University of New Hampshire Scholars\u27 Repository
Abstract
The present impacts from coastal storms and high tides grow significantly over time due to SLR even over the relatively short period to 2060. Hydrodynamic model simulations of storm surge with and without sea level rise scenarios show that although flooding and inundation increases with increasing subtidal forcing and higher sea level, dissipation of the tide and storm surge in the estuary channel somewhat limits the maximum inundation that might otherwise be expected in the back marsh areas. The estuary is dominated by high marsh, which lies high in the intertidal zone and by 2060 it will convert to mostly low marsh unless it can build very rapidly (greater than 5 mm/year). The marsh supports fisheries and many charismatic birds, some marsh dependent, and provides a culturally significant view-scape across the estuary. The Sea Level Affecting Marsh Model (SLAMM) was used to predict habitat changes due to 0.73 m SLR by 2060 under different accretion rates and levels of protection for developed areas that became intertidal. Although the relative amounts of high marsh and low marsh varied dramatically, the overall marsh area remained within 5% of the current levels and mostly increased if marsh accretion rates exceeded 2 mm/year. Limited areas of intertidal flats supporting shellfish exist in present day, but in the near future culturally important recreational shellfish areas will convert to open water. However, areas that are currently low marsh will drown and may provide future shellfish areas. The open water harbor is important for boating, access to coastal waters and recreational fishing. Currently, the open water area is small, but may double in size by 2060 and the greater tides relative to the marsh elevation will create a different feel for the estuarine landscape in the future because high tides will regularly cover larger areas of the marsh with seawater.
Outside of the estuary on the oceanfront, beaches and dunes support tourism and intensive recreational use as well as federally protected nesting shorebirds (piping plover, least tern). Most of the outer beach is exposed to Gulf of Maine waters. Where there are existing floodwalls, rising sea levels will worsen the storm danger and damage to the integrity of the walls. Unless walls are raised, storms will also transport massive amounts of beach sediments over the walls and across the barrier system. Beaches will have less width and steep ramping to the walls will severely decrease the value of the beach for tourism. In areas with dune systems, very little change is predicted for this area because the dynamic equilibria of the dune-beach system will allow the beach to build in elevation as sea level rises and wind-driven dune building will continue. Bedrock outcrops (at Plaice Cove, Great Boars Head and the inlet) help reduce landward erosion.
The socio-economic impacts result in more people flooded as sea level rises, particularly of socially vulnerable populations, and more anchor institutions flooded. Residents presently living in the most socially vulnerable census blocks were 8.6 times more likely to be located in the flood zone, compared to those living in blocks with low social vulnerability. Under climate change,census blocks with high percentages of the population living in poverty were 17.7 times more likely to be located in the flood zone. This analysis more likely reflects the winter/spring population than the summer population. The estimated annual expected value damages in the present are approximately 0.90M.In2060withSLRtheyare4.8 M. Using a 7 % discount rate, the present expected value of these damages between 2018 and 2060 is approximately 27M.Therearemanysourcesofpossibleerrorinthisvalueduetomissingdata,andnotincludingdamagestoinfrastructure,humanmortalityandmorbidity,lostbusiness(particularlyrecreation),andothercascadingandmultiplierevents.Wealsodonotincludethevalueofecosystemservices.Theadaptationgoalfocusedonprotectingthesocio−economicsystemsofthebarrierbeachareas.Engineeringapproachesthatonlyusehardstructuresorgreysolutionsmayweighthecommunitiesdownwithseveredebt,resultinlongtermdamagetotheenvironmentanddegradethecharmandattractivenessoftheareatotourists.Ontheotherhand,allowingover−washandmarshmigrationeverywherewillreducethenumberofpeopleandtouriststhatbenefitfromthebeachesanddunes,shellfishflats,marinas,fishingandmarshes.Becausemarshesoflimitedareadonotsignificantlydecreasestormsurgeandthereislimitedwaveactivityonthewestern,inlandsideofthebarrierbeaches,themarshesmaynotdirectlycontributetoreducingthefloodingonthewesternsideofthebarrierbeaches.Regionalsolutionssuchasbuildingabermorafloodwall(smallerfootprintlimitsdirectmarshlosses)tolimitlandlosswillpreventmarshmigration.Floodprotectionbermstoprotectallresidentialdevelopmentborderingtheestuarywillresultinsignificantmarshloss.Individualsitefloodmanagementactionssuchelevatingbuildingsmustbeemployedthere.Themostexpensiveadaptationsareneededonthecoastalsidewherethebeach−dunesystemhasbeenreplacedbyanarmoredshoreline(seawalls)designedforpedestriansandautomobiles,butnotbeachgoers.Thesewallsneedtobefortified;theirexpansionopensanopportunitytoprovidealternatetransportationpathwaysthataresafe(bikelane),greenspace,andamoreattractivepromenade(increasingecosystem/culturalservicesforresidentsandvisitors).Thebeachesneedtobenourishedtoprovidesandyareasathightide(especiallyinthenorthernareas)tobettersupportthetourismindustry.Thetwooceanfrontsectionswithoutwallsordunesystemswereespeciallyvulnerable;thesecouldbenefitfromgreenadaptationsolutionsthatconstructandmaintainsacrificialdunesatrelativelylowcost.Residentialareaslandwardofexistingdunefieldsweredeemedthebestprotectedandonlyrequiredlowcostadaptationdecisions(e.g.,buildingsandbarriersatbeachaccesscutsandmaintainingdunehealth).Heavilyusedroadsthatcrossmarshesoncausewayswillneedtoberaised.Althoughmoreexpensive,roadwayselevatedabovethemarshsurfacewillreduceimpactsfromdirectfillingandprovidebettertidalexchange.ThusthroughouttheHSE,therearelimitedreasonablegreenoptionsforcoastalfloodmanagementhere.Thepresentvalueadaptationcostsin2018includingcapitalandmaintenancecostsdiscountedat7149 M. This adaptation cost is more than the previously estimated damage avoided cost or benefit of 27M.Because,asnotedearlier,thisbenefitestimateissignificantlyunderestimatedduetodataandmethodologicallimitations,wecannotreallystatethisprojectisnotcost−effective;itactuallymaybecosteffective.Alowerdiscountratewouldalsoincreaseitcost−effectiveness.Adaptationwouldmitigatesomeofthedirectimpactstosocialvulnerablepopulations,butinsomeareaswouldrequiretownsinvestingintheprotectionoftheirindividualresidentsinsteadofbeingpartofapossiblylesscostlyregionalsolution.Theadaptationplancouldprovideimportantpublichealthbenefitsthroughtheadditionofthegreenelevatedwalkway(inplaceofcurrentparkingspaces)alongthefloodwall.Wemetwithseverallocalnon−governmental(NG)andmixedgovernmentalandNGorganizationsoverthegrantlifetime.Theygenerallysupportourfindings.Onepossibletroublingpossibilityisthat75Asdescribedabove,themarshesthemselvesarenotmajorcontributorstopresentandfuturefloodprotectioninthearea.Theyare,ofcourse,valuableforotherreasons.Examplesincludehabitat,runofftreatment,recreation,tourism,andcarbonstorage.AnestimateoftheannualvaluesoftheseservicesinHSEareapproximately370 M under present and future SLR conditions. Thus, their preservation should be a priority