JUNE 2012 Ipswich Surface Water Management Plan...IPSWICH SURFACE WATER MANAGEMENT PLAN – PHASE 3 REPORT 7.2 Detailed Assessment - London Rd / Lavenham Road Priority area. 41 7.3

JUNE 2012

Suffolk Flood RiskManagement Partnership

Ipswich Surface WaterManagement PlanPhase 3 Report – May 2012

Produced for Suffolk County Councilby Ipswich Borough Council

IPSWICH SURFACE WATER MANAGEMENT PLAN – PHASE 3 REPORT

Produced by: Ipswich Borough Council, Drainage Engineering.Grafton House, 15–17 Russell Road, Ipswich IP1 2DE

For further information on anything in this document, please contact:

Denis Cooper: [email protected] Telephone 01473 432854

Jason Wakefield: [email protected] Telephone 01473 432853

Acknowledgements: Members of the Project Board

Jane Burch – Suffolk County Council, Flood & Coastal Policy Manager

Matt Hullis – Suffolk County Council, Chairman of SFRMP

Graham Day – Anglian Water, Senior Asset Planner

Briony Tuthill – Anglian Water, Flood Risk Manager

Mike Tee – Ipswich Borough Council, Head of Planning Transport and Regeneration

Phil Camamile – Water Level Management Alliance, Chief Executive

Giles Bloomfield – Internal Drainage Board

Mick Whiley – Environment Agency, Technical Specialist, Asset System Management

Richard Houghton – Environment Agency, Flood Risk Manager

Jeremy Bloomfield – Environment Agency

Suffolk County Council Consultants

Roger Edgson – Principal Consultant, AECOM

Anglian Water

Andy Bird – Norfolk & Suffolk Asset Planning, Asset Management

David De Rosa – Clear Environmental Consultants Ltd

Penny Blatch – Clear Environmental Consultants Ltd


Issue & Version List

Date Version To

Phase 1 report 4 Sharepoint

Phase 3 report 4 Mike Tee 31/1/2012

Phase 3 report 5 Sharepoint & Project Boardmembers 10/2/2012

Phase 3 report 6 blanked out modelling/AWcosts and red text indicatingrecent changes

Jane Burch 27 April 2012

Phase 3 report 7 revised modelling/AW costs

LimitationsInformation contained or used has been gathered from a range of different sources, the providers of suchinformation should be contacted before making any irreversible decisions based upon it.

Neither Suffolk County Council nor Ipswich Borough Council provide warranty as to the accuracy,completeness, or suitability for any purpose of the information contained herein.


1 LIST OF APPENDICES 6

2 ABBREVIATIONS 7

3 POLICY CONTEXT 9

4 EXECUTIVE SUMMARY, CONCLUSIONS & RECOMMENDATIONS 10

5 INTRODUCTION 135.1 Purpose, History and Guidance on use of Report 135.2 SWMP process 135.3 Geographic Extent of the Report 145.4 Overview of Ipswich drainage system 145.5 Highway or railway drains 205.6 SUDS and Soakaways 205.7 Overview of Local Sources of Flood Risk 215.8 Overview of Recent Activities or Projects 225.9 Overview of Future flooding 23

6 PREPARATION 256.1 Establishment of the Partnership 256.2 Ipswich SWMP aims as Agreed by the Project Board 256.3 Information Held by Ipswich Borough Council 266.4 Data and Information Available from other Partner Organisations 276.5 National Data 276.6 Public information 276.7 Systems for storing & sharing flood risk data, maps and Information 276.8 Quality Assurance, security and licensing 276.9 Preliminary Assessment of Flooding Across Ipswich 276.10 Identification of Level of Assessment 316.11 Prioritisation of Areas for Detailed Assessment 316.12 Priority Areas For Detailed Assessment 356.13 Brief Description of Top 10 Drainage Areas for Detailed Assessment 356.14 Planning the Detailed Assessment 366.15 Communication and Engagement plan 38

7 RISK ASSESSMENT 397.1 Detailed Assessment by Anglian Water 39


7.2 Detailed Assessment - London Rd / Lavenham Road Priority area. 417.3 Food Maps for 2011 and 2040 467.4 Comparison with EA’s Flood Map for Surface Water (FMfSW) 517.5 Predicted Numbers of Properties at risk at 2011 51

8 OPTIONS 538.1 Short-listing of Options and Planning Option Evaluation 538.2 Options for Lovetofts Drive/ Lagonda Drive 538.3 Options for Worsley Close / Ellenbrook 538.4 Options for Ancaster Road / Burrell Road 538.5 Options for London Road / Lavenham Road / Hadleigh Road 538.6 Flood maps for Options 538.7 Evaluation of options for Lovetofts Drive/ Lagonda Drive 588.8 Evaluation of Options for Worsleyy Close / Ellenbrook 598.9 Evaluation of Options for Ancaster Road / Burrell Road 608.10 Evaluation of Options – London Road / Lavenham Road /

Hadleigh Road. 638.11 Ipswich Wide Options – Control of Creep 64

9 FLOODD AND COASTAL RESILIANCE PARTNERSHIP FUNDING BID FOR SUDS SCHEME AT LOVETOFTS 66

10 IMPLEMENTATION AND REVIEW 6710.1 Existing Ipswich Wide Actions 6710.2 Proposed Action Plan – Capital and Maintenance 6810.3 Proposed Action Plan – Public Information and Advice 7110.4 Proposed Action Plan – Future SWMPs for Ipswich 7210.5 Prioritisation of SWMPs across Suffolk 7310.6 Advice and Information for Planning and Highway Authorities 7410.7 Advice for The Local Resilience Forum 7410.8 Water Framework Directive 7710.9 Review 7810.10 Lessons For Future SWMPs 78

11 PROPOSALS FOR MANAGEMENT AND MAINTENANCE OF DATA 80

12 APPENDICES 81

13 REFERENCES 82


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12.1 Local Flooding, Watercourses, ASTSWF & Groundwater Flooding map.

12.2 Geology, General Topography & Minor watercourses map.

12.3 Highway and Railway Drains and SUDS Catchments map.

12.4 High risk areas where gulley cleaning should be focused map.

12.5 Highway and IBC community improvements 2010/11

12.6 IBC Outstanding Highway Drainage Works

12.7 1953 Tidal Flood map.

12.8 1939 Fluvial Flood map

12.9 1947 Fluvial Flood map

12.10 Historic Flooding records table

12.11 Historic flood record (Spreadsheet)

12.12 Historic flood records - map

12.13 Prioritisation Spreadsheet

12.14 Communication and Engagement plan

12.15 IBC Enforcement Policy for paving of gardens

12.16 Lovetofts 100 yr DN 2011 map

12.17 Lovetofts 100 yr DN 2040 map

12.18 Ellenbrook 100 yr DN 2011 map

12.19 Ellenbrook 100 yr DN 2040 map

12.20 Ancaster Burrell 100 yr DN 2011

12.21 Ancaster Burrell 100 yr DN 2040

12.22 London Rd, Lavenham Rd to Hadleigh Rd 100 yr DN 2011 map

12.23 London Rd, Lavenham Rd to Hadleigh Rd 100 yr DN 2040 map

12.24 Lovetofts 100 yr DN 2011 Hazard Map

12.25 Ellenbrook 100 yr DN 2011 Hazard Map

12.26 Ancaster Burrell 100 yr DN 2011 Hazard Map

12.27 London Rd, Lavenham Rd to Hadleigh Rd 100 yr DN 2011 Hazard Map

12.28 Lovetofts Cost Benefit Analysis Workbook

12.29 Ellenbrook Cost Benefit Analysis Workbook

12.30 Ancaster Burrell Cost Benefit Analysis Workbook

12.31 London Rd, Lavenham Rd to Hadleigh Rd Cost Benefit Analysis Workbook

1 List of Appendices


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2 Abbreviations

AEP Annual Exceedance Probability. The probability associated with a return period. Thus anevent of return period 50 years has an AEP of 0.02 or 2%

AOD Above Ordnance Datum

AONB Area of Outstanding Natural Beauty

ASTSWF Environment Agency map showing areas susceptible to surface water flooding (ignoringunderground drainage)

AW Anglian Water

CFMP Catchment Management Flood Plan (for East Suffolk)

DEFRA Department for Environment, Food and Rural Affairs

EA Environment Agency

FDGiA Flood Defence Grant in Aid

FDMS Flood Defence Management Strategy (for Ipswich – tidal and fluvial)

FMfSW Environment Agency flood map for surface water (takes account of drainage)

FRM Flood risk management

FRMP Flood risk management plan

GIS Geographic Information System. A software framework that captures, stores, analyses,manages, and presents data that is linked to location

IBC Ipswich Borough Council

IDB Internal Drainage Board

LDF Local Development Framework. Ipswich Borough Council’s planning framework

LiDAR Light Detection and Ranging. Method for collecting high-resolution topographic data

LLFA Lead Local Flood Authority

PFRA Preliminary Flood Risk Assessment. Requirement under the Flood Risk Regulations (2009)

PH IBC Portfolio Holder

PM Project manager

RAMSAR Wetlands of International Importance

RBMP River Basin Management Plan. Environment Agency plan for delivery of the WaterFramework Directive within the Anglian Region

RMS Root Mean Square error of LiDAR data. Difference between values predictedand observed


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RP Return Period – average time between reoccurrences

PV Present Value. The PV of the scheme is the notional sum of money that needs to be putaside (invested) now to fund the scheme. PV is affected by inflation and investmentinterest rates and may include capital and ongoing maintenance costs

SCC Suffolk County Council

SFRA Strategic Flood Risk Assessment

SFRMP Suffolk Flood Risk Management Partnership

SPA Special Protection Area. Area protected under the EU Birds Directive

SSSI Site of Special Scientific Interest

SW Surface Water

SuDS Sustainable Drainage Systems

SWMP Surface Water Management Plan

WFD Water Framework Directive


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Defra’s intention is to use Surface WaterManagement Plans (SWMPs) as the primary vehicleto manage surface water flood risk in England. Thisintention was published in the Future Water Strategy(2008). The SWMP concept was recognised andpromoted within Planning Policy Statement 25(PPS25) and is implicit in the new National PlanningPolicy Framework.

A SWMP will fit within the existing policy frameworkand can provide the evidence base to inform

Preliminary Flood Risk Assessments and fulfil therequirement for Flood Risk Management Plansunder the Flood Risk Regulations (2009).

The Floods and Water Management Act 2010requires lead local flood authorities to develop astrategy for local flood risk management for theirarea. SWMPs make an important contribution toinforming the development of this strategy andidentifying ways to implement it.

3 Policy context

Figure 3.1 Flood and coastal erosion management policy framework


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1. The Surface Water Management Plan processhas been found to be cost beneficial; the costs ofthe study and likely costs of flood relief measuresput together should easily exceed the expectedreduction in flood damage costs.

2. 34 sub catchments in Ipswich have beenidentified and prioritised. The top fourcatchment areas:

a) London Road to Lavenham Road andHadleigh Road

b) Ancaster Road/Burrell Road

c) Lovetofts Drive to Lagonda Drive

d) Worsley Close/ Ellenbrook Green

These areas have been studied in detail.The resulting action plan (see section 10) proposesmeasures for alleviating flooding in these fourareas and suggests ways to reduce the effects ofurban creep (paving of gardens, small extensions,etc) which should also have Suffolk wide benefits.

3. Actions include: further modelling by AnglianWater, subsequent updating of option evaluation,design and implementation of retrofit SuDS systemsand maintenance work.

4. The plan suggests four further sub-catchmentareas are studied in detail following the processes

described later in this report. These are: SwinburneRoad, Coltsfoot Road, Portman Road andMaidenhall Approach. If this second stage SWMP isalso found to be cost effective, then the SWMPshould be extended again to include lower priorityareas and so on, until the costs of SWMPdevelopment plus the cost of flood managementmeasures exceed the benefits.

5. The cost of the second stage SWMP is estimatedto be £77,000.

6. A methodology for prioritising detailed SWMPstudies across Suffolk is described and an Excelworkbook provided for future use. This made use ofthe Environment Agency’s Areas Susceptible toSurface Water Flooding (ASTWF) maps.The spreadsheet should be updated to use themore recent Flood Map for Surface water (FMfSW)which was used for the Suffolk Preliminary FloodRisk Assessment.

7. Another workbook has been developed forestimating damage costs and benefits in eachdetailed study area. A similar methodology couldbe used for future SWMP studies.

8. The following numbers (below) of properties arepredicted to flood internally in each of the fourpriority areas:

4 Executive Summary, Conclusions& Recommendations

Return period (yrs) 1 2 5 10 30 50 100 200

Ancaster Internal 1 1 1 2 2 10 15 19

Chantry Park Internal 0 0 0 0 0 0 3 6

Ellenbrook Internal 0 2 7 11 21 25 34 42

Lovetofts Internal 0 0 0 2 13 13 18 21


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9. The SWMP predicts fewer properties at risk thanthe PFRA and the EA’s Flood Map for Surface Water(FMfSW). Based on SWMP figures above, asexplained in section 7.5, the total number ofproperties likely to suffer internal flooding in Ipswichis estimated to be 1,525, whereas the estimatequoted in the PFRA was 5,628 (external + internal).These figures are for an extremely intense and rarerainfall, with a 0.5% annual probability in any year(occurs on average once every 200 years)

10. Smaller numbers of properties will flood in morecommon events as shown on the following graph:

11. Across Ipswich an average of about 360properties are predicted to suffer some floodingper year, of which 130 would suffer internalflooding. In a 1-year return period event48 properties are predicted to flood of which18 would suffer internal flooding.

The above Ipswich-wide estimates are likely to behigh because they were extrapolated from thehighest priority areas. The average number ofproperties recorded to flood per year is 124.Flooding, especially external, often goesunreported, so the results give credence to theSWMP predictions.

12. There are inherent inaccuracies in predictingwhether homes will flood. For example, no accountis taken of individual property protection, wash fromvehicles, fences, etc. Blockages affecting pipes,grilles or grates and deposits of flood debris willhave varying affects. Ground level data used formodelling has an accuracy of + or -150mm,whereas thresholds are often only 50mm aboveground levels.

The methodology used therefore providescatchment-wide figures and should not berelied on for predictions of flooding atindividual properties.

13. New flood maps are included for the fourstudy areas. Flood hazards to people aregenerally low apart from in a few parts of theEllenbrook area – notably at Gusford CommunitySchool. It is recommended the schoolmanagement are made aware of these hazardsand encouraged to put appropriate measuresin place.

14. It will be for Suffolk County Council to decidewhether the newly mapped areas are significantflood risk areas (Flood Risk Regulations 2009).

15. The new flood maps should be incorporated orcross referenced in the Strategic Flood RiskAssessment (SFRA) and made available to plannersand developers.

16. The Ipswich Local Development Framework(LDF) already includes requirements for controllingdevelopment in local flood risk areas. The newmaps should be used to assist in associated FloodRisk Assessments (FRAs).

17. Lessons for future SWMPs are listed in section10.10. The following are considered to beespecially important at this time:

l Representation of highway gullies in models inflood risk areas is important and will provide anunderstanding of flood mechanisms and aiddecisions on flood management and design ofsolutions involving SuDS.

l Simulation results from a wide range of returnperiods are needed to properly assess options.Interpolation from a limited range of results issometimes possible but time consuming.

18. The measures proposed in the SWMP areexpected to have a very small but beneficialaffect on water quality in local watercourses.

19 .The following information and advice should beprovided to the public.

Initially this will be at the exhibition/drop in sessionused for consulting the public on the action plan:

l Flood maps – with limitations explained.


l Information on the developing Suffolk LocalFlood Risk Management Strategy.

l Responsibilities – changes in regard to privatedrainage made in October 2011.

l Responsibilities – who and how to reportflooding or blockages.

l Advice on flood risk management and waterquality improvement measures that residentsand businesses can take:

w Not to obstruct flood paths, ditchesor watercourses.

w Not to wash cement, fat, oil, or pesticidesdown drains.

w Paving of gardens e.g. – minimise pavedareas, use permeable construction orrain gardens.

w Flood resilience barriers, etc.

w Flood warnings and awareness /preparedness.

20. No formal environmental or equalityassessments have been undertaken. These will beapplied to individual flood management schemesthat result from the plan.

21. Under the Floods and Water Management Act,Suffolk County Council will be responsible for

monitoring and reporting implementation of theaction plan. This will be done through the SuffolkFlood Risk Management Partnership in line with theannual review proposed in the Suffolk Local FloodRisk Management Strategy.

It is recommended the Suffolk Flood RiskManagement Partnership continues to worktogether to discuss and monitor progress.According to Defra’s guidance, the action planshould be reviewed and updated once every 6years as a minimum but there are circumstanceswhich might trigger an earlier review such as:

l Occurrence of flooding.

l Additional data or modelling becomingavailable which may alter the understandingof risk.

l Outcome of investment decisions by partnersrequiring revisions to the action plan.

l Additional development or other changeswhich affect surface water flooding.

Long term recording, monitoring and review offlood records will be undertaken, as planned inthe draft Local Flood Risk Management Strategy.This should make use of Ipswich Borough Council’shistoric records to set the base line data.

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5.1 Purpose, history andguidance on use of this reportThe main purpose of this report was to inform theproject board members and enable them toendorse both the work undertaken so far and theproposed action plan prior to public consultation. Itwill also inform the development of future SWMPsand future investment decisions.

Consultation responses will be considered andappropriate amendments may need to beincorporated. Following formal approval andadoption by partners, the report will be madeavailable to the public, developers and plannersalongside the Ipswich Strategic Flood RiskAssessment (SFRA).

The report is expected to be a living document,updated in line with changing circumstances suchas when further detailed studies or flood riskmanagement measures are planned orimplemented or if major flooding problems occur.

