st mary’s island flood risks to cmt estates€¦ · the flooding in uckfield in 2000 (upstream...
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Chatham Maritime Trust. The Coach House, Pembroke, Chatham Maritime, Kent ME4 4EU Telephone 01634 891888 | Fax 01634 890808 | E-mail info@cmtrust.co.uk | 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.
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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.
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