lock. Discharge rates from the development have also been improved;with the 2-year event peak site run-off having been reduced by two thirds.

All flap valves should be subject to regular maintenance to prevent floodwaters entering thesystem.

See Appendix C for the IH124, existing Brownfield and proposed run-off calculations.


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GEOENVIRONMENTAL






OVERALL ASSESSMENT OF FLOOD RISK AND RECOMMENDATIONS

The site does lie within the 1 in 200 year tidal zone.

To mitigate the flood risk to the property and the occupiers the following should be put inplace as part of the planning proposals:

Surface water run-off should be disposed of using SUDS Techniques includingpre-treatment of surface water run-off where ground conditions and ground waterlevels permit. After initial treatment a direct discharge to the tidal estuary can beachieved by connecting to a Wessex Water manhole to the west of the study areain land under control of the Gaia Partnership;

A flood defence embankment wall is required to a level of 8.90m to alleviate tidalflooding on the site. This can be delivered in two parts linked to phased planningapplications given the current level of flood defence is so close to the 1 in 200year design event;

Minimum finished floor levels to be 9.05m AOD in the south and west on the topof the flood defence bund and 300mm above ground levels locally in the northand east. Site ground level / parking behind the frontage tidal defences canremain at the existing ground levels.

Any proposed outfalls are to be fitted with flap valves or similar to prevent theingress of flood waters, and should the valves become hydraulically locked duringtimes of high water, adequate storage should be provided to prevent the systemsurcharging. All flap valves should be subject to regular maintenance to preventfloodwaters entering the system.

We recommend that the phased development option be constructed and that theplanning authority and EA use their powers to enforce delivery of the defencebetween points 1 and 8 on drawing EE00431/Fig4 when a planning application islodged on the boatyard site.

Reviewed by Date RevisionN A Smith 03/11/06 Original issueN A Smith 06/11/06 AN A Smith 14/11/06 BN A Smith 01/10/07 C


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Appendix A

Drawings

Fig. 1 Site Location PlanFig. 2 Existing Site Layout, Topographical Survey and Existing Flood RegimeFig. 3 Proposed Site LayoutFig. 4 Proposed Flood Alleviation OptionsFig. 5 Flood Flows and StorageFig. 6 Enlarged Flow Regime at Site


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Appendix B

Third Party Records

EA Flood Risk MapWessex Water Sewer Records


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EE00431 - Newtown Road, Highbridge September 2007

IoH 124 Greenfield & Existing Brownfield run-off

IoH 124 Greenfield

QBARrural = 0.00108 AREA0.89

SAAR1.17

SOIL2.17

AREA (km

2

) = 0.50SAAR* = 750

SOIL* = 0.3

Catchment area = 50.000 ha

Total Site area = 2.039 ha

Brownfield area = 0.958

Greenfield area = 1.081

MAF / QBAR Greenfield = 0.099 m3/s for 50 ha

Windes Wallingford Procedure Brownfield flow

0.958 ha - 2yr storm 195 l/s

Summary

Return Periods Growth factors Greenfield

1.081ha Site

area flow

(m3/s)

Brownfield

0.958 ha

site area

flow (m3/s)

Total

existing site

run-off

(m3/s)

Mean Annual Flood (MAF) 1.00 = 0.099 m3/s 0.0021 0.2216 0.2237

1 yr 0.78 = 0.077 m3/s 0.0017 0.1521 0.1538

2 yr 0.88 = 0.087 m3/s 0.0019 0.1950 0.19695 yr 1.23 = 0.122 m

3/s 0.0026 0.2399 0.2425

10 yr 1.49 = 0.147 m3/s 0.0032 0.2906 0.2937

25 yr 1.84 = 0.182 m3/s 0.0039 0.3588 0.3627

30 yr 1.90 = 0.188 m3/s 0.0041 0.3705 0.3746

50 yr 2.12 = 0.209 m3/s 0.0045 0.4134 0.4179

100 yr 2.42 = 0.239 m3/s 0.0052 0.4719 0.4771

Climate Change 100yr+ 30% = 0.311 m3/s 0.0067 0.6135 0.6202

Greenfield

50 ha flow

(m3/s)

plus

EE00431\IoH 124 Somerset.xls\IoH 124


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H17 Newtown Road, Highbridge

