st mary’s island flood risks to cmt estates€¦ · the flooding in uckfield in 2000 (upstream...

Chatham Maritime Trust. The Coach House, Pembroke, Chatham Maritime, Kent ME4 4EU Telephone 01634 891888 | Fax 01634 890808 | E-mail [email protected] | Website www.cmtrust.co.uk

StMary’sIsland

FloodRiskstoCMTEstatesUpdatedFebruary2013

ExecutiveSummary

WhilsttheproximityofChathamMaritimetothetidalMedwaytendstoleadpeopletoworryaboutfloodingfromtheriver,theriskofthishappeningisconsideredtobeverylow.

PartsofChathamMaritimearebelowboththepredictedand,moresignificantly,thehistorichighwaterlevelintheRiverMedwayandtheseareasaredependentupontheflooddefences.Maintenanceofflooddefencesisthereforeofconsiderableimportance.

ThisbriefingpaperdescribesthefloodrisksatCMTandattemptstoputthemintocontext.Theconclusionisthattheriskofpluvialflooding(fromrainfall)ismuchhigherthantheriskofcoastalorfluvial(river)flooding.

Theincidenceofextremeintenselocalisedrainfallappearstobeontheincreaseandtheestateisdependentuponthestoragecapacityofadesigneddrainagesystem,whichwouldneedtobeeffective.

ThisreportconcludesthattheflooddefencesaroundtheEstateshouldprovideanadequateandappropriatelevelofprotectionagainstcoastalorfluvialfloodingforsomeyearstocome.

Thispaperwillneedtobereviewedfromtimetotimetoensurethatitreflectsanynewinformationorguidancemadeavailableonthissubject.

StMary’sIslandFloodRiskstoCMTEstateUpdatedFebruary2013

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1 IntroductionTheChathamMaritimeEstateissituatedadjacenttothetidalRiverMedway.Thiscloseproximitytowaterbringswithitobviouspotentialforfloodrisk.

Thispaperconsiderstheextentoftheriskandtheactionstakentominimiseanydangertolifeandproperty.Itisbasedonareviewofpublishedreferencepapersandplanninginformation,detailedinAppendix1.

Thepaperwasfirstpreparedinthesummerof2011andwascirculatedtoTrusteesinFebruary2012.ItwasupdatedinFebruary2013.

2 GeneralFloodRiskIngeneralfloodingcouldoccurfromthreemaincauses:tides(coastal),river(fluvial)andrainfall(pluvial)flooding.

Coastalfloodingcanoccurnearthesea,includingtidalriverestuaries.Tidesaretheresultofsolarandlunargravitationalpullonbodiesofsurfacewaterandtothatextentarepredictable.Extremehightidesoccurduetoacombinationoffactorsincludingdistantevents;off-shorestorms,lowatmosphericpressureormoredramatically,seismicevents(tsunamieffect).Intidalriverestuariestheheightofwaterexperiencedcanalsobeinfluencedbyheavyrainfallup-streamresultinginhighriverflows.Thefactthatthistypeoffloodingoccursduetoacombinationofeffectsmeansthatitisnotpossibletoforecastlongtermpreciselywhenitmighthappen.(Althoughshorttermitwouldonlyoccurduringhightideevents).ThefloodinginLewesinOctober2000isanexampleofthesefactorscomingtogetherwiththeresultthatthetidalRiverOusebrokeitsbanks(overtoppedtheflooddefences).

Fluvial floodingiswhenarivercannotcopewiththequantityofwaterflowingthroughitanditbreaksitsbanks.ThefloodinginUckfieldin2000(upstreamfromLewes)isanexampleofriverflooding.

Pluvial floodingfromlocalisedexcessiverainfallismoredifficulttopredict.InrecentyearsthistypeoffloodingcausedsignificantproblemsinHastingswhereextremelocalisedandrelativelyshortdurationrainfallwasexperienced.TheincidentinHastingsoccurredbecauseapoorlymaintainedculvertrestrictedtheflowofsurfacewater.

AtChathamMaritimeadjacenttothetidalRiverMedwaythereistheobviousriskofcoastalorfluvialflooding;lessobviousistheriskofpluvialflooding,whichcouldoccurifthesurfacewaterdrainagesystemfailstooperateeffectively.


