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Frankwell Flood Alleviation SchemeShrewsbury
Published by: Environment Agency Midlands Office Sapphire East, 550 Streetsbrook Road Solihull, West Midlands B91 1QT Tel: 0870 8506506 Email: [email protected] www.environment-agency.gov.uk
GEMI0304BHES-E-E
February 2002 flood © Shropshire Star
February 2004 flood © Shropshire Star
SHREWSBURY
Frankwell, September 2002
IntroductionShrewsbury is the county town of Shropshire with apopulation of approximately 100,000 people. It has a richarchitectural and historical heritage and is one of the bestpreserved medieval towns remaining in England. The towncentre stands on a hill in the middle of a narrow neckedhorseshoe loop of the River Severn with the riversurrounding most of the town’s historical centre.
The River Severn has always played a central role in the developmentof the town, and it remains an important asset. Over the centuries, theriver has helped to build the town’s character and its prosperity.
However, it also brings the damaging threat of flooding.
The River Severn is approximately 354 km long, from its source in theCambrian Mountains in mid-Wales to its outflow into the BristolChannel, making it the longest river in Great Britain. By the time itreaches Shrewsbury, it has combined with other rivers including theVyrnwy and Perry and effectively acts as the main river draining thenorthern part of the Cambrian Mountains and the agricultural plain ofnorth Shropshire. This is an area of approximately 2,500 km2.
History of floodingOver the years, development in Shrewsbury has encroached onto the floodplain, resulting inextensive areas at risk from flooding. There are approximately 400 residential and commercialproperties at risk in Shrewsbury. In addition, transport links are severely disrupted duringfloods, thereby isolating the town centre.
The town has a long history offlooding problems with notableevents occurring in 1795, 1941,1946, 1947, 1960, 1964, 1965,1968 and more recently in 1998and 2000. The largest recordedflood was in 1795 whenfloodwater reached about twometres deep in the Frankwell area.The largest flood in living memorywas in 1946, although theNovember 2000 floods camewithin 230mm of this depth.
Historically, a major flood hascaused significant damage onaverage once every ten years,but time between floods can varysignificantly. There has been arecent and dramatic increase inthe number and severity of floodsin Shrewsbury. Since 1998 therehave been eleven flood eventscausing serious property flooding.Records indicate that our winters
are getting wetter, and summersdrier. Whilst this may be a resultof the long-term natural cycle, wecannot exclude that it could be aconsequence of global warming.
In the autumn of 2000, the worstflooding for over 50 years causedwidespread damage along thelength of the River Severn.Shrewsbury was badly affectedand the town was extensivelyflooded three times in the spaceof six weeks. As a result, theEnvironment Agency accelerateda feasibility study to investigatethe provision of flood defences forthe town.
Since the 1950’s a number offlood alleviation options havebeen proposed for Shrewsbury.These included increasing the sizeof the existing river channel,diversion channels, flood storage
areas and flood walls andembankments in the town. In theearly 1990’s, a proposal for ascheme was rejected due toconcerns over the visual impact offloodwalls in the town. However,innovative systems incorporatingdemountable barriers haveprovided a new alternative toovercome such problems.
Frankwell, 1946
Welsh Bridge, 1946
Development of possible solutionsFollowing the autumn 2000 floods, theGovernment pledged additional funds to allowwork to start on flood defences for prioritylocations. The Agency subsequently carried outconsultations and began identifying possiblesolutions to the flooding problem inShrewsbury. Whilst dams, dredging andbypass channels were considered, it wasconcluded that floodwalls and embankmentswould provide the only environmentallyacceptable, economic and technically feasibleoption. By using an innovative demountablebarrier system, where necessary, the visualimpact could be minimised and environmentalimprovements achieved.
Site investigations were undertaken throughoutShrewsbury and studies carried out as part of awider catchment strategic study to help establishpractical solutions. This work confirmed thatconstructing defences to keep out river water wasonly part of the problem. The groundwater andsewerage system in Shrewsbury also needed to beconsidered. Any solution had to consider the effectof flooding from underground strata and from thesewers. Severn Trent Water Ltd, the companyresponsible for the sewerage system in Shrewsbury,worked closely with the Agency to identifypossible solutions.
Frankwell, 2000 Floods © Shropshire Star
View across the river from Frankwell, Shrewsbury, September 2002
Frankwell, 2000 Floods © Shropshire Star
The Frankwell schemeFeasibility studies identified the Frankwell area of the town as the area most likely to beeligible for a flood defence scheme. Frankwell is the first area in Shrewsbury where asubstantial number of properties are affected by flooding. In addition, much information onthis area was already available from earlier investigations, allowing an accelerated programmeto progress.
Prior to the scheme, there were no flood defences in Frankwell. The heights of the existing riverbanks andriverside buildings varied. It was estimated that many properties in Frankwell had no more than a 1 in 3 yearstandard of protection1.
The Frankwell area of Shrewsbury was at riskof flooding through the action of four principalmechanisms:
• river overtopping• groundwater flow/seepage• breaching of existing riverside walls and buildings• inadequate drainage.