Impermeable Area Impermeable Area Impe

Proposed Site All Proposed Site Brownfield Pro

Total site area = 27121 m2 Total site area = 8602 m2 Tota

2.712 ha (Not all positively drained) 0.860 ha

Roof Area 4710 m2 Roof Area 1228 m2 Roof

0.471 ha 0.123 ha

Imp % = 90 0.424 ha Imp % = 90 0.111 ha Imp %

Block Paving 1442 m2 Block Paving 301 m2 Block

0.144 ha 0.030 ha

Imp % = 70 0.115 ha Imp % = 70 0.024 ha Imp %

Access Road & Footpath 7496 m2 Access Road & Footpath 2136 m2 Acce

0.750 ha 0.214 ha

Imp % = 100 0.750 ha Imp % = 100 0.214 ha Imp %

Greenfield contribution = 13473 m2 Greenfield contribution = 4937 m2 Gree

1.347 ha 0.494 ha

Imp % = 20 0.269 ha Imp % = 20 0.099 ha Imp %

Total = 1.289 ha For Windes Design Total = 0.348 ha For Windes Design

+ 30% climate change = 1.547 ha For Windes Design + 30% c limate change = 0.418 ha For Windes Design

Current Site Total Current Site Brownfield Cur

Total site area = 27121 m2 Total site area = 19344 m2 Tota

2.712 ha 1.934 ha

Roof Area 4032 m2 Roof Area 1203 m2 Roof

0.403 ha 0.120 ha

Imp % = 90 0.363 ha Imp % = 90 0.108 ha Imp %

Concrete 3605 m2 Concrete 3277 m2 Conc

0.361 ha 0.328 ha

Imp % = 90 0.324 ha Imp % = 90 0.295 ha Imp %

Access Road + Parking 13514 m2 Access Road + Parking 11087 m2 Acce

1.351 ha (50% Positively Drained) 1.109 ha (+50

Imp % = 100 1.351 ha Imp % = 50 0.554 ha Imp %

Greenfield contribution = 5970 m2 Greenfield contribution = 3777 m2 Gree

0.597 ha 0.378 ha

Imp % = 20 0.119 ha Imp % = 20 0.076 ha Imp %

Total = 2.039 ha For Windes Design Total = 0.958 ha For Windes Design

+ 30% climate change = 2.446 ha For Windes Design + 30% c limate change = 1.149 ha For Windes Design

AREAS Rev A.xls\AC Data


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(c)1982-2006 Micro Drainage

Clarke Bond Structures Ltd Page 1

Malvern House Newtown Road

Yeoford Way Highbridge

Exeter EX2 8LB Brownfield 2yr Flow

Date 28 September 07 Designed By AMC

File Brownfield.sws Checked By

Micro Drainage System1 W.10.4

STORM SEWER DESIGN by the Modified Rational Method

Network Design Table

PN

Length

(m)

Fall

(m)

Slope

(1:X)

Area

(ha)

T.E.

(mins)

DWF

(l/s)

k

(mm)

HYD

SECT

DIA

(mm)

1.000 10.00 0.100 100.0 0.958 2.00 0.0 0.600 o 375

Network Results Table

PN

Rain

(mm/hr)

T.C.

(mins)

US/IL

(m)

E.Area

(ha)

E.DWF

(l/s)

Foul

(l/s)

Infil.

(l/s)

Vel

(m/s)

CAP

(l/s)

Flow

(l/s)

1.000 75.0 2.1 5.000 0.958 0 0 0 1.81 200 195


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Appendix D

Tidal Calculations

Hinkley Tidal Calculations 1 in 200 year eventTidal Inflow -1 in 200 year eventTidal Curve 1 in 200 year + 30% Climate ChangeTidal Inflow -1 in 200 year event + 30% Climate Change


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EE00431 - H17 Newtown Road, Highbridge FRA September 2007