This report documents the SWMP process inchronological order and has evolved from andreplaces the phase 1 report.

Sections 5.2 to 6.13 are essentially unchanged.

Section 6.9 was written to inform the Suffolk PFRAin 2010 - this has not been updated followingthe detailed modelling and option evaluationdescribed in later parts of the report.

5.2 SWMP processDevelopment of the SWMP followed Defra’sguidance, issued March 2010www.defra.gov.uk/publications/files/pb13546-swmp-guidance-100319.pdf

This describes the general process fordeveloping an action plan for SurfaceWater Management.

The action plan includes cost effective measureswhich reduce flooding, manage theconsequences of flooding, or prevent increasedflooding – e.g. by preventing obstruction of flood-paths. Other benefits should include water qualityimprovements and more joined up working withpeople or partners, influencing planning anddevelopment control and emergency plans.

The production of the plan was funded by Defrawith additional input from Anglian Water, IpswichBorough Council and Suffolk County Council. Thebudget was limited and detailed study had tofocus on priority areas - only 4 of the 34 sub-catchment areas forming the study area.

The prioritisation process concentrated on floodingissues with greatest emphasis put on historicflooding rather than areas identified as susceptibleto surface water flooding (ASTWF) on theEnvironment Agency’s map.

The following diagram from Defra’s guidanceillustrates the process of developing a SWMP.

5 Introduction


5.3 Geographic extent ofthe reportThe areas covered by the SWMP report are outlinedin green. These denote catchment boundaries orwater sheds.

The purple areas indicate valleys or low spotswhere flooding is most likely.

Pinewood within the Babergh District Council areato the South East of Ipswich has been included inthe SWMP study at the request of the Project Board.

Babergh District Council and the EnvironmentAgency had no records of flooding in Pinewood.However Ipswich Borough Council do have somerecords of flooding here as the Borough arehighway agents for this area

5.4 Overview of Ipswich drainage Ipswich Borough Council has always hada drainage engineering team who have

developed and operate the Council’s drainageand flood defence policy – effectively a qualitysystem aiming to manage flooding problemswith record keeping and monitoring of floodinga core activity. As a result Ipswich has an arrayof records, GIS and software used to developthe SWMP.

The overview includes descriptions of tidal flooddefences. Whilst tidal flooding is outside the scopeof the SWMP, tidal defences will have some effect.In some areas the defences may act as barrier tosurface water flows, in others defences will aidsurface water drainage.

5.4.1 Main River Gipping and Orwell Estuary

Ipswich is sited where the freshwater River Gippingbecomes the tidal River Orwell. The Orwell Estuarydownstream from Ipswich is a wildlife site ofinternational importance designated SSSI,RAMSAR, SPA and within an Area of OutstandingNatural Beauty.

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The catchment of the River Gipping includes thetowns of Stowmarket, Needham Market, Bramford,Claydon and western parts of Ipswich, but ispredominantly rural.

At Horseshoe Sluice (Yarmouth Road) the riverdivides, with the tidal Orwell on the west side andthe freshwater Gipping on the east side of anisland. The island is defended against tidalflooding but not fluvial flooding. The Gipping spillsover Handford sluice (off West End Road) to jointhe Orwell.

The active port area and parts of the urban areaadjacent to the Orwell are low lying and at risk oftidal flooding (see map on next page).

Following the 1953 tidal surge the River Gippingand Orwell flood defence walls were upgradedin a comprehensive scheme between1970 and 1983. The river channel was improvedand 15 km of flood defence walls and5 control structures were constructed. These walls

are generally at least 2m above inlandground levels.

By 2006 these defences had deteriorated andwere in need of improvement. In response theIpswich Flood Defence Management Strategywas adopted.

The first stages of the Strategy were constructedbetween 2008 and 2010. These replaced andraised the level of the defences on the East andWest banks of the Orwell downstream of the WetDock by about 1.5m.

The final major part of the strategy is to install theBarrier across the New Cut. The EnvironmentAgency plan to start building the barrier in 2014, with the work expected to take twoyears to complete.

The barrier is a gate, which will normally not affectriver levels. The gate would be raised at low tide, inadvance of predicted surge tides >3.6m AOD.


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It will be operated in a way to avoid increasingupstream river flood levels.

The strategy also proposes future repairs to thedefences upstream of the barrier to keep them attheir original design levels.

The strategy is designed to provide a standard ofprotection against tidal and fluvial flooding,including combinations, of 0.33 % annualexceedance probability (1 in 300 years returnperiod) allowing for the increased sea levelsexpected in 100 years time.

The Wherstead Road area is protected, mainly bythe high ground of the West Bank Terminal andsome local raising of the main road. A local floodlevy-funded scheme has recently been completedto raise the level of this defence to 4.4m AOD.

5.4.2 Wet Dock and Dock Sewer

The Wet Dock, completed in 1840, is connected tothe Orwell by a lock. Water levels are normally

maintained at about 1.5m AOD. The OrwellNavigation Service closes a movable floodgatesited between the lock gates, when the tide levelreaches 2.6m AOD.

The Wet Dock Lock gates normally retain water inthe Wet Dock; however each leaf gate includes2 sluices, each 1.1m x 0.4m located close to thebase of the gate. These might be opened to assistdrainage should flooding occur when the lockgates are closed. The level of the top of the lockgates is 3.1m AOD.

The Dock sewer, owned by the Port Authority andskirting the North and East of the Wet Dock,originally intercepted the polluted water from oldculverts and streets thus keeping the encloseddock clear of pollution. The Dock sewer has twooutfalls into the Orwell. The Port Authority hasresisted the connection of piped drainage systemsinto the dock and as a consequence theenclosed salt water in the Dock is of good quality.

However, every year or two, surface water flooding(resulting from overloading of piped drainagesystems) affects Duke Street, Fore Street, CollegeStreet and Key Street - the lowest roads surroundingthe Dock. The floodwater overflows into the dockhelping to reduce flood levels and consequences.

5.4.3 Belstead Brook

Belstead Brook (main river) joins the Orwell Estuaryat Bourne Bridge to the South of the town.The catchment is mainly rural but includesCopdock and the extreme south west of Ipswich.The brook has a largely undeveloped flood plain,with three properties known to be at risk of flooding.

5.4.4 Alderman Canal

The Alderman Canal (ordinary watercourse & LocalNature Reserve) originally fed water mills atAlderman Road and Stoke Bridge, with flows fromthe River Gipping. In about 1880 the channeldownstream of Alderman Road was filled in andreplaced with part of the “Low Level Trunk Sewer”.Apart from a small diameter penstock, river flowsare now prevented from entering the canal by anembankment across the old channel. There is noknown formal outlet.

Water is retained at a high level by another earthembankment, with crest level 3.7m AOD, along thesouth side of the canal. Any leakage is interceptedby a counter ditch, which drains the low-lyingmeadows and playing fields back into the tidalOrwell via a culvert and surface water sewer atConstantine Road. The water level in the canal isnormally the same as the River Gipping.However, during periods of flood risk the

Environment Agency close the penstock to preventovertopping of the embankment, which has only200mm freeboard in normal conditions.

There are a number of trees along theembankment of the canal, which could increasethe likelihood of a breach if they were to fall dueto high winds.

A survey was carried out in February 2010as follows:

The extent of flooding that would result froma breach of the embankment was determinedin the SFRA.

5.4.5 Mill River – east of Ipswich

Mill River flows eastwards from the extreme east ofIpswich towards the Deben Estuary. The upstreampart in the urban area has been replaced with asurface water sewer, which outfalls into a SSSI wetland area known as Bixley Heath.

Upstream of the wetland area, large sections ofthe original valley have been filled. However, the


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original valley remains in two areas – upstream ofBixley Road and just off Bucklesham Road.Drainage of these areas is reliant on the surfacewater sewer. In extreme events the remaining lowareas are liable to flood.

Water leaves the wetland area and flows throughseveral ponds at Purdis Heath. The enmained partof Mill River starts downstream of the ponds andflows through a rural area.

5.4.6 Westerfield Watercourse

This flows westwards from Westerfield villagetowards the River Gipping at Claydon. Areas ofundeveloped land including the Council’sMillennium Cemetery in the North of Ipswich fallwithin its catchment.

5.4.7 Other watercourses, springs andland drainage

Due to geological conditions many other smallerwatercourses exist. As the town has been urbanisedsome have become fragmented, piped or onlyflow in exceptional conditions.

During heavy rainfall, runoff and overflow fromoverloaded or blocked drainage systems inevitablymakes its way towards the minor watercourses andthen the low areas adjacent to the Orwell andGipping, including the Wet Dock.

During 2009 the Environment Agency undertooka national exercise to map areas that may besusceptible to surface water flooding (ASTSWF).These maps ignore the presence of undergrounddrainage and relate to only one storm event.Effectively these highlight valley bottoms or hollowswhere flooding may occur.

A plan in Appendix 12.1 shows the areassusceptible to surface water flooding, overlaid onareas where historic local flooding, major floodpaths and watercourses have been recorded byIpswich Borough Council.

Away from the main valley of the Orwell andGipping the ground rises steeply to a flattish,predominantly residential, area at about 30-40mAOD. Boulder clay (diamicton) caps the veryhighest areas to the north of Ipswich. Below this,sands and gravels overlay London Clay. Many ofthe minor watercourses are fed by springs issuing

from the base of the sands and gravels. Over timesome watercourses have eroded steep sidedtributary valleys, cutting into the higher areas.(A map showing geology, minor watercourses andgeneral topography is included in Appendix 12.2).

As Ipswich developed many of these watercourseswere used for water supply or culverted where theyflowed through streets towards the Orwell.Examples are in Northgate Street, Lower BrookStreet, Spring Road and Upper Orwell Street.

Some watercourses were used to create the pondsin Christchurch Park, Holywells Park and ChantryPark. Along the eastern boundary of Holywells Park,a canal, with water retained by an earthembankment up to 3m high, originally fed theCliff Brewery. This is now drained via an AnglianWater storm overflow sewer into the Orwell.Problems have recently arisen with high waterlevels or falling trees threatening to breach theembankment, with leaks flooding across parkingareas in adjacent premises.

In several locations land drainage systems(intended to drain ground water using porouspipes) have been installed in valley bottoms tohelp drain gardens. Examples can be found atTuddenham Avenue, Cavendish Street, AncasterRoad, Gippeswyck Park and Cliff Lane.

Land drains were also incorporated in the mainriver flood defences – these drain ground on thelandward side and at intervals these outfall throughthe sheet piled walls with flaps intended to preventreverse flow.


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5.4.8 Sewerage system

Circa 1880 the Low Level Trunk Sewer was installedand tributary sewers were added as the town grewrapidly. The original system is still in use and carriesfoul and surface water runoff from North West andcentral Ipswich around the Wet Dock and to theCliff Quay Waste Water Treatment Works.

In the lowest parts of the town, the low-level seweris extremely shallow and pumping stations wereinstalled to lift foul/combined flows into the sewer.Some of these areas also have separate surfacewater systems draining to the estuary by gravity.Flap valves were intended to prevent reverse flowwhen tide levels exceed ground level. In someareas, such as Bath Street and Wherstead Road,oversized pipes or storage tanks are includedto store runoff when rainfall coincides with hightidal conditions.

By 1939 the system had to be reinforced by theaddition of the High Level Trunk Sewer

(600–1500mm diameter) constructed on a roughlyparallel route to the north of the Low Level Sewer.This permitted development of the Croftsresidential area to the northwest of Ipswich.

Later, flows from villages outside Ipswich atBlakenham, Bramford and Claydon were pumpedinto the system. Storm water overflow sewers,(from the trunk sewers to the river), were added torelieve flooding. Even so, both trunk sewers floodduring severe weather, especially where they crossthe tributary valleys. The water then flows over landalong the valleys and watercourses towards thelowest parts of the town.

Many other sewerage improvements and additionswere made as the town expanded, the mostrecent being “Project Orwell” a £33million2.4m diameter tunnel and a series of pumpedtanks which provided further relief andreduced emissions from the overflow sewers tothe river/estuary.


Foul and combined flows from northwest andcentral Ipswich are finally pumped into the CliffQuay wastewater treatment works.

Much of the east of Ipswich drains via combinedsewers to either the “Eastern Area Trunk Sewer” builtin 1960, or the “South East Area Sewer” built in1983. As they enter the Cliff Quay treatment works,large storm overflow structures allow surplus flows tospill via screens to the Orwell.

There are now some 40 major outfalls through theflood defence walls into the Orwell or Gipping.Most have flap valves intended to prevent reverseflow and tidal flooding. Some of these are verylarge – twin 2.7m square flap valves at Stoke Bridgeand two pairs of 2.4m diameter flaps atToller Road.

The Anglian Water system in Ipswich now includes15 pumping stations, a further 4 pumped tanks,at least 6 attenuation tanks and an openattenuation pond at Ransomes Europark.The sewerage system serving northwest and centralIpswich is therefore complex.

Anglian Water (AW) uses “InfoWorks” computermodels to enable them to understand theoperation of the sewer network, and modelpossible improvement schemes in detail.Coverage of Anglian Water’s models is shown ona map in section 7.1

Much of the Chantry area, south of the river, isserved by separate foul and surface watersewerage systems. Surface water systems drain toBelstead Brook. Foul sewage is drained by gravityto Chantry wastewater treatment works. AW has anunverified model of the surface water system.

5.5 Highway or railway drainsIn a few areas of Ipswich, highway or railway drainsdischarge to watercourses. In the Dales Road areathe railway, in cutting, drains rural runoff from fieldseast of Henley Road towards Norwich Road.

Highway or railway drains are unlikely to be shownon Anglian Water’s sewer maps. Some have beenmapped – see plan in Appendix 12.3.

5.6 Sustainable Drainage Systems(SuDS) and soakawaysAs a result of policy changes during the last fewyears (PPG25, PPS25, the Ipswich Drainage andFlood Defence Policy and Building Regulations),SuDS, soakaways or attenuation systems havebeen increasingly used to reduce adverse impactson watercourses and the sewerage network.Examples are at the Park and Ride and AngliaParkway sites north of Bury Road and St Mary’sConvent. Areas of the town served by such systemsare recorded by Ipswich Borough Council.See plan in Appendix 12.3.

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In parts of Ipswich, soakaways are used for surfacewater drainage. These are usually the propertyowners’ responsibility. However some 82soakaways, adopted by the Highway Authority, areknown to exist and have been mapped, (see planin Appendix 12.3) and others probably exist.During the past few years many of the older ones,installed 1950–1970, have been found to be totallyinadequate and several have been replacedor enlarged.

Ravenswood, a 1200 home development currentlyunder construction, uses landscaped infiltrationbasins (SuDS), flood paths and soakaways forsurface water drainage – all designed to protecthomes from a 1 in 100 year rainfall event.These features do not affect the springs andwatercourses in Braziers Wood.

Some recent developments, located in low areas,where attenuation storage has been installed,have suffered from flooding because surchargingof the sewerage system prevents discharge at thedesigned rates. Anglian Water typically specifies anallowable discharge and designers erroneouslyassume the sewer has capacity, for that dischargerate, without surcharging.

Other recent developments have included low-level basement car parking or buildings belowwater levels (surcharge levels) that commonlyoccur in adjacent sewers. Some of these aresituated in flood risk zones. Private pumpingsystems are increasingly being used in an effortto avoid flooding of such low areas.

5.7 Overview of local sources offlood risk1

Ipswich Borough Council is unusual in having about30 years of detailed records of local floodingresulting from heavy rainfall, not attributed toovertopping of river or tidal defences.

As the town grew and more surfaces were paved,runoff increased. Flooding resulted and was oftensubsequently alleviated by drainage

improvements. Thus the oldest records are unlikelyto be of much significance.

Such flooding results mostly from surface runoff,overloading of piped systems, soakaways andordinary watercourses (ditches, streams or valleybottoms) or ground water.

Local flooding occurs much more frequently thantidal or fluvial flooding, generally with relatively lowconsequences, however repeated flooding cancause much distress and expense, especiallywhere floodwater (often with sewage) enters orcomes close to entering homes.

Records show homes have suffered internalflooding in 88 locations.

Basement and subway flooding has occurred.

Highways flood most often but the extent is difficultto define. Some roads become impassableregularly, for example Holywells Road andAncaster Road.

In a few locations, manhole covers are blown offfollowing heavy rainfall, sometimes along withroad surfacing, and foul debris is deposited onstreets. The open manholes represent a serioushazard to people, resulting in Councillors and MPsbecoming involved and petitions being received.

Recent changes in property conveyance practisesand insurance are believed to have resulted inunder reporting of flooding.

Many factors can influence this type of flooding,such as whether manhole covers are stuck andblocking of grilles on outfalls or gully grates.

Ground water and springs affect gardens in manyareas including Tuddenham Avenue, Spring Road,Springfield Close, Cavendish Street/Back HamletAllotments, Birkfield Drive, Heatherhayes, PembrokeClose, Lavender Hill, Coltsfoot Road, LavenhamRoad, Worsley Close, Manchester Road and RitaBrook Road. These are mostly at the crag/clayinterface (see geological map, Appendix 12.2)and associated with minor watercourses.


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1. Note, this section was written in 2010 at the start of the SWMP process


Currently the most serious local flooding problemsare at Lovetofts Drive, Daimler Close, SwinburneRoad, Norwich Road, Coltsfoot Road, Monton Rise,Bridgewater Road, Ellenbrook Road, Bixley Road,Hadleigh Road, Holywells Road, Duke Street andMaidenhall.

The most frequently flooded areas are the roadsaround the Wet Dock - Bridge Street, Key Street,College Street and Duke Street. However, the depthof floodwater is currently limited since it can easilyoverflow overland into the Wet Dock.