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3 KeyIssuesandHistoricalEvidenceThekeyissuesare:

• Theheightoffuturefloodevents• FlooddefencelevelsaroundStMary’sIsland

• Flooding‘bythebackdoor’throughMedwayPort

• Thedesignofthesurfacewaterdrainagesystem

• Thecorrectoperationofthesurfacewaterdrainagesystem

3.1 TheheightoffuturefloodeventsLookingatknownhistorytheNorthSeaFloodingof1953isthemostsignificanteventinthepast100years.Itwasprobablythemostsignificantfloodingeventforseveral1000years(Anythinggreaterwouldhaveleftevidence,evenifonlyanecdotal).ItwascausedbyaheavystormthatoccurredonthenightofSaturday31January1953andmorningof1February1953.ThefloodsstrucktheNetherlands,Belgium,EnglandandScotlandwithadeathtollinexcessof2000.InMedwaytheeventwasrecordedasalowleveltidalfloodwithlevelsatRochesterBridgereaching4.84MaboveOrdnanceDatum(meansealevel).

Otherrecordedhighlevelswere3.93MODin1927,4.54MODin1949,3.8MODin1960,4.6MODin1965and4.51MODin1978.

InrecentyearstheRiversideWalkhasfloodedoccasionallyandthisindicatesthathighwaterlevels,above+4.0MODanduptoabout+4.2MODareexperiencedfromtimetotime.

Historicallythemajoreventin1953achievedabout300mmhigherwaterlevelthanrecordedbeforeorsince.The1949,1965&1978levelswerecirca600mmhigherthanotherrecordedevents.Recentevents(last10years)havebeenabout300mmlowerthanlevelsrecordedin1965&1978.

Usingthishistoricdataitcouldbesuggestedthatafuturefloodevent300mmor600mmhigherthaneverrecordedbeforewouldbeanextremeevent.

Inrecentyearsfloodpredictionhasbeenthesubjectofsignificantstudy.Concernsoverglobalwarminganditspossibleeffectsonsealevelrise,plustheuseofstatisticalmethods,means


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thatfloodprotectionlevelsaresetsignificantlyhigherthanwouldbedeterminedbyhistoricevidence.

3.2 FlooddefencelevelsaroundStMary’sIslandTheflooddefencelevelsaroundStMary’sIsland,protectingagainsttidalfloodingaredeterminedtosuitapredicted1:1000yearevent,plusestimatedsealevelriseto2050plusanallowancefor“greenhouse”effectsandthedevelopmentofwavesabovethistheoreticalstandingwaterlevel.

The1:1000yeareventisdeterminedbystatistics,beingthe0.1%probabilitylevelbaseduponastandarddeviationcurve.Thisisafactualexercisebasedondatafromthepastandthereshouldbenothingwrongwiththemathematicsinthisrespect.Towhatextentthisisa“real”riskisanothermatter.Itisusinginformationfrompasteventstopredictthefuture.

ThecurrentflooddefencelevelaroundStMary’sIslandis+6.2MOD.Thisis1.36metresabovethehighestlevelexperiencedintheareaandthereforeaverydramatic,unprecedentedstepchangewouldbenecessarytoresultinover-toppingofthecurrentflooddefences.“Globalwarming”orthe“greenhouseeffect”mightcreatesuchastepchangebutthatisyettobeseen.Theriskofover-toppingisthereforeconsideredtobelow(lessthan0.1%forthe“hereandnow”,possiblyrisingto0.1%probabilityincirca2090).

ItisrelevanttonotethatfloodprotectionlevelsarenotstandardisedandmanyareasintheUKaresubjecttofloodriskfrom1:100yearor1:200yearevents.InthisrespectStMary’sIslandisverywellprotected.ItisinterestingthatStMary’sIslandflooddefencesareatthesamelevelasthoseprotectingtheMedwayTunnel,whichislocallyaverysignificantmajorelementofinfrastructure.

StMary’sIslandastidal-washedmarshespre-1858,dividedfromthemainlandbyStMary’sCreek.

StMary’sIslandasitexiststoday,showing3basinsbuiltonthelineofStMary’sCreek.TheredlineindicatestheChathamMaritimeTrustflooddefences.


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3.3 Flooding“bythebackdoor”Accordingtothe1993OveArup&PartnersreporttheflooddefencelevelatMedwayPortisgenerallyatabout+5.6MOD(about600mmlowerthantheStMary’sIslanddefences)butatthedockentrancetheprotectionisonlyatabout+5.15MODforalengthofabout100M.Thislowerdefenceleveloveradefinedlengthcouldberegardedastheweaklinkbeingovertoppedearlierthanotherdefencesinthearea.Thearrangementcouldleadtoflooding“bythebackdoor”andthisscenariowasconsideredbyOveArupbymodellingthelowerdefencesasawidecrestedweir.