Date Time ACD ODN 1 in 200 year

Peak of 7.86m

10/03/01 00:15:00 1.464 -4.34

10/03/01 00:30:00 1.173 -4.63

10/03/01 00:45:00 0.928 -4.87

10/03/01 01:00:00 0.706 -5.09

10/03/01 01:15:00 0.644 -5.16 LP -5.156

10/03/01 01:30:00 0.742 -5.06 -5.077

10/03/01 01:45:00 0.972 -4.83 -4.864

10/03/01 02:00:00 1.334 -4.47 -4.516

10/03/01 02:15:00 1.764 -4.04 -4.097

10/03/01 02:30:00 2.22 -3.58 -3.647

10/03/01 02:45:00 2.72 -3.08 -3.149

10/03/01 03:00:00 3.302 -2.50 -2.564

10/03/01 03:15:00 3.943 -1.86 -1.913

10/03/01 03:30:00 4.611 -1.19 -1.229

10/03/01 03:45:00 5.324 -0.48 -0.494

10/03/01 04:00:00 6.069 0.27 0.280

10/03/01 04:15:00 6.83 1.03 1.076

10/03/01 04:30:00 7.603 1.80 1.891

10/03/01 04:45:00 8.373 2.57 2.708

10/03/01 05:00:00 9.095 3.30 3.481

10/03/01 05:15:00 9.792 3.99 4.232

10/03/01 05:30:00 10.448 4.65 4.945

10/03/01 05:45:00 11.058 5.26 5.614

10/03/01 06:00:00 11.604 5.80 6.218

10/03/01 06:15:00 12.08 6.28 6.752

10/03/01 06:30:00 12.468 6.67 7.194

10/03/01 06:45:00 12.756 6.96 7.531

10/03/01 07:00:00 12.942 7.14 7.759

10/03/01 07:15:00 13.01 7.21 HP 7.860

10/03/01 07:30:00 12.919 7.12 7.736

10/03/01 07:45:00 12.69 6.89 7.463

10/03/01 08:00:00 12.348 6.55 7.070

10/03/01 08:15:00 11.882 6.08 6.546

10/03/01 08:30:00 11.354 5.55 5.958

10/03/01 08:45:00 10.782 4.98 5.327

10/03/01 09:00:00 10.144 4.34 4.630

10/03/01 09:15:00 9.465 3.67 3.894

10/03/01 09:30:00 8.782 2.98 3.158

10/03/01 09:45:00 8.102 2.30 2.430

10/03/01 10:00:00 7.412 1.61 1.696

10/03/01 10:15:00 6.705 0.91 0.949

10/03/01 10:30:00 6.025 0.23 0.235

10/03/01 10:45:00 5.37 -0.43 -0.448

10/03/01 11:00:00 4.74 -1.06 -1.100

10/03/01 11:15:00 4.132 -1.67 -1.726

10/03/01 11:30:00 3.563 -2.24 -2.307

10/03/01 11:45:00 3.025 -2.78 -2.852

10/03/01 12:00:00 2.527 -3.27 -3.352

10/03/01 12:15:00 2.062 -3.74 -3.816

10/03/01 12:30:00 1.634 -4.17 -4.238

10/03/01 12:45:00 1.286 -4.51 -4.577

10/03/01 13:00:00 1.011 -4.79 -4.839

10/03/01 13:15:00 0.709 -5.09 -5.126

10/03/01 13:30:00 0.555 -5.25 -5.263

10/03/01 13:45:00 0.518 -5.28 LP -5.28

Hinkley 2001 data.xls\10-03 2001 extrap to 7.86


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EE00431 - Site H17 - Highbridge, FRA September 2007

1 in 200 Tidal Inflow

Time mAOD

mAOD Time

Spill Level 7.81 07:07

Tidal peak 7.86 07:15

Ponding Invert level 7.22

Width of breach = 4.63 m

Cross sectional flow area = 0.139 m2

Distance from defence to Low Spot = 0.00 m2

Area of ponding "A" below 7.59m = 2347.00 m

m

Max depth based on total volume over area = 0.012 m

or = 7.232

* Assumed velocity = 0.5 m/s

Flow Volume

(m3/s)* (m

3)

07:07:25 7.810 0.0000

07:15:00 7.860 0.0694 15.7

07:21:03 7.810 0.0000 12.6

Total volume = 28.3

Flow Calcs Highbridge 1in200.xls\Tidal Inflow - 1 in 200


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EE00431 - H17 Newtown Road, Highbridge FRA

Flow Calcs Highbridge 1in2

-8.000

-6.000

-4.000

-2.000

0.000

2.000

4.000

6.000

8.000

10.000

00:00:00 01:00:00 02:00:00 03:00:00 04:00:00 05:00:00 06:00:00 07:00:00 08:00:00 09:00:00 10:00:00 11:00:00

Spill Level (7.81mAOD)

1 in 200 + CC tide curvederived from Hinkley

Peak 8.874mAOD

Tidal Lock (1.70mAOD)


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EE00431 - H17 Newtown Road, Highbridge FRA September 2007

1 in 200 + CC Tidal Inflow

Time mAOD Width of breach = 70.00 m

01:15:00 -5.156 mAOD Time

01:30:00 -5.107 Marsh level 7.81 08:35 Cross sectional flow area 7.810 = 6.670 m2