Major newsworthy flooding events occurred on 22occasions between 1976 and 2007.

Traditionally pipes were typically designed to run fullin a rainfall event with a return period of 1 to 2years. Modern designs are generally for no floodingin a 30 year return period. Local flooding isinevitable in extreme events which exceed thesedesign standards.

5.8 Overview of recent activitiesor projectsAnglian Water (AW) most recently completedsewerage flood relief schemes in Hadleigh Roadand Larchcroft Road (2007). Further improvementsare being considered at Meredith Road, LovetoftsDrive and Bridgwater Road and Ellenbrook Green.Such projects are normally triggered by floodinginside buildings, which occurs more often thantwice in 10 years. However, in any circumstanceprojects need to be cost beneficial.

Highway drainage schemes were recentlycompleted at Hadleigh Road, Bixley Road,Campbell Road, Birkfield Drive, Hawke Road andWhitton Church Lane.

A prioritised programme of outstanding highwaydrainage works is included in Appendix 12.5.

A highway maintenance operational planspecifies maintenance programmes.These include cleaning road gully pots andsoakaways. Pots are routinely cleaned everynine months and soakaways annually, at present.Gully connection pipes and footwaydrains are maintained as and when complaintsare received.

Grates are cleaned when Ipswich BoroughCouncil Road Space sweeps roads. The frequencyvaries with location and time of year.More cleaning is required in areas with treesduring the autumn. The drainage team hasprovided maps indicating high-risk areas wheresuch road sweeping should be focussed (seeAppendix 12.4). Recently advanced warnings fromthe new National Flood Forecasting Centreenabled pre-emptive cleaning of grates incertain areas.

Highway projects such as speed humps or raised‘tables’ can divert surface water flows intoproperties. Resurfacing can raise carriagewaylevels causing similar but more widespread affects.On the other hand small changes in design mayenable improvements to flood risk management.These could involve extra gullies, raised kerbs orlowered carriageway levels.

A map showing current highway and boroughcouncil community improvements programmedfor implementation in 2010/2011 is included inAppendix 12.5.

Ipswich is included in the Essex and SouthSuffolk Shoreline Management Plan (SMP).The consultation draft dated 12 February 2010confirms the policy of “Hold the Line” upstream ofthe Orwell bridge (West bank) and the Cliff QuaySewage treatment works (East Bank).

The £70 million Ipswich Flood Risk ManagementStrategy (FDMS) is the Environment Agency’s 100-year plan for tidal and fluvial flood defences.This will benefit surface water drainage in the floodplain once the tidal barrier is installed – expectedby 2016.

Ipswich Borough Council operates a Drainage andFlood defence Policy (13) that explains theborough’s current flood risk management activities.These include development control in accordancewith PPS25 and promotion of SuDS, followingstandards set in the policy. The Borough Council’spolicy was developed following widespreadconsultation and approved by the Council’sExecutive Committee in 2001.

Ipswich Borough Council’s Strategic Flood RiskAssessment (SFRA) recommended risk associated

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with minor watercourses is managed throughvarious Local Development Framework policies:

l Known ordinary watercourses and majorfloodpaths have been mapped (no assessmentof flows has been undertaken).

l It is the Council’s intention (6) to inspect openwatercourses from time to time and ifnecessary use its powers to ensure they arekept clear by their owners.

l Ipswich Borough Council’s Drainage and FloodDefence policy (6) will “not permit drainage ofsurface water into land drains or pipedwatercourses unless they have beenconstructed to an acceptable standard andhave adequate capacity.”

l The Environment Agency’s consent is required topipe in watercourses2. Their policy is that this isnot normally permitted. Ideally they should beretained as open space, encouraging wildlife.

Further mitigation measures are suggested asrisks associated with watercourses are expectedto increase:

l In the LDF Green Corridors should includewatercourses sited in gardens or open spaces.

l The existing embankment which retainsHolywells Canal adjacent to LDF sites 70 and 44is in poor condition and is not owned by theCouncil. People and property downwhill,including users of Holywells Road, will be at riskshould a breach occur. It should bestrengthened to adequate standards beforeother works on either site commences.

l Where spring fed watercourses discharge intothe sewerage system, the abstraction and useof the water for irrigation could reduce sewerflows and so provide several benefits – savingmains water as well.

IBC’s drainage engineering team currentlyresists any plans to raise paving levels aroundthe Wet Dock.

Recently the Planning Inspectorate (Nov 2011)approved the Borough’s Local DevelopmentFramework (LDF) (13). The Core Strategy andPolicies Development Plan Document includepolicy DM4 which confirms the council will applythe PPS25 hierarchy3:

l Assess flood risk

l Avoid flooding

l Substitute

l Control – using SuDS and implement SWMP tomanage flood risk.

Paragraphs 9.32 to 9.42 of DCM4 refer to theStrategic Flood Risk Assessment (SFRA), SWMP,the Ipswich drainage and flood defencepolicy, the future national SuDS standards andsafety of developments in flood risk areas.

Where developments are affected by mappedflooding shown in the SFRA (the EnvironmentAgency’s areas susceptible to surface waterflooding is currently included in the SFRA), a floodrisk assessment is required even if the developmentis less than 1 hectare.

Other relevant controls relate to basements andraising of ground levels around the wet dock.Specific relevant extracts are provided in section10.1 of this report.

5.9 Overview of future floodingFactors likely to increase flooding includeclimate change and ongoing increases inimpermeable areas which are typically due tohouse extensions and paving of gardens. Some ofthis increase may be controlled by theimplementation of controls on paving of frontgardens. Conversely flooding may locallydecrease when major sewerage or drainageimprovements are made.

There has been a growing trend to pave vergeswhere they become damaged by parkingvehicles. This can have a major effect on local


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2. Since April 2012 the County Council has taken over responsibility for consents in ordinary watercourses. The Environment Agencyretains responsibility for consents in main rivers

3. Now incorporated into the technical guidance associated with the National Planning Policy Framework


flooding, especially where large areas offront gardens have also been paved and soilsare permeable.

The Borough Council’s drainage and flooddefence policy has set standards for SuDS since2001 which generally mean new developments willnot increase flood risk (for events with an AEP >1%).Thus there could be an increase in the severity offlooding for rarer, more severe events.

However, where soakaways are used for brownfield sites, which previously drained surfacewater to the sewerage system, there should bea reduction in flood risk.

When the relevant section of the Flood and WaterManagement Act has come into force, theresulting National SuDS Standards (determiningpeak discharge rates and preferential use of use ofinfiltration type SuDS) are expected to be similar tothose currently applied by Ipswich BoroughCouncil. These National Standards are thereforelikely to have a similar effect on local flood risk.

Increasing sea levels (resulting from both climatechange and isostatic change) may increaseflooding from sewerage systems that drain surfacewater from the lowest parts of the town into thetidal Orwell. When tide levels are above the soffitoutfall pipes the hydraulic gradient, and hencecapacity of drainage systems serving the lowestareas, is reduced. If the tide exceeds upstreamground levels then discharge to the Orwell isnot possible.

The operation of the proposed tidal barrier at theNew Cut will help mitigate this effect for sewers thatout fall upstream of the barrier.

A further improvement would result if the barrierwas raised at low tide in advance of expectedpluvial events when predicted by the newEnvironment Agency/Met Office floodwarning service.

However, the performance of sewers draininginto estuary downstream of the barrier atWherstead Road will reduce unless futureimprovements such as the addition of storagecapacity are implemented.

Growth in population would increase foul flows andhence flood risk for combined systems but waterefficiency measures and polices such as meteringcould reduce the trend for increasing waterconsumption and sewage production. With thegrowth in population forecast for Ipswich, theoverall affect on flooding and pollution is likely tobe neutral.

Increasing sea levels will increase the risk of groundwater flooding in lower areas. Some isolated lowareas have been identified close to theGipping at Yarmouth Road and Gatacre Roadwhere ground levels are below between 3.8m and3.4m AOD

Retro-fitting of infiltration type drainage for existingdevelopment may increase the risk of groundwater or minor watercourse flooding in some areas.

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6.1 Establishment ofthe partnershipThe Suffolk Flood Risk Management Partnership(SFRMP) was set up in 2009. It comprises: SuffolkCounty Council, all district/borough councils,Internal Drainage Boards, Anglian Water, theEnvironment Agency, Suffolk Resilience Forum,Highways Agency and the Broads Authority. There isalso a joint flood scrutiny panel comprising districtand county councillors and supporting officers inthe county.

A sub-group of the SFRMP comprisingrepresentatives from Suffolk County Council,Ipswich Borough Council, the Environment Agency,the East Suffolk IDB and Anglian Water was set upas a Project Board in December 2009 to overseethe management of the Ipswich SWMP.

The Board was managed by Jane Burch (SuffolkCounty Council’s Flood and Coastal PolicyManager). Board members report to and representtheir organisations. The Board oversees the SWMPwork and reports to the full SFRMP and ScrutinyPanel. The SFRMP will continue to manage andmonitor the plan once in place.

In September 2009, at Suffolk County Council’srequest, Ipswich Borough Council agreed tomanage the production of the SWMP. Funds wereprovided by Defra.

6.2 Ipswich SWMP aims as agreedby the Project Board 1. To provide an improved understanding of the

surface water flood risk in Ipswich.

2. To produce a detailed plan of action tomanage surface water flood risk for high-riskareas, and a less detailed plan forremaining areas.

3. To develop practical experience and guidelinesfor undertaking SWMPs, for the benefit of otherpartners in Suffolk.

4. To implement some practical flood alleviationwithin existing budgets or using funding fromexternal sources.

5. To provide information for use in spatial andemergency planning.

6. To replace/update Ipswich Borough Council’sDrainage and Flood Defence Policy(in conjunction with National SuDS Standards).

7. To provide public information and advice onflood protection to improve customer service.

8. To seek solutions that help deliver the objectivesof the Water Framework Directive.


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6 Preparation

Flooded garden in Ipswich


6.3 Information held by Ipswich Borough Council

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Format Notes

Flood database pre 2000 Vp Info/ CSV Carefully designed database

Flood database post 2000 Excel

Indicative local flood map MapInfo GIS tab Polygons outline max recordedextent of floods, no indicationof return period

Internal and externalflooding points

MapInfo GIS tab Produced for SWMP

Paper Files – flooding Paper 6 lever arch files

Flood pictures JPG

Flood files Word, excel, outlook files in folders

Various Drainage plans Tif Scanned in 2007/8

Public sewer maps 1882,1950’s, 1992

Tif Scanned in 2007/8

Locational Sewer surveydata (sewer manhole cards)

Tif Scanned in 2007/8

Sustainable DrainageSystems catchments

MapInfo GIS tab

Sustainable DrainageSystems designs

Excel spreadsheets, paperfiles/plans/planning applicationdrawings and calculations

Soakage Test results MapInfo GIS tab IBC’s tests and extracted fromplanning applications

Highway carrier drains MapInfo GIS tab Does not include gullyconnection pipes

Highway Gullies MapInfo GIS tab (+ now visibleusing Google Earth)

Some critical gullies mapped

Planning applicationdocuments

Anite Database, and paper files Will include drainage designsfor larger developments

Highway Drainage schemes Paper and electronic files

AW’s sewer map 2009 MapInfo GIS tab IBC internal Intranet/GIS

AW water mains 2009

6.4 Data and informationavailable from otherpartner organisations Anglian Water supplied public sewer records andoutputs from their hydraulic models.

6.5 National data LiDAR and ASTWF from the Environment Agency.

During the process of developing the SWMP theEnvironment Agency published its Flood Map forSurface Water (FMfSW). This was only used forcomparison purposes. It is recommended that thisdata is the starting point for future SWMPs.

6.6 Public information Anglian Water public sewer map.

Ipswich Borough Council indicative watercourseflood map.

Ipswich Borough Council Strategic Flood RiskAssessment, level 2.

Suffolk County Council’s Preliminary Flood RiskAssessment was prepared during the course ofdeveloping the SWMP – informed by the Phase 1SWMP report.

The Suffolk Local Flood Risk Management Strategywas in production during the later part of theSWMP process.

6.7 Systems for storing andsharing flood risk data, mapsand information Collated flood record extracts and data relatingto priority assessment is contained in anExcel workbook.

During the course of the study a web basedSharePoint was set up on the Borough Council’sserver to aid data transfer between partners.

6.8 Quality assurance, securityand licensing Ipswich Borough Council’s engineering team hasachieved accreditation to ISO 9001:2008 andpassed the surveillance audit for ISO 14001:2004.

Data provided by Anglian Water is subject to aLicence Agreement.

Spot checks using audit trails have beenundertaken on collated flood data – see theworkbook.

6.9 Preliminary assessment offlooding across Ipswich4

6.9.1 Historic tidal and fluvial flooding

Tidal and fluvial flooding is relevant since high riverlevels can aggravate operation of surface watersystems. Minor flooding may occur from thedrainage systems when tidal flaps leak in dryweather when the tide exceeds ground level.


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Format Notes

Gas, electricity

Priority Area Assessment andcollated historic flood data

Excel work book

Guidance on SWMPs

4. This section was written in 2012 to inform the Suffolk Preliminary Flood Risk Assessment.


Rarely will flash flooding occur at the same time astidal/fluvial flooding. Research has shown there islittle or no correlation between fluvial flows andsurge tides for East Anglia. The Ipswich Tidal BarrierReport April 2009 addresses this issue.

The most recent serious tidal flood was in 1953.A plan in Appendix 12.7, copied from IpswichBorough Council’s contemporary paper record,shows the extent mapped against 1950’sbackground Ordnance Survey plans.

The level 2 Strategic Flood Risk Assessmentdescribes and maps the extent of tidal flooding forvarious tide levels and defence failure scenarios.

The most recent severe fluvial events were in 1947and 1939. These were partly caused by flooddebris that obstructed the old Seven Arches Bridgeat London Road. The current replacement bridge issingle span and no longer obstructs the flow.

It appears that during these events, floodwaterfollowed the original path of the River Gipping(it was filled in 1882) through the Ipswich Villagearea, and spilled across Bridge Street into the WetDock at Albion Wharf. Floodwater was reported tobe five feet deep in Princes Street and cars wereswept away.

Contemporary paper record plans showing the1939 and 1947 floods on relevant OS surveybackground are reproduced in Appendices 12.8and 12.9.

6.9.2 Historic pluvial or local ‘flash’ flooding

The extent of known local flooding has beenmapped by Ipswich Borough Council – seeAppendix 12.12.

Flooding is only shown where repeated complaintsare received that does not appear to be due toblocked road gullies. The map shows 88 locations,the extents of flooded areas are based oncontours, photographs and reports (not generallyLiDAR). Reported incidents are monitored.Between 2001 and 2009 annual numbers rangedfrom 68 to 200 with no apparent trend.

No indication of frequency is provided on the map,however since the flooding has occurred and byinspection of the records and newspaper cuttings itis regarded as ‘likely’ - typically occurring with returnperiods between less than 1 year to 25 years.

Flooding particularly affects buildings lower thanadjacent roads, especially basements andsubways. These are not shown on the map.Some have been fitted with flood boards, non-return valves or pumps in an effort to alleviatethe problem.

Ipswich Borough Council has 3,150 detailedrecords of flooding incidents, including one fromthe 1950s. Older records include paper plans.692 photographs and some videos also providevaluable evidence. All relevant records are now inExcel and GIS format. Records are kept wheneverflooding is reported (small puddles due to blockedhighway gullies are not included).

Some records have been added following onsite,post-flooding investigations. These often includedoor knocking or fire service records of attendance

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1939 Floodwater from river Gipping, spillinginto wet dock at Albion Wharf

1939 Floodwater in Princes Street


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at flooding incidents as well as media coverageand questionnaires. These show flooding is oftennot reported and people seem to move housemore often in flood risk areas.

The database structure was carefully planned anddeveloped back in 1988. An incident is defined asone property flooding in one event. Thus twoproperties flooding in one event would count astwo incidents. Similarly one property flooding in twoevents would be two incidents.

A distinction is made between internal and externalflooding. Internal flooding is likely to causedamage, external flooding is usually a temporaryinconvenience but it can lead to anxiety and,especially if foul sewage is present, health risk.Past work provides some evidence of a spatial linkbetween deprivation and flooding.

As part of the SWMP process these records havebeen extracted and copied to an Excel workbook– see Appendix 12.11. Incidents due to fouldrainage blockages have been removed andrecords sorted by drainage area.

The workbook shows where recent flood reliefprojects are believed to have reduced flood riskand denote which records were removed toaccount for this. It is important to note that pastflood relief will not stop flooding, just reduce thefrequency. Typically newer projects are designed toreduce flooding to 1–10% annual exceedanceprobability (AEP).

Anglian Water supplied records of flooding that donot include full addresses (due to confidentiality).A few records have been included as shown onthe worksheet.

A GIS map showing the location of the finalhistoric internal and external incidents is includedin the appendices. The plan also shows locationswhere manhole covers are known to blow duringheavy rain.

The final workbook shows relevant historic floodingat about 500 different addresses.

No attempt has been made to allocate returnperiods (frequency) to the flooding data.

The most severe events tend to be localised. That isheavy rain most often affects only small areas, and

the intensity of moving storm cells does not remainconstant as these build and collapse.

Rainfall records needed to assign return periodswould have to be based on a network of gaugesthat record intensity every minute or so.Weather radar makes such analysis a possibilitybut is not realistic at present.

Sewer model verification typically involves theuse of perhaps 10 rain gauges being set up fora period of several months. These are very unlikelyto catch a severe event.

6.9.3 Historic flooding of critical infrastructure

There are no records of flooding affectingcritical infrastructure.

