Long Island's Dynamic South ShoreDocument Transcript
Long Island’s Dynamic South ShoreA Primer on the Forces and TrendsShaping Our CoastJay Tanski
2Long Island’s Dynamic South ShoreA Primer on the Forces and Trends Shaping Our CoastIntroductionLong Island’s Atlantic coastline is a special place for many reasons. The south shore is home toa wide variety of habitats which support a vast array of plants and animals, some threatened orendangered. It is also the place where millions of people live, work, and play. The 120-mile coaststretching between Coney Island and Montauk is remarkably diverse in terms of its physicalcharacteristics, use, and development. This shore contains everything from heavily developedurbanized barrier islands to New York State’s only federally-designated wilderness area. Areabeaches are a prime recreational resource, attracting millions of visitors every year and serving asthe foundation of a multibillion-dollar regional tourism industry.Long Island’s coast is also extremely dynamic, constantly changing in response to naturalprocesses associated with wind, waves, and tides as well as human activities. The dynamicnature of the shoreline coupled with people’s desire to use and enjoy the shoreline presents uniquechallenges in managing this resource. Making decisions that balance conservation of the naturalenvironment with significant demand for use of the shore requires a sound understanding of theprocesses shaping and impacting the coast.This primer provides a brief overview of what we know about coastal processes and erosion onLong Island’s south shore, based on the best available scientific information. While by no meansan extensive treatment of the subject, the information presented here is intended to familiarize thereader with the major shoreline trends and technical issues associated with erosion and erosionmanagement on the south shore.Jay TanskiNewYork Sea Grant Extension ProgramJay Tanski is the New York Sea Grant coastal processes specialist.He can be contacted at: firstname.lastname@example.org or 146 Suffolk Hall, Stony Brook University, Stony Brook, NY 11794-5002.Since 1971, New York Sea Grant, apartnership of the State University of NewYork, Cornell University, and the NationalOceanic and Atmospheric Administration(NOAA) has been “Bringing Science to theShore” through research, extension andeducation to improve the environmental andeconomic health of New York’s marine andGreat Lakes coasts.Acknowledgements: The author would like to thank technical reviewers HenryJ. Bokuniewicz (Stony Brook University), Mary Foley (National Park Service), CherylHapke and S. Jeffress Williams (U.S. Geologic Survey), and Norbert Psuty (RutgersUniversity). Their input and comments greatly improved the document but theauthor takes full responsibility for the content and any errors or omissions. Theauthor is also grateful to Paula Valentine and Robin Lepore of the National ParkService and Cornelia Schlenk and Barbara Branca of New York Sea Grant for theireditorial review. This publication was made possible with funding from theNational Park Service. Figures by Jay Tanski unless noted otherwise.Tanski, J. 2007. Long Island’s Dynamic South Shore — A Primer on the Forces and Trends Shaping Our Coast.New York Sea Grant. 27 pages.
3ContentsIntroduction............................................................................................................................. 2Long Island’s South Shore .................................................................................................... 4The Dynamic Shore..................................................................................................................... 5The Beach.................................................................................................................................. 5Effects of Storms........................................................................................................................ 7Long-Term Shoreline Changes.................................................................................................... 10Historical Changes — Sea Level Rise and Barrier Island MigrationA Look at the Past .................................................................................................................... 11Barrier Island Migration............................................................................................................. 12Sea Level Rise and the Future.................................................................................................... 12Sand — A Valuable ResourceLongshore Sediment Transport: Not Quite a “River of Sand”........................................................ 14Tidal Inlets — An Important Part of the SystemStabilized Inlets........................................................................................................................ 16Breaches and New Inlets........................................................................................................... 18Impacts of Human Responses to Shore ErosionStructural Responses................................................................................................................ 19“Soft” Responses..................................................................................................................... 21DunesDune Characteristics................................................................................................................. 23Dune Dynamics......................................................................................................................... 24Humans and Dunes................................................................................................................... 25In Conclusion... .................................................................................................................... 27Selected References................................................................................................................. 27Selected On-Line Resources...................................................................................................... 27Metric Conversion Factors.......................................................................................................... 27
