Origin and stability of tidal inlets in Massachusetts

1 Origin and Stability of Tidal Inlets in Massachusetts Duncan M. FitzGerald Abstract The origin,morphologyandsedimentation processes of tidal inletsalongthe Massachusetts coastare highly variabledue to a wide rangein physicalsettings.The factorswhichhavegovernedtheirdevelopmentandcontributedto thesedifferentmorphologies includewave andtidal energy,sedimentsupply, origin of the backbarrier,bedrockgeology, sea level history, storms,and modificationsby man.Someof thevariabilityof theseindividualparameters can be relatedto the glacial historyof thisregion.With the useof examples, physicaldata setsand morphologicalcasehistories,this paper examinesthe evolutionandstabilityof inlets,primarily onthemainlandcoastof Massachusetts. Introduction The morphologyandphysicalenvironmentof the Massachusetts coastare as diverseas any comparablestretchof shorelinealong North America. To a large extentthe diversityhasresultedfrom the varying effectsof glaciation on a pre-existing,fluvially-erodedlandscape.This hasproducednumerous Formationand Evolutionof Multiple Tidal Inlets Coastaland EstuarineStudies,Volume 44, Pages1-61 Copyright1993 by the AmericanGeophysicalUnion 2 1 OriginandStabilityof TidalInletsin Massachusetts large and small embayments,a wide range in shorelineorientations,and highlyvariablesedimentsupplies.This regionencompasses thebedrock/till dominatedshoresof northwestern BuzzardsBay andMassachusetts Bay and thesandycoastalplainof southeastern Massachusetts andtheregionnorthof CapeAnn. Tidal inletsin thisareaareassociated with manydifferentcoastal settingsincluding large, well-developedbarrier islands,long sandy spit systems,andnarrow,transgressive sandand gravelbarriers(Fig. 1). These inletsexhibita widerangeof sizesandmorphologies whicharerelatedto their hydrographicregime, sedimentabundance,bay size, tidal prism, and manmade modifications(Fig. 2). The origin of tidal inlets in Massachusetts is equally diverseencompassing fivefine processes,barrier breaching,spit accretion, and other mechanisms. During the past 10 yearsmanyharborsalongthe Massachusetts seaboard havefilled to nearcapacitydueto the dramaticincreasein numberof boat ownersandtheirdemandfor boatslips.The existingovercrowded conditions coupledwith futureneedsfor harborageexplainswhy theentrances toharbors arebeingmaintaineddespiteconsiderable costto individualtownsandState andFederalgovernments.More thanhalf of thetidal inletsin Massachusetts haveundergonemodificationprojectsto improvetheirnavigation.In additionto navigationconcerns, it is importantto considerhow the shoalingand closureof tidal inletsimpactshellfishing,theexchangeof nutrientsbetween thenearshore andbaysandmarshes, andthewell-beingof juvenilespeciesof manyfin fishthatusetheinletsandbaysasnurserygrounds.Knowledgeof inletprocesses asdetermined fromfieldinvestigations andhistoricalanalyses is vital in managingtheseresources andplanningfor futurecoastaldevelopment. Tidal inlets are definedas openingsin the shorelinethroughwhich water penetratesthe land therebyprovidinga connectionbetweenthe oceanand bays,lagoonsor marshandtidalcreeksystems.The main channelof a tidal inletis maintainedby tidalcurrents(BraunandGerdtsen,1955).The second half of thisdefinitiondistinguishes tidal inletsfrom large,openembayments or passageways alongrockycoasts.Tidal currentsatinletsareresponsible for the continualremovalof sedimentdumpedinto the main channelby wave action.Thus, accordingto this definitiontidal inletsoccuralongsandy(or sandandgravel)barriercoastlines, althoughonesideof an inlet may abuta DuncanM. FitzGerald 3 7•0 o 7•ø i!t•Salisbury Beach •-.••..•,, Meremack River • _/ "'""'"•'• Plum Island /• Parker '!•'••••al,/E ss ex R•ver /x Ann•squam River Cape Ar ATLANTIC OCEAN Saugus R•ver Massachusetts Bay BOSTON NewInlet MASSACHUSETTS n Harbor CapeCodL•ght 0 I 0 42 ø-- Pamet R•ver•, •,, Plymouth Bay Cape Cod 10 M•les , Harbor Manomet 10 Kms. Bay Wellfleet •,, Nauset HerringR•ver Sandy Barnstable Nauset Spit • -. I• H•11 Buzzards Bay New Inlet Chatham •CotultRiver Harbor Popponessett Waquo•t Bay Harbor , //• Edgartown Nantucket AI / Cuttyhunk Nantucket •, Westport R•ver Harbor Sound lens Pond Slocum River Island I Harbor•• I. Figure 1. Location ofmajortidalinlcts inMassachusctts andothcrsites discussed in thcpaper. Numbers 1-5refertolocations where inlets haveclosed: 1.Shirley Gut,2.South RiverInlet, 3.Scussct MillsCreekInlct,4. EastHarbor Inlct,and5. Katama BayInlct. 1 Origin andStabilityof Tidal Inlets in Massachusetts ß ß ..: , .... .., .:• ....... :.,.. ..... . .. ß•;•;• -•.:.-:•:s::: .%..: Figure2. Obliqueaerialphotographs of: A. New Inlet,Scituate, B. WestportRiverInlet,Westport, C. ParkerRiver Inlet, Ipswich,D. NausetInlet, Eastham,E. PametRiver Inlet, Truro,F. GreenPond Inlet, Falmouth,G. BassRiver Inlet, Dennis/Yarmouth,H. Green Harbor Inlet, Marshfield. Duncan M. FitzGerald 5 bedrockheadland.Sandremovedfromtheinletchanneliscarriedintothebay during the flood cycle forming flood-tidaldeltasor transportedseaward during the ebb phaseforming ebb-tidaldeltas(Fig. 3). The presenceor absenceof thesesandshoals,their size,andhow well they are developedare related to the region's tidal range, wave energy, sedimentsupply, and backbarriersetting.The generalmorphologyof tidal inletsandtheir associatedsandbodiesandtheprocesses thatcontrolsedimenttransportpatternsare discussed in a reviewchapterby Boothroyd(1985) andin a recentvolumeon tidal inletseditedby AubreyandWeisbar(1988). Thispaperwill discusstheoriginandvariabilityof tidalinletsin Massachusettsandwill demonstrate hownaturalandman-madechangestoinletsaffect theirstability.Tidal inletterminologywill follow thatof Hayes(1975, 1979). PhysicalEnvironment To understand thevaryingmorphology, processes, andbehaviorof tidalinlets in Massachusetts, it is importantto evaluatethem in termsof the physical environment in whichtheyhaveevolved.The morphological variabilitythat existsalongthe Massachusetts coastcan be explainedin termsof an area's geologicalsettingandhydrographic regime(Fig. 4). The glacialhistoryof a particularshorelinesegmentdictatesthe sedimentsupplyto the regionand whetherthe coastis rocky or not. Wave energyand tidal rangeof the area influencehowthe sedimentwithintheshorelinesegmentis dispersed. Major stormsandthe windregimeof the areaalsoaffectthepathwaysof sediment transport.Wave and tidal energyalong the Massachusetts coastis largely controlledby the exposureof the shorelineand where it is situatedwith respectto major coastalbays. Tides The coast of Massachusetts can be divided into a number of shoreline segmentsandembaymentsbasedon similartidal range(NOAA, 1991; Fig. 4). The regionincludingCapeCod andMassachusetts Bays and extending northwardto theNew Hampshireborderismesotidal(2.0 < TR < 4.0 m) with 6 1 OriginandStabilityof Tidal Inletsin Massachusetts Figure3. 1976verticalaerialphotograph of EssexRiverInletillustrating themorphology of ebband flood-tidal deltas. meanrangesbetween2.5 and3.1m andspringrangesincreasing to asmuch as3.5m atWellfleetHarbor.Alongtheoutercoastof CapeCodthemeantidal rangegraduallydiminishes to the southfrom 2.7 m at CapeCodLight to 2.0 m at ChathamHarborInlet.Thistrendcontinues alongMonomoyIsland suchthatatMonomoyPointthemeantidalrangeis 1.1m. WithinNantucket andVineyardSounds, includingalongtheislandsof Martha'sVineyardand Nantucket,the tidesare microtidal(TR < 2.0 m) and generallythe range decreases from eastto west. At HarwichPortthemeanrangeis 1.0 m andat FalmouthHeights0.4 m. The shoreline in BuzzardsBay is alsomicrotidal DuncanM. FitzGerald 7 Glaciolacustrine Deposits including Deltas and Lake Bottom Sediments :.2•'•'"'.-?• Glaciofluvial Deposits I•1 • Glaciomarine Deposits Moraines andKames "• .... "'"'"'"'"' TillCovered Bedrock I Holocene Deposits 2.6 Mean Tidal Range 0.29 Mean Wave Height Mean Wave (m) (m) Approach Longshore Transport Direction 2.6 .28 N .3 .49 .25 10 0 Miles CAPE 10 Km COD .49 • BAY 1.8 2.7 0.9 0.6 1.1 NANTUCKET S 0 UND .56 • •.o• 0.9•56 .75 0.8 -• .72 ß Figure 4. Physical setting oftheMassachusetts coast including surficial deposits (fromLarson, 1980; StoneandPeper,1980),meantidalrange(fromNOAA'stidaltables ofNorthAmerica), shallow water meanwaveheights anddominant waveapproach direction (fromJensen, 1983),andnetlongshore transport directions determined fromspitgrowth,erosional-depositional trendsin thevicinityof coastal stmctttres and other coastal features. 8 1 OriginandStabilityof TidalInletsin Massachusetts with slightlylargerrangesthanthe soundsto theeast.At GreatHill nearthe entranceto theCapeCodCanalthemeantidalrangeis 1.2rnandatCuttyhunk Islandnearthe entranceof BuzzardsBay the rangeis 1.0 m. Given thisdistributionin tidalranges,it canbe expectedthattidal inletsare larger,deeperandmore stablealongsandyshorelineswheretidal rangesare large and bay areasare expansive.A comparisonof the large, deepinlets northof CapeAnn versusthe shallowinletsof NantucketSoundillustrates this relationshipwell (Tables 1 and 2). Waves The highlyvariableorientationof theMassachusetts shorelinecoupledwith itsnumerousembayments causesdifferentexposures to incidentwaveenergy (Fig. 4). Temporalvariationsin waveenergyaredueto theseasonal distribution of storms and changingprevailing wind regime. Deepwater wave energiesfor 'thiscoastareknownfrom a wave hindcaststudyfor theregion offshoreof NausetBeach,CapeCod (U.S. Army Corpsof Engr., 1957) and from a wavegaugelocatedwestof CuttyhunkIsland(Thompson,1977).The shallowwaterwaveenergy(depth= 10.0m) for theMassachusetts coasthas been determinedfor 19 stationsusing20 yearsof hindcastdata (Jensen, 1983).The deepwaterhindcastdataindicatethattheoutercoastof CapeCod andthe shorelineto the northaredominatedby east-northeast waveenergy associated with the passageof extra-tropicalnortheaststorms.The shallow waterwavedatacorroborate thisgeneraltrendwith someexceptionsdueto shelteringandwave refractionprocesses. The wavegaugeoff Cuttyhunk,whichrecordedthreepartialyearsof data, indicatesthatthedeepwatermeansignificantwavefor thisregionis0.9 rnand the wave periodis 7.5 sec(Thompson,1977). The shallowwaterwavedata for the southwardfacing shorelinesshowthat the dominantwave energy comesfrom the southand that the southshoresof Martha's Vineyard and Nantucket experiencethe largest waves along the Massachusetts coast (Jensen,1983). Duncan M. FitzGerald 9 Wave energywithin Cape Cod Bay, BuzzardsBay, NantucketSound,and VineyardSoundis low duetolimitedfetch.Thus,waveprocesses alongthese coastsare tied closely to local wind conditions.The northernshoresof BuzzardsBay andNantucketandVineyardSoundsexperiencegreatestwave energywhenextratropicalstormsor hurricanespassto thewestof Massachusettsgeneratingstrongsoutherlywinds. Prevailing southerlywinds also occurin theseregionsduringthespring,summer,andearlyfall months.The southeastern coastsof CapeCodBay,BuzzardsBay,andVineyardSoundare influencedby wavesgenerated by prevailingnorthwest windsduringthelate fall, winter,andearly springmonths(MageeandFitzGerald,1980). The magnitudeand directionof longshoresedimenttransportalong the Massachusetts coastarehighlyvariableandhavebeenestimatedfrom local erosional-depositional patternsaroundcoastalstructures, migrationof inlets, growthof spits,andgrainsizetrends.Net longshoretransportdirectionsare summarizedin Figure 4. SedimentSupply The PleistoceneEpoch dictatedthe sedimentdistributionand abundance alongtheMassachusetts coast(Fig.4) (Larson,1982;StoneandPeper,1982). Reworkingof theglacialdepositsproducedthe sandsupplythatwasresponsiblefor the developmentof the presentday barrierand tidal inlet system. North of CapeAnn, the major sourceof sandfor the coastalzonehasbeen reworkingof theMerrimackRiverdeltathatwasdeposited duringtherelative sea-levellow stand,approximately10,500yrs BP (Edwards,1988). These sediments haveformedan extensivebarriersystemthatextendsfrom Great BoarsHead, New Hampshireto CapeAnn. The coastalregionfrom CapeAnn southto Manometis mostlysedimentstarvedcontainingexl•osures of bedrockwith thintill covers(1 to 3 rn thick) andsomeglacialmarinedeposits.Sedimentis slightlymoreabundantin the vicinity of BostonHarbor and the SouthShorewheredrumlinscomprise muchof the shoreline.The drumlinshavea sandcontentof 30-40% (Newman et. al., 1990). 