6.9.4 Historic groundwater flooding

The council has recorded locations wherecomplaints have been received about localflooding, watercourses, areas susceptible tosurface water flooding and groundwater floodingmap in Appendix 12.12.

6.9.5 Future flood risk

Areas susceptible to surface water flooding mapsare a useful guide to where extreme or futureflooding might occur and highlight naturaldrainage paths.

These maps ignore the presence of undergrounddrainage altogether. Secondly, it relates to onlyone rainfall event – a 0.5% AEP (annualexceedance probability) of 6.5 hour duration.Shorter or longer duration storms may cause moreor less flooding at different locations.

During the course of the development ofthe SWMP the EA published its Flood Mapfor Surface Water. This takes account ofthe effect of buildings and makes anallowance for underground drainagesystems. These were not used for theSWMP development but are comparedwith SWMP flood maps.

Ipswich Borough Council’s indicative local floodmaps show a much smaller extent of flooding thanthe areas susceptible to surface water flooding


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mapping. This is to be expected due to theseverity of the event modelled – an event unlikelyduring the past 30 years, and also because ofsome of the assumptions the current (2009)mapping makes.

In an urban situation such as Ipswich, where thereis a well developed, large capacity undergroundsystem, it is likely there will generally be much lessflooding than shown on the areas susceptible tosurface water flooding maps.

To assess extreme flooding, GIS was used tohighlight Ordnance Survey address points within‘intermediate risk’ areas susceptible to surfacewater flooding. These are mapped and shown onthe plan along side historic flooding.

4,429 address point where found to be within theintermediate mapping.

8,857 address points lie within ‘less susceptible risk’areas on the areas susceptible to surface waterflooding map. Most of these would be protectedby thresholds /floor levels being above ground level(depth of water for less is >100mm).

Bearing in mind the above factors and the beliefthat flooding is under reported, the conclusion isthat the actual number of addresses likely to be“at risk” in severe events was estimated at between500 and 5,000.

Note: these figures are superseded byestimates in section 7.5

Future flooding of critical infrstructure

At present Ipswich Borough Council has no recordsof flooding affecting the operation of electricitysub stations, hospitals, fire stations or emergencyrest-centres however 29 were found to be withinthe ASTSWF.

Future developement

Locations and areas for planned futuredevelopment are from the March 2010 LocalDevelopment Framework (LDF) submissions stage.Surface water from these is likely to be drainedSuDS and so development is unlikely to affectdownstream flooding.

However, where new developments are withinASTWF, there is a risk that new buildings could beflooded by floodwater originating off the site.

Based on LDF proposals there could be 35 Ha ofdevelopment site within intermediate ASTWF areas.Assuming a density of 30 Units/Ha this would meanthat there could be up to an additional 1050addresses at risk.

At present Ipswich Borough Council plans tomanage this flood risk issue following the principlesin PPS25: where a development site is within anASTSWF, the developer should be required toproduce a flood risk assessment (FRA), which mayinvolve using SWMP output if it exists. Alternativelythe developer might be required to undertakedetailed modelling which could be used toupdate the SWMP.

Climate change is expected to increase risks.PPS25 provides guidance (summarised in theStrategic Flood Risk Assessment) on allowances tobe made when planning developments in floodrisk areas. Sea levels are predicted to rise about1m over the next 100 years and peak rainfallintensities increase by 30%.

6.9.6 Results of preliminary Ipswich-wide assessment

Historic records indicate circa 500 properties are atrisk. ASTWF show 4429 at intermediate risk.

Between 500 and 4,400 properties may be at riskof surface water flooding in severe events.

Note: these estimates are superseded byfigures in section 7.5

Unless controls are maintained circa say 1,000additional properties in 35 Ha of planneddevelopment may become at risk.

Flooding from blown manholes is known at12 locations.

More detailed flood risk assessment was proposedas follows:

6.10 Identification of levelof assessment To ensure major elements of the SWMP action planare cost effective it is necessary to firstly estimatethe costs of damage caused by flooding in a ‘donothing’ scenario. Damage costs are defined bya graph, gradually increasing as the severity ofrainfall and flooding increases.

The effectiveness of a particular flood riskmanagement option or group of options is thenassessed by determining the frequency anddepths of flooding with flood risk managementmeasures and re-calculating damage costs.The benefit – cost ratio is the ratio of the damagecost prevented by the option divided by the costof implementing that option.

Revenue (maintenance) options can be includedif the approach considers expenditure anddamage over a long time period, by using NPVs(Net Present Values).

For a flood risk management measure to beincluded in the action plan it will need to havea benefit-cost ratio great than 1. Flood riskmanagement measures may be refined improvingthe benefit-cost ratio by considering differentstandards of protection (varying the designfrequency of flooding). There are currently no fixedflooding standards for SWMPs.

Historic records of flooding cannot be used fordamage costs estimation, as there is insufficientdata to establish storm frequency and they will nottake into account climate change or other factors,and the effectiveness of flood risk managementoptions will not be known.

It is therefore necessary to use appropriatemodelling techniques to establish flood depths,frequencies and extents and then damage costs.

Because modelling and associated survey work isresource intensive and resources limited, it wasnecessary to focus detailed assessments in certainpriority areas.

In some other SWMPs, priority areas for furtherassessment have been decided simply byinspection of maps showing where flooding has

been recorded. However for Ipswich, the largenumber of records makes it difficult to decide onthat basis.

6.11 Prioritisation of areas fordetailed assessmentAn Excel workbook was developed which alsoprovides a framework for future use. The workbookshould be seen as an aid to make the finaldecisions on priorities for further assessments.The results of the subsequent detailed assessmentsenable a more accurate prioritisation of drainageareas based on flood risk.

The process aims to:

l Focus detailed assessment in areas where it ismost cost effective – i.e. where flood risks aregreatest, costs of the study are lowest andpotential benefits are highest.

l Start detailed work on a relatively simple andsmall area not requiring sewer model data.

l Develop a methodology which may be usedacross Suffolk

The Ipswich area was divided into 34 catchments(drainage areas) based on topography /watercourses. Flood risks, assessmentmethodology and hence study costs are differentin each area.

Criteria, which broadly and simply describe floodrisk, have been identified and are describedbelow. These are effectively the drivers for moredetailed future assessment.

Within each drainage area, each driver is scoredand assessment costs estimated. Scores allocatedto each driver are adjusted by the use ofweightings applied consistently across all areas toallow for the varying significance of thecrtieria/driver. A high degree of judgement wasnecessary and reached after discussion amongstBoard members. Some of the reasoning behindthe weightings used is discussed below.

The variation in costs of detailed assessment is likelyto broadly reflect the variation in costs of flood riskmanagement. This is because both are expectedto depend to a large extent on the size of the area


31


within which most of the flooding occurs and whichneeds to be modelled to determine the flood riskmanagement measures and damage costs.

Final scores and priorities are based on the ratio ofdrivers to cost of the detailed assessment.

6.11.1 Criteria for describing flood risk – drivers

A wide range of criteria has been carefully chosento cover the range of risks and consequencesexperienced in Ipswich. A simpler approach wasconsidered at first however this resulted in someinconsistent priorities. The method suits the datareadily available for Ipswich.

Data availability will influence how priorities can beassessed in the future across Suffolk, however theworkbook can be easily adapted by changingthe weightings. For example if long-term historicflood records do not exist in all drainage areasthen weightings for historic data would need tobe reduced and weightings for the number ofaddresses in areas suceptible to surface waterflooding increased.

As SWMP work progresses and data sharing isestablished, priorities might be reconsidered,perhaps using predicted flood volumes from thesewerage model or using the EnvironmentAgency’s more recent flood map for surface waterrather than the areas suceptible to surface waterflooding map.

The workbook includes:

l Number of addresses within areas suceptible tosurface water flooding.

l Future development site area.

l Number of critical infrastructureinstallations within areas suceptible to surfacewater flooding.

l Historic highway flooding (not blocked gullies).

l Historic property flooding incidents – internal.

l Historic property flooding incidents –external (gardens).

l Whether foul sewage debris is deposited ingardens or in streets.

l Consequences (varies from inconvenience tobuilding/contents damaged/cars damaged/road traffic accidents).

Cost of assessment depends on:

l Area to be surveyed or modelled.

l Degree of interaction between tidal, fluvial,sewerage, highway drainage, SuDS.

l Assessment method.

6.11.2 Data details and weightings

Number of addresses within areas susceptibleto surface water flooding areas – The numberof addresses within the areas suceptible to surfacewater flooding map was the basis of Defrafunding allocation and is useful where there are nohistoric records or where there has been no recentmajor rainfall.

There is more certainty attached to historic recordsthan to addresses in areas suceptible to surfacewater flooding. Taking the above issues intoaccount a much higher weighting is applied tohistoric flooding as described below (about 10times higher).

The scoring/weighting applied means this driveraccounts for 3% of the total driver score.Including this criterion may allow, to some extent,for climate change.

In areas outside Ipswich, where there is nounderground drainage, the weighting foraddresses in areas suceptible to surface waterflooding could be increased, as this becomesa more accurate indicator.

Areas of future development sites – Locationsand areas are from the March 2010 LocalDevelopment Framework (LDF) submissions stage.These will be served by SuDS and are unlikely toaffect downstream flooding. However, where newdevelopments are within areas susceptible tosurface water flooding there is a risk that newbuildings could be flooded by floodwateroriginating off the site. For this reason only siteswithin areas susceptible to surface water floodingare viewed as drivers.

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The weighting used means this driver accounts for3% of the total driver score.

Number of critical infrastructure installations –At present the council has no records of floodingaffecting operation of these sites so a lowweighting has been applied. The weighting usedmeans this driver accounts for 3%.

If historic records had indicated large numbers ofproperties were affected by flood induced failureof critical infrastructure then the weighting wouldhave been increased.

Historic highway flooding – the Borough Council’sGIS maps show areas commonly affected but donot show flood depths. For the last few years atleast £100,000 per annum has been spentalleviating highway flooding to varying standards.A simple approach is used based on localknowledge, supported by records where a score of1-3 is applied depending on whether any roadssuffer minor flooding (score 1) or are impassable ordamaged (score 3). The weighting used meansthis driver accounts for 6% of the total driver score.

Historic property flooding incidents – Dataprocessing is described in 6.9.2

Data from the first sheet of the workbook(see appendix 12.13) automatically fills theappropriate cells of the second sheet – the priorityscoring sheet.

Scores for external flooding account for 22% of thetotal driver score and scores for internal floodingaccount for 43%.

In areas outside Ipswich, where there is nounderground drainage, the weighting foraddresses in areas susceptible to surface waterflooding could be increased, as this becomesa more accurate indicator.

Foul sewage debris deposited – Much of theIpswich sewerage system is combined i.e. it carriesboth foul sewage and surface water. In somelocations floodwater deposits sewage debris ingardens, on verges, and on roads. This causesmuch distress and complaint and a potentialhealth hazard. Flood records indicate whether foulsewage is involved. However the scores express ourexperience of complaints and how much debris is

left. Score 0 for no sewage, 1 for little visible signand 3 for large amounts of sludge or obvioussewage items.

Scores for foul sewage debris deposited accountfor 8% of the total driver score.

Consequences – This was added to take accountof any potentially very damaging floods – i.e. deepwater and areas where high damage costs arepossible. An example is Coltsfoot Road wherefurniture and carpets are ruined every few years, orPortman Road where the football pitch is thoughtto be at risk. Score 1 for low consequences and 3for high.

Scores for this criterion account for 9% of the totaldriver score.

The following factors affect the cost of undertakingdetailed assessments.

Area to be surveyed and/or modelled – This isassumed to be the major influence on cost of thestudy. A GIS map has been developed whichassumes the area to be surveyed and modelled isthe intermediate ASTWF and ignores areas remotefrom known flooding locations. The area to bemodelled in each drainage area was multiplied by£2,000 to give a comparative cost of surveyingand building models.

In one location (Lovetofts Drive) a 2-dimensionalmodel was thought to exist and the costs ofbuilding this has been manually overridden.In another, (Chantry park) a simple ISIS or Infoworksmodel could be used, and again costs have beenreduced manually.

No account has been made here for running themodels, as a similar set of simulations is required foreach drainage area.

Across the 34 drainage areas the estimatedmodelling and surveying costs range from £2,000to £128,000. These figures were subsequentlyadjusted as described below:

Degree of interaction between tidal, fluvial,sewerage, highway drainage, SuDS and easeof assessment of potential flood riskmanagement measures – High water levels inrivers or ordinary water courses coincident withheavy rainfall may cause temporary backing up


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along drainage systems increasing flooding.Sewers may surcharge with similar affects onhighway drainage. SuDS may overflow and addflows to downstream drainage systems.Conversely some flooding may relate toinadequate highway drainage.

Where such interactions occur there will beadditional modelling work required and costs areincreased by a varying percentage up to 30%.

At the initial stage there was a high degree ofuncertainty in modelling costs and seweragesystem model availability. Models can sometimesbe sensitive to small changes in input dataresulting in instabilities and errors that can be timeconsuming to resolve.

Across the 34 drainage areas the final estimatedmodelling and surveying costs ranged from£2,066 to £150,000. These costs are forcomparison only and should not be usedfor budget purposes.

Costs and benefits of flood risk managementmeasures – Cost benefit ratios guide the finalpriorities of flood risk management measures.However, these require information that can onlybe gained by modelling. Even so, the idea ofincorporating cost benefit ratios at this stage of thepriority assessment has been considered:

Back in 2003 Ipswich Borough Council producedsome “heroic” cost estimates for alleviating orproviding temporary emergency flood protectionfor the known historic flooding locations shownon the indicative local flood map. These floodareas are much smaller than the areas susceptibleto surface water flooding. The estimates werebased on judgement alone and did not accountfor additional properties that may be at risk in thefuture or a pre set standard of performance.

Typically the cost for alleviating flooding in onearea was £1 million, whereas the cost of providingtemporary flood barriers was only a few £1,000s.Each option would provide a very differentstandard of protection.

The exercise illustrated the magnitude of total costneeded to substantially reduce flooding bytraditional methods, circa £24 million, and

explained that some of the long standingproblems remained because only a few propertieshad been positively identified to benefit froma traditional flood relief solution. This is typicalof a risk-based approach to floodrisk management.

Anglian Water’s flood relief projects are typicallyjustified if it costs less than £100,000 to alleviateinternal flooding that has occurred at least twice in10 years.

No further attempt at cost benefit analysiswas made.

Thus there are a very wide range of flood riskmanagement costs and standards ofperformance and insufficient data on floodfrequency and damage costs.

The idea of assuming damage costs of say£30,000 per historic internal flooding incident wasconsidered, however without flood frequencyinformation before and after flood relief, thebenefits could not be estimated.

The final worksheet is included in appendix 12.13.

6.11.3 Use for other SWMPS

The table below shows the final priorities and howthis varies depending on availability of historic dataand weightings applied.

The workbook has been tested for use where thereis little or no historic flood data by settingappropriate weightings to zero to simulate thefollowing scenarios:

1 no records for internal or externalhistoric flooding.

2 no records for any historic flooding

3 no records for any historic flooding but theweightings for number of addresses in areassuceptible to surface water flooding set at 50%;critical infrastructure in areas suceptible tosurface water flooding set at 33% and futuredevelopment in areas suceptible to surfacewater flooding set at 17%

The scoring system and resulting prioities wereagreed by the Board members in July 2010.

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6.12 Priority areas fordetailed assessment The table below shows the resulting priorities fromthe workbook. The top 10 have been reconsideredand adjusted to bring forward priority areas where:

l modelling is largely independent of data fromAnglian Water.

l a simple approach could be taken.

l a large amount of appraisal work has alreadybeen undertaken.

l it was considered likely that real improvementscould be implemented.

The scoring system and resulting prioities wereagreed by the board members in July 2010.

6.13 Brief description of top10 drainage areas fordetailed assessment1. London Road to Lavenham Road and

Hadleigh Road: Runoff from the dualcarriageway drains to SuDS in Chantry Park,which can overflow across the park into a pondand watercourse which drains into the seweragesystem. The likely solution here is cheap andprobably achievable within 12 months, and willreduce peak flows into the sewerage system.The risk of the sewer backing up will need to beconsidered. The watercourse, in a strip of land ofunknown ownership, needs maintenance.Records indicate flooding has beenreported twice.


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2. Ancaster Road/Burrell Road: The most recentevents affected a small, very low area ofhighway which floods from surface water andcombined sewerage systems and runoff froman unadopted road. Floodwater overflows intoa low-level car park under a new residentialblock. The likely solution is a new surface watersewer to adjacent river. The SWMP couldinfluence planning of new buildings or thesurfacing of unadopted road. There is aninteraction between surface and tidal waters.Anglian Water has a model of undergroundsystems but not the surface water drainageserving Burrell Road. There are many floodrecords for Burrell Road, but none since 1988.The reasons for this is unknown andresidents/owners may need to be contactedto verify if any solution has been implemented.

3. Lovetofts Drive to Lagonda Drive: Floodingfrom the combined sewerage system since atleast 1977. Anglian Water had a premlimary 2-d model. Recent development close by isnominally designed to allow floodwaterthrough. A range of partial relief measures arepossible. However, since at least 1987 anAnglian Water project has not been costbeneficial. A petition from residents to the MPwas received in respect of flooding here.

4. Worsley Close/Ellenbrook Green: Surfacewater flooding. Anglian Water has a model ofthe local underground surface watersewerage system and has supplied someflood predictions and undertaken a recentappraisal. Ipswich Borough Council hasconducted a door-to-door survey. The localresidents’ group submitted a petition about theflooding issue. There are several possible floodalleviation measures.