The flooding also had other impacts including:
• misery and despair for affected home andbusiness owners
• damage to property and possessions• reduction in property values and problems with
obtaining insurance • social impacts• loss of business and trade in the town centre• traffic and public transport disruption.
The main objectives of the flood defencescheme in Frankwell were to:
• find suitable ways of keeping the higher river floodwater from the land and property that requiredprotection
• prevent flooding of areas by surface waterbackflow of river water up existing outfalls andsurcharging drainage systems
• address the groundwater flow problem by theinclusion of suitable ‘cut-offs’ and drains tominimise and control seepage of water beneathflood defences during times of flood
• minimise the impact on, and where possibleenhance, the local environment.
1 Floods are categorised by their size and the frequency with which they can be
expected to occur. A 1 in 5 year flood is one that has a 20% chance of happening
in any year – this is a relatively minor flood. A 1 in 100 year flood has only a 1%
chance of happening in any year, but its effects can be enormous.
Frankwell, 2000 Floods © Shropshire Star
ConsultationDetailed consultation with all interestedparties took place throughout the durationof the scheme. This included direct liaisonwith individuals likely to be affected, publicmeetings, questionnaires, newspaperarticles, exhibitions, leaflets, press releasesand presentations.
Extensive consultations were also carried out witha wide range of organisations, groups andindividuals including local authorities, Governmentagencies, public utilities, landowners, wildlifegroups, local interest groups, members of thepublic etc.
The Agency also had to apply to Shrewsburyand Atcham Borough Council for planningpermission for the scheme. As part of this process,an Environmental Statement was published forpublic consultation.
Elliot Morley MP, Minister for Environment and Agri-environmentvisiting the town to understand the problem, Frankwell, Shrewsbury,November 2000
The Agency adopted a creativeand innovative approach becauseheritage and aestheticconsiderations were paramount.The resulting scheme combinedtraditional stone clad floodwallsand embankments with newdemountable defences. These areonly erected in the event of aflood. For the majority of the year,when the river is not a threat, theywill simply not be there.The Agency is responsible for thestorage and erection of thedemountable defences and whennot in use, the barriers arestored locally.
The flood defences principallyconsist of steel sheet pileunderground walls up to 16metres deep, which limit the flowof groundwater under thedefences. They also act as afoundation for reinforced concretewalls up to 3 metres high anddemountable aluminium barriers.The demountable defences havebeen designed so that they can beerected progressively. This meansthat when a flood warning isissued the defences are erected ina particular sequence therebyincreasing efficiency and providinga longer time to erect the barriersin advance of a flood. If flooding isimminent, only the demountablebarriers necessary to stop floodingwill be erected. Shrewsbury isfortunate in that the Agency isusually able to provide at least6 – 12 hours warning time.
The longest single section ofdemountable defences is along the‘promenade’ at Frankwell Quay, anarea that has been significantlyenhanced as a result of the scheme.The demountable elements havebeen standardised as far as possible.This means that most of theelements are fully interchangeable.The post spacing is at 3 metreintervals, the same as in Bewdley,and the heights of the posts areeither 1.5 metres (eg. along thepromenade) or 3 metres (eg. acrossthe car park entrance). There is alsoa new exit from the Frankwell carpark that will allow vehicles to leavethe car park even when all thedemountable defences are in place.
The works have also included majorimprovements to the drainagesystem, including the constructionof two new large pumping stationsby Severn Trent Water Ltd. Thesewill reduce the risk of flooding dueto backing up of the drains. Under normal circumstances, thepumping stations will operateautomatically. However, they aremonitored remotely from theSevern Trent Water control roomand can be operated manually if required.
Flood risk can never be completelyremoved. The Frankwell schemehas been designed to provideprotection from a 1 in 100 yearflood. It should be borne in mindthat a more severe flood couldovertop the new defences.
The Frankwellscheme in detailThe Agency submitted the planning application for theFrankwell scheme just seven months after the Autumn2000 floods, whilst much of the detailed design was still inpreparation. This approach allowed our main contractor tostart work on site in January 2002, just over a year afterthe floods.
Building the flood defences, Frankwell, April 2002
September 2002
September 2002
There has been closeco-operation with Shrewsburyand Atcham Borough Councilthroughout. This has resulted inconsiderable attention being paidto the aesthetics of the schemein order to minimise itsenvironmental impact and toimprove the riversideenvironment. The walls have beenclad with carefully selected brickor stone facings and the site hasbeen re-landscaped.
Environmental impacts weremitigated as far as possible andthe scheme offered a number ofenvironmental benefits in additionto the alleviation of the flooding
problem in Frankwell. Theseincluded opportunities to enhancethe local riverside environmentthrough improved appearance ofwalls, higher amenity andecological value throughmitigation planting, maintainingthe pedestrian link betweenFrankwell and the town duringtimes of flood, and improvedstandard of living and generalwell-being for residents andbusiness owners.