1OriginandStability ofTidalInletsinMassachusetts 10 Table 1. Characteristics of selectedtidal inlets along the Massachusetts mast. Name of Associated Inlet Location SettinS Barriers Merrimack River Newburyport BetweenBarrier Islands Salisbury Beach, andPlum L•land Parker River Ipswich Between Island and Barrier Spit, and Controlled by Dmmhns Plum Island and Castle Neck Preset Backbarrier Associated Environm •t Rivers andStreams Inlet Mode of Form ation North Shore MerrimackRiver Development of Regressive Marsh and Tidal Creeks Parker and with Op• Water Ipswich Rivers Axeas Development of Regressive Estuaryand Marsh System Bm'riers Between Barrier Spit Castle Neck, Marsh and Tidal Essex and CastleandBedrockOutcrop and Coffin Beach Creekswith OpenWater neck Rivers Areas AnmsquamRiver Gloucester Betwe• Bedrock Outcrops Barriers Axeas Wingaersheek Beach Barriers Development of Regressive Marsh and T•dal Creeks Anni•quamand JonesRivers Development of Regressive Barrier Revere/Lynn BetweenMainland SaugusRiver Point of Pines Spit Marsh andT•dal Creek Saugusand and Barrier Spit Beach Spit Accretion Pines Rivers $oqth $hor• New Inlet Scituate Between Barrier Spit and Drumlin Third Cliff Spit, and Hummarock GreenHarbor P1)mouthBay Cave Co• Marsh and T•al Creeks North and South Rive• Beach Storm Breachingof Barrier, 1898 Marshfield Betwe,mBedrock Outcrop and Barrier Spit Gre•mHarbor Spit Marsh and T•dal Creeks Gre•mHarbor Rivers Spit Accretion Plymouth Betwe,mDrumlin and Barrier Spit DuxburyBeach, SaquishNeck, and Plymouth Spit Spit Accretion Barnstable Betwe,re Barrier Spit SandyNeck Bay with Peripheral Marsh and Some T•al Flats JonesRiver Bay Barnstable Harbor Searot ttarbor East Dennis Between Mainland Op,m Water Areas No Major Streams Spit Accretion and Marsh and Tidal Creeks and Mainland and SesuitBeach Olin Water Areasand Marsh and T•dal Creeks Small Barrier Spit Herring River Eastham Between Mainland and Barrier Spit First Encounter B each Creeks Parnet River TYuro Between Two Harbor Bar Beach Tidal Flats, Marsh, Marsh andTidal Barrier Spits and Tidal Creeks Between Two Marsh and Tidal with somc Open Assessment Spit Accretion Creek No Major Streams Pamet River Spit Accretion Spit Accret•on Outer Cape Cod Nauset Inlet Eastham Barrier Spits No Major Streams Sp,t Accretion Water Areas New Inlet Chatham Betwere Barrier Spit and Barrier Island Nauset Spit/ Nauset L•land Bay with Intemdal No Major Streams Flats Storm Breachingof Barrier, 1987 Chaiham Harbor Monomoy Breach Chatham Chatham Between Two Nauset Island/ Barrier Islands Monomoy Island Between Two Monomoy Island Barrier Bay with •ome Intertidal No Major Streams Spit Accret•on Flats Bay No Streams Islands Storm Breachingof Barrier, 1978 Nantucket Sound Stage Harbor BetweenTwo Barrier Spits Harding Beach, Morriz Island Dike and Spit West D•mniz/ Between Mainland West Dennis Yarmouth and Barrier Spit Beach Spit Cotuit Inlet Barnstable BetweenMainland and Barrier Spit OysterHarbor Beach Spit Bay PopponessetBay Barnstable/ Mashpee BetweenMainland and Barrier Spit PopponessetSpit Bay Santrotand Mashpee Rivers Spit Accretion WaquoitBay Mashpeel BetweenTwo SouthCapeBeach Bay QuashnetRiver Spit Ac•.n'etion UnnamedSpits Bay No Major Streams Artificial Breach, 1951 Slocum Spit Bay with Peripheral Marsh Slocum River Spit Accretion Bay with Marsh and Tidal Flats No Major Streams Artificial Breach, 1986 Ba•s River GreenPond Chatham Falmouth Barrier Spits Falmouth BetweenTwo Bay with Tidal Flats Marsh and Tidal No Major Streams Artificial Breach, 1945 Bass River Spit Accretion Mills River Spit Accretion Creeks Barrier Spits Buzzards Slocum Bav River South Dartmouth BetwetmBedrock Outcropand Barrier Spit Aliens Pond Westport River South Between Two Little Beach/ Dartmouth Barrier Spits Aliens Pond Spit Westport Betwe,mBedrock Outcropand HorsmeckBeach Barrier Island Estuarywith some East and West Branch Marsh and T•dal Flats of WesVportRiver Development of Regressive Barrier Duncan M. FitzGerald 11 Table 2 Name of Structure Inlet Location MerrimackRiver Newburyport ParkerRiver Morphology and stability of selectedtidal inlets along the Massachusettsmast Ipswich and Inlet Improvements Stability Double Jetties Dimensions D•th Prior to Jetty Construction History of Southerly Mtgratton and BreachingBack to North 10 1 Outer Cl•annelMigrates 9.7 None Width 323 Essex None EbbI•ltas Well-Developed, Subtidal Intertidal 926 South, Throat Stable EssexRiver FloodDeltas Well-Developed, Intertidal Stable 12.2 3:54 Well-Developed, Stable 9.4 343 None Stable 20.0 350 None Intertidal Anmsquam River GloucesterDredgedOut• Cl•annel Well-Developed, Intertidal Well-Developed Sub/Intertidal Subtidal/ Intertidal SaugusRiver RevereJLynn Revetmentsalong Inner Cl•annel None, Modification by Man New Inlet Scituate None Stable in Present GreenHarbor Marshfield Double Jettim and DredgedChannel Channel Shoaling Plymouth None Stable 8. • 230 None 8.0 140 None None 20.0 2000 Well-Developed, Well-Developed, Location PlymouthBay Intertidal Barnstable Barnstable None Stable Moderately Well, Subtidal 12.8 1400 Harbor Well-Developed, Intertidal Sub/Intertidal Well-Developed, Sits on Shallow Shelf SesmtHarbor East Dennis Double Jetties and DredgedChannel Channel Shoaling 2 8 83 None None Herring River Eastham None Stable 1.0 35 Well-Developed, Well-Developed, PametRiver 'IYum 2.3 90 Well-Developed, Intertidal Well-Developed, Intertidal History of Northerly and SoutherlyMigration 3.2 26:5 Well-Developed, Intertidal Well-Developed, Intertidal Still Equilibratin8 (see other papers 5.0 150 Well-Developed, Intertidal Well-Developed, Mostly Intertidal Double Jettiesand Significant Channel Dredged Channel Shoaling Sub/Intertidal Outer Cape Cod this volume) Chatham Harbor Monomoy Breach Subtidal Chatham None History of Southerly Migration 60 700 ModeratelyWell, Intertidal Well-Developed, Sub/Intertidal Chatham None ContinuedShoaling 2.0 220 Well-Developed, Intertidal Poorly Developed, Subtidal . Nantucket Sound StageHarbor Chatham DredgedOuter Channel Shoaling 3.0 80 None Well-Developed, West Dennis/ Double Jetties Channel Shoaling Yarmouth andDredgedChannel 3.0 130 None Subtidal, Sits 3.4 240 Well-Developed, Moderately Subtidal Well, Subtidal Channel Bass River CotrotInlet Barnstable Interttdal DredgedOuter Channel Shoaling on Platform Channel PopponessetBay Barnstable/ Mashpee None History of Northerly Migration 2.0 and BreachingBack to North 75 Well-Developed, Intertidal WaquoaBay Mashpee/ Falmouth Double Jetties Channel Shoaling GreenPond Falmouth Double Jettiesand DredgedChannel SlocumRiver South Dartmouth None Allms Pond South OccasionalArtificial Dartmouth Breachingof Spit Poorly Developed, 3.0 35 ModeratelyWell, Sub/Interttdal Poorly Developed Channel Shoaling 2 1 80 Intertidal Subtidal WideningDue to Spit Erosion 34 90 Mod•ately Well, Sub/Intertidal Intertidal, Sits on Intenidal Migrating Westward 1.0 60 None Stable 7.6 260 Intertidal Intertidal Platform when Inlet Westport River Westport Closes Dredged Channel Poorly Developed, Subtidal Subtidal 12 1 OriginandStabilityof Tidal Inletsin Massachusetts Southof Manomet,includingmostof CapeCodandmuchof theMartha's VineyardandNantucket shorelines, sandis abundant duetothepresence of extensive glacialoutwash deposits. Areaswithlesssandresources coincide with coastscomposed of glaciolacustrine deposits(e.g.,partsof southern CapeCod Bay).or morainedeposits(e.g., northernshoreof Martha's Vineyardandthe ElizabethIslands). Thenorthernshoreof BuzzardsBayisalsosediment starvedandischaracter- izedbytill covered peninsulas separated bydeepembayments (FitzGerald et. al., 1987).Sediment isslightlymoreabundant alongthesouthwestern halfof theshoreline dueto thepresence of someglaciofluvialandglaciolacustrine deposits in additionto somethickertill deposits suchasGooseberry Neck,a drumloidal feature offshore of Horseneck Beach. Occurrence of Tidal Inlets Introduction The formationof a tidalinletrequiresthepresence of anembayment andthe development of barriers.In coastalplainsettings, oftentheembayment or backbarrier isformedthrough theconstruction ofthebarriers themselves, like much of the East Coast of the United States or East Friesian Islands of the NorthSea. In Massachusetts, theoriginof theembaymentmayberelatedto drownedfivervalleys,rockyor sandyirregularcoastfines, kettles,groundwatersapping channels, ortheformation ofabarrierchain.In thesesettings, tidal inletsareformedwhentheopeningtotheembayment becomes constricted by barrierconstruction acrossthe embaymentor when an existingbarrieris breachedduringa stormor cut artificially. Varioussettingsof tidal inlet development in Massachusetts arelistedin Table 1 anddiscussed below. Drowned River Valleys Thebestexampleof tidalinletdevelopment in a drownedfivervalleysetting is MerrimackRiver Inlet locatedbetweenSalisburyBeachandPlumIsland Duncan M. FitzGerald 13 (Fig. 1). The MerrimackRiver, whichdrainsmuchof New Hampshireand northeastern Massachusetts, delivereda largequantityof sandto thecoastal regionduringdeglaciation.Much of thissedimentwasdepositedin theform of threemajordeltasat 33 rn and 16 rn abovepresentmeansealevel and50 rnbelowmeansealevel (Edwards,1988). The lastdeltawasdepositedduring