5. Swinburne Road to Bramford Lane:Combined sewer flooding since at least 1956.It requires water company investment but haslacked priority. Both the Borough Council andAnglian Water have attended residents’group meetings

6. Coltsfoot / London Road / Campion Road:Surface water sewer flooding since 1960s.Anglian Water has a model of the system there.

7. Portman Road area: Recent shallow floodingwith combined sewage has affected thehighway, part of the Ipswich Football club andtwo garages. This area is very low lying and iscrossed by the low level trunk sewer. The areais served by surface water sewers, which drainvia non-return valves into the tidal River Orwell.The lowest parts are parkland and these drainto a minor watercourse. A simple LiDAR-based‘InfoWorks’ ground model has been built byIpswich Borough Council to look at drain-downtimes for tidal or fluvial floodwater as part ofthe Strategic Flood Risk Assessment.

8. Maidenhall Approach / Rapier Street /Belstead Avenue / Wherstead Road:Combined sewer flooding since at least the1960s. Lacked priority for Anglian Waterinvestment.

9. Chesterton Close / St Catherine’s Court:Surface water flooding. Anglian Water believesthis is not due to sewer flooding. Availability ofmodels unknown. Some simple flood riskmanagement measures have been identified.

10. Belstead Road / Lanercost Way: Floodinghere seems to relate to surcharging of thesurface water sewer resulting in occasionalinternal flooding. More usually affects theroads and gardens. Sporadic reports indicateflooding of highway has occurred 3 timessince 2000. Blockage of outfall grille or flapmay be a contributory factor. Unlikely thatAnglian Water has a model of this systemwhich drains to Belstead Brook.

6.14 Planning thedetailed assessmentPlanning carried out between July and October2010 included a series of technical meetingsbetween Ipswich Borough Council’s drainageteam, Anglian Water and their consultant,Clear Environmental.

The general methodology was approved by theProject Board on 1 October 2010.

Estimates indicated there was likely to be sufficientresource available to undertake detailedassessments for the top 4 priority areas:

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1. London Road to Lavenham Road andHadleigh Road

2. Ancaster Road / Burrell Road

3. Lovetofts Drive to Lagonda Drive

4. Worsley Close / Ellenbrook Green

Anglian Water confirmed that they agreed with thetop ten drainage areas prioritised for detailedassessment. These are mainly areas where thewater company is already aware of flooding andtheir existing hydraulic models are largely suitablefor looking at local flood relief.

However, Anglian Water did not have a model ofthe London Road to Lavenham Road andHadleigh Road system. It was agreed IpswichBorough Council would develop a model forthis area.

Anglian Water agreed to fund and undertake workfor areas 2, 3 and 4 above. Model developmentwould include the addition of 2-d meshes and theinclusion of features that did not belong toAnglian Water (e.g. major SuDS, watercourses,ponds and highway drainage networks) wherethese influence flooding.

Consideration was given to the management ofdata. Data for modelling features such as SuDSwas available from a variety of records held byIpswich Borough Council. These could either havebeen supplied directly to Anglian Water or suppliedas ‘mini models’, which might also double asworking records.

The Floods and Water Management Act requiressignificant assets to be recorded. There will be anincreasing amount of non-water companydrainage assets such as SuDS which need to berecorded and included in models.

In addition, the SWMP aims included deliveringSuffolk wide benefits and learning.

The decision was therefore made to obtainInfoWorks ICM (integrated catchmentmanagement) to model and record the assets forthe London Road to Lavenham Road andHadleigh Road system. Both the hardware andsoftware sit within Ipswich Borough Council but arethe property of Suffolk County Council and will also

be available for future use across the Suffolk FloodRisk Management Partnership.

Resulting model data could be included in AnglianWater’s models simply by ‘copying/pasting’.Similarly Anglian Water’s assets could be includedin the InfoWorks model if necessary.

Anglian Water’s consultant, Clear Environmentaldeveloped the models used. Clear Environmentalwere thus involved in the subsequent developmentof the SWMP methodology.

Clear Environmental were expected to obtainmuch of the detailed information needed for the2-d mesh generation from Google maps andvisual studies to establish locations of droppedkerbs, garden walls or fences.

Site surveys were planned to determine floorlevels of buildings and road gulley locations.Ipswich Borough Council was to arrangesurveys and supply data direct toClear Environmental.

Site surveys to determine precise floor levels aboveOrdnance Datum were subsequently found to beunnecessary. See 8.7.1

Once models had been completed and verifiedagainst historic records the consultants wouldundertake flood mapping for a range of stormreturn periods and durations either side of thecritical duration.

The output mapping would include floodlevels/depths to enable flood damage costs to beestimated for each property affected.

It was decided to follow the Defra guidance andundertake simulations for the 100, 50, 30, 10, 5, 2and 1 year rainfall return periods.

Damage costs would be estimated using the floodmapping results including ground levels, depthsand water levels in the form of GIS tab files.

Initially, modelling results were needed for existingand future ‘do nothing’ scenarios, where futurescenarios include allowances for growth, climatechange and urban creep expected over the next30 years.


Flood mapping and damage costs would bepresented to project partners to decide on whichpossible flood risk management measures wouldbe modelled in detail.

Chosen options would be modelled and benefitcost options assessed for all priority areas.

For each option there was no pre-set designstandard in terms of acceptable flood frequency.Standards would inevitably vary from property toproperty. Modelling would test how well eachoption performed. For some options furtherrefinement of designs would be needed (postSWMP) in order to optimise the benefit to cost ratio.

As there would be a lot of data sharing/transfera SharePoint site was proposed and set up.This would help members of SWMP team to sharecontrolled information and large files.

6.15 Communication andengagement plan Engagement of stakeholders and partners wasplanned. A formal engagement plan wasapproved by the Board on 21 July 2010. The plan,included in Appendix 12.14, is closely linked to theapproach proposed above.

It is a self-contained document summarising theSWMP process, key messages and details plannedstakeholder engagement including a projectprogramme and planned meeting schedule.

The plan proposed:

l 5 board meetings.

l 3 less formal Ipswich Borough Council,stakeholder and Anglian Water team meetings.

l Letter drop and door knocking.

l A public exhibition/drop in session. This would besimilar in format to those organised for theEnvironment Agency’s Ipswich Flood DefenceProject and Anglian Water’s ‘Project Orwell’.

l County and Borough Councillor involvementwould initially be by email and briefings to thePortfolio Holders.

Correspondence with the stakeholders was addedto this document as the SWMP progressed alongwith general updates.

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7 Risk AssessmentAnglian Water’s consultant, Clear Environmental,undertook detailed 2-d modelling using InfoWorksCS for priority areas at Ancaster Road / Burrell Road,Lovetofts Drive to Lagonda Drive and WorsleyClose / Ellenbrook Green.

Ipswich Borough Council used InfoWorks ICM tocreate a 2-d model for the London Road toLavenham Road and Hadleigh Road catchment.

7.1 Detailed assessment byAnglian WaterAnglian Water’s existing verified model of thecombined sewage system draining to Cliff QuaySewage Treatment Works was used for the AncasterRoad / Burrell Road and Lovetofts sub-catchments.This model is large, extending as far as GreatBlakenham and Martlesham and inlcudes manyadvanced features such as real time control.It can make allowances for tidal and fluvialinteractions. (The model was originally developedbetween 1986 and 2000).

A second much smaller un-verified model of theundergound surface water sewerage systems inthe Bridgewater Road and Coltsfoot Road areaswas used for the Worsley Close / Ellenbrook Green

priority area. This model had been previouslydeveloped by Anglian Water to look at floodingissues but had no 2-d mesh.

Between September and October 2010 ClearEnvironmental were engaged on updating andimproving the models, and adding the 2-d mesh.The updates and improvements were in part forthe water company’s own purposes. That is anassessment of development plans and generalmodel updating. Addition of the 2-d mesh wassolely for the purposes of the SWMP.

Ipswich Borough Council supplied the followinginformation to Anglian Water and their consultant:

l Plans showing the probable extent of 2-d meshrequired – i.e. the area where overland flowshad been observed or was expected based onlocal knowledge and experience.

l Various historic plans showing SuDS andhighway drains.

l Flood records / prioritisation spreadsheet.

l Rainfall records for a flood event in 1998.

l Estimated land drainage flows.

The resulting models included muchlarger mesh zones than originallyintended. This probably increasedcomputer run times and the risk ofmodel instabilties.

Initial flood mapping for the 1998 rainfall event butusing existing (2010) models was presented inOctober 2010. These seemed to be reasonablyrealistic bearing in mind the limitations /assumptions made in the use of rainfall data froma single rain gauge.

This mapping confirmed the area where 2-d meshwas required and where modelling of highwaygullies may be needed to confirm causes offlooding, responsibilities possible evaluation ofoptions or to improve accuracy. Anglian Water’s Sewerage models


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In reality gullies (grates, gully pots and pipes) arethe interface between surface flows andunderground sewer flows. However, they are notnormally included in sewer models. Gullies are notsimple to model accurately and can berepresented in various ways. In reality, as the flowalong road channels increases, the flow by-passinggullies increases. Debris may completely or partlyobscure grates. Road cross falls and longitudinalgradients are also important factors affecting flowinto gullies.

Ipswich Borough Council undertook a survey torecord the location of 384 highway gulliesconsidered to be critical to flooding issues and withClear Environmental researched availablemodelling techniques.

The following was extracted from ClearEnvironmental’s modelling report.

“The gullies were modelled as nodes using thegrate size as the shaft plan area; no account ofthe grate itself has been made. The depthof the gullies has been set at 150mmadding 0.075m3.

The cover level for all the gullies has beenbased on the modified ground model.Whilst much simplification of the gully openinghas been undertaken for inclusion in themodel, it is documented that in many instancesthe connection link between the gully pot andthe sewer network is a greater restriction.

The available guidance suggests the maximumflow rate that can be accepted by a gully potwithout surcharge is about 10 l/s if the outletpipe has a diameter of 100mm.

The nature of the connection links is unknownacross the model, consequently theconnections have been assumed to be100mm diameter pipes. The gradient of eachconduit has been set to ensure a pipe fullcapacity of 10 l/s through the link.

To avoid adding model instabilities all gully pipelengths have been set to 5m. For the purposesof the model the gullies have been connectedto the nearest available modelled node.If a suitable node was considered to be too far

away a dummy manhole has been created topermit the connection of the gully to thenetwork. Again, this has further increased thestorage volume of the network, althoughthe overall impact is considered negligible.”

It should be noted that Ipswich BoroughCouncil believe gully connection pipesare 150mm diameter – as required bycurrent highway design standards.The consultant’s lack of inclusion of thehead loss at grates may counter thiserror. Whatever method is used there willremain uncertainties about the degreeof blockage of grates which could havea considerable affect on flooding.

There remains the problem that thisrepresentation does not enable the effectof blockages to be considered.

In extreme events, when most of thesewerage system surcharges to groundlevel, flows into gullies may cease oreven reverse. In the case of reverse flowfrom the sewerage system, theconnection pipe size will makea difference to flood depths. IpswichBorough Council’s method of modellingavoids this issue. Either way care needsto be taken, if drawing on modelpredictions, to make conclusions onresponsibilities for highway flooding.

Flood maps for 2011 were shown to the projectboard and SFRMP in April 2011.

The Board agreed allowances for future growth,climate change and creep to be incorporated infuture models:

l Horizon 30 years – i.e. 2040

l Growth (planned development) included onlywhere it reduces flooding at Lovetofts Drivearea. Here a brownfield site currently drainssurface water runoff to the combined seweragesystem. This will be redeveloped and drainedvia SuDS to the surface water system. For other

sites SuDS would be required to avoidincreased flows for all events up to a 100-yearreturn period.

l Climate change – a 10% increase in peakrainfall intensity is expected up to 2055. So a7.5% increase is assumed to 2040.

l Sea level rise – 203mm by 2040.

l Creep (Paving of gardens or house extensionsnot requiring planning permission) two mainsources of information are:

w The UKWIR 2003 report: Allowance variesfrom 0.4 to 1.1 sq m per house per year fordevelopment <40 years old and

w R Allitt’s WAPUG paper (2009); researchbased on 4 cities. Average creep was about0.75 sq m per house per year; more fordetached and less for terraced homes.

w For the SWMP, creep was based on housingdensity, regardless of age with a ceiling of5% added impermeable area.Typical range from 0.86 to 0.1 sq m peryear. The total addition for 82,176 propertieswas 73 Ha. (8.9 sq m/property or 0.3 sq mper year per property)

Options for flood alleviation, which had been putforward, were agreed by the Board and simplisticrepresentations of the various flood risk measuresmodelled. Further details of options and theirevaluation follow in Section 8.

Clear Environmental’s Hydraulic Modelling ReportCL491/005/002 dated May 2011 describes thework undertaken and assesses the results. This wasreceived on 24 June 2011.

Results data which included flood maps andGIS files, showing pipe surcharge status, peak flowand hazard ratings, was supplied on a disc due tofile size.

The report highlighted areas of potentialinaccuracies, some of which related to instabilitiesin the models in some, but not all, simulations.These result in volume imbalances or peculiarflood predictions such as floods on the slopingembankment of the A14.

Subsequently the report was reviewed anda spreadsheet detailing and prioritising missing

or unstable simulations was produced and usedas a basis for discussions held on 12 Sept 2011.Anglian Water decided to curtail modelling workat this point as costs were rising.

Ipswich Borough Council subsequently found it waspossible to interpolate water levels from goodsimulation results for some missing or unstableresults as described in Section 8.7.

7.2 Detailed assessment: LondonRoad to Lavenham Road andHadleigh Road priority area.Project Board decisions were taken as describedfor the previous section. This section deliberatelydescribes the modelling process in some detail asit is not documented elsewhere.

The InfoWorks model, developed by IpswichBorough Council, includes the highway drainagesystem serving part of London Road, theassociated SuDS system, the Milden Road pondand ditch, headwall, gullies, highway drains andsewers in part of Lavenham Road. Overland flowfrom London Road across Chantry Park anddownhill through back gardens as far as a steepdrop at Norris House, Hadleigh Road is includedusing 2-d mesh. The modelling process issummarised below:

2-d mesh zones were generated by ICM fromLiDAR data. Gullies were surveyed in Dec 2010and the ditch in 2011 (following clearance ofsome vegetation).


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At the upstream end of the catchment a meshzone represents the London Road highway witha maximum triangle area of 7.5m2 and aminimum element of 5m2. Here 100% runoffand a Manning’s roughness “n” value of 0.015are assumed.

Gullies are represented as nodes with the “floodtype” set to “2d gully”. Head discharge curves forgullies were derived by IBC, assuming noblockages of the grates but including for headlosses due to grates. “Rainfall” is applied directly tothe mesh. Resultant flood mapping shows howrunoff is routed along the road channels to thegullies and then into the piped network andthe SuDS system.

A “dummy” overflow weir is included to simulateany flow from the road, which bypasses the gullies;the crest is set at the lowest kerb height.

The SuDS system is in fact a series of undergroundsoakaways. These were readily modelled usingstandard ICM features.

For historic verification three ring soakaways withzero infiltration were included in the 2006 model.For ‘do nothing’ scenarios new soakaways including2 trench soakaways, which were installed in 2006,are represented with an infiltration rate of 20mm/Hr.Data representing the soakaways was obtainedfrom Ipswich Borough Council’s drawing LONHW/05.

Flood predictions were found to be relativelyinsensitive to changing soakage rates. This isbecause the SuDS are relatively small and theyoverflow regardless of the soakage rate, even incommon events.

Flows from the SuDS system and the overflow weirdischarge onto a second 2-d mesh zone, whichroutes overland flows across the park. The meshrepresenting the park has a triangle size of 25–100sq m and Manning’s roughness n = 0.035 (shallowdepths of flow through long grass)

Various ways of modelling runoff from thepark were tested, including applying apercentage of rainfall directly to themesh (this usefully helped confirm thecatchment area that needs to be meshedas well as the flood path).

A wide range of flows results from differentmethods of rural flow estimation.

The final method used was to divide the parkcatchment area into three polygons and use theRe FH method to derive flow hydrographs resultingfrom each polygon for a range of storm durationsand frequencies. Appropriate sets of thesehydrographs were then applied to the mesh in thevalley floor as point inflows.

To represent the pond and ditch various methodswere tested. Initially reliance was placed on themesh however it was soon apparent that the meshgenerated from LiDAR did not represent eithercorrectly. This was due to LiDAR data beingunreliable in areas with dense trees/undergrowthand because the ditch was very narrow comparedto the LiDAR resolution.


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Attempts were made to lower the mesh but resultswere not visibly different. The model softwarecompany was contacted and eventually anupgrade was issued which should have solved theproblem. However, to maintain progress, beforethe upgrade was issued, the ditch was representedas a trapezoidal channel with cross sections basedon a site survey and the longitudinal sectionbased on LiDAR. The ditch bed was found to fall3m in 160m; however the section near the pondwas relatively flat. Obstructions held back flow atseveral locations – as shown on the picture above.These were finally represented by assuming theditch was only 100mm deep.

“Dummy” (imaginary) nodes were included in themodel along the channel at changes in gradient.The flood type for the nodes was set to 2-d, whichallows floodwater to escape from the channelonto the mesh and water on the mesh to enter the

channel. The ground level at each node is set tomesh ground level – If the model is re-meshedthese ground levels may need amending.

Innovyze advise these dummy nodes canbe replaced with a linear connectionbetween the channel and 2-d mesh.This should prove more realistic.

The upstream dummy node for the ditch is sited atthe lowest mesh element within the pond.

Representation of the pond followed carefulconsideration of LiDAR and ditch levels andassumed the standing water in the pond was atthe ditch outlet invert level when the surface areaof water was 450 sq m (from OS mapping).Some of the storage capacity of the pond isrepresented in the node and part on the 2-d mesh.