01:45:00 -4.921 Tidal peak 8.874 09:30 Cross sectional flow area 8.015 = 16.661 m2

02:00:00 -4.595 Marsh level 5.5 10:25 Cross sectional flow area 8.428 = 47.617 m2

02:15:00 -4.191 Cross sectional flow area 8.722 = 71.681 m2






03:45:00 -0.522

04:00:00 0.297 Distance from defence to Low Spot = 0.00

04:15:00 1.149 Area of ponding at 5.5mAOD = 2762979 m

04:22:39 1.638 Outfall Area considered deducting buildings at 20% = 2210383 m

04:30:00 2.028

04:45:00 2.919 Max depth based on total volume over area = 0.068 m

05:00:00 3.770 or = 5.568 m

05:15:00 4.606

05:30:00 5.408 Flow Volume Level after 3 tide cycles = 5.70 m

05:45:00 6.168 (m3/s) (m

3)

06:00:00 6.864 * Assumed velocity = 0.5 m/s

06:15:00 7.488

06:24:11 7.810 0.0 0.0

06:30:00 8.015 8.3 1453.8

06:45:00 8.428 23.8 14462.607:00:00 8.722 35.8 26842.1

07:15:00 8.874 42.1 35059.5

07:30:00 8.699 34.9 34635.6

07:45:00 8.358 20.9 25130.5

08:00:00 7.885 5.0 11672.3

08:01:50 7.810 0.0 274.5

08:15:00 7.270

08:30:00 6.589

08:45:00 5.866 Total volume = 149531

09:00:00 5.077

09:15:00 4.251

09:30:00 3.432

09:45:00 2.629

10:00:00 1.827

10:26:30 1.638 Outfall

10:15:00 1.017

10:30:00 0.251

10:45:00 -0.476

11:00:00 -1.164

11:15:00 -1.816

11:30:00 -2.416

11:45:00 -2.972

12:00:00 -3.476

12:15:00 -3.937

12:30:00 -4.351

12:45:00 -4.674

13:00:00 -4.917

13:15:00 -5.181

13:30:00 -5.292

13:45:00 -5.28

Flow Calcs Highbridge 1in200 CC.xls/Tidal Inflow - 1 in 200 CC


28/48

Appendix E

WinDes Calculations

WinDes Result File Pipe System 100yr + 30% CC under Tide Lock ConditionsWinDes Result File Pipe System 2yrWinDes Result File Pipe System 30yr

WinDes Result File Pipe System 100yrWinDes Result File Pipe System 100yr + 30% Climate Change


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Exeter EX2 8LB CC Tide Lock Pipe SystemDate September 07 Designed By AMC

File CC Test Tide Lock.src Checked By

Micro Drainage Source Control W.10.4

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

Storm

Duration(mins)

Maximum

Control(l/s)

Maximum

Outflow(l/s)

Maximum

Water Level(m OD)

Maximum

Depth(m)

Maximum

Volume(m)

Status

15 Summer 0.2 0.2 3.9868 0.9868 279.5 O K30 Summer 0.2 0.2 4.1693 1.1693 375.7 O K60 Summer 0.2 0.2 4.3662 1.3662 482.1 O K120 Summer 0.3 0.3 4.5817 1.5817 597.1 O K180 Summer 0.3 0.3 4.7182 1.7182 666.5 O K240 Summer 0.3 0.3 4.8187 1.8187 715.3 O K

360 Summer 0.3 0.3 4.9822 1.9822 787.4 O K480 Summer 0.3 0.3 5.1342 2.1342 842.4 O K

600 Summer 0.3 0.3 5.3342 2.3342 886.5 O K720 Summer 0.0 0.0 3.0000 0.0000 0.0 O K960 Summer 0.0 0.0 3.0000 0.0000 0.0 O K

1440 Summer 0.0 0.0 3.0000 0.0000 0.0 O K2160 Summer 0.0 0.0 3.0000 0.0000 0.0 O K2880 Summer 0.0 0.0 3.0000 0.0000 0.0 O K4320 Summer 0.0 0.0 3.0000 0.0000 0.0 O K5760 Summer 0.0 0.0 3.0000 0.0000 0.0 O K7200 Summer 0.0 0.0 3.0000 0.0000 0.0 O K8640 Summer 0.0 0.0 3.0000 0.0000 0.0 O K