theHolocenelowstandandwasformed,in part,throughthecannabalismof the 16 rn elevationdelta (Edwards, 1988). Subsequentdrowning of this erosionalvalley duringthelateHoloceneformedthepresentdayembayment at thefiver mouth.Later,theembaymentwasconstrictedduringtheevolution of Plum IslandandSalisburyBeach,resultingin theformationof Merrimack River Inlet. The major sandsourcefor theseregressivebarriersand the barriersto the southwas the onshorereworkingof the topportionof the 50 rn deltaduringtheHolocenetransgression. The formationof PlymouthBay andlocationof its entranceare alsoclosely relatedto deglaciationprocesses (Fig. 5). As theBuzzardsBay Lobe of the continental ice sheetretreated northward acrosssoutheasternMassachusetts, sometimeafter 15,300 yrs BP (Larson,1982), glacial Lake Tauntonwas formedcovetingan areaof approximately 140 km2. Duringmuchof its existencethe lake drainedto the souththrougha spillwayjust northof Fall River (Larson, 1982). However, after the Cape Cod Bay Lobe retreated northeastward removingthelake'seasterndam,thewaterdrainedthroughthe JonesRiver valley, which was 4 to 7 rn lower than the lake level (Larson, 1982). Presently,thefiver formstheestuarineheadwaters of KingstonBay withinPlymouthBay (Fig. 6). The greatestthicknessof sediments abovethe acousticbasement(> 20 m) in thePlymouthBay areais alongtwo troughs; onecoincidingwith thenorth-south longdimensionof thebay andtheother definingthepresentcourseof PlymouthInlet's mainebbchannel(Hill et al., 1990;Fig. 6). This inferredpaleodrainage systeminsidethe bayjoins with the Postglacialdrainagepatternsoutsidethe bay asreportedby Oldale and O'Hara (1977). Thus,it wouldappearthatdrainageestablished duringthe earlyHolocenehasdictated,to a largeextent,thegeometryof PlymouthBay and the positionof its inlet. The barriersthat front PlymouthBay have evolvedfrom landwardmigratingtransgressive barriersand throughspit accretion fromsediment erodedfromnearbydrumlinsandtill cliffs(Hill and FitzGerald,in press). 14 1 OriginandStability ofTidalInletsinMassachusetts GLACIAL TAUNTON BOSTON LAKES AND CAPE COD -:-/_•GLACIAL LAKE • ICEMARGIN :"'"'• ß MORAINE • DELTA • Truro & Weftfleet Plains ß Brockton Jones R•ver OUTLET Duxbury Provlncetowr •qouth Lake: Eastham Outwash Plain ! / __ Wareham -'j_-::jGlaczal Plain Lake-- C_•_p• __ Harwzch Fall R•ver Plain -'i Mashpee I Whites Brook Barnstable ParkerRiver Plain 0 utlet Woods Hole Nantucket _ Moraine N Martha's Vineyard Plain 0 5 0 10 ml .• 10 km Nantucket Plain Figure5. Mapof southeastern Massachusetts depicting deposits andfeatures of lateWisconsinan glaciafion(modifiedfromLarson,1982). Duncan M. FitzGerald 15 Green Harbor • Greater Depth toAcoustic Basement -- Bathymetr•c Contour Than 20 meters •UXBURY, KINGSTON Drainage of Gurnet Point Glacial Lake Taunton PLYMOUTH 2000 ,0pq Meters Figure6. Map of PlymouthInlet andembayment showingbathymetry andstructure contours of the depthtotheacoustic basement. Notethatthenorthern portionof thebayandtheinletchannelcoincide withwheresediment thickestis greatest.Thetwoarmsarebelievedtobemajorchannels thatwere activewhenGlacialLakeTauntonwasdrainingto theeast(afterHill et al., 1990). 16 1 OriginandStabilityof TidalInletsin Massachusetts RockyIrregular Shorelines The Cape Ann promontory and the northerncoastof Buzzards Bay are themajorrockycoastlinesin Massachusetts.Along theseshorelines pocket beachesare the dominantaccretionarylandformand tidal inletsexistonly where the sedimentsupplywas abundantenoughto develop significant barriers(Fig. 3). The lack of sandalongCapeAnn hasprohibitedbarrierand inlet developmentexceptfor a smallinlet associated with the pocketbarrier of Good Harbor Beach. Sedimentis slightly more abundantalong the northwestcoastof BuzzardsBay andtidal inletsaremorenumerous(Fig. 3; Table 1). This shorelineis characterizedby deeplyincisedembayments frontedmostlyby thin, sandand gravelbarriers. In this regioninletswere formed by spit accretionderivedfrom sedimentthat had beenerodedfrom adjacentshorelines,as well as sedimentmoved onshorefrom nearshore glacial deposits(FitzGerald et al., 1987). The larger inlets in this area, includingSlocumandWestportRiver Inlets,arepositionednextto bedrock outcrops.Severalinletsalongthiscoasthaveclosedin historicaltimesdue to the transgression of the barriers. SandyIrregular Shorelines The originalCapeCod shorelinethatwasformedby risingsealevel during late Holocene(approximately3,000 to 4,000 yrs BP) was probablyhighly irregulardueto thenonuniformtopography of themoraines,outwashplains andotherglacialsedimentsthatcompriseCapeCod. It is likely thatprotoCapeCodhadthesamegeneral"arm"formbutwith numerousembayments andsmallislands(Davis, 1896). This shorelinehasbeensmoothedthrough erosionof headlands, disappearance of someof theislands,anddevelopment of spitsacrossthebays. Numeroustidalinletswereformedasa resultof spit accretionincludingBarnstableHarborInlet, NausetInlet, ChathamHarbor Inlet, andmany others(Fig. 1; Table 1). A basalpeat samplecollectedat a depthof 5 rn belowthe presentmarsh surfaceat ScortonNeck nearthebeginningof SandyNeck wasradiocarbon datedat 3,170 yrsBP (Fig. 7; Redfield,1967). Thisdateandotherswereused Duncan M. FitzGerald 17 by Redfield(1967)to hypothesize thatSandyNeckbeganformingnotmore than4,000yrsBP. Thesandthatcomprises SandyNeckwaserodedfromthe Wareham Pitted Plain, Ellisville Moraine and other surroundingglacial deposits (Fig.5) andtransported southby littoralprocesses. Thesandycliffs alongtheManometandSagamore shorelineareevidenceof thiserosion.It is likelythattheformationof othertidalinletsalongCapeCodalsooccurred approximately 3,000to4,000yrsBP,coincident withrisingsealevelandspit accretion.However, the barriersthat front the otherinletsare considerably youngerthanSandyNeck dueto the transgressive natureof mostof them. This hasbeendocumentedat variousinletson CapeCod by FitzGeraldand Levin (1981), Aubrey and Gaines(1982), Aubrey and Speer(1984), and Giese (1988). Kettles A uniquemeansof coastal bay developmentand tidal inlet formation occurredalongthe CapeCod Bay shorelinein Eastham. This portionof CapeCod iscomposed of theEasthamOutwashPlain(Fig.5) whichcontains numerouskettles. One of the largestof thesekettles(1,200 rn across)is locatedon the coastand formsthe embaymentbehindHerring River Inlet (Fig. 8). Thirty-sixaugercorestakenthroughout themarshsystemlandward of FirstEncounterBeachindicatethatthebaseof thekettleis at least8 rn deep (Fig. 9). The coresrevealthatthemarshpeatsandorganicmudsarethickest in the easternsideof the embaymentand thin towardthe inlet mouthand barrierspit.Themarshdeposits areunderlainbymedium-to-coarse sandsthat are moderatelywell-sorted. The westernthird of the embaymentcontains little or no marsh depositsat the surfaceand is coveredby supratidal vegetation(Fig. 9). The shallowness of thecoresin thisregiondoesnotallow for a determinationof the presenceof marshpeatsat depth(> 2.5 m). The stratigraphy of thekettleandmorphologyof thepresentbarrierspitand tidal inlet systemsuggestthatduringtheHolocenetransgression, rising sea level floodedthe kettleforminga largeembayment(Fig. 10). Sanderoded fromthecoastto thenorthandtransported southbuilt a spitacrossthemouth of theembaymentformingHerringRiver Inlet. A scenariofor the filling of the bay beginswith the contemporaneous depositionof sedimentalongthe fringeof thebay with marshgrowthtowardthecenter,andthedepositionof 18 1OriginandStability ofTidalInletsinMassachusetts 'x I I / I / / x . i \ \ \ \ \ \ I ) • Z C• Duncan M. FitzGerald 19 Figure8. Obliqueaerialphotograph of HerringRiverInlet andmarshsystem.Thisinletislocatedon thepittedEasthamOutwashPlain. Thebayof thisinletwasoriginallya kettlethatbecameconnected to the sea. sheetsandsandflood-tidaldeltasalongthe seawardsideof theembayment. Storm waves overwashing FirstEncounterBeach duringevents like the Blizzardof 1978wouldhaveintroduced largequantities of sandintothebay and may explainthe lack of surfacepeat and organicmud depositsin the easternthirdof theembayment.Sedimentdeposited alongthemarginof the baywouldhavecomefromoverlandsources andfromfine-grainedsediments cardedin suspension by tidal currents.As the bay wasconvertedto high marshwith smalltidalcreeks,thetidalprismwasgreatlyreduced,resulting in a smallerequilibrium inletcrosssectionandelongation of thespitsystem. Spartinamarshpeatscroppingout in the intertidalzone seawardof First EncounterBeachsuggest thatthedecrease in bayareais alsoa resultof the transgression of First EncounterBeach. 20 1 OriginandStabilityof TidalInletsin Massachusetts GroundwaterSappingChannels One of the noteworthycoastalmorphologiesalongthe southshoresof Cape Cod, Martha's Vineyard, and Nantucketis the north-southward trending, floodedvalleysthat form the inlet-associated baysof this region(Fig. 11). The depressions that resultedin the formationof theseelongatedbayswere once consideredto have originatedfrom meltwater streams(FitzGerald, 1985) dueto permafrostconditions(Oldale andBarlow, 1986); recentwork of Marsh Peat :::::::::::::::::::::::::::::::::::::::::::::::::::: '• -"2m ..................................................... zm '•••'•e (srgan,c Muds ::::::::::::::::::::: 0 D.i•i•i•i•i•::•::•::•::•i• iiii::i::::::ii!iBA ::::::::::::::::::::::::::::::: .......................... ::::::::::::::::::::: .......... • • •- / _k_ 0 200 400 meters Figure9. Locationof augercoresandthicknessof marshpeatsandbay-fill muddepositsat Herring River backbarrierregion. Duncan M. FitzGerald 21 Longshore Sand Transport STAGE I Spit Peripheral Accretion Marsh shee•', sands", Development Storm Deposits STAGE 2 Continued Spit Accretion FIRST ENCOUNTER BEACH STAG E 3 HERRING RIVER INLET Figure10. Conceptualmodelof tidal inlet formationandmarshdevelopmentof HerringRiver 22 1 OriginandStabilityof TidalInletsin Massachusetts suggeststhat they developedthroughthe processof groundwatersapping (Caldwell, pers.comm.; D'Amore, 1983). Topographicmapsshowthat the baysare fairly evenly spacedalonga given stretchof shorelineand have a pinnatedrainagestructurewhichis unlikethepatternthatwouldhaveresulted if the valleys formed from a braided streamnetwork associatedwith an outwashplain. Secondly,it is reasonableto assumethat the hydraulichead producedby GlacialLake CapeCod(Fig. 5), whichwasatleast29 rn(Larson, 1982;Oldale,pers.comm.),coupledwith thecoarse-grained Mashpeeoutwash plain would have causedpiping as groundwaterflowed toward the depression whichis now NantucketSound. This processis knownto move sand-sizedmaterialand createchannelsthat migrateheadwardas they develop(D'Amore, 1983). The drainingof Lake Cape Cod would have terminatedthisprocessandrisingsealevelwouldhaveeventuallyfloodedthe streamvalleys.Duringthesameperiod,sandthaterodedfromtheintervalley headlandswould have fed spit systemsthat built acrossthe floodedbays forming tidal inlets. Barrier Chains The Massachusetts coasthastwo major barrierchains;one extendingfrom GreatBoarsHeadin New Hampshireto CapeAnn andanotherthatstretches alongthe outercoastof CapeCod from CoastGuardBeachto Monomoy Island(Fig. 12). Themodeof inletformationalongthesetwochainswasquite differentandrelatedto differences in barrierdevelopment andfiverdrainage patterns. OuterCapeCod Chain ThebarriersformingtheNausetSpit-Monomoy Islandchainformedthrough spitaccretionfrom sedimenterodedmostlyfrom the glacialcliffs northof CoastGuardBeach(Fig. 12) (Fisher,1987; Giese, 1988 andthisvolume). Periodically,stormbreaching hassegmented thesebarrierspits,suchthatat varioustimesthereare two or more quasi-stable inlets. Quite recently MonomoyIslandwasbreached duringthe6-7 FebruaryBlizzardof 1978and NausetBeachwasbreachedduringthe northeast stormof 2 January1987. DuncanM. FitzGerald 23 BOSTON ••.