The resultant model was easily edited to replicatethe effects of a larger pond or ditch.

From the headwall at the downstream end ofthe ditch a 300mm diameter pipe drains to the


Anglian Water surface water sewer in LavenhamRoad. The headwall includes a grille, which isfrequently partially blocked with debris.

The effect of the blockage is represented in the ‘donothing’ scenarios as an orifice with a diameter of100mm.

The pipe from the headwall and a 110m length ofthe downstream sewer at Lavenham Road isincluded in the model together with highway gulliesand connecting pipes. However connectingsewers and flows from the urban area are notincluded in the model.

The model predicts flows can overtop the ditchhead wall and flow onto Lavenham Road.The predictions were confirmed followingdiscussions with residents. The 2-d mesh zonetherefore extends further downstream, includingpart of Lavenham Road, where modelled gulliesutilise the same head discharge curve as those atLondon Road.

A roughness zone (n = 0.016) represents thesmoother highway surface at Lavenham Road.A small part of the mesh here was dropped by34mm to represent a low kerb line.

It was found that the gullies at Lavenham Roadcould only intercept a small flow before the waterlevel overtopped the low kerb and overflowed intogardens, along the valley floor and on towardsHadleigh Road. Records confirm occasionalflooding in gardens and more frequent flooding atHadleigh Road.

The 2-d mesh extends from Lavenham Roadfurther down along the valley to Hadleigh Roadwhere there is a natural break point in the form ofa 2m drop at Norris House.

The ReFH was used to estimate runoff from gardensassuming the sewerage system carried runoff fromall paved and roof areas within 10m of buildingsand pavement (as per WAPUG paper 21 – March2009). Runoff was applied as a single point sourceto the mesh in the bottom of the valley abouta quarter of the way down hill from LavenhamRoad towards Hadleigh Road.

For the ‘do nothing’ scenario pre-2006 and, a 100year return period, simulations were undertaken for

7 storm durations between 15 and 1440 minutes.The critical duration for flooding of properties wasfound to be 360 to 720 minutes.

For highway flooding at London Road the criticalduration is 15 minutes.

7.2.1 Ipswich Borough Council model –historic verification

Flooding of properties in Milden Road (built 1982)has only been reported once in the past 29 years –during 1998.

Rainfall data recorded at Cliff Quay SewageTreatment Works (3 km to SE) was available forthis event but could not be directly used ina simulation, because the ReFH methodcalculates hydrographs based on rainfall returnperiods and durations. Processing revealed the1998 event approximated to a 2-year returnperiod 720 min duration event albeit withsubstantial rainfall over several days before themain event. However, no flooding was predictedfor the main event.

Flooding of properties in Milden Road waspredicted in a 30-year return period, 720 minuteduration event.

Research into old correspondence revealed the1998 flooding was blamed by residents onobstructions. It is considered that over time theseobstructions may have been cleared or reformed,so for historic modelling there is a difficulty indeciding what ditch section is appropriate.

The apparent discrepancy may also be explainedbecause the return period of the rain is notnecessarily the return period of the flood.

In 1998 the catchment must have been very wetbefore the main event however ReFH estimationsdo not take into account specific antecedentconditions. It is also likely that prolonged overflowfrom the London Road highway soakaways duringthe preceding days would have waterlogged thevalley down hill.

Another possibility considered was that surchargingof the sewerage system increased water levels inthe ditch. The sewerage system serving Lavenham

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Road flows eastwards and under London Road as itcuts into higher ground. This means it is possible forsewer flooding to occur in Lavenham Road.

The surface water sewer here is included inAnglian Water’s sewer model of the BridgwaterRoad and Coltsfoot catchments, but is consideredto be un-verified.

Anglian Water’s modelling indicates surchargelevels reach 16.9m AOD in a 100 year returnperiod of 60 minute duration in 2040 (consideredto be the critical duration for flooding at BridgwaterRoad). A transient reverse flow of 37 l/sec ispredicted towards the upstream sewer manhole inthe garage area at Lavenham Road, which hasthe lowest cover level of 16.6m.

Ipswich Borough Council’s model was tested to seewhat effect surcharging of Anglian Water’s systemmight have on the ditch system, by applyinga constant 16.9m AOD level hydrograph to themodelled section of the Anglian Water sewer.The test showed sewer surcharging would notaffect flooding further up the ditch at MildenRoad but will have some affects downhill ofLavenham Road.

7.2.2 Other model findings

Overflow from the London Road highwaysoakaways onto park: The pre-2006 base modelpredicts overflow more often than annually, whichwas as expected.

The post-2006 ‘do nothing’ model (with extendedSuDS serving London Road) with 2011 rainfall alsooverflows at least annually. Site inspection revealsoverflow has occurred recently.

Flood depth across park <50mm even in a 200year return period event.

London Road dual carriageway flooding: Flooddepth is limited by kerb and verge height.Floodwater which overtops the kerb bypasses theSuDS system. Critical duration here is 15 minutes.

Milden Road: Some shallow garden flooding10–50mm deep in rear gardens of Milden Road ispredicted in a 5–10 year return period event andinternal flooding at three properties is predicted ina 100 year event.

Flood depth did not exceed 50mm, even in a 200year return period event.

Lavenham Road: Gardens flood 10-50mm deepat a 2–5 year return period. Water <10mm deepoverflows into the garage area and then into reargardens along the valley floor.

Gardens between Lavenham Road andPickwick Road and Hadleigh Road: Floodwaterflows towards Hadleigh Road with depths up to560mm predicted in a localised hollow at reargarden of number 85, in a 200 year return periodevent. Flood depths <10mm in a 50-year returnperiod. In a 100 year return period, depthsexceed this.

85 to 89 Hadleigh Road: ‘Do nothing’ and ‘base’models predict gardens adjacent to three homesto flood 50–100mm deep in a 200 year returnperiod event. Internal flooding is predicted ata 50–100 year event. A land drain is shown onrecords plans here. However, it is not readilyaccessible and is not represented in the model.If the land drain is functional the flooding would beless frequent.

There are many records of historic flooding hereattributed to sewer overloading and / orinadequate highway drainage. Since a flood reliefproject was installed by Anglian Water in 2007 noreports have been received.

Norris House: Water overflowing from the roadover or through the retaining wall was last recordedin 2006.


45


7.2.3 Conclusion for Chantry Park

Bearing in mind various factors (including the lowcost of potential solutions) the model with a 100mmdeep channel, which predicts flooding in a 30 yearreturn period is considered to be acceptable eventhough no verification involving a flow and rainfallsurvey, in accordance with the Code of Practise forthe hydraulic modelling of sewer systems (WAPAGNov 2002), has been undertaken.

7.2.4 Growth and creep

No planned developments were considered likelyto affect surface water flows. Additional roof orpaved areas were assumed to drain to thesewerage system. Allowances for climate changebut not growth or creep were therefore included inthe 2040 ‘do nothing’ model.

7.3 Flood maps for 2011and 2040The results for 2011 ‘do nothing’ scenarios aremapped and included in appendices 12.22and 12.23 and at a smaller scale on thefollowing pages. 2040 results are available butare very similar.

The maps show where water depths are greaterthan 0.01m. This depth was chosen because therewill inevitably be some depth of water onimpervious surfaces.

Flows of runoff along properly designed roadchannels will exceed 0.017m deep once a year.However most roads will have surface irregularitiesand gully spacing is often more than ideal, sodepths may often be greater. Some shallowflooding will be shown on many roads whereas,in reality, this is not a problem.

Normally house floor levels are about 50–100mmabove the lowest surrounding ground and sofloodwater is most likely to enter homes in areaswhere flooding is greater than 50mm deep.Even then floodwater may not enter if the durationof flooding flood is short; if doors are sealed orairbricks/door thresholds are above the floor levelor the floor levels raised.

Flood mapping assumes gullies are not blocked,ignores the effects of wash caused by vehiclesand may not take into account all garden fences,walls, etc.

Another issue is the accuracy of ground levels usedin the modelling. The average error of LiDAR is + or– 150mm. Model results are also simplifications, inparticular the predicted depths of water assumean average ground level at each mesh element.

Flood maps are therefore indicative.

The maps are held in electronic MapInfo format,associated data fields for each 2-d elementinclude maximum flood depth, water level,average ground level and peak hazard rating.

46

7.3.1 Flood map for Lovetofts 100 year return period, ‘Do Nothing’


47


48

7.3.2 Flood map for Worsley Close / Ellenbrook 100 year return period ‘Do Nothing’


49

7.3.3 Flood map for Ancaster Road / Burrell Road 100 year return period ‘Do Nothing’


50

Flood map for London Road to Lavenham Road and Hadleigh Road. 100 year return period. ‘Do Nothing’

7.4 Comparison with theEnvironment Agency’s flood mapfor surface water (FMfSW)SWMP flood maps have been compared to theEnvironment Agency’s flood map for surface waterfor a 30 year return period. These maps show onlypredicted flooding of greater than 100mm deep.

The SWMP maps generally show much smallerareas of flooding and some differences in thelocation of the flooding. This is mainly becausethe SWMP models more accurately represent sub-surface flows and include interfaces betweensurface and underground systems. SWMP modelsrealistically predict locations not shown on theFMfSW maps (e.g. Bridgwater Road), where waterescapes from the underground systems to flowoverland and vice versa.

7.5 Predicted numbers ofproperties at risk at 2011 Numbers of properties predicted to flood in a 200year return period event in each study area wereestimated using both the areas susceptibleto surface water flooding (ASTSWF) and the floodmap for surface water (FMfSW) maps andcompared to those predicted to flood using the

SWMP modelling. Section 8.7 describes themethodology used.

The FMfSW is judged to be the most accurate basisfor projecting Ipswich totals from SWMP predictions.9,025 properties are within areas shown to havemore than 100mm of floodwater in a 200 yearreturn period event.

In the four study areas the SWMP predicts 231properties would flood whereas the FMfSWpredicts 515.

Assuming the four study areas are representative ofthe whole town, the total for Ipswich should beabout 4048. i.e. (231/515 = 0.45) times the totalpredicted by the FMfSW (9025).

Similarly 1,525 properties are likely to sufferinternal flooding.


51

Conclusions

Estimated number of properties flooded inIpswich study area in a 200 year return period

Internal flooding 1,525External 2,523Total 4,048

PFRA total was 5,626


Using the same principles the numbers of properties predicted to flood at various return periods are shownin the graph below:

Numbers of properties predicted to flood in Ipswich based on SWMP study at 2011

52

Some idea of the likely accuracy can beconcluded by comparison with records:

The predicted annual average number ofproperties flooded was internal flooding 131 + external flooding 226 = 357 per year(estimated from the area below a graph ofnumber of properties flooded againstannual probability.)

Ipswich Borough Council records

(An incident is a property flooding on oneoccasion, 10 incidents could be 10 propertiesin one event or 1 property in 10 events)

It is known that flooding, especially external,often goes unreported, so the results of thecomparison are not surprising and give credenceto the SWMP predictions.

Year No. of Incidents

2001 68

2002 200

2003 119

2004 121

2005 138

2006 110

2007 109

Average 124

8.1 Short-listing of options andplanning for option evaluationThe project team short-listed flood alleviationoptions which were subsequently agreed by theBoard in April 2011. This meeting includeda presentation of early flood maps together withdetails of the methodology planned for evaluatingoptions and reviewing likely outputs against theagreed SWMP objectives. e.g. all options putforward should reduce pollution.

Decisions on which options to investigate in moredetail were taken at this early stage to ensure the‘do nothing’ models could easily be edited torepresent the options chosen, and any differencesin predictions resulted only from inclusion ofremedial measures rather than other differencesbetween models.

A rigorous approach (described in detail in section8.7) to identifying flood receptors, flood damages,and cost benefit analysis was agreed to ensureconsistent approach was applied. To facilitate this,Excel workbooks were developed, suitable for usein future SWMPs.

8.2 Options for Lovetofts Drive /Lagonda DriveThe following flood alleviation measureswere considered:

1. Offline tank.

2. Divert gullies from combined system to newSuDS along verges of Lovetofts Drive. SuDS todrain to the surface water system.

3. Removal of surface water fromcombined system.

4. Possible extension of rising main.

5. Strategic SuDS including attenuation of BuryRoad highway drainage to the north ofBury Road.

8.3 Options for Worsley Close /Ellenbrook1. Grille cleared more frequently on outfall –

assume 10% blocked.

2. SuDS system constructed in Stonelodge Parkbehind the bund wall.

3. Underground storage in Stonelodge Park.

8.4 Options for Ancaster Road /Burrell Road1. High-level reinforcement of surface water sewer

draining to river. This would operate infrequentlyand be unlikely to affect water quality.

2. More frequent maintenance: clearing highwaygullies in Ancaster Road.

3. Bolting down of private drain cover in footwayof Pooleys Yard.

8.5 Options for London Roadto Lavenham Road andHadleigh Road1. Maintain SuDS, outfall grille and ditch.

2. Larger pond, including maintenance as aboveoption 1.

3. Larger SuDS plus option 1.

8.6 Flood maps for optionsFor The Chantry Park – Hadleigh Road priority areaa full range of flood maps are available.

For the other three locations only results for a 30year return period were available.

Process for evaluating options

Defra’s SWMP technical guidance (March 2010)has been followed. The prime measure forevaluation being the cost / benefit ratio. This is inline with the emerging national partnership funding


53

8 Options


(FDGiA) funding for flood riskmanagement schemes.

Other issues also affect option choice.For example environmental benefits such ascreation of water dependant habitat.

The steps used for evaluation are:

1. Define receptors (those things that will beaffected by flooding)

2. Estimate flood depths at receptors

3. Estimate average annual damage costs in2011 based on flood depth in each receptor

4. Estimate average annual future damage costs(for ‘do nothing’ scenario)

5. Design and model options

6. Estimate average annual damage costsfor each option for both the future and2011 scenarios

7. Calculate the financial benefit for each option= reduction in damage cost

8. Calculate benefit / cost ratio

Receptors include buildings and, at AncasterRoad, the highway as flooding closes the road.At Lovetofts Drive damage to the highway, causedwhen a manhole cover blows, along with roadsurfacing, is also taken into account, assumingdamage occurs when the pressure head at themanhole exceeds ground level.

Flood damage costs for various forms ofdevelopment and assessment techniques are laidout in the Multi Coloured Handbook (MCH)5

Damage cost data is available for:

l Residential sector average – basis for SWMP

l Detached houses (split into ages andsocial classes)

l Semi-detached houses

l Terraced houses

l Bungalows

l Flats

l Closed roads, non residential buildings – e.g.shops, schools

The MCH relates flood damage cost to flooddepth based on historic floods. The damage costfor the average residential property is shown asa constant £948 for flood levels from 300mmbelow ground floor level up to floor level.

For floods exceeding the floor level, damagecosts increase rapidly. A depth of 300mm isshown to cause on average £22,944 ofdamage. The figures include for damageto gardens/fences/sheds.

Estimated damage costs cannot be used forspecific properties but, over a wide area, willprovide the best estimate of total damage costs.

Most floor levels are 0 to 100mm above groundand flood mapping does not show areas wherefloodwater is below ground. The decision wastherefore taken to ignore damages resulting fromflooding below ground floor level and assumeflood receptors would be buildings where adjacentflood depths were >50mm in the 100 year eventin 2040.

To simplify the approach the damage costsrelating to the residential sector average wasassumed for all residential properties.

Gardens, sheds, summerhouses and remotereceptors unaffected by options were notregarded as receptors as damage costs for thesewere already included. Buildings with a past historyof flooding were included.

Enhancements were added:

5.6% uplift for emergency services costs

£200 per flood per property for injuries

£99 per flood for investigation/record by thelocal authority (IBC costs)

54

5. “The Benefits of Flood and Coastal Risk Management Handbook of Assessment Techniques – (Flood Hazard ResearchCentre 2010.)”

At Ancaster Road the cost of traffic delaysare estimated in accordance with the MCH. It was assumed:

l The diversion route is 2.5km

The peak traffic flow is 1416 vehicles per hour(2004 data)

The road becomes impassable with >150mm offloodwater (as suggested in the MCH).

Flooding occurs at peak traffic time when theaverage speed of traffic is 20 km/hr.

At Ancaster Road, flooding affects highway andcan overflow over a very low kerb (10mm) intoa basement car park sited beneath a building,which comprises 85 flats. There is no flooding insidehomes. In reality a mixture of foul and surfacewater overflows onto the highway then into the carpark. The car park was designed to be floodresistant and includes an extensive drainagesystem. The storage capacity and drainage systemwithin the basement are not represented in themodel. The threshold polygons used to extractdata from GIS flood maps were positioned on thehighway under the bridge and also alongsidethe entrance to the car park.

Damage costs for flooding of the car park wereassumed to apply when the depth of adjacentfloodwater reached 10mm or when combinedsewer surcharge levels exceed ground level(apart from the sealed cover option). The MCHhas no appropriate examples, so damage costswere assumed to be the same for each eventand included cleaning up foul matter,intangibles (anxiety, health, etc) and localauthority costs.

At Lovetofts Drive a £50,000 damage cost wasagreed with Anglian Water for repairing the roadsurface and manhole when the sewer surchargelevel exceeded ground level at manhole 5601(at the Kerry Road junction).

8.6.1 Estimation of flood depths in receptors

2-d models represent the ground surface asthousands of triangular mesh elements. The outputGIS flood maps include average ground level andpeak flood level at each element.

In some locations (as illustrated) topography isrelatively steep and flood depths vary around theperimeter of buildings.

The location of doors or other openings wherewater can enter will therefore be critical. Thesethresholds were mapped (shown red on aboveplan) following a site inspection or by using Google‘Streetview’.

Threshold tab files include fields for predicted waterlevel and average ground level for each event.One threshold file was produced for eachmodelled scenario.