Shrewsbury is well known for itshistorical and archeologicalheritage and there are a numberof sites in the Frankwell area,which are of both local and
Environmental considerationsAn Environmental Impact Assessment was undertaken in order to consider the likelyenvironmental effects a flood defence scheme would have on the town. A range of positiveenvironmental improvements have been incorporated into the scheme.
national importance, includingWelsh Bridge. None of thehistorical sites were adverselyaffected by the scheme.
During the construction process acomplex pattern of mediaevalposts were found set into the riversediments south west of WaterLane, downstream of WelshBridge. The posts form parts ofvarious alignments spanning theperiod from circa 1250 – 1300 toperhaps as late as 1600. Manyinterpretations of these posts arepossible, but it seems likely thatthey formed part of a mediaevalfish weir.
Partnershipsand fundingThe Frankwell Flood Alleviation Scheme was approved andfinanced by the Regional Flood Defence Committee. This is anexecutive committee through which the Environment Agencyis required to discharge its Flood Defence activities. Itsmembers are appointed by the Department for Environment,Food and Rural Affairs (Defra), local authorities and theEnvironment Agency. The committee supervises all mattersrelating to flood defence including funding requirements forcapital and maintenance works.
The Defra has been very supportive during all stages of the Frankwellproject. They contributed to the scheme by way of Grant Aid.
The scheme was prepared in close consultation and co-operation withShrewsbury & Atcham Borough Council and Shropshire County Council.
The viability of the scheme hinged on an agreement between theEnvironment Agency and Severn Trent Water Ltd, which was necessaryto ensure that flooding from both the river and the sewers would bealleviated simultaneously. River Severn, Frankwell, Shrewsbury, September 2002
At the beginning of February 2004, thenew demountable defences at Frankwellwere erected and put to the test in a floodsituation for the first time.
Three days of torrential rain in the Welsh mountainscaused the River Severn at Shrewsbury to rise tolevels similar to those of October 1998.
At the peak of the flood on 6 February 2004, thedefences were successfully holding back about 1.9metres of flood water. The new defences preventedthe flooding of up to 74 properties which wouldpreviously have flooded.
The scheme in operation
The futureThe Agency has extended the flood alleviation investigation across the whole of Shrewsbury.This has helped identify costs and benefits of potential solutions in the remaining flood riskareas. Works have to be prioritised nationally on the basis of economics, people and theenvironment. At present, the Agency is not in a position to actively promote permanent flooddefences for other parts of Shrewsbury. However, the situation is under regular review and theAgency will actively promote schemes for Shrewsbury if the opportunity arises in the future.
In the interim, the Agency is undertaking trials oftemporary flood defences at suitable sites inShrewsbury, as part of a wider trial on the RiverSevern. Although temporary defences do not offerthe same degree of protection and robustness aspermanent and demountable flood defences, theycan offer an interim solution in certain locations.
The Environment Agency, Severn Trent Water Ltd,Shropshire County Council and Shrewsbury andAtcham Borough Council have agreed that a JointAction Plan is the most sensible way to deal with anyfuture flooding elsewhere in Shrewsbury. Such a planmakes the most effective use of scarce resources sothat everyone can act together as a single agency.
The aim of the plan is for the four organisations towork in partnership in order to reduce the impact offlooding. This inter-agency partnership successfullyprotected 42 properties in the Abbey Foregate areaof the town during the February 2004 flood, using amobile dam and associated pumps.
Regular river maintenance and monitoring is essentialto ensure that the benefits of the improvement worksare achieved. Environment Agency staff are availablearound the clock to ensure that the defences workproperly. The Environment Agency will continue toissue flood warnings to the public via local FloodWardens and Automatic Voice Messaging.
February 2004
Shropshire County Council aims to maintainexisting road access arrangements into the towncentre with the minimum of disruption. Shrewsbury and Atcham Borough Council aim toprovide sandbags to domestic properties in theaffected areas on a priority basis. They will alsoprovide advice on environmental health issues andco-ordinate the clean up after any flooding.
Severn Trent Water will act upon flood warningsissued by the Environment Agency by sealing offparts of the sewerage system from the river andoperating pumping equipment in order to preventflooding from the public sewers.
The Agency is working on the Severn CatchmentFlood Management Plan and the Fluvial SevernStrategy. This approach will enable a catchment-wide, long-term (50 years) strategic approach to betaken looking at possible solutions along the fulllength of the River Severn. The Agency recognisesthat to reduce the risks associated with flooding, orto combat possible climate change effects, there isa need to address wider issues such as changes inland use, farming practice, development controland urban drainage.
Frankwell flood alleviation scheme – key facts
Number of properties protected from a 100 year flood event: 74
Cost of Agency work: £3.5 million
Scheme commenced: January 2002
Scheme completed: November 2003
Length of ‘demountable’ flood defences: 155 metres
Length of permanent flood defences: 700 metres
Maximum depth of sheet piling: Up to 16 metres
Design and supervision consultants: Black & Veatch (UK) Ltd
Main contractor: Dean & Dyball Construction Ltd
April 2002
September 2002
February 2004