• CAPE COD ..• Cotult• NANTUCKET •½• •"--•• SOUND x'••Oak B/uffs /("•'.•-•Oak• Hbr Inle• Vigeyard• J Bluffs• Haven• • Figure 11. Groundwater sapping channels along thesouthern shores of CapeCodandMartha's Vineyard. Inlets areunstable along thesouth shore ofMartha's Vineyard duetosmall bayareas, small tidal ranges andmoderate wave energy. Onthesouthwest coast ofCape Codsimilar conditions have necessitated theconstruction ofjetties tokeepinlets open andnavigable. 1OriginandStability ofTidalInletsinMassachusetts Coast Guard Beach Great Boars Head Nauset Inlet Hampton Beach • Hampton RiverInlet Seabrook Beach ORLEANS SALISBURY Sahsbury _l Beach Merrimack Pleasant Ba•• River Inlet NEWBURYPORT _,,• -- CHATHAM • Inlet New ChathamInlet Parker • River Inlet •Monomoy Breach IPSWICH Essex River _ _ Annisquam N N 0 I 0 1 Mile i Km 0 0 1 M•le 1 Km Figure12. Majorbardercoasts in Massachusetts. Duncan M. FitzGerald 25 The segmentationof the barriersand the developmentof inlets along this coastarerelatedto a gradualrestrictionof tidal flow throughexistinginlets due to spit accretionand inlet migration(Giese, 1988; this volume). This produces differencesin tidalrangeandtidalphasebetweentheoceanandbays whichcanproducea substantial hydraulicheadacrossthebarrier.Underthese conditionsthe barrieris susceptible to breaching,particularlyduringstorms when the hydraulicheadincreasesdue to the stormsurge. Thinningof barriersisalsoakey factorin controllingwhenspitsarebreached. If the barrier is wide and has a well-developedfrontal dune ridge and secondarydune system,breachingis difficult regardlessof the hydraulic head.In contrast,destructionof theforeduneridgeandthinningof thebarrier allowsbarrieroverwashing,channelization of thereturnflow, andsubsequent inlet formation.Historicalshorelinechangedataof the glacialcliffs northof NausetSpit indicatethat for the periodbetween1938 and 1974 therewere significanttemporalandspatialvariationsin shorelinelocation(Gatto,1978). Thus,it canbereasonedthatduringthesameperiodof time thesupplyof sand to the southernbarriersystemmay havebeenequallyvariable,whichmay haveinfluencedtheretreatandadvanceof the barriershoreline.Changesin the trend of shorelineretreatand advanceare probablyrelatedto natural variationsin waveenergyandthefrequencyof majorstorms.Thus,breaching of theouterCapeCodbarriersystemoccurswhena sufficienthydraulichead has been establishedand the barrier has sufficientlythinnedto facilitate overwashing duringa majorstorm(seeFriedrichset al., thisvolume). Northern Massachusetts Chain Thebarrierchainnorthof CapeAnncontainsfive majorbarriersandfive tidal inlets(Fig. 12). Althoughvariousworkershaveproposed littoralcurrentsand spit accretionas responsiblefor the formationof thesebarriers(Nichols, 1941; McIntire and Morgan, 1964; Rhodes, 1973), these authorswere unawareof the large accumulationof sandthat existsin the Merrimack River delta(vol. = 1.4 x 10 m) 6 km offshoreof thepresentfiver mouthin 50 m of water (Edwards,1988). It is now believedthat sandwhich formedthisbarrier chaincameprimarilyfrom a reworkingof the50 m depthMerrimackmarine deltaandto lesserextentfromthereworkingof otherglacialdepositsonthe 26 1 OriginandStabilityof Tidal Inletsin Massachusetts continentalshelfandsomesedimentdischarged from theMerrimackRiver. Using the shallowseismicreconstruction of the 50 rn delta by Edwards (1988), Som (1990) calculatedthaterosionandonshoretransportof the top 2.5 rn of the deltaduringthe Holocenetransgression couldaccountfor the entirevolumeof sandcomprising thebarrierchainandtidaldeltashoals.It hasbeenwidelyreportedthatmarinedeltascanbe a significantsourceof sediment in development of barriers,includingthecoastsof Maine(Belknap, 1987; FitzGerald et al., 1990), North Carolina (Hine et al., 1979), Georgia (Oertel, 1979), andLouisiana(Penlandet al., 1988). It is believedthatthepresentbarrierchainbeganformingduringthe MidHolocenefrom transgressive barrierscontainingnumerous ephemeraltidal inlets. In a stratigraphicstudyof northernNew England,Mclntire and Morgan(1963) datedthe initial stageof Plum Islanddevelopmentasoccurringsometime priorto 6,300yrsBP. Theotherbarrierstothenorthandsouth probablyformedshortlythereafterfrom sanddeliveredonshorefrom the shelf and from sandmovedalongshore by wave action.As the barriers stabilizedandincreasedin width,tidal inletsprobablydecreased in number andalsobecamemorestable.Tidal inletsalongthischainareassociated with oneor morefiver systems, althoughwith the exceptionof the Merrimack River, they are small and dischargelittle freshwatercomparedto their saltwatertidalprisms(Table 1). Althoughtheriversaresmall,theirvalleys provided ideal locationsfor inlets to stabilizeand the developmentof backbarrier marshes and tidal creeks. The association of tidal inlets with formerfivervalleysis commonalongmanybarriercoastlines (Mortonand Donaldson, 1•973;Oertel,1975;Halsey,1979).Inletsalongthischainare also partially stabilizedor anchorednext to bedrockoutcrops(Hampton, Essex,AnnisquamRiver Inlets). Thus,theinletsalongthesetwo chainsdiffer in thatthe CapeCodinletsare associated with spitsystems, areformedasa resultof barrierbreaching, and tendto migrate. In contrast,inletsnorthof CapeAnn areassociated with a barriercoastthatevolvedfrom transgressive barriers,formedin paleo-river valleys, and are relatively stable. Duncan M. FitzGerald 27 MorphologicalVariability Variability in tidal inlet morphologyalongthe Massachusetts coastis a productof the vastly differentphysicalsettingsunderwhich inlets have formedand evolved. Tidal inletsmay differ from one anotherin size and channelgeometry,shorelineconfiguration,associated sandshoals,backbarriersettingandothercomponents. Many of themajordifferencesamong the inletscan be explainedin termsof varyingwave and tidal conditions (Hayes, 1975; 1979). Sedimentsupplyand tidal prism are otherimportant variablesthatgoverninletmorphology(DavisandHayes,1984).Characteristics of the tidal inlets discussed in this section are listed in Tables 1 and 2. Inlet Size The cross-sectional areaof an inlet is dictatedby its tidal prism (O'Brien, 1931; 1969) which, in turn, is primarily a functionof bay size (openwater area) and bay tidal range. The largestinletsin Massachusetts occuralong mesotidalshorelineswherebackbanSer areasare expansiveandcomposed chieflyof openwater. PlymouthInlet (Fig. 6) is suchan inlet,havingthree largecontiguous bayscomposed of openwaterareasandtidalflats. It hasa springtidal rangeof 3.3 m. Theseconditionscombineto producea spring tidalprismof 1.2x 106m3andaninletcross-sectional areaof 9,160m2(Hill et al., 1990). Otherlargetidal inletsoccuralongthe barrierchainnorthof CapeAnn (Merrimack,Parker,EssexInlets) andin CapeCod Bay (BarnstableHarborInlet). Theseinletshavemesotidalranges(TR = 3.0 m) and largebackbarrierareas(Table 1). Tidal inletsarerelativelysmallalongthemicrotidalshorelines of Buzzards Bay (TR = 1.0 to 1.3 m) andNantucket(TR = 0.4 to 1.2 m) andVineyard Sounds(TR = 0. 5 to 0.8 m). In theseregionsthe low tidal rangesaddedto the diminutivesizeof mostof the inlet associated baysresultin smalltidal prismsand small equilibriuminlet channels(Table 2). Even at Westport River Inlet (Fig. 2) which drains both the East and West Branch of the WestportRiverEstuary,theinletthroatisonly250 rnwidewith a stablecrosssectionalareaof 850rn andanaveragedepthof 3.4 rn(MageeandFitzGerald, 1980). In comparison, thebayareasof boththeParkerRiverandEssexRiver Inlets, north of Cape Ann, are smallerthan that of WestportRiver Inlet, 28 1 OriginandStabilityof TidalInletsin Massachusetts howevertheir inlet throatcrosssectionsare more thantwice aslarge (3,097 m2and 1714m2,respectively; FitzGerald,unpub.data). Along the southerncoastof CapeCodtheeffectof smalltidalrangesoninlet sizeis particularlywell illustrated.Despitetherelativelyprotectedenvironment within NantucketSound,whichproduceslow wave energyand small longshoretransportrates(4,400 m• net easterlytransportin thevicinityof BassRiver Inlet; SlechtaandFitzGerald,1982), mosttidal inletsarejettied and/ordredged.Tidal prismsandtidalcurrentsareinsufficientalongmostof thismicrotidalshorelinetomaintainnavigableentrancechannelsfor pleasure craft. AssociatedSand Shoalsand Backbarrier Settings Sandwhich is dumpedinto the inlet channelby littoral processesandflood tidal currentsis transportedseawardby ebbcurrentsto the ebb-tidaldeltaor movedlandwardinto baysforming flood-tidal deltas. Ebb-tidal deltasare links and short-termrepositoriesin the littoral transportsystemthat allow sandto bypassinlets. Flood-tidaldeltasmy buildverticallyto form intertidal sand shoalswhich subsequentlymay be colonizedby marsh vegetation, resultingin thefilling of thebay (Lucke,1934). Modelsdepictingtidal deltas andinletsettingswerefirstputforthbyHayeset al. (1973) andHayes(1975), originallybasedon the tidal rangeof the region. Later, thesegeomorphic modelsweremodifiedto includetheinfluenceof waveenergy(Hayes,1979; Nummedal and Fischer, 1978). Ebb-tidal deltas In Massachusetts ebb-tidaldeltasarewell developedalongmesotidalshorelinesatmedium-to-large inlets(Table2). At theselocations,likeEssexRiver Inlet (Fig. 3), theebbdeltahasa mainebbchannelthatincisesa broadarcuate accumulation of sandcalledtheswashplatform(Hayes,1975). On topof the swashplatformarewavebuilt swashbarswhichmigrateonshoreeventually attachingto the beach (Hine, 1975; FitzGerald, 1976). The main channel shoalsin a seawarddirectionandisoftenborderedby linearbars.In mesotidal Duncan M. FitzGerald 29 settings wheresandis abundant, swashbarsandchannelmarginlinearbars areoftenexposed atlowtide.At largejettied inletslikeMerrimackRiverInlet the ebb-tidal delta forms too far offshore for intertidal bars to develop. Likewise,at New Inlet alongtheSouthShore(Fig. 2) thepaucityof