GIS tools were used to rapidly import flood levelsand average ground levels from the appropriatecells of the flood maps into the threshold file.

An Excel workbook was developed to calculate theflood depths in each receptor, calculate damagecosts and count the properties flooded.This required the height of each floor aboveground level.

Consideration was given to surveying properties todetermine floor levels relative to ordnance datum.


55


However since the 2-d mapping uses averageground levels, and produces average water levelsat each element, and because the error for LiDARdata was +/-150mm, it was decided to estimatefloor heights above ground level. This was done,in most cases, by inspection either on site, orusing Google (using brickwork courses asa measure).

50mm appeared to be a realistic average heightof threshold above ground and was generallyadded to the workbook apart from where theinspections showed a lower height.

Four workbooks were used to evaluate options, onefor each priority area. Each workbook containsworksheets for each modelled scenario.These calculate damage costs for each modelledevent. The workbook can be readily reused forfuture SWMPs.

The workbooks ensure the same floor heights areused for all scenarios and events. Sensitivity to floorheights can also be investigated easily.

The average annual damage cost (AAD) for eachscenario is calculated from the damage costs for

each event (normally 8 events). However, ClearEnvironmental had only modelled a single event(30 Year Return Period) for each option.An additional sheet was therefore used tointerpolate flood levels at receptors for themissing events.

The method used for interpolation was based ona full set of results for the 2011 or 2040 scenarioswhich enabled flood levels at each receptor to berelated (by a factor) to the flood level at a keygauge point for each return period. Similartechniques are described in WAPUG IUD ModellingGuide Rev 1 v28.

A check was made to ensure the original modelpredictions and damage costs for the 30 yearreturn period events for each scenario wereconsistent and that any zero damage figures reallydid mean no flooding in the 30 year event.The check revealed some errors – some options inthe Ellenbrook area worsened performance,contrary to any reasoning. Detailed checks on themodel output and the consultant’s report revealedinexplicable increases in flow volumes. In this casethe interpolation technique can generate small

56

apparent changes in damage costs that are notreal and such results were treated with caution orignored. Interpolation will not work when storagesolutions overflow in events with a return period<100 years.

8.6.2 Cost benefit analysis

A final worksheet (see the example extract above)brings together AADs from 2011 and 2040 ‘donothing’ and option scenarios and calculatesbenefits for each year between 2011 and 2040(by linear interpolation).

Scheme costs and benefits are recalculated aspresent values (PVs) to enable options forschemes with varying capital (design/construction)and revenue elements (e.g. maintenance) to becompared, assuming a 30 year lifetime ofthe scheme.

The present value of the scheme is the notionalsum of money that needs to be put aside(invested) now to fund the scheme. PV is affectedby inflation and investment interest rates. The SWMPmakes no allowance for inflation and uses a 3.5%investment rate, as suggested by Defra, over thenext 30 years.

For modelling, notional AADs at 2011 for optionswere estimated from 2040 option AADs assumingthe same ratio between 2011 and 2040 ‘donothing’ AADs.

Capital costs were generally estimated using theSuffolk County Council highways partnershipschedule of rates (current at 2011). These wereadded at the appropriate date. Revenue costs areassumed to be annual.

Finally the net present value for benefits andcosts and the benefit cost ratio is calculated foreach option.


57

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60

Option PV cost PV benefitsmin max

BCRmin max

Comments

Option 1 – Ensure outsidegrille is <10% blocked

£40,967 ? ? ? Small cost but mightbe very beneficial

Option 2 – SuDS in StoneLodge Park

£31,690 ? £523,826 ? 16.5 Low costs but mayhave high benefits

Option 3 – Offline StorageTank in Stone Lodge Park

£123,223 ? £501,866 ? 4.5

More simulations are needed to properlydetermine benefits. The benefits may besubstantially less than shown since someflooding from the surface water sewer is expectedto remain downstream of the SuDS or storagetank. However this would be reduced furtherby improvements arising from the grillemaintenance. (1m lower surcharge levels ina 30 year return period)

The table indicates:

l Maintenance of the grille may be worthwhile ascosts are small and a relatively small reductionin damage costs would be cost beneficial.The assumption is that a particularmaintenance regime will reduce blinding of thegrille from 50% to 10% (on average).

l The SuDS system in Stone Lodge park appearsto have the most potential. Further simulationsfor a range of events are needed and thebenefit cost ratio and design could beoptimised by examining a range of capacitiesand considering the use of SuDS combined withthe outfall grille option.

8.9 Evaluation of options forAncaster Road / Burrell RoadNo options for relieving flooding in Burrell road wereconsidered and some amendment to the optionsput forward for flood relief at Ancaster Road havebeen made as described later.

The table below appears to indicate very littlebenefit is achieved by any option. However this isnot the case in reality.

Summary of Damage costs for 30 Year Return Period only

Scenario DoNothing2011

DoNothing2040

Option 1 –AdditionalSWS

Option2 –Clean SWOutfall

Option 3 –Bolt downcovers oncombinedsystem

Option 4 –Bolt downcovers oncombinedsystemandprovidenew SWS

Option 5 –Bolt downcovers oncombinedsystem &Clean SWOutfall

30 Yr RPdamagecosts

£10,636 £10,993 £9,335 £10,362 £10,933 £10,362 £10,362

When wider ranges of return periods are considered (using interpolation techniques) a different pictureemerges as in the following table. This is because the options are expected to have the most effect ondamages at return periods <30 years. For example, bolting down covers halves the frequency of floodingof the car park.


61

Option PV cost PV benefits BCR Comments

Option 1 – Additional SWS £135,141 £4,342 0.03

Option 2 – Clean SW Outfall £9,615 £2,647 0.3

Option 3 – Bolt down covers oncombined system

£9,335 £45,841 4.9 Most costy beneficial

Option 4 – Bolt down covers oncombined system and providenew SWS

£144,477 £82,654 0.6 Greatest benefits

Option 5 – Bolt down coverson combined system & CleanSW Outfall

£18,950 £71,409 3.8

Cleaning sediment from the surfacewater system

This has replaced the agreed option ‘morefrequent maintenance – clearing highway gullies inAncaster Road’.

Blockages of road gully grates due to sedimentswashed down from the unmade part of AncasterRoad can have a significant major effect onflooding. The gullies have been modelled verysimplistically as the impact of varying degrees ofgully serviceability cannot be accuratelyrepresented in the model.

Modelling partially blocked gulliesmay have provided useful informationon responsibilities.

The consultant has, however, modelled the effectof sediment removal from the existing surfacewater outfall sewer from Ancaster Road,assuming the depth of silt is reduced from200mm to 0.

Sediments tend to build up gradually in pipes withslow speeds of flow. As sediment builds up, thespeed of flow eventually increases and a steady

state depth of sediment is reached. It is likely the200mm depth was the steady state and soafter cleaning, the sediment is likely to return,especially at this location where sediments arewashed down from the unmade road further upthe catchment.

Ideally the option model should have includedsome sediment to be more representative of thesituation between successive cleaning operations.However there is no meaningful data to base therate of build up on.

When estimating the cost of maintenanceoptions there is a problem decidinghow frequently maintenance is neededand how to represent the effects ofmaintenance in a model. Recordsof sediment depths or grilleblockages and maintenance (includingcosts) undertaken would assist inevaluating options

Based on experience, Ipswich BoroughCouncil has assumed that an initial clean wouldcost £2,000 and thereafter £1,000 everytwo years.


Reinforcement of surface water sewerdraining to river

Clear Environmental modelled this as a 375mmdiameter outfall pipe with 300mm and 225mmdiameter branches, to which 10 new highwaygullies connect – each set to carry up to 15 l/sec.The report seems to indicate this pipe is too large ina 30 year return period event. The GIS mappingshows it carries a peak flow of 215 l/sec from thenew pipe to the Orwell, whereas the expectedmax flow is 150 l/sec.

This is probably explained because flood waterin the model is entering at manholes as wellas gullies.

The consultant considered that due to shortcutstaken in modelling, the results are limited and donot reflect the true impact a new surface watersewer would have.

To refine the design and performance further, moredetailed modelling is needed. Nevertheless, theresults have been used in the benefit cost analysis.

Bolting down of private drain cover in footwayof Pooley’s Yard

The modelling for ‘do nothing’ and all optionscenarios assumes the combined sewer manholesshown below are sealed and that there is aneffective non return valve installed on the foulconnection from the flats.

The consultant describe this private drain, howeverAnglian Water adopted such lateral drains underthe public highway in October 2011.

During a heavy shower on 6 January 2012 floodingoccurred and the following photographs show howsewage / surface water escaped from a boltedcover as well as the manholes on the connectionfrom the flats.

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The effect of sealing these two manholes wasconsidered by assuming, in the ‘do nothing’scenarios, sewage could escape wheneversurcharge level exceeded ground level. A nominalestimate of £10,000 has been assumed for sealingthese covers.

The option ‘bolt down covers’ option was carriedforward to the action plan along with ‘cleansurface water outfall’. This is included since it is notknown when the sediment depth was recorded, orhow much is in it now.

Gully maintenance and road sweeping would beundertaken by the highway authority wheneverreports from the public are received, so anotheraction would be to ask local residents to reportblocked gullies or gravel deposits on the highway.

8.10 Evaluation of options forLondon Road to Lavenham Roadand Hadleigh RoadA full range of simulations were undertaken with noapparent model instabilities, so there was no needto interpolate or check the 30 year return periodpredictions.

One slight change in options was made. Options 2and 3 assume the ditch was not deepened and is100mm deep.

Options 1 and 1a are almost the same. 1a doesnot include costs for maintaining the SuDS or grille.SuDS should be maintained routinely by thehighway authority.


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It is suggested residents could ensure the grille is clear.

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Option 1 – Deepen ditch to 300mmdeep, maintain grille (10% blinded),maintain SuDS (Infiltration rate50mm/Hr)

£23,020 £6,315 0.27 Cost includes annualmaintenance of SuDSand monthlyinspection/cleaningof grille

Option 1a – Deepen ditch to300mm deep + maintenance ofgrille & SuDS – not including costs formaintaining SuDS or grille

£9,848 £6,315 1.28 Cost of ditchdeepening included,SuDS or grillemaintenance costnot included

Option 2 – Enlarge pond (to store500 cum) + Maintenance of grille &SuDS – not including costs ofmaintaining SuDS & Grille

£10,558 £6,197 0.6

Option 3 – Larger SuDS (+200 cum)& maintenance of SuDS and grille.Costs of maintaining SuDS and grillenot included

£70,013 £425 0.0

The larger SuDS option has little effect because theoverflow from the SuDS peaks earlier than therunoff from the park, which is predominant.

It is suggested option 1a is taken forward with nofurther refinement or modelling.

8.11 Ipswich wide options –control of creepClear Environmental has provided full sets ofsimulations for ‘do nothing’ scenarios

l At 2011

l At 2040 with climate change and creep

l At 2040 with climate change

l At 2040 with creep

Flood mapping for the Ancaster Road andEllenbrook areas has been used to derive thebenefits that may accrue across Ipswich ifincreased flooding due to creep could becontrolled. These areas were chosen becausegrowth has no affect in them.

From section 7.5 the number of propertiesestimated to flood internally across Ipswich in a200 year return period event at 2011 was 1525.The average annual damage (AAD) costs perproperty calculated for Ancaster and Ellenbrookwere assumed to apply to 1525 properties acrossIpswich and costs and benefits for the whole ofIpswich estimated as follows on the next page:

For replacement or new paving >5 sq m atdomestic properties planning permission is requiredunless the applicant drains the area to apermeable surface.

Assuming 50% of creep is due to increasingpaved areas, the maximum benefit that might begained is to reduce creep damage by 50% asshown in Option 1.

However, this is unlikely to be possible in practicesince the local planning authority will not knowwhere non-compliant paving is taking place;a publicity campaign is unlikely to be fully effectiveand not all paving will be correctly drained.

Some enforcement is taking place at present –Ipswich Borough Council’s enforcement policyincluded in Appendix 12.15. However, this is verymuch on a reactive basis only when reports ofnon compliance are made by neighbours orhighway inspectors and no specific records arebeing made. No publicity or spot checks havebeen undertaken.

Option 2 assumes what is probably a more realisticreduction. Cost benefit analysis shows it is

worthwhile spending £12,000 per year on a Suffolkwide publicity campaign, monitoring andincreased enforcement.

A rough estimate indicates perhaps 450 propertyowners per year are likely to pave areas >5 sq m inIpswich resulting in increased AAD of £100,000.

If the planning authority needs to become involvedin 30% of these then about 130 site inspections/checks would be needed – costing circa£12,000 pa.

Monitoring and recording would enable theeffectiveness of enforcement/ publicity to bemanaged and the above estimates to be refined.

The publicity campaign would reduce damagesacross Suffolk.

The Draft Local Flood Risk ManagementStrategy for Suffolk6 (consultation draft April 2012 –see link in section 13) includes an objectiveto ‘prevent increased flood risk as a resultof new development by preventing additionalwater entering existing drainage systems,wherever possible’.


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Option 1 – Enforcement results in50% creep i.e. fully effective controlof paved areas

£392,232 £402,260 1.0 Cost set to £22,000per year to giveBCR = 1

Option 2 – Enforcement results in75% creep is partially effectivecontrol of paved areas

£204,835 £203,949 1.0 Cost set to £12,000per year to giveBCR = 1

6. Available on www.suffolk.gov.uk/floodrisk


On 28 June 2011, Suffolk County Councilrequested the project to consider making a bid forFlood and Coastal Erosion Resilience PartnershipFunding (more usually known as FDGiA) for theretrofit SuDS option at Lovetofts Drive. Anglian Wateragreed to this as a possible OFWAT pilot study intothe use of SuDS to relieve sewer flooding andreduce the numbers of properties at risk. The initialbid was duly completed and an outline submittedto the Environment Agency in July 2011.

The project manager documented the basis forthe bid and summarised comments / conclusionsregarding the validity and model stability problems

and the missing results needed to improve thebenefit cost ratio and chances of success.

A revised bid was formulated using interpolatedflood levels derived as described earlier insection 8.7. This was submitted in October 2011and confirmed by the Regional Flood andCoastal Committee.

At the time of writing (April 2012) the detailedproject design and appraisal is being undertakenby Anglian Water, prior to construction planned in2012/13. The funding bid will cover part of the cost– the remainder being funded by Anglian Water.

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9 Flood and Coastal ErosionResilience Partnership Funding Bidfor SUDS Scheme at Lovetofts Drive

This section includes:

l A description of existing actions tobe implemented.

l A new action plan for capital andmaintenance works.

l A new action plan for providing publicinformation and advice.

l A proposed action plan for future SWMPsin Ipswich.

l Proposals for prioritisation of SWMPsacross Suffolk.

l Advice and information for planning andhighway authorities.

l Advice for the Suffolk Resilience Forum andemergency planning.

l Advice regarding the WaterFramework Directive.

l Recommendations for future review.

l Lessons for future SWMPs.

10.1 Existing Ipswich-wide actions The above assume the Ipswich Drainage andFlood Defence Policy (2001) (13) remains in force.This and the measures listed below apply to varioussub-catchments (not just the 4 detailed studyareas) throughout Ipswich.

Ipswich Borough Council’s policy (commenced2001) promotes SuDS to ensure new developmentsdo not increase flood risk, and sets standards forflood protection and SuDS. The policy is effectivelya quality system that also includes monitoring,responding to and recording flooding.

As a result of the policy the borough alsoundertakes inspection and maintenance of itsadopted SuDS (mostly at Ravenswood).


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10 Implementation and Review

Grass filter strip and shallow infiltration basin at Ravenswood. No gully pots or grates to block


Ipswich Borough Council’s policy is likely to needreviewing when the developing Suffolk Local FloodRisk Management Strategy is adopted inOctober/November 2012 and when the nationalsustainable drainage systems (SuDS) standardscome into force and the SuDS Approval Body (SAB)is set up (likely to be 2013).

The review should include proposals for designationof significant flood assets.

Other policies, practises or strategies currentlyin place should remain. These were described insection 5.8

l Anglian Water’s capital programme.

l Prioritised list of highway drainage schemes(in Appendix 12.5) should be maintained inthis report.

l Road sweeping focused on flood risk areaswith trees.

l Pre-emptive cleaning of critical gully gratesfollowing receipt of flood warnings.

l Gully pots cleaned regularly by thehighway authority.

l Ongoing education / liaison with highwaydesign teams regarding new speedhumps, other road improvements to ensurethese do not worsen flooding or perhapsalleviate flooding.

l Development Management Policy DM4 of IBC’sCore Strategy and Policies Development Plan(Dec 2011) (13) described in section 10.6.

Other recommendations from the SFRA (June2011) (pages 112 –120) (Italics denote those whichhave not generally been put into practise) include:

l Regular inspection of ordinary watercourses.

l Policy not to permit drainage of surface waterinto land drains or piped watercourses unlessthey have been constructed to an acceptablestandard and have adequate capacity.

l Policy not to allow piping or filling in ofwatercourses.

l In the LDF Green Corridors should includewatercourses sited in gardens or open spaces.