4Long Island’s South ShoreThe south shore of Long Island can be divided intotwo distinct regions based on the physical character-istics of the coast (Figure 1). Stretching almost 100miles from Coney Island in New York City to South-ampton in the east, the shore is composed of nar-row, sandy islands and peninsulas separated fromthe mainland by shallow bays. These features arecalled barrier islands and barrier spits because theyform a barrier between the ocean and the bays andthe mainland. There are five barrier islands (fromwest to east: Coney, Long Beach, Jones, Fire andWesthampton) and two spits (Rockaway and South-ampton). Six openings or tidal inlets separate thebarriers and connect the bays with the ocean. Allof the inlets are artificially stabilized with structuresand are dredged to allow for navigation by commer-cial and recreational boats.East of Southampton, the barrier island systemgives way to what is known as the headland region.Here, the mainland directly abuts the ocean all theway to Montauk Point. In the western portion ofthis 30-mile stretch of coast, sandy beaches sepa-rate the ocean from a low-lying plain that is madeof material laid down by waters melting from glacierstens of thousands of years ago. To the east, theflat plains are replaced by 40- to 60-foot high bluffsformed when the glaciers stopped their advancesouthward and dropped the material they were carry-ing which ranged from large boulders to fine clays.Development and use of the coast also changesfrom west to east along the south shore (Figure2). Heavily urbanized barrier islands and mainlandshores are common in the west. Not many peoplerealize it, but Coney Island in New York City is (orwas) a barrier island. The western barriers (ConeyIsland, Rockaway and Long Beach) are home toyear-round communities with residences, commercialbusinesses and industry. Beaches in the easternand central sections of the south shore are heavilyused for recreation due to their proximity to densepopulation centers. For example, Jones Beach StatePark, created in 1929 on Jones Island, receivessome six to eight million visitors per year. Fire Islandis less densely developed with federal (Fire IslandNational Seashore), state (Robert Moses) and county(Smith Point) recreational park facilities interspersedwith 17 primarily seasonal communities.The Otis Pike Fire Island High DuneWilderness, the only federally desig-nated wilderness area in New York,occupies seven miles of this islandand another 14 miles of the nationalseashore is undeveloped. FromWesthampton to Montauk Point, theshore is characterized by summer resortand residential communities. The well-known “Hamptons” are found here.Despite the development found alongthe coast, Long Island’s south shore,like many ocean coasts, is subject tochange. Sand comes and goes from thebeaches. Some areas are lost to thesea while in other areas beaches areactually building seaward. Most peopleare aware erosion problems exist onLong Island’s south shore beaches.But exactly how is the coast changingand what causes these changes?Figure 1. Long Island’s south shore includes a variety of different shoreline types including an ex-tensive barrier system with islands separated by tidal inlets in the west and a headlands section withhigh glacial bluffs in the east.(Satellite photo:NASA Visible Earth http://visibleearth.nasa.gov/)
5Figure 2. The south shore is characterized by a variety of different land uses. (Jones Beachphoto: U.S. Army Corps of Engineers Beach Erosion Board, Satellite photo: NASA Visible Earthhttp://visibleearth.nasa.gov/)The Dynamic ShoreAlthough Long Island’s coast contains a variety ofshore types (barrier islands and spits, mainlandbeaches and glacial bluffs), they are all primarilycomposed of small, loose materials such as grav-els, sands and clays. Most of these sediments caneasily be moved and reworked by wind and water,so the shorelines are inherently unstable and con-stantly changing in response to natural and humanforces. The actual behavior of Long Island’s shoreis dependent on four major factors:1) the amount of wave and current energy striking..... the coast, which is largely related to storm..... intensity and frequency;2) the supply of sand available for building thebeaches or shoreline;3) short- and long-term changes in sea level;and4) human activities in the coastal zone thatalter or disrupt natural processes andmovement of sand.While simple in concept, thesefactors interact in complex waysand over different time scales.The relative magnitude and impor-tance of each factor in determiningshoreline behavior varies depend-ing on the particular stretch ofcoast being considered and the pe-riod of interest, making erosion adeceptively difficult process to fullyunderstand, predict and manage.The BeachWhen many people think of thecoast, they automatically visual-ize the beach since this is wherethey spend most of their time atthe shore. But the beach is notjust that sandy strip of land be-tween the waterline and the toeof the dune (or bluff, as the casemay be) where you put your towelduring the summer. Technically,beaches are usually defined as theaccumulation of material (usually sand) moved by theaction of waves and currents. Comprised of differentparts (Figure 3), the true beach really includes every-thing from the dune toe seaward to the outermostpoint where waves begin to break which can be inwater 20 to 30 feet deep or deeper in major storms.The breaking waves exert force on the sea floor andcreate currents which move material on the bottom.Larger waves start breaking in deeper water so thebeach extends even further seaward.Inland BeachBackshore ForeshoreDuneor Bluff BermCrest of BermHigh WaterLevelLow WaterLevelBarFigure 3. Beach terminology. (Illustration by John Norton)