Thereport(paragraph6.3.3)considerstheimpactofthelowerflooddefencelevelsintheporttakingaccountofthemitigatingeffectsofstoragewithinthebasins.OvertoppingthroughthegapintheflooddefenceswouldonlyoccurforalimitedperiodatthetopofthetidalcycleandtheovertoppingwaterwouldinitiallyfillBasin3aboveitsnormaloperatingwaterlevel.ThereportsuggeststhatthestoragecapacityofBasin3,aboveitsnormaloperatinglevel,couldcaterforaneventwithriverlevelsupto+5.75MOD.FormoreextremeeventsBasin3wouldoverflowintoBasin2andultimatelyintoBasin1.ThiswouldprovidesignificantadditionalstoragecapacityandtheOveArupreportsuggeststhatthiswouldprovidebufferstorageagainstafloodof+5.95MOD(N)intheriver.

Itcouldbededucedfromtheabovethattheprotectionaffordedbytheeffectsofstorageismorethan1.0metresabovethehighestriverlevelexperiencedinthearea.Thereforetheriskofcoastal/fluvialfloodingremainslow(lessthan0.1%forthe“hereandnow”,possiblyrisingto0.1%probabilityincirca2065).

Theaboveassumesthattheexistingflooddefencesareingoodconditionandwellmaintained.Failureoftheflooddefencescouldaffecttheassessmentdramatically.

Sealevelriseandtheeffectsofglobalwarmingareforwardpredictions.Toadegreetheyare“crystalballgazing”andcouldbewrongineitherdirection.Thereforetheassessmentsneedtobereviewedandup-datedfromtimetotime.


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PhotocourtesyofCountrysideMaritime

3.4 ThedesignofthesurfacewaterdrainagesystemThesurfacewaterdrainagesystemsatChathamMaritimearetidal-lockedandonlydischargetotheRiverMedwayatmid-tidelevelandbelow.Forapproximately6hoursoutofevery12thesystemsrelyuponthestoragecapacityofover-sizedpipes.

ThedrainagesystemsshouldhavebeendesignedtoBritishStandardsandshouldthereforecopewithnormalworstcasepredictedrainfall.Thedesignsshouldhavetakenaccountoftheareaofimpermeablesurfacingandthetimetakenforthewatertorunoffthesurfacingandtravelalongthepipes.GiventhesizeofChathamMaritime,thenatureofthedevelopmentsandthefactthatthesystemsarelockedabovemid-tidelevelthestoragecapacityofthesystemsshouldhavebeendesignedtocaterforquitesevereandprolongedstormconditions.

Thedetailsofthedesignsarenotknownbuttheywerecarriedoutinmorethan12yearsago,beforeChathamMaritimehadbeendeveloped.Thecatchmentareasofactualdevelopmentsand/ortheintensityofrainfallcouldthereforevaryfromthedesigncriteriausedatthetime.Thismightnotbecomecriticaluntilallpartsoftheestatehavebeenfullydeveloped.Giventherisksandtheincidentsofextremerainfallthathaveoccurredinrecentyears(e.g.atBoscastle,Cumbia&Hastings),itwouldbesensibletoverifyandconfirmthesuitabilityofthesystemsfortheestateasitnearscompletion.

TheinstallationofthesystemsisnotcompleteasfurtherlargediameterpipeworkandanotheroutfallarestilltobeconstructedonthewestsideofStMary’sIsland.Anopportunitystillexiststhereforetovarytheprovisiontosuitthedevelopmentifnecessary.


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3.5 ThecorrectoperationofthesurfacewaterdrainagesystemInadditiontoverifyingthatthedesignofthesystemsremainsvaliditisimportanttoverifythattheyhavebeeninstalledcorrectlysothattheyfunctioneffectively.TheimminentadoptionofthedrainagebySouthernWatershouldgoalongwaytoverifyingtheconditionofthesystems.

Itisvitalthatthestoragecapacityinthesystemsisavailableforfivetosixhourperiods,twiceadayeveryday.Ifasystemdoesnotemptythroughthelowtidecycleorifitjustrefillsasthetiderises(duetofailureofaflapforexample)thenadequatecapacitymightnotexisttocaterforanextremerainfallevent.Maintenanceisthereforeveryimportant.ItisworthrememberingthatincidentsoffloodingfromheavyrainfallareoftenfoundtohavebeencausedorexacerbatedbypoormaintenancevistheexampleoffloodingintheHollingtonareaofHastingsinJuly2009,whereanobstructedculvertcouldnotcopewiththeintensityofrainfallexperienced.ThisexampleisevidencethatextremelocalisedrainfalleventsarenotrestrictedtotheWestCountry&Cumbia.