sandin thisregionprobablyprevents barsfrombuildingverticallyto an intertidal exposure.In contrast, atthestructured PametRiverInletwhereanabundant sandsupplyleaksaround theupdriftjetty, intertidalbarsarewellformed(Fig. 2). Thesmalltidalprismandrelativelyweaktidalcurrents of thisinletresult in theebbdeltaformingin shallowwatercloseto theinletmouth(FitzGerald and Levin, 1981). Ebb-tidal deltas are much more poorly developedalong the microtidal shorelines of BuzzardsBay andNantucketSound(Table2). In theseregions theebbdeltais completelysubtidaldueto relativelysmalltidalprismsand smallertidalrangeto exposethesandshoals.At manyinlets,like Westport RiverInlet(Fig.2), theebbdeltais bestdefinedduringlargewaveconditions which serveto outline its extent. At other inlets, suchas Slocum River Inlet andseveralinletsalongthesouthern coastof CapeCod(BassRiverInlet,Fig. 2), the ebb-tidal delta is moderatelywell developedand visible in aerial photographs becauseit hasformedon a shallownearshore platform. Ebb deltasatsmalltidalinletsalongmicrotidalshores aremostlyabsent(Table2). Flood-tidal deltas MosttidalinletsinMassachusetts havesingularormultipleflood-tidaldeltas, providedthereis enoughspacein thebackbarrier for themto form(Table2). Flooddeltasdeveloplandwardof theinletthroatwheretidalcurrentvelocities diminishdueto an increasein channeldimensions.At inletswherefilling of the backbarrierhasproducedmarshislandsandtidalcreekswith little openwaterarea,flood deltasmay be absent(e.g.,New Inlet, Scituate;Fig. 2). In someinstances, deltasbecomecolonizedandmodifiedby marshgrowthand areno longerdiscernibleasflood-tidaldeltalandforms(cf., FitzGeraldet al., 1990). At jettied inletsandinletswith boatmarinas,flood deltasare often removedtoprovidebetternavigationor spacefor boatmoorings(e.g.,Green Harbor, Scituate,Fig. 2). 30 1 OriginandStabilityof TidalInletsin Massachusetts Flood-tidaldeltasarenormallyhorseshoe-shaped andconsistof a floodramp thatbifurcatesintofloodchannelsthroughwhichsandis transported ontothe deltaplatform(Fig. 3). The ebbshieldwhichdefinesthe landwardextentof the deltais thehighestpartof the deltaandis commonlypartiallyvegetated by Spartinagrasses.This part of the delta shieldsthe restof the shoalfrom effectsof the ebb currents. Sanderodedfrom the ebb shieldby ebb tidal currentsis carriedseawardformingebb spitswhich extendtowardthe inlet throat(BoothroydandHubbard,1975;Hayes, 1975). On the Massachusetts coast,flood deltasare bestdevelopedat large inlets along mesotidalshorelines(Table 2). For instance,flood deltas are well formed with intertidal exposuresat Merrimack, Parker, and EssexRiver Inletsnorthof CapeAnn andat PlymouthInlet andBarnstableHarborInlet in CapeCodBay. TherearemultipleflooddeltasatNausetInlet (Fig.2); their presenceinfluencestheflow of waterthroughtheinlet andthepatternof inlet migration(AubreyandSpeer,1984). Multiple flood deltasare still evolving landwardof thebreachthroughNausetBeachandtheirresultingconfiguration andlocationwill stronglyaffectthepatternsof flow in ChathamHarbor andPleasantBay (FitzGeraldand Montello, 1990; seeotherarticlesin this volume). A largeflood deltaon the westernsideof the MonomoyBreachis presentlyundergoingmodificationdue to the recentclosureof this inlet. Along the microtidalshorelinesof BuzzardsBay and NantucketSound, flood-tidaldeltasareusuallysmallcomparedto thosefoundalongmesotidal shorelines.Commonly,muchof thedeltais subtidalandirregularlyshaped (e.g.,WestportRiver Inlet andGreenPondInlet; Fig. 2). Their diminutive natureprobablyis relatedto smallertidal prismsand weakertidal currents. Stormsarea majorcauseof flooddeltadevelopment alongmicrotidalcoasts resultingfrom the processof barrierbreaching(Pierce,1976) or increased sedimentbeingdeliveredto the inlet coupledwith elevatedflood current strengthassociated with stormsurgedevelopment (FitzGerald,1988). BackbarrierSystems Therearetwo majortypesof backbarrier environments associated with tidal inletsin Massachusetts andthesecorrelate wellwithtidalrange(Hayes1975, DuncanM. FitzGerald 31 1979).Tidalinletsalongmesotidal coasts havebackbarrier areas composed primarilyof hightidemarsh(Spartina patens) incised by majorandminor tidalcreeks.At inletsnorthof CapeAnnandNewInletin Scituate, riversform themajortidalchannel(s) in thebackbarrier (Fig. 12 and2, respectively; Table 1). In mesotidal settings the percentage of openwaterareaand intertidal flatsdecreases awayfromtheinletmouth whilethepercentage of marshincreases (Fig. 13; Som, 1990). Inmicrotidal settings tidalinletsconnect theocean toshallow baysorlagoons (e.g.,GreenPondandBassRiverInlet;Fig. 2, Table 1). In the caseof Westport RiverInlet,thebaysaredrowned fivervalleyswithsomeintertidal flats andmarshareas. Most of themarshislandsoccurbehindthe middle of Horseneck Beachatthesiteofanoldtidalinletandprobably represent floodtidaldeltashoals thatweredeposited beforetheinletclosed(Mageeand •oo• • 80- Lu 60- • 40- ß ,.... • 20- Distance from Inlet INLET (Idlometers) UPLAND THI•OAT Figure13.Distribution ofbackbarrier environments associated withEssex RiverInlet.Notethatwith increasing distance away from theinlet mouth, thepercentage ofmarsh increases while open water areasandintertidal flatsdecrease (afterSom,1990). 32 1 OriginandStabilityofTidalInletsin Massachusetts FitzGerald, 1980; Ibrahim, 1986; FitzGerald et al., 1987). The difference betweenthe marshand tidal creek backbarriersettingof mesotidalinlets versusthe openwater and fringingmarshof microtidalinlets is probably relatedto thelargertidalprisms,strongertidalcurrents,andgreaterpotential of bringingsedimentintothebayatinletswithlargertidalranges.Thegreater tidalfluctuationin thebayalsoproduces largerintertidalareaswhichpromote marshformationand stabilizationof fine-grainedsediment.One exception to this trendis the mesotidalbackbarriersystemof PlymouthInlet (Fig. 1) wherethe bayqscomposedprincipallyof intertidalsandandmud flats and openwater areas(Fig. 6). A derailedstratigraphicand sedimenttransport studyof thisr.egion hasrevealed thatPlymouth Bayhasbeena sediment sink sinceits forrfiation duringtheMid-to-LateHoloceneandthatthebayfill consistsgenerallyof a finingupwardsequence of sandsandmuds(Hill et al., 1990; Hill and FitzGerald, in press). The presenceof intertidalflats and absence of marshes indicate that these environments are not suitable for marshdevelopment.This conditionis likely the resultof the sizeof thebay andtidalrangewhichallowtidalandwave-generated currentsathightideand especiallyduring stormsto inhibit colonizationof the flats by halophytic grasses.Ice gougingof flats duringthe winter may alsobe an operative process.The absenceof saltmarshesdespitethepresenceof expansivetidal flatshasbeennotedalongtheFriesianIslandsin theNorth Seaandbehindthe CopperRiverDelta barriersystemin Alaska(FitzGeraldandPenland,1987). Tidal Inlet Stability Stabletidalinletsarein dynamicequilibriunwith the scoutingactionof tidal currentsandtheinfilling of sedimentdeliveredby longshorecurrents(Inman andFrautschy,1965). However,theequilibriumof theinletchanneldoesnot imply stabilityin positionof theinlet,ratheronly in its cross-sectional area. The sizeof aninlethasbeenshownto beproportional to thevolumeof water flowingthrough it duringahalftidalcycle(tidalprism).Thisrelationship was quantifiedby O'Brien (1931, 1965) andlater refinedby Jarrett(1976) for structured versusunstructured inletsandinletswith varyingwave energy (i.e., Pacific,Gulf andAtlantic Coasts). Duncan M. FitzGerald 33 The stabilityof inletsalongthe shoresof Denmark, NetherlandsandUnited Stateswas examined in detail by Bruun and Gerritsen (1959) and Bruun (1967) and found to be governedby shearstressalongthe channelbottom. Theynotedthatthemagnitudeof shearstressandmaximumcurrentvelocities in thechannelnecessary to flushtheinletof sedimentvariedaccordingto inlet geometry,rate of littoral drift delivered to the inlet, and concentrationof suspended versusbedload.Later investigators suggested thatinletspossess a critical cross-sectional area and if inlet size is reduced below this critical value throughthe influx of sand,it will close (O'Brien and Dean, 1972). While thesevariousrelationships wouldbe usefulin interpretingthe evolution andclosureof certaininletsin Massachusetts, thelack of hydraulicand morphologicdata concerningtheseinlets make theseanalysesimpossible. Therefore, the stability of Massachusettsinlets will be evaluated using historicalinformationandotherdata sources.Effects of varying tidal prism and wave energy,changesin sedimentsupply,inlet closuresandopenings, andjettied inlets will be examined. Tidal Prim and Wave Energy The influenceof tidal prism and wave energy on the equilibrium cross sectionalareaof tidal inletsis illustratedwell alongthe northwestcoastof BuzzardsBay. Thisshorelineconsists of elongatedbaysfrontedbythintransgressivebarriers;the beachridge barrierof HorseneckBeach at Westport River Inlet is a majorexception. As shownin Figure 14, thereis a close correspondence betweenbay size and inlet width, with large bayshaving wider inlets. This relationshipexistsbecausetidal rangeis fairly constant alongthiscoastandbay areacanbe takenasa firstorderapproximation of inletcross-sectional area. The factthatmanyof thebayshavenopermanent connection to theseais a functionof a limitedsedimentsupplyin a regimeof rising sea level (Fig. 4). During the ongoingtransgression the lack of sedimentalongmostof thiscoasthasresultedin a landwardmigrationof the barriers,a process whichisdecreasing bayareaata fasterratethantheupland hasbeeninundatedby risingsealevel. This hasreducedthe tidal prismsof many of the bays causingthe closureof the smaller inlets. This same phenomenonexplainsthe lack of tidal inlets along the elongatedpond 34 1 OriginandStabilityof TidalInletsin Massachusetts shorelines of Martha'sVineyardandNantucket(Fig. 11). If jettieshadnot beenconstructed at manyof the inletsalongthe Cape'sNantucketSound, several of them would have closed. The relationshipdepictedin Figure14 alsoillustratestheimportanceof wave energyin influencingthe stabilityof inlets. Note thatwhile tidal inletsexist at SaltersPondandLittle RiverInlet, thelargerbaysof QuicksandPondand BriggsMarshPond,whichpotentiallywouldproducelargertidalprismsthan theothertwo, maintainno permanentinlets. This apparentconu'adiction in the aforementionedarea/inletwidth relationshipcan be explaineddue to differencesin wave energy. The easterntwo bays that have tidal inlets (SaltersPondandLittle RiverInlet) arepartiallyshelteredfromwaveenergy by headlandsand the offshoreElizabeth Islands. In contrast,the barriers fronting Quicksandand BriggsMarsh Pondsare directly exposedto the prevailing southerlywave climate (Fig. 4). Thus, for inlets along the BuzzardsBay coastthat arecloseto the conditionwhich producesinstability and closure(O'Brien andDean, 1972), it appearsthat slightdifferencesin 012Kilometers '•:. SLOCUM • OU•CKSAND • RIVER • :•. • .. ) •.WESTPO RT : ].' .... • •••[•LITTLE "• '•'• '/A=Sk•= •6'•'(•' POND •MARSH ',', BUZZARDS BAY • A=ekm 2 Figure14. Plotof bayareaversustidalinletwidthfor thenorthwest BuzzardsBaycoast.7hisdiagramillustrates therelationship between tidalprismandinletcross sectional area.Largerbayareasproducelarger tidalprisms which requirelargertidalinletopenings andconversely (fromFitzGerald,1988). Duncan M. FitzGerald 35 waveenergyandlongshore sediment transport ratescancontrolthefateof the inlet (FitzGerald et al., 1987). ChangesIn SedimentSupply Along barrierislandcoastsa decreasein sedimentsupplyleadsto beach erosionanda thinningof thebarrier.Usually,thisconditionmakesthebarrier moresusceptible to stormbreachingandtidalinletformation.As speculated earlier,sucha situationmay havefacilitatedtherecentbreachingof Monomoy Islandin 1978 andNausetBeachin 1987. Along the BuzzardsBay coastin SlocumRiver Embaymenta spit with associated tidalinletwasformedandsubsequently destroyed in a periodof lessthan50 years.The construction of thespitsystemthatformedtheinlet andits laterdestruction werea consequence of a periodof sandabundance followedby sedimentstarvation(FitzGeraldet al., 1986;Fig. 15). Before 1941theinnerembaymentwasopenanda channelexistedalongDeepwater Figure15. The construction anddestruction of SlocumSpitasdetermined fromverticalaerial photographs andfield studies(fromFitzGeraldet al., 1986). 