DuneBermStorm wavesEroded profileEroded profileEroded profileOriginal profileOriginal profileAccretion(bar formation)BarStorm waves6number of factors including the size and direction ofthe waves, the size and shape of sand grains on thebeach, the level of the water at the time the wavesstrike the shore, and the initial shape of the beach,just to name a few.Waves play a major role in controlling the form, posi-tion and size of the beach. They are the primaryagents responsible for picking up and moving sandalong the coast. The beach responds quickly tochanges in wave energy (Figure 4). In general, verylarge, choppy waves, like those associated with bigstorms, tend to pick up and remove sand from thebeach berm (that relatively flat part of the beachwhere you sunbathe in the summer) and, if the stormis strong enough, the dunes behind the beach. Thislowers the elevation, flattening the beach profile,and causes the berm and shoreline to move land-ward. (For the purposes of this primer, shoreline isthe boundary between the land and the water.) Thematerial picked up by the waves can move in a varietyof directions (landward, seaward or along the coast)depending on a number of factors. Frequently, mate-rial is moved offshore and is deposited in a bar dur-ing storms. As this bar grows, it causes bigger wavesto break and dissipate their energy before they reachthe landward beach berm. In this way, the beachactually helps protect itself. Although you may not beable to see it standing on the shore, the sand in thebar is still part of the beach and has not been lostfrom the “system.”In calmer weather, long, gentle waves can actuallypick up much of the sand that had been transportedto the bar and bring it back onshore, building up theberm, raising the height of the backshore and movingthe beach berm and shoreline back seaward.Thus, there is a cycle where the beach erodes andbuilds back up in response to wave action. In somecoastal areas, this is referred to as the winter/sum-mer seasonal beach cycle, because beaches tend tobe narrower in the winter when there are more stormsand wider in the summer when weather conditions(and waves) are generally calmer. However, researchhas shown this seasonal cycle is not as regular forLong Island ocean beaches as it is in some otherregions. Here, the width of the beach depends moreon the amount of time since the last storm ratherthan the season. You often find wide beaches in themiddle of winter and narrow beaches in the summerdepending on recent weather conditions.Figure 4. Beach response to storm waves. (Illustration by John Norton)Although not technically part of the beach, dunesare closely linked with the beach and are oftenconsidered as part of the beach system. In naturalsettings, dunes are the mounds of sand depositedlandward of the active beach, usually by the wind.Dunes may be artificially created by either placingsand or creating obstacles (sand fencing or vegeta-tion) to trap sand blown by the wind. Dunes are acommon feature along the south shore. They takemany forms and can be an important component ofthe beach system. You will learn more about duneslater in this primer.Day-to-Day Changes: The beach is constantlychanging from day-to-day, week-to-week, month-to-month and year-to-year, primarily in response tothe waves. The size and even the presence of anypart of the beach at a given time is influenced by a
7Figure 5. Erosion along the shore can be highly variable, even overshort distances, as evidenced by this erosion “hot spot” adjacent to awide beach. (Photo: Atlantic Coast of New York Monitoring Program,http://dune.seagrant.sunysb.edu/nycoast)Year-to-Year Changes: Measurements madealong the south shore show the position of the wa-terline on some ocean beaches may move back andforth by as much as 270 feet over the course of ayear as the beach alternately grows and erodes inresponse to wave action. These changes are largelycontrolled by the frequency and intensity of stormshitting the coast.Storms not only generate high waves, they also causethe water level to increase above the elevationsexpected with the normal tides. This difference inactual or observed water height from the predictedtide level is known as storm surge. Storm surgesallow the waves to attack higher up on the beach andcause erosion. As a result, storms can move largeamounts of sand from the visible beach very quickly.In some cases, one stretch of the shoreline may beseverely eroded while adjacent beaches will have re-mained stable or even gained sand (Figure 5). Whilethese erosion “hot spots” are frequently observed,the underlying causes are not well understood but arethought to have something to do with the presence orabsence of the bar offshore.Even after relatively modest events, beachgoers oftensee scarps cut by the waves on the beach (Figure 6).Much of the sand removed from the beach above thewaterline is still in the beach system and may returnto the upper portion of the beach under the rightconditions. Surveys of some beaches on the southshore show they usually rebuild fairly quickly,generally within a month after most storms.While the beach may be constantly changing and thewaterline moving back and forth, the position of theshoreline fluctuates around an “average” position thatwon’t change very much on a yearly basis as long asthe sand is not lost from the beach system. However,this may not be the case if the storms are verysevere and sand is being removed from an area with-out being replaced.Effects of StormsStorms play a major role in shaping our shoreline.Long Island experiences both hurricanes and thewinter storms known as nor’easters. Hurricanesare usually smaller in size but more intense thanFigure 6. Storms can remove sand from the beach leaving steep scarps(top). In most cases, much of the sand returns to the beach (bottom),usually within a few weeks after the storm.