10080 Summer 0.0 0.0 3.0000 0.0000 0.0 O K15 Winter 0.2 0.2 4.0513 1.0513 313.1 O K30 Winter 0.2 0.2 4.2533 1.2533 420.9 O K60 Winter 0.3 0.3 4.4742 1.4742 540.0 O K120 Winter 0.3 0.3 4.7227 1.7227 668.9 O K180 Winter 0.3 0.3 4.8872 1.8872 746.7 O K

240 Winter 0.3 0.3 5.0177 2.0177 801.4 O K360 Winter 0.3 0.3 5.3067 2.3067 882.3 O K

480 Winter 0.0 0.0 3.0000 0.0000 0.0 O K600 Winter 0.0 0.0 3.0000 0.0000 0.0 O K720 Winter 0.0 0.0 3.0000 0.0000 0.0 O K960 Winter 0.0 0.0 3.0000 0.0000 0.0 O K

1440 Winter 0.0 0.0 3.0000 0.0000 0.0 O K

Storm

Duration(mins)

Rain

(mm/hr)

Time-Peak

(mins)

15 Summer 89.02 1930 Summer 59.85 3460 Summer 38.41 64120 Summer 23.81 124180 Summer 17.74 184240 Summer 14.29 244360 Summer 10.50 364480 Summer 8.44 484600 Summer 7.12 604720 Summer 0.00 0960 Summer 0.00 0

1440 Summer 0.00 02160 Summer 0.00 02880 Summer 0.00 04320 Summer 0.00 05760 Summer 0.00 07200 Summer 0.00 08640 Summer 0.00 0

10080 Summer 0.00 015 Winter 89.02 1930 Winter 59.85 3460 Winter 38.41 64120 Winter 23.81 124180 Winter 17.74 184240 Winter 14.29 244360 Winter 10.50 364480 Winter 0.00 0600 Winter 0.00 0720 Winter 0.00 0960 Winter 0.00 0

1440 Winter 0.00 0


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31/48


32/48





Exeter EX2 8LB CC Tide Lock Pipe SystemDate September 07 Designed By AMC

File CC Test Tide Lock.src Checked By


Circular Pipe Details

Diameter (m) 2.100 Length (m) 260.000 Cover Level (m) 6.400

Slope (1:x) 500.0 Invert Level (m) 3.000

Orifice Outflow Control

Diameter (m) 0.010 Discharge Coefficient 0.600 Invert Level (m) 3.000


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Exeter EX2 8LB 2yr Pipe SystemDate September 07 Designed By AMC

File 2yr New Pipe.src Checked By


Summary of Results for 2 year Return Period

Storm

Duration(mins)

Maximum

Control(l/s)

Maximum

Outflow(l/s)

Maximum

Water Level(m OD)

Maximum

Depth(m)

Maximum

Volume(m)

Status








1440 Winter 12.7 12.7 3.1333 0.1333 2.6 O K

Storm

Duration

(mins)

Rain

(mm/hr)

Time-Peak

(mins)

15 Summer 36.37 1330 Summer 24.05 2260 Summer 15.35 38120 Summer 9.61 70180 Summer 7.27 100240 Summer 5.95 130

360 Summer 4.48 188480 Summer 3.66 248

600 Summer 3.12 308720 Summer 2.74 368960 Summer 2.24 490


10080 Summer 0.42 504015 Winter 36.37 1430 Winter 24.05 2360 Winter 15.35 40120 Winter 9.61 72180 Winter 7.27 102

240 Winter 5.95 130360 Winter 4.48 190

480 Winter 3.66 248600 Winter 3.12 308720 Winter 2.74 368960 Winter 2.24 490

1440 Winter 1.68 734


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Storm

Duration(mins)

Maximum

Control(l/s)

Maximum

Outflow(l/s)

Maximum

Water Level(m OD)

Maximum

Depth(m)

Maximum

Volume(m)

Status

2160 Winter 9.6 9.6 3.1163 0.1163 1.9 O K2880 Winter 7.8 7.8 3.1073 0.1073 1.5 O K4320 Winter 5.9 5.9 3.0918 0.0918 1.1 O K5760 Winter 4.8 4.8 3.0823 0.0823 0.8 O K7200 Winter 4.1 4.1 3.0763 0.0763 0.7 O K8640 Winter 3.6 3.6 3.0718 0.0718 0.6 O K

10080 Winter 3.2 3.2 3.0678 0.0678 0.5 O K

StormDuration

(mins)

Rain

(mm/hr)

Time-Peak

(mins)