36 1 OriginandStabilityof TidalInletsin Massachusetts Point.Between1941and 1951a spitbeganformingat DeepwaterPointand accretedeastwardacrossthe bay. By 1974 the spithaddeflectedthe main channelto a positionalongPotomskaPointproducinganinlet approximately 100 rn wide. After the mid-1970's the spit began to erode and storm overwashingcauseda landwardmigrationof the barrierand onshoredisplacementof theinletthroat.Thespitwasbreachedin November1984during a springtide andsecondinletwasformedadjacentto Deepwaterpoint(Fig. 16). Sincethat time, the barriercontinuedto migrateonshoreuntil it was transformed into anintertidalbar50 rnlandwardof its 1985position(Mello, pets.comm.). As thisprocessproceeds,the innerbay will rerumto an open waterembaymentandthe tidal inlet will disappear. The sedimentation historywithin SlocumRiver embaymentsuggests thata discretesupplyof sandwas responsiblefor forming the spit systemand SlocumRiverInlet. FitzGeraldetal. (1987) speculate thatthe 1938Hurricane transported sandfrom theAllensPondbarriersintotheembayment(Fig. 14). Once'thesedimentwasinsidethebay,low waveenergygraduallymovedthe sandtowardDeepwaterPoint. A seriesof partiallybuffedbeachridgesin the marshsystemlandwardof the shorewardmigratingbar suggests thatspitand tidal inlet formationprocesshasoccurredseveraltimes in the pastin this embayment. ClosureAnd Openingsof Tidal Inlets NumeroustidalinletshaveopenedandclosedalongtheMassachusetts coast in historictimesandmorewouldhaveclosedif dredging projects hadnotbeen undertakenandengineeringstructures hadnotbeenconstructed. A partiallist of inlet openingsandclosuresis givenin Tables1 and3, respectively.To illustratethe conditionsthatled to inlet openingsandclosureseveralcase studiesare presentedbelow. Shirley Gut Priortothemid 1930's,ShirleyGutwasa tidalinletthatseparated DeerIsland andPointShirleyalongthenortheast shoreof BostonHarbor(Fig. 17A). The earliestsurveysandchartsof thisregionindicatethattheinletwas146rnwide DuncanM. FitzGerald Figure16. Obliqueaerialphotographs of SlocumRiverInletin: A. 1983andB. 1985. 37 38 1 OriginandStabilityof TidalInletsin Massachusetts Table 3. Inlets which have closed (partial list). Location INLET Shirley South Cut River along shown LOCATION Massachusetts Figure 1. Closed field in Closed 1934-36 During Northeast in Scusset Mills Sagamore Closed Cape was East Harbor coast HISTORY Winthrop Marsh the in Province town Closed a Storm 1898 when God the Canal Built in Forming !869 Pilgrim Lake Katama Bay Martha's Vineyard Closed 1915, in 1869, 1934, 1969 at theinlet throatin 1860 and 10.7m deepin 1847, shoalingto 7.2 rn by 1861 (Nichols, 1949;Figs. 17B andD). During the next70 to 75 years,the inlet narrowedand shoaledandby 1934 the inlet channelwas barely subtidaland only 25 rn wide at meanhigh water. A narrowisthmus(30 rn wide) joined Point Shirley and Deer Island in 1936 (Nichols, 1949). Eventuallythe isthmuswasbroadenedandfilled toprovidebetteraccessto facilitiesonDeer Island. The inlet closedsometimebetween1934and 1936andit hasbeensuggested thatstormprocesses contributed greatlytofillingtheinlet(FitzGerald,1980). The short-termstabilityof thechannelcrosssectionduringvariousperiodsof theinlet'shistory(1861 to 1869,Fig. 17B) suggests thatundernormalwave conditionsthe inlet was probablystableand tidal scourwas sufficientto removesedimentdumpedinto the channelby longshoresedimenttransport. However, during stormsstrongerwave energieswould have substantially increasedthe transportof sandandgravelfrom alongDeer IslandandPoint Shirleytowardtheinlet. DuringtheBlizzardof 1978thisregionwasthesite of considerable deposition,includinglargegravelwashoversgreaterthan1 rn thick (FitzGerald,1981).Althoughsomeof thesedimentdumpedintothe inlet duringstormswouldhavebeenremovedby increasedcurrentscaused by theaccompanying stormsurge,muchof thesedimentprobablyremained. The reasonfor thisis thatduringstormsmostof theincreasedflow into and cutof northernBostonHarborwasaccommodated throughPresidentRoads Duncan M. FitzGerald 39 channel.Thus,atShirleyGuta disequilibrium wasestablished duringstorms betweenthe volume of sedimentdeliveredto the inlet and the quantity of sedimentscouredby tidalcurrents;thisconditionled to closureof theinlet. Katama Bay Inlet KatamaBay Inlet on the southeastern shoreof Martha'sVineyard(Fig. 11) hasopenedandclosednumeroustimesduringthe past150 years(Ogden, 1974). As seenin Figure18,breachingof NortonPointspitnormallyoccurs in the middleof the barrierandis commonlyassociated with major storms B. C H ELS E A.-2:.::' _,0.... ,n 1739 MEANHIGHWATER 1860 U S Coast Survey 1898 Harbor and Land Commission 1934•lass Dept--of Pubi:•-Wo•<• 2• 4oo O. Point Shirley Profiles of Shirley Gut Deer Island Approximate ••.• -............__•• / • "•'. '-.. '• .. x-x.'-.,\ , ,.,• / ",xt, ,. \ _ ..!,.,' _ ./..............-:,,. x, ........• ............ ß ß ß ':. • High Wate? • A. ppro,x.•rnate -+1o ........ 0 / '" •o • 1934•- Mass Dept of PubhcWorks 2-3o ...... 1865 ........... 1861 Harbor andLand Commission --4o 1•7 Harbor and LandCommission • I(• feet 1869 SostonWater Works - Harbor and Land Commission Figure17. Historicalaccountof theclosureof ShirleyGut atBostonHarbor:.A. earlymap of the regionin 1739, B. andD. morphologicalchangesof the inlet channel (from Nichols, 1949), and C. stormprocesses responsiblefor closingtheinlet (from FitzGerald,1980). 40 1 OriginandStabilityof TidalInletsin Massachusetts (Ogden,1974). The openingwhichformedin 1886 was causedby a severe Januarynortheaststorm (Whiting, 1887). Breachingsof the barrier, in approximately thesamelocation,wereproducedby the 1938Hurricaneand HurricaneCarol in 1954. The FebruaryBlizzard of 1978 openeda small breachalong the westernpart of the barrier, but this incipientbreach immediatelyclosed(HansonandForrester,1978). Man-madecutsthrough thebarrierwereattemptedin 1871,1873, 1919and1921;onlythelastof these wassuccessful (Ogden,1974). After aninletis cut,it migratesto theeastand eventuallyclosesasthespitattaches toWasquePoint. Inletclosureoccurred in this manner in 1869, 1915, 1934 and 1969; since 1969 it has remained closed. Norton Point"• '"'""'•'::' '-' ' 1942 1951 KATAMA BAY\ Wasque ,.::-" .\ "::":': ..... u•" 0 1000 Meters -1776 --- 1846 Chff " -- 1948 ,-,.--, 1969 Figure18. Shorelinechanges at KatamaBay, Martha'sVineyard(fromOgden,1974). Duncan M. FitzGerald 41 Theinstabilityof KatamaBayInletisrelatedtoa numberof factorsincluding: a strongeasterlylongshoretransportsystem,a smalltidalrange(TR - .8 m), the shallowness of the southernendof KatamaBay thatincludesnumerous intertidalshoals,and a northerndeepchannelopeningto KatamaBay at Chappaquiddick Point(Fig. 11). The easterlymovementof sandalongthe southernshorelineof Martha's Vineyardproducesan eastwardextensionof theNortonPointspitandaneasterlymigrationof KatamaBay Inlet. As the inletmovesfartherto theeast,themaintidalchannelin thebayelongatesand flow at theinlet becomeslessefficient(cf., Keulegan,1967). Also, repeated historicalmigrationsof the inlet haveproducednumerousflood-tidaldelta depositswhich obstruct flow andprovidean intertidaleast-westbarrier betweenthe northernandsouthernportionsof thebay. The mostimportant factorwhichhasled to thehistoricalinstabilityof theKatamaBay Inlet is the presenceof therelativelydeepinlet channelwithin EdgartownHarbor (Fig. 11). Most of theKatamaBay tidal prismis exchangedthroughthispassage. If thisopeningdid notexist,theephemeralKatamaBay Inlet wouldbe much largerandwould probablyremainopen. Allens Pond Inlet The historyof AllensPondInlet on the northwestern coastof BuzzardsBay is depictedover a 46-year periodin Figure 19. During this time the inlet repeatedlymigratedwestwardat an averagerate of 100 m/yr, slowing considerably whenit reached afarwesterlyposition.Thewestward migration of the inlet is in oppositionto the dominanteasterlylongshoretransport directionandis causedby theconfiguration of thebackbarriermainchannel, whichrunsparallelto theeasternspitbeforeturningsouthward at theinlet mouth (Fig. 19). Ebb flow in this channelis directedat the westerninlet shoreline,causingerosionof thewesternchannelbankin a mannersimilarto thatof thecutbanksideof afivermeander.Thewesterlyaccretingspitis the point bar sideof the meanderbendandreceivesits sedimentfrom sandthat hasbeenerodedat theinletmouthandtransported seawardto the eastward longshoretransportsystem(FitzGeraldet al., 1987). This modeof updrift inletmigrationhas'•eendesoribed by AubreyandSpeer(1984)asoneof the mechanisms reponsible for thenortherlyupdriftmigrationof NausetInlet. 