4 EffectsoffloodingandforwardplanningThecopelevel(basinedgelevel)atBasins1&2andthedrydocksisapproximately+4.5MOD(N).Thisdictateslocallevels,isbelowtheflooddefencelevelandbelowthehighestwaterlevelsexperiencedintheRiverMedway.Therearedevelopmentsbothnorthandsouthofthebasinswithgroundfloorlevelsatorbelowthislevel.ThereforepartsofChathamMaritimearedependentupontheflooddefences.

SomegroundfloorlevelsatChathamMaritimeareatcopelevelbutmostresidentialgroundfloorsareaboveabout+5.15MOD(N).

GroundlevelsinthecentralpartofStMary’sIslandarelowerwithpartsofCentralWalkat+3.6MOD(N).Itisbelievedthatfloodingwouldaffectthesepartsfirst.Thisisparticularlythecasewithpluvialflooding.

Assumingtheflooddefencesremaineffectivethestoragecapacityofthebasinsshouldproveadequateevenforextremeevents.IfextremeeventsbecomemoreregularcontingencyplanscouldbeexploredtolowerthewaterlevelsinBasins1to3onthelowtidepriortoanimpendingspringhightide,associatedlowpressureandpredictedNorthSeasurge.Thiswouldprovideincreasedstoragetocoverthepeakevent.ThismightnotproveadequateinitselfanditmightbenecessarytoclosethegapintheflooddefencesattheentrancetoBasin3–astrategicactionoutsidethescopeofCMT.


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5 Conclusions&RecommendationsWhilsttheproximityofChathamMaritimetothetidalMedwaytendstoleadpeopletoworryaboutfloodingfromtheriver,theriskofthishappeningisconsideredtobeverylow.

PartsofChathamMaritimearebelowhistorichighwaterlevelintheriverandaredependentupontheflooddefences.Maintenanceofflooddefencesisthereforeveryimportant.

Theriskofpluvialfloodingisconsideredmuchhigherthantheriskofcoastalorfluvialflooding.Theincidenceofextremeintenselocalisedrainfallappearstobeontheincreaseandtheareaisdependentuponthestoragecapacityofadesigneddrainagesystem,whichwouldneedtobeeffective.

Itisrecommendedthatthedesignofthesurfacewaterdrainagesystemsshouldbereviewedandverifiedasbeingadequateforthedevelopmentasimplementedandnowenvisaged.Thereviewshouldtakeaccountoftheincreasedoccurrenceofhighintensitylocalisedrainfall.ThisreviewshouldbecompletedbeforefurtherdevelopmentiscarriedoutandbeforeconstructioncommencesonthenewOutfall15,sothatthestoragecapacitycanbeincreasedifnecessary.

Itisrecommendedthatthesurfacewaterdrainagesystemsaresubjecttoregularinspections.Itisparticularlyimportanttocheckandconfirmthatflapvaluesareeffective(thatthesystemsemptythroughthelowtidecycleandthattheyremainemptythroughthehightidecycle).Suchsimplechecksshouldbepossiblefromgroundlevelwithouttheneedtoentermanholesandthereforeitwouldnotbeunreasonabletocheckthesystemsatleastonceperyear–probablybestdoneintheearlyautumneachyear.

28thFebruary2013


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Appendix1

1 SourcesofInformation

1.1 “MedwayStrategicFloodRiskAssessment(SFRA)”,MainreportAugust2006,AddendumFebruary2011.Thisreportprovidesanexcellentreferenceforthissubject.ItprovidesbackgroundhistoryandexplainshowtheSFRAwaspreparedanddescribesthefindingsconciselyforthemaingeographicareas.Thestudyrelatestotidal(coastal)orfluvialflooding.

1.2 ChathamMaritime,FloodProtectionReportbyOveArup&PartnersforEnglishEstatesMarch1993.ThisprovidesonoverviewcoveringthewholeofChathamMaritimeincludingtheHistoricDockyardandMedwayPort.

1.3 RecorddrawingsshowingthedesignofthesurfacewaterdrainagesystematChathamMaritime(butsomeareknowntobeinaccurate).Thedesigncriteriaorthedesignintermsofcatchmentareasisnotknown.ThedrawingsbyParkmanaredatedJanuary2001butsomeworkshadalreadybeencompletedbythatdate.

1.4 Recordsandsurveysshowinggeneralsitelevelsasavailable.

1.5 “MedwayFloodDefenceStrategyStrategicFloodRiskAssessmentAddendum”,FinalReportFebruary2011.ThisreportupdatestheSRFAbytenyears,tobaseline2010.

st mary’s island flood risks to cmt estates€¦ · the flooding in uckfield in 2000 (upstream...

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