(Core strategy 9.35 refers to safeguarding andprotecting flow routes)

l The existing embankment which retainsHolywells Canal adjacent to LDF sites 70 and 44is in poor condition and is not owned by theCouncil. People and property downwhill,including users of Holywells Road, will be at riskshould a breach occur. It should bestrengthened to adequate standards beforeother works on either site commence.

l Where spring fed watercourses discharge intothe sewerage system, the abstraction and useof the water for irrigation could reduce sewerflows and so provide several benefits – savingmains water as well.

l Policy to resist any plans to raise paving levelsaround the Wet Dock (IBC Core Strategy 9.42).

l Requirements for site specific floodrisk assessments.

l Restrictions on basements in areas susceptibleto surface water flooding (SFRA page 114 andIBC Core strategy 9.41).

l Flood risk management measures using SuDSdescribed in the SFRA include guidanceon what types of SuDS should be used anda geological map.

l Opportunities for strategic SuDS – need toidentify areas suitable for retrofit SuDS beforespaces for water are lost e.g. verges alongsideHeath Road (SuDS scheme implemented)and Bixley Road (highway authority aware butnot implemented).

l Need to identify sites for additional storage forsurface water in flood zone 3 probably close toAnglian Water tanks at Stoke Bridge, AldermanRoad recreation ground and Yarmouth Road.

10.2 Proposed action plan –capital and maintenancel The present value (PV) costs shown below are

based on capital works being implemented in2013. Costs include design fees and 25%contingencies and, if appropriate, operatingcosts for 30 years.

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l PV benefits are the expected reduction in flooddamage costs over 30 years. PVs assume adiscount rate of 3.75% p.a.

l For further information on methodology seesection 8.6.2. For details of scheme costs orbenefits see appendices 12.28 to 12.33.

l Pictures and plans showing some of theproposals follow the table.


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Proposed SuDS in Stone Lodge Park

This plan shows outline proposals super-imposed on the 30 year return period floodextent for the ‘do nothing’ scenario.

Flows from the SuDS are limited to 3 l/sec bya bar screen and orifice.

This plan shows lines representing proposedearth banks superimposed on the 30 yearreturn period flood extent for the optionscenario at 2040.

The lines indicate grassed banks up to 800mm high.

This plan shows floodwater overtopping thebanks. An emergency bypass and weirs wouldbe required to prevent breaching of the banksand ensure the orifice control can easily bemaintained in the event of blockage.

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Grille and ditch at Milden Road

It is suggested the ditch and grille would bebest maintained by residents. (IpswichBorough Council does not own the ditch).Alternatively, the council could undertakeclearance of vegetation and use of anexcavator – subject to agreementand funding.

10.3 Proposed action plan: publicinformation and adviceAn agreed aim for the SWMP was “To providepublic information and advice on flood protectionto improve customer service”.

The agreed engagement plan also proposed apublic exhibition/drop in session to consult localresidents on the proposed action plan. This eventwill be used to provide:

l Flood maps – with limitations explained.

l Information on the developing Suffolk LocalFlood Risk Management Strategy.

l Responsibilities – Anglian Water’s take over ofprivate drainage from October 2011.


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Grille at Ellenbrook

To work effectively the grille needs to be nomore than 10% blocked Do nothing modelsassume 50%.

It is proposed local residents assist in keepingthis clear using a rake or by contactingAnglian Water.


l Responsibilities – who to report flooding orblockages to.

l Advice on flood risk management and waterquality improvement measures that residentscan take:

w Not to obstruct flood paths

w Not to wash cement, fat, oil, or pesticidesdown drains

w Not to increase impermeable pavingof gardens

w Use flood resistance and resiliencemeasures. E.g.: barriers, flood resistantmaterials, flood plans and insurance

w Utilise weather warnings

10.4 Proposed action plan: futureSWMPs for IpswichThe overall cost effectiveness of the SWMPapproach has been considered by adding into theprevious action plan table the costs of developingthe SWMP:

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Overall the SWMP is found to be costbeneficial, even if the benefits forOption 2 at Worseley Close are foundto be marginal.

The SWMP should be extended to look atthe next priority areas:

l Swinburne Rd to Bramford Lane

l Coltsfoot / London Rd / Campion Rd

l Portman Road area

l Maidenhall Approach / Rapier St. /Belstead Avenue / Wherstead Rd

These are all areas where sewerage systemspredominate and so it is assumed Anglian Waterwould wish to be involved and contribute toundertake 2-d modelling.

Anglian Water’s model already includes 2-d meshfor the Swinburne Road area, but this will need tobe revised to allow floodwater to flow fromMeredith Road under the canopy of a filling stationand on to Norwich Road.

Costs for modelling and evaluating options forthese four additional study areas are expected tobe less than for the current SWMP since there wouldbe no development costs or learning curve, nosoftware purchase and no study area prioritisation.Use can be made of the current methodology,workbooks and this report.

Estimated cost is £ 77,000.

If this second stage SWMP is also found to be costeffective, then the SWMP should be extendedfurther to include lower priority areas and so on,until the costs of SWMP development plus thecost of any flood alleviation measures exceedthe benefits.

In each case this report should be updated ratherthan completely re written.

10.4.1 Possible Ipswich or Suffolk-wide action:raised floors

Existing data could be used to assess the likelyaffects of requiring floor levels in new buildings tobe a certain minimum height above ground level.

Floor levels are normally only raised in newdevelopments following a flood risk assessment.Enforcement could prove difficult, as developmentcontrol and building control teams do not routinelycheck floor levels on site. Access requirementsunder part M of the building regulations may alsobe an issue in some circumstances.

10.5 Prioritisation of SWMPsacross SuffolkThe map below shows the four detailed studyareas, subject of this report, coloured in red,superimposed on the coloured squares from thePreliminary Flood Risk Assessment. The squares arecoloured according to the number of propertieswithin the flood map for surface water areas.

There appears to be little correlation between thetop priority study areas and the coloured squares.This is because the SWMP prioritisation processconsidered many other factors.

The process for prioritising study areas described insection 6.11 should be followed across Suffolk,initially concentrating on areas where colouredsquares are clustered together.

The PFRA identifies such clusters at Ipswich,Lowestoft, Bury St Edmunds, Haverhill, Newmarketand Sudbury. The first stage will be to map andidentify study areas / catchments in these townsstarting where the population at risk is highest.

The process for prioritising study areas detailed insection 6.11 should be followed for each cluster.Consideration should be given to updating theprocess using the newer flood maps for surface


water instead of the areas susceptible for surfacewater flooding maps.

10.6 Advice and information forplanning and highway authoritiesThe Planning Inspectorate recently approvedIpswich Borough Council’s Local DevelopmentFramework (LDF). It includes policy DM4 whichconfirms the council will apply the PPS257 floodpolicy hierarchy:

l Assess flood risk

l Avoid development in flood risk areaswherever possible

l Substitute

l Control – using SuDS and implement SWMP tomanage flood risk

Where developments are affected by floodingshown in the strategic flood risk assessment (SFRA)a flood risk assessment is required even if thedevelopment is less than 1 Ha. Note – theEnvironment Agency’s areas susceptible to surfacewater flooding is included in the current SFRA.

Developers preparing flood risk assessments shouldnow make use of the new SWMP mapping. This will be more accurate than either of theEnvironment Agency surface water maps, but isonly complete for localised areas. This report andthe maps should be made available to plannersand developers.

It appears the new flood map for surface watershould replace the areas susceptible to surfacewater flooding map in the SFRA.

The SWMP report and flood maps should beincluded with other planning documents on thecouncil’s website.

Obstructions to the path of floodwater should notbe permitted. Obstructions such as raised groundlevels, speed humps, walls across the flow of watershould not normally be permitted. Effectively thismeans designating mapped flood areas as ‘bluecorridors’ (a concept promoted in the Suffolk LocalFlood Risk Management Strategy).

SuDS designs may need to take into account thenew SWMP flood maps. The SuDS Approval Body(when in place) or borough council will need tocheck that SuDS will not be affected by floodwaterfrom off site.

10.7 Advice for the LocalResilience ForumFlood hazard maps have also been produced foreach of the four priority areas.

“Flood hazard” describes the conditions in whichpeople are likely to be swept over or drown basedon depth and velocity of floodwater (not the rateof rise of floodwater) in a particular event.

The formula below is used to calculate hazardratings across flooded areas. The variation inhazard rating is mapped and can be usedin considering the safety of developments.

The danger classifications of ‘danger to all’,‘danger to most’ and ‘danger to some’ referencesare from HR Wallingford (2005) Flood Risks toPeople Phase 2, The Flood Risk to PeopleMethodology, Environment Agency\DEFRA R&DTechnical Report FD2321/TR1, March 2005.

The following hazard map is for a 100 year returnperiod at 2040 (‘do nothing’ scenario) at GusfordCommunity School, Sheldrake Drive where thehighest hazards to people are predicted.

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7. Now largely incorporated into the National Planning Policy Framework technical guidance


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Below: Putting in new drainage pipe


A visit to the school revealed flooding has beena problem even though no reports have beenreceived by the borough council. The school

management has prevented water entering theschool buildings by blocking off doors on the westside of the building.

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The maps show small areas where the flood depthreaches 300mm with a speed of flow 0.5m/secequating to a hazard rating of 1.3 or a “danger tomost people”.

This flooding will most likely result from a storm witha short duration – an hour or less – so floodwaterwill rapidly appear and disappear in minutes ratherthan hours.

It is suggested the school management are madeaware of the severity of the hazard that floodingmay occasionally present, and be encouraged toplan evacuations to avoid this area. The schoolmanagement should also be encouraged toreact to severe weather warnings now availableover the web.

No critical infrastructure was found to be at risk offlooding within the areas studied by the SWMP.

10.8 Water Framework DirectiveEEC directive 2006/60/EC is designed to integratethe way water bodies are managed acrossEurope. It requires all inland and coastal watersto reach good ecological status by 2015 througha catchment based system of River Basin

Management Plans. These incorporatea programme of measures to improve the status ofall water bodies (seas, rivers, lakes, etc).

The Anglian Region River Basin Management Planprovides example actions for local authorities –‘implement SWMPs’ and ‘promote use of SuDS ,including retrofitting’.

Water quality standards and targets are listed in theplan for the local water bodies shown below:


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Water body Referenceon map

Current Status Target

WesterfrieldWatercourse

R57 Good Good

Orwell R15 Good Good by 2015

Orwell R16 Moderate Target is Good by 2017 but disproportionatelyexpensive, technically “infeasible”

Orwell R17 Moderate Target is Good by 2017 but disproportionatelyexpensive, technically “infeasible”

Orwell R19 Good Good

Mill River R10 Moderate Good chemical by 2015Good rco 2027

Finn R13 Poor Good by 2027

Finn R11 Moderate Good by 2027


No relevant specific measures required to improvewater quality are shown in the RBMP for thesewater bodies.

The SWMP Action plan does not include schemesspecifically designed to improve water quality.However an agreed aim of the SWMP was to “SeekWater Quality Improvements” to assist with the RiverBasin Management Plan.

Most of the actions involve the use of SuDS, whichwould be cost-beneficial. The ditch improvementproposed at Chantry is more akin to maintenancethan capital works and would very slightly increasepeak flows into Anglian Water’s surface watersystem. However, this would also reduce the floodflow that enters the combined sewerage system atHadleigh Road. Here there is a combined seweroverflow structure (which occasionally dischargesdilute sewage to the Gipping) so this wouldprobably also result in a small improvement inriver quality.

Control of creep, put forward as a SWMP action,could also prove beneficial.

10.9 ReviewUnder the Floods and Water Management Act,Suffolk County Council will have responsibility formonitoring and reporting implementation of theaction plan.

Progress will be regularly reviewed to checkwhether actions have been undertakensatisfactorily or new legislation or data require theplan to be updated. As with the Suffolk Local FloodManagement Strategy, this review will take placewithin a year and on a regular basis thereafter,particularly as additional SWMP studies areundertaken and the action plan is expanded.

The Suffolk Flood Risk Management Partnership willcontinue to work together to discuss progress andthe Suffolk Joint Flood Scrutiny Panel will assist themin this respect.

According to the Defra guidance on SWMPs theplan should be formally reviewed and updatedonce every six years as a minimum but there arecircumstances which might trigger an earlier reviewsuch as:

l Occurrence of flooding

l Additional data or modelling becomingavailable which may alter the understandingof risk

l Outcome of investment decisions by partnersrequiring revisions to the action plan

l Additional development or other changeswhich affect surface water flooding

Long term recording, monitoring and reviewof flood records should also be undertaken, asplanned in the draft Suffolk Local Flood RiskManagement Strategy. This should make useof historic records to set the baseline data.

10.10 Lessons for future SWMPs1. Simulation results from a wide range of return

periods are needed to properly assess options.Interpolation of results is sometimes possiblebut time consuming.

2. Modelling partially blocked gullies bysetting flood type in ICM to ‘gully 2d’’ enablesmore accurate modelling of highwayrunoff and may provide useful informationon responsibilities.

3. When estimating the cost of maintenanceoptions there is a problem deciding howfrequently maintenance will be needed andhow to represent the effects of maintenancein a model.

4. Records of how sediments build up or grillesblock during maintenance intervals, togetherwith costs, would assist in evaluatingmaintenance options.

5. The workbooks used in this SWMP for costbenefit analysis should be used forfuture SWMPs.

6. LiDAR data is unreliable in areas covered bytrees and dense undergrowth and is unlikely torepresent small watercourses narrower thanthe LiDAR resolution.

7. Small watercourses should be represented aschannels in 2-d models, with channel depthdetermined by site survey relative tosurrounding valid LiDAR levels. The use of

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a linear connection between mesh andchannel should be investigated rather thanusing dummy nodes.

8. Floor heights above ground level are difficultto assess and there is little point in being tooprecise due to inaccuracies of LiDAR andbecause 2-d models output average groundand water levels at each mesh element.Floor heights above average ground level atthe relevant mesh element were estimatedbased on brickwork courses.

9. Estimated flood depths and damage costsshould not be provided or published forspecific properties but over a wide area.These will provide the best estimate of totaldamage costs.

10. Where areas are identified as being atsignificant flood risk, hazard maps shouldbe produced and residents/emergencyplanners informed.


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Anglian Water will archive and keep the sewermodels used for this study.

All other data is currently saved on Ipswich BoroughCouncil’s Nas06 Server. GIS maps and workspacesare saved on the GIS server – backed up daily.This will be transferred to Suffolk County Councilas appropriate.

Copies of the final version of this report andappendices will be saved on the SharePoint systemand available on both the Suffolk County Councilwebsite (www.suffolk.gov.uk/floodrisk) andIpswich Borough Council’s website alongside theSFRA and LDF documents.

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11 Proposals for Management andMaintenance of Data

Appendices will be available on websites and the SharePoint site.


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12 Appendices


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13 References

1 DEFRA SWMP Guidance, March 2010 www.defra.gov.uk/publications/files/pb13546-swmp-guidance-100319.pdf.

2 Planning Policy Statement 25 (PPS25),Communities and Local GovernmentRevised March 2010

www.communities.gov.uk/documents/planningandbuilding/pdf/planningpolicystatement25.pdf

3 Ipswich Drainage and Flood DefencePolicy, 2002

www.ipswich.gov.uk/downloads/Ipswich_Flood_Defence__Drainage_Policy_rev_Aug_20091.pdf

4 Building Regulation Part H

5 Ipswich Local Development Framework www.ipswich.gov.uk/site/scripts/services_info.php?serviceID=72

6 Ipswich Tidal Barrier Modelling Report,Halcrow, April 2009

7 IBC Level 2 Strategic Flood Risk Assessment,May 2011

www.ipswich.gov.uk/site/scripts/download_info.php?downloadID=1103

8 WaPUG User Note 40, Couple 1D – 2DModelling in Urban Areas

www.ciwem.org/media/144958/WAPUGUser Note 40 Overland flow v01 091104.pdf

9 UKWIR Report, 10/WM/07/14, Impact ofUrban Creep on Sewerage Systems

www.ukwir.org/web/ukwirlibrary/93273

10 M Allitt WaPUG Paper, Investigations intoUrban Creep at 5 Cities, 2009

www.ciwem.org/media/53157/Autumn2009_Paper_9.pdf

11 CIRIA Report 156, Infiltration Drainage –Manual of Good Practice

12 Revitalisation of the FSR/FEH rainfall-runoff method

www.ceh.ac.uk/feh2/FEHReFH.html – defra

13 WaPUG User Note 21, Catchment Data forthe Wallingford Procedure PercentageRunoff Model

www.ciwem.org/media/144861/WAPUG_User_Note_21.pdf

14 WAPUG Code of Practice for the HydraulicModelling of Sewer System v3.001, Nov 2002

15 Suffolk Preliminary Flood Risk Assessment, 2011

16 Flood Defence Grant in Aid (FDGiA)Guidance

www.environment-agency.gov.uk/research/planning/135234.aspx


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17 Multi Coloured Handbook – The Benefits ofFlood and Coastal Risk Management: AHandbook of Assessment Techniques, FloodHazard Research Centre 2010

18 WaPUG IUD Modelling Guide Rev1 v28 www.ciwem.org/media/44495/WaPUG_IUD_Modelling_Guide_Draft_Rev1_v28_(June_09)_v01-001.pdf

19 Flood Risks to People Phase 2 FD2312/TR2.DEFRA and Environment Agency(March 2006)

http://sciencesearch.defra.gov.uk/Default.aspx?Menu=Menu&Module=FJPProjectView&Location=None&ProjectID=12016&FromSearch=Y&FieldOfStudy=12&SearchText=FD2321&ShowDocuments=1&SortString=ProjectCode&SortOrder=Asc&Paging=10&FJP=1

20 EU Water Framework Directive, EEC directive2006/60/EC

http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32000L0060:EN:NOT

21 Flood Risk Regulations 2009 www.legislation.gov.uk/uksi/2009/3042/contents/made

22 Anglian River Basin Management Plan,December 2009

www.environment-agency.gov.uk/research/planning/124725.aspx

23 Suffolk Local Flood Risk ManagementStrategy Draft Feb 2012

www.suffolk.gov.uk/assets/suffolk.gov.uk/Emergency and Safety/Civil Emergencies/2012_01_23 Suffolk LFRMS.pdf

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