42 1 OriginandStabilityof TidalInletsin Massachusetts 1934 <•G_•--" • :,/•-•,/•__.,• Figure19. Historicalshorelinechangesat AllensPondInlet. This inlet migratesupdriftand periodicallyclosesuntil a channelis cutat theeasternendof thespit(from FitzGeraldet al., 1987). Duncan M. FitzGerald 43 Astheinletmigrates tothewest,flowin theelongating inletchannelbecomes increasingly inefficient(Keulegan,1967). Whenit reachesa far westerly positionasit did in 1934,1951,1962,and1977(Fig. 19),theinletnarrows andeventuallycloses.Becausethe bay behindthe inlet, AllensPond,is a productive shellfishing area,theTownof SouthDartmouthdredgesa new inletattheeastern endof thespitwhenever thishappens. Since1980,aninlet hasbeencutthroughthespitin 1985andagainin 1989.Whentheinletclosed mostrecently,arainyautumnresultedin themarshbeingfloodedby approximately30 cm of waterandsalinityin thebaydroppingto 7%0(Mello, pers. comm.). Thus,to maintainanopeningat AllensPondInlet, a newinletmust be cutevery5 to 10 years. BriggsMarshPond andQuicksandPond BriggsMarshand QuicksandPondsare elongatedbayssituatedon the northwestern shoreof BuzzardsBay (Fig. 14). Althoughthe pondsare actuallylocated ontheRhodeIslandcoast,theinletsthatopenandclosealong their shores are similar to the behavior of some inlets on the southern coasts of CapeCod,Martha'sVineyardandNantucket(Fig. 11) andthus,theyare includedin thediscussion here.Asexplained earlier,thesmallpotentialtidal prismsof thesepondsandtheexposure of thiscoastto waveenergyprohibit permanentinletsalongtheir barriershores.However,significantinflow of freshwaterduring late winter and early spring,which is derivedfrom precipitationandmeltingsnowandice, producesoutletsat thesesitesand otherpondsalongthiscoast(FitzGerald etal., 1987).Thedischarge channels arenarrow (<20m')andformwhenthepondovertops theheight ofthebarrier, usuallyatthesiteof a previousinlet. Thedepthof theoutletchannel,which seldom exceeds meanlowwater,andoveralldimensions of thecutthrough thebarrieraredependent on thevolumeof waterdischarged andhydraulic headbetweenthepondandoceanlevel. Becausethebarriersthatfrontthese pondsarecomposed of sandandgravelandarecommonlyporous,some wateris continuallydischarged throughthebarriersthemselves. Astheoutletsbecome fullydeveloped, tidalexchange between theoceanand bayestablishes tidalinletprocesses including theformation ofrecured spits, inlet migration,andthe development of flood-tidaldeltas(Greacenet al., 44 1 OriginandStabilityof TidalInletsin Massachusetts 1983;Fig. 20). Similarprocesses havebeendescribed atothertidalpondson theRhodeIslandcoast(Boothroydet al., 1985). Tidal inletsalsodevelopat thesepondsduringintensestorms suchasthe1938HurricaneandtheBlizzard of 1978. Regardless of how the inletsformed,theyareephemeralfeatures whichclosesoonaftertheyopen,usuallywithinseveralmonths.As seenin themapsof QuicksandandBriggsMarshPonds,theconstruction of floodtidal deltasis an importantprocessin impedingflow throughtheinletsand causingtheirclosure(Fig. 20). { : Salt Marsh •j• Supratidal Grasses and Shrubbery *•rr Beach Grass (Ammophila) "'"'½• Unvegetated Sand and Gravel ß I•.•, ',•....:•BeachRidges ....... '"'• Flood-Tidal Delta Complex Tombolo Components '"' ":"'•Sand Bedrock Island •t Quicksand Po : 0 100 200 Meters Figure20. Sedimentaryenvironments of thebarriersfrontingBriggsMarshandQuicksandPonds. Emphemeralinletsform alongtheseshoresdueto stormsandthedischargeof freshwater.Note the development of flood-tidaldeltaswhichrestrictflowthroughtheinlets(fromFitzGeraldetal., 1987and modified after Greacenet al., 1983). Dtmcan M. FitzGerald Scusset Mill 45 Creek Inlet Priorto theconstruction of the CapeCod Canalin theearly 1900's, Scusset Mill Creek emptiedinto Cape Cod Bay throughScussetBeach (Fig. 1). However, after the canal was completedand jetties were constructedto stabilizethe entrancechannel,the inlet closed. In digging the canal the headwardportionof ScussetMill Creekwasconnected to thewaterwayand thusitstidalprismwassimplydivertedtothecanal.Theaccumulation of sand updriftof the northjetty causeda 200 rn progradation of ScussetBeachand all tracesof ScussetMill CreekInlet disappeared. New Inlet, Scituate New Inlet is locatedat theconfluenceof theNorthandSouthRiversalongthe Massachusetts southshore(Fig. 21). The presentinlet is anchorednext to FourthCliff, oneof severaldrumlinsalongthis sectionof shoreline.In its earlyhistoryNorth River wasknownfor its shipbuilding andan American piratewho preyedon unsuspecting merchantshipsjourneyingto andfrom Bostonduringthe 1700's. At thattimetheentranceto theSouthandNorth riverswas5.5 km southof FourthCliff alongthesouthernendof Humarock Beach(Fig. 21). Theinletremainedin thatlocationuntil 1898,whena winter stormbreachedthesandybarrierthatjoinedThirdandFourthCliffsandNew Inlet wasformed. After theoriginalbreach,theinletmigratedslightlyto the southandstabilizedagainstFourthCliff. Much of the sandthat originally comprisedthe barrier beachbetweenThird and Fourth Cliffs has been reworkedonshorein theformof twotransgressive spitsthatextendsouthwest from Third Cliff and northeastfrom Fourth Cliff (Fig. 21). New Inlet is presentlyborderedon the northby a wide, low beachand intertidalshoal complex(Fig. 2). The stabilityof New Inlet will dependon thelongevityof FourthCliff andthe futureof HumarockBeach.The thin partsof Humarock Beachandits overallerosionalhistoryhaveleft thisbarrierhighlysusceptible to breaching,especiallythe neckregionwhereSouthRiver impingesalong the backsideof the barrier(Fig. 21 ). 46 1 OriginandStabilityof Tidal Inletsin Massachusetts Scituate 3rd \ Cliff \ Inlet Opening 1898 • / ) ATLANTIC NEW INLET OCEAN \ \ \ Marshf•eld 1000 Meters Inlet Closed 1898 Figure21. Thenortheast stormof 1898causedthebreachingof thebarrierbeachbetweenThird and Fourth Cliffs. As New Inlet became established,the old inlet at the southernend of Hummarock Beach closeddue to its greatlyreducedtidal prism. Duncan M. FitzGerald 47 Jettied Inlets Thereare morethan}20jettiedinletsin Massachusetts, mostof them concentrated alongthemicrotidalshorelines of NantucketSound,Vineyard Sound,and BuzzardsBay (Table 1). Jettiesare constructed to improve navigation through tidalinletsanddepending upontheirengineering design, theymaystabilizeamigrating inletoritsmainchannel, preventorrestrictthe longshore transport of sandfromenteringtheinletchannel, provideprotection from wave action,andconstrictor directthe ebbflow of an inlet to scour a deeperentrance channel.Jettyprojects in Massachusetts havehadmixed results,but in few instanceshave they providedthe ultimatesolutionto navigation problems andinmanycases theyhavecreated additional sedimentationanderosionalproblems.The effectsofjettiesalongtheMassachusetts shorelineare discussed usingseveralinlet casehistories. Merrimack River Inlet Tidal inlets situatedalong coastswhere the longshoretransportrate is comparable or greaterthanthe potentialsedimentdischarge in the inlet channel,oftenmigrateor havemain ebb channelsthat are deflectedin a downdriftdirection(BruunandGerritsen,1959;Oertel, 1975). This conditionwasprevalentatthemouthof theMerrimackwheretheinlethada history of southerly migrationfollowedbyabreaching backtothenorthpriortobeing stabilizedby jettiesin 1881(Fig. 22; Hubbard,1975;U.S. Army Corpsof Engr.,1976). Since1881,thejettieshavebeengraduallyextendedsothat now the northjetty is 1,250m longandthe southjetty is 745 rn long.The shoreline hasprograded on bothsidesof theinletdueto trappingof littoral drift moving southalong SalisburyBeachduringnortheaststormsand movingnorthalongPlumIslandby wavesthatrefractaroundtheebb-tidal delta (Hubbard,1975). After the initial progradationof the beach,the northernshorelinestabilizedas furthersandaccumulationagainstthejetty wasremovedby stormwavesandtransported overthejetty into the inlet channel (Hubbard, 1973). The northernendof PlumIslandhasa peculiarshape,in thata basinexistsin themiddleof thebarrier(Fig.22). Bathymetric surveys indicatethatthebasin 48 1 OriginandStabilityof TidalInletsin Massachusetts ! Salisbury Beach t•\ I ! ß ß ß I I .1' ! I ........ 1827 .... 1851 ----- 18 9O 1931 I MœR ' "' " • • ,t ... R/M, Zl'"• '' ,,,,-x'••_.--,, : RIVER '•' / I. "'.....';::.,-.•. • ',, Jj i ""; •......... . ' \\ ..' !(, '). ,,\ ..' ß ' ,•, ""4, ..•.•,, ß ß.. •,, lOOO i ' 'X:•...-.•... \ \ ".. : t x,, ,,..., '•,, ,,•.,, .:•.. ß \ \• \\!' \ ' ....,•• •,,,•.-•• Feet "',,,j ß •... PlumIsland . Figure22. Shorelinechanges at MerrimackRiverInlet (fromNichols,1941). Duncan M. FitzGerald 49 is almost9 rn deep(at meanlow water). Althoughsomeof thedepthmay be attributedto dredging,it appearsas thoughthe basinrepresentsa former positionof theMerrimackRiver Inlet channelthatexistedsometimebetween 1827and1851. Nichols(1964) suggested thatthebasinformeddueto a retrogrationof northernPlum Island,followedby developmentof a spit along northernPlum Islandwhichbuilt northward.This scenarioseemsunlikely becausethedominantlongshoretransportdirectionis to thesouthin thisarea (Hubbard,1975), andthusthe spitwouldhavebeenaccretingin opposition to this. It appearsmore likely that ebb-tidaldelta breachingtook place at this inlet (Fig. 23) (cf., FitzGerald, 1988). In this model, the southerlylongshore transportof sedimentcauseda preferentialadditionof sandto the northern, updriftsideof theebb-tidaldelta(Fig.23, Stage1). In turn,thisaccumulation produceda southerlydeflectionof the main ebb channelanderosionof the northeastend of Plum Island (Fig. 23, Stage2). In this configurationthe circuitouspathwayof themainchannelproducesinefficientflow throughthe inlet, resultingin a new channelbeingbreachedthroughthe ebb-tidaldelta sometime before1851(Fig.23, Stage3). The sandthathadbeenontheupdrift sideof theebbdeltamigratedonshoreforminga largearcuatebar(U.S. Army Corpsof Engr., 1976). In time thebar attachedto Plum Islandat its southern endandfurthersandsupplyfromtheebbdeltabuiltthebarabovemeanhigh water (Fig. 23, Stage4). This mechanismof inlet sedimentbypassingand basindevelopmentalongthe downdriftinlet shorelinealsohasbeenidentifiedontheSouthCarolinacoastatPriceandCapersInlets(FitzGerald,1982). The major effects of the Merrimack River Inlet jetty project have been a stabilization of the inlet channel and initial accretion of the shoreline on both sidesof the inlet. However, subsequentshoalingat the inlet has required severalemergencydredgingprojectsby theU.S. Army Corpsof Engineers. Erosionof the shorelinesouthof thejettieshasresultedin beachnourishment projectsand the constructionof sevengroins by the Commonwealthof Massachusetts. Bass River Inlet Jettiesinterruptthenaturalmechanisms wherebytidalinletsbypasssandand in doing so, causea progradationof the updrift shorelinewhile the sand- 50 1 OriginandStabilityof TidalInletsin Massachusetts 1. DEFLECTED MAIN EBB CHANNEL 2. ERODING DOWNDRIFT SHORELINE Dominant I Longshore • Transport ;.'• '.' .L \ • '"'-..• • ••Spillover Channels • ".•. ", • II '•;:• '", ß.•? ', ', / ",,,, ß •:• 3. EBB-TIDAL DELTA / BREACHING 4. BAR WELDING-BASIN t!:'i'1 FORMATION "-, Subtidal to ,•;'••./..ntertidal ".':• .;,.