2160 Winter 1.26 1100

2880 Winter 1.03 14604320 Winter 0.77 2128

5760 Winter 0.63 29127200 Winter 0.54 35928640 Winter 0.47 4352

10080 Winter 0.42 5024


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Rainfall Details

Region ENG+WAL Cv (Summer) 0.750 Summer Storms Yes

Return Period (years) 2 Cv (Winter) 0.840 Winter Storms YesM5-60 (mm) 19.000 Shortest Storm (mins) 15 Climate Change % +0Ratio-R 0.350 Longest Storm (mins) 10080

Time / Area Diagram

Total Area (ha) = 1.289

Timefrom:

(mins)to:

Area(ha)

0 4 1.289


36/48














37/48









Storm

Duration(mins)

Maximum

Control(l/s)

Maximum

Outflow(l/s)

Maximum

Water Level(m OD)

Maximum

Depth(m)

Maximum

Volume(m)

Status








1440 Winter 21.9 21.9 3.1918 0.1918 6.3 O K

Storm

Duration

(mins)

Rain

(mm/hr)

Time-Peak

(mins)


360 Summer 8.11 196480 Summer 6.54 256




240 Winter 10.95 142360 Winter 8.11 200


1440 Winter 2.87 728


38/48









Storm

Duration(mins)

Maximum

Control(l/s)

Maximum

Outflow(l/s)

Maximum

Water Level(m OD)

Maximum

Depth(m)

Maximum

Volume(m)

Status


10080 Winter 5.0 5.0 3.0838 0.0838 0.9 O K

StormDuration

(mins)

Rain

(mm/hr)

Time-Peak

(mins)

2160 Winter 2.11 1096

2880 Winter 1.69 14644320 Winter 1.24 2136


10080 Winter 0.65 5008


39/48








Rainfall Details



Time / Area Diagram


Timefrom:

(mins)to:

Area(ha)

0 4 1.289


40/48


41/48









Storm

Duration(mins)

Maximum

Control(l/s)

Maximum

Outflow(l/s)

Maximum

Water Level(m OD)

Maximum

Depth(m)

Maximum

Volume(m)

Status








1440 Winter 27.6 27.6 3.2208 0.2207 8.8 O K

Storm

Duration

(mins)

Rain

(mm/hr)

Time-Peak

(mins)


360 Summer 10.50 202480 Summer 8.44 260




240 Winter 14.29 148360 Winter 10.50 208


1440 Winter 3.62 730


42/48









Storm

Duration(mins)

Maximum

Control(l/s)

Maximum

Outflow(l/s)

Maximum

Water Level(m OD)

Maximum

Depth(m)

Maximum

Volume(m)

Status


10080 Winter 6.0 6.0 3.0928 0.0928 1.1 O K

StormDuration

(mins)

Rain

(mm/hr)

Time-Peak

(mins)

2160 Winter 2.64 1100

2880 Winter 2.11 14684320 Winter 1.53 2136


10080 Winter 0.78 5072


43/48








Rainfall Details



Time / Area Diagram


Timefrom:

(mins)to:

Area(ha)

0 4 1.289


44/48














45/48





Exeter EX2 8LB CC Pipe SystemDate September 07 Designed By AMC

File CCyr New Pipe.src Checked By



Storm

Duration(mins)

Maximum

Control(l/s)

Maximum

Outflow(l/s)

Maximum

Water Level(m OD)

Maximum

Depth(m)

Maximum

Volume(m)

Status








1440 Winter 35.8 35.8 3.2607 0.2607 13.3 O K

Storm

Duration

(mins)

Rain

(mm/hr)

Time-Peak

(mins)


360 Summer 10.50 206480 Summer 8.44 266




240 Winter 14.29 152360 Winter 10.50 214


1440 Winter 3.62 734


46/48









Storm

Duration(mins)

Maximum

Control(l/s)

Maximum

Outflow(l/s)

Maximum

Water Level(m OD)

Maximum

Depth(m)

Maximum

Volume(m)

Status


10080 Winter 7.8 7.8 3.1068 0.1068 1.5 O K

StormDuration

(mins)

Rain

(mm/hr)

Time-Peak

(mins)

2160 Winter 2.64 1100

2880 Winter 2.11 14684320 Winter 1.53 2200


10080 Winter 0.78 5048


47/48








Rainfall Details



Time / Area Diagram


Timefrom:

(mins)to:

Area(ha)

0 4 1.289


48/48












Documents

010-158-42395 Flood Risk Assessment