• •, '• Extent/on .:.. :• / / • •II .., Figure23. Conceptualized fourstagemodelof basindevelopment atnorthern PlumIsland. Duncan M. FitzGerald 51 starveddowndrift shorelineretreats. Bass River Inlet, located along the southern CapeCodshoreline, isa goodexampleof thiscondition (Figs.2 and 24). A doublejettysystem, whichwasconstructed attheinletbetween1866 and 1889,interrupts theeasterlylongshore transport systemthatdominates thiscoast.A netlongshore transport rateof4,400m3/yrwascalculated forthis shorelinefromthevolumeof sandthatwastrappedby thewestjetty (177,000 m3) duringa 40-yearperiodfrom 1938 to 1978 (SlechtaandFitzGerald, 1982).This is a minimumestimatebecauseit doesnot accountfor the sand thatbypassed thejettyduringthistime. Evenatthisrelativelylow transport rate, sanddepositionalongthe Yarmouthshorelinenecessitated a 30 rn extensionof the westjetty in theearly 1950's(Fig. 24 ). "•J /• :. _• • _• '• •.• / . • ,5 Dennis Yarmouth '.. . .1.•' .•.•.:,19 :, :::[ ,,•' I ,," . •.!..'• eters .,.:,L" . .,.ß:'•': '"' ' / . 9•8_• .1978•.; "-•"• ....... BASS RIVER INLET Figure.24. Shoreline changes atBassRiverInletastheresultofjettyconstruction (fromSlechta and FitzGerald, 1982). 52 1 OriginandStabilityof TidalInletsin Massachusetts In conjunctionwith the construction of thejettiesin the 1800's, theinterior of BassRiverInletwasdredgedtoprovidea straightseaward pathfor themain channel. The jetties were designedto stabilizethe entrancechanneland alleviateshoalingproblems. However, the small tidal prism of the inlet, coupledwith periodicleakageof sandaroundthe westjetty andtransportof sandover the eastjetty, have causedcontinuousshoalingproblems. The shallowness of theoffshorein thisregion,whichis lessthan2 rn deep(mlw) at a distanceof 1.5km fromtheshoreline,exacerbates theshoalingproblems seawardof thejetties (SlechtaandFitzGerald, 1982). Green Harbor Inlet One considerationwhen constructing jetties is their orientationwith respect to the approachof stormwavesandthe dominantwave swellof theregion. For instance,atWellsInlet alongthecoastof southern Mainethedoublejetty systemis orientedinto thedirectionof dominantwave approach(Byrneand Zeigler, 1977) makingnavigationthroughthejettied channelduringstorms and evenperiodsof large swell extremelytreacherous.In addition,wave actionin thechannelhasbeenshownto contributeto thelandwardtransport of sandthroughtheinletwhichhascausedshoalingproblemsin theharborage area (Mariano and FitzGerald, 1991). GreenHarborInlet,locatedalongtheMassachusetts SouthShore(Figs.1and 2), has doublejetties orientedto the southeastaway from the dominant northeaststormwaves(WeisharandAubrey, 1988). Currentmeasurements takenat threesitesacrossthechannelat themouthof thejettieson 31 August and 2 September1981 indicatethatthe tidal currentsare weak (max. vel. < 40 cm/sec), and that ebb velocities are consistentlystrongerthan flood currents(FitzGerald, unpub. data). Based on tidal current measurements alone,thedatasuggestthattheinlet shouldnot importsediment.However, theentrancechannelhashada longhistoryof shoalingproblemsandhasbeen dredgedon severaloccasions.Interestingly,thematerialthataccumulates in the entrancechannelconsistsof sandand gravel. Someof this sedimentis transported overthenorthernjetty duringmajorstormsandevidenceof this processcan be seenin Figure 2. However, most of the sedimentmoves landwardintotheinletthroughthemouthof thejetties(WeisharandAubrey, Duncan M. FitzGerald 53 1988). It wouldappearthatthesituationat GreenHarborInlet is comparable to Wells Inlet, whereby shoalingwaves in the main channel generate instantaneous, landward-directed currentsthataugmentfloodcurrentswhile retardingebbcurrents.Thisprocess,especiallyduringstorms,wouldaccount for sedimenttransportinto the inlet. Construction of dikes,roadways, andretainingwallsin thebackbarrier region of Green Harbor have contributed to weak currents in the inlet channel. These structures cordonedoff portionsof the bay, marshsystem,and tidal flats, therebydecreasingthevolumeof waterenteringandleavingthe inlet. The tidalprismwasreducedasa consequence of thesemodifications,sothe size of the equilibriumentrancechanneldecreased. Pamet River Inlet Prior to stabilization,Pamet River Inlet in northernCape Cod Bay had a historyof northerlymigrationwithperiodicbreachings of thespitto thesouth (Figs.2 and25). A newinletwascutthroughthespitin 1919andstabilized 1919 f•. 1933 J• 1938 Morphological Changes Due to Jetty Construction N 1919 s. ..... •':'•,..•.M Lw--'•---_2m-,,?'"::'"":'"-'• .... N 1920 S ß'...:?:v-.:-,2m N 1934 S N1951 Olm 25m 50m 75m I Figure25. Morphological changes atPametRiverInletduetojettyconstruction (fromFitzGeraldand Levin, 1981) 54 1 OriginandStabilityof TidalInletsin Massachusetts with two stonejetties. The new inlet was70 m wide anddredgedto a depth of 4 m below mean low water. When the inlet was dredged,the spoil was placed landward of the inlet in the form of a dike which partitioned the backbarrierinto two bays (Fig. 25, Year 1919). The jettied inlet quickly shoaled and it was not until the old inlet was closed and the dike severed that the combinedtidal prismsof the two baysdeepenedthe channelto 3.1 rn by 1950 (FitzGerald and Levin, 1981). In 1951 the jetties and channel were widened to 100 m to accomodate increasedtraffic throughthe inlet. The equilibriumchannelcrosssection responded by shoalingmorethana meter(Fig. 25, Years 1950 and1951). At the sametime thejettieswerewidened,theywere alsolengthened50 m. In this configurationthe southjetty was an efficient sedimenttrap of the northerlylongshoretransport,resultingin a 50 rnprogradation of itsshoreline and a retreat of the downdrift northern shoreline. This retreat continued such that the northjetty was separatedfrom the dunesystemandpresently10 to 20% of the tidal prismleaksbetweenthe northjetty andthe adjacentbeach. On thesouthsideof theinlet,thebeachhasbuilt to theendof thejetty andsand is readily transportedinto the inlet. Shoalingwavescombinedwith flood currentsmove lobes of sandinto the backbarrierregion. Little of this sedimentis removedby ebb tidal currentsasevidencedby the beachridges that are forming inside the inlet on the southernside of the bay (Fig. 2). Presently,accessthroughthejettied channelcanonly be accomplished near high tide (FitzGeraldandLevin, 1981). The shoalingproblemsat PametRiver Inlet haveresultedfrom poorjetty designandconsauctionprojectsin thebackbarrier.During theperiodfrom themid-1800'sto theearly 1900's,thebuildingof roadsandrailroadtrestles hasrequiredportionsof the marshandbay to be diked. Thesedikeshave isolatedpartsof themarshin someareasandrestrictedtidalflow in others.As tidalstrength wasdiminished,sedimentation in thebackbarrier proceeded at a fasterrate. Theseprocesses combinedwith thelandwardtransportof sand into thebayhavereducedthetidalprismanddecreased theinletequilibrium crosssectionalarea (FitzGerald and Levin, 1981). Duncan M. FitzGerald 55 Summary Tidal inletsoccuralongtheentireMassachusetts coastbutaremostcommon wherethe sandsupplyis mostabundant,includingthebarriercoastnorthof Cape Ann and the sandycoastsof Cape Cod. Inlets are lesscommonalong thesand-starved coastsof CapeAnn andBuzzardsBay. Developmentof tidal inletsiscloselyrelatedto barrierformationandconstriction of anembayment. In Massachusetts, the constrictionof embayments hasoccurredmostcommonlythroughspitaccretiondueto thepresenceof irregularcoastlines(e.g., SandyNeck andNausetSpit), however,theformationof beachridge barriers in frontof bays(e.g.,HorseneckBeach)is alsoan importantprocess.Bays associated with Massachusetts inletshavedevelopedin a varietyof different settings.Drownedfiver valleys,groundwatersappingchannels,kettles,and the enclosureof rocky and sandyirregularcoastsare examples. Tidal inlets in Massachusetts exhibit highly variable morphologiesand sedimentation processes asa resultof varyingphysicalparameters including wave and tidal energies,sedimentsupply, and size of the backbarrier. Generally,inletsalongmesotidalshorelineshavewell-developedebb- and flood-tidaldeltasandtheirbackbarrierareasusuallyconsistof marshandtidal creekswith someopenwaterareasneartheinletmouth. PlymouthBay Inlet is an exceptionin thatits backbarrieris dominatedby intertidalflats. Inlets onmicrotidalshorescommonlyhavepoorlydevelopedebb-tidaldeltas,poorto-well developedflood-tidaldeltasandbackbarrierareasconsistingof open bayswith peripheralmarshes(Hayes, 1975). The sizeof inletsis governed by bay areaand tidal range. Stabilityof unstructured tidalinletsalongtheMassachusetts shoreis dependentonbaysize,physicalsetting,andtheerosional-depositional historyof the associated barriers. Tidal inletsthat haveclosedduringthe past 100 yrs generallyarethosealongmicrotidalshoresthathadsmallbayareasor those wherethe tidal prismwasreroutedthroughanotherinlet. Severalbarriers along the BuzzardsBay coastand the shoresof Martha's Vineyard and Nantuckethave ephemeralinletswhich openduringstormsor when snow meltandprecipitation causethepondleveltoexceedtheheightof thebarrier. Draining of the pondproducesa cut throughthe barrierleadingto inlet 56 1 OriginandStabilityofTidalInletsin Massachusetts formation.Inletsthatoriginatein thismannertypicallyexistfor a periodof lessthansixmonths.Breachingofbanfiersandformationof semi-permanent tidal inletshaveoccurredpredominantlyalongtheoutercoastof CapeCod. Them are numerousjettied inlets on the Massachusetts coast;they are particularlyprevalentalongmicrotidalcoastswheresmallbaysand small oceantidal rangesgeneratesmalltidalprisms. Jettiesconstructed at these sitesdisruptthe littoral transportsystemcausingupdrift accretionand downdrifterosion.In mostcases,periodicdredgingof the inlet channelis requiredto provideadequatenavigation. Acknowledgments Thecollectionandanalyzesof datapresented in thispaperweresupported by grantsand contractsfrom Massachusetts CoastalZone Management,U.S. Army Corpsof Engineers,U.S. MineralsManagementService,LloydCenter for Environmental Studies, and numerousindividual towns. The arduous fieldworkof severalgraduatestudents, includingAndrewMagee,Andrew Bakinowski,JamieGreacen,Alan Saiz,Rapi Som,andCynthiaThomlinson contributedto thematerialpresentedin thispaper. RobertChambers,Larry Nelson,and KennethTobbinprovidedthe augercore data presentedfor HerringRiverInlet. SteveGoodbredandWendyQuigleyhelpedpreparethe manuscriptandEliza McClennenof BostonUniversityCartographic Lab draftedthe figures. References Aubrey,D.G.,andA.G.Gaines,1982.Recentevolution of anactivebarrierbeachcomplex: Popponesset Beach,CapeCod, MA. TechnicalReport# WHOI-82-3, WoodsHole, MA, 75.pp. Aubrey,D.G. andP.E.Speer,1984. Updriftmigration of tidalinlets.J. Geology, v. 92,p. 531- 546. Aubrey,Twichell, D.C., andS.L. Pf'mnan, 1982. Holocenesedimentation in the shallow nearshore zoneoff NausetInlet,CapeCod,MA. Mar. Geology,v. 47, p. 243-259. Duncan M. FitzGerald 57 Belknap, D.F., B.G. Andersen,R.S. Anderson,W.A. Anderson,H.W. Borns, Jr., G.L. Jacobson, J.T. Kelley,R.C. Shipp,D.C. Smith, R. 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