8nor’easters, with stronger winds and higher stormtides. Hurricane storm surges can increase sealevel more than ten feet above the normal tide level.These storms usually pass through this area in amatter of hours but, if they happen to coincide with ahigh tide, the abnormally high water levels threatenhuman life and can cause extensive damage to thebeach and properties along the shore. The Sep-tember 1938 hurricane, known as the “Long IslandExpress,” passed over Westhampton and reportedlyhad winds of 96 miles per hour and a storm surge ofnine feet. This storm caused more than 50 fatalitieson Long Island and destroyed hundreds of homes onthe coast (Figure 7).More recently, Hurricane Gloria struck our coast in1985. However, this storm moved very fast andpassed quickly over the south shore close to lowtide. Although the storm surge was seven feet insome areas, the actual storm tide or water level el-evation was only two or three feet above normal hightide levels. As a result, most of the damage fromGloria was caused by the wind rather than the water.The situation could have been considerably differentif the storm had hit six hours earlier or later, nearerto high tide. Fortunately, because New York is fairlyfar north, we have not seen very many hurricanes.Only nine have actually made landfall in the LongIsland and New York City area since 1858 (Figure 8).While not as powerful as hurricanes, nor’eastersoccur much more frequently in this area. Becausethey cover a bigger area and are slower moving thanhurricanes, nor’easters usually affect a larger por-tion of the coast (hundreds of miles of shoreline asopposed to tens of miles) for a longer period of time(days versus hours). Nor’easters can also producewaves larger than those generated by hurricanes.During the 1992 December nor’easter, gauges offthe south shore of Long Island measured waves over30 feet high. Storm surges associated with winterstorms, while generally lower than those of hurri-canes, are still substantial. Measurements taken atthe Battery in New York City showedthe December 1992 nor’eastercaused water levels to rise morethan 4.5 feet above normal, al-lowing waves to reach dunes andbluffs behind the beach. Statisti-cally, storms with similar tide levelshave a high probability of occurringover any 30 year period and aresometimes referred to as “30 yearstorms.” (This does not mean thattwo or more storms of this magni-tude could not occur in a shortertime interval.) Because of theirlong duration, large waves and highstorm tides, these intense stormscan have a devastating impact onthe coast.The worst hurricanes andnor’easters move vast quantitiesFigure 8. Tracks of hurricanes making landfall in the New York City/Long Island area since 1858.(Storm data from: http://maps.csc.noaa.gov/hurricanes/)Figure 7. Damage in Saltaire,Fire Island,caused by the 1938 hurricane.(Photo: U.S.Army Corps of Engineers 1958)
9of sand, rearranging the beach which can havelong lasting effects on the shoreline. During majorstorms, the elevated water levels and big waves canerode large volumes of sand from the shore andattack the dunes or bluffs behind the beach. Thestorms move material along the shore to adjacentareas, but some of the sand eroded from the beachand the dune may be carried seaward and depositedin water too deep for it to be brought back by thegentler waves during calmer conditions. This sand islost from the beach system. If enough sand is trans-ported into deeper water, the beach will not be ableto fully recover and the shoreline will move landwardresulting in long-term erosion or recession.If the storm surge is high enough, the waves power-ful enough, and the beach and dunes low enough,storms can erode the beach and dunes and causean overwash. Water carries sand over the beachand through the dune depositing it on the landwardside in a feature known as a washover fan (Figure9). The 1962 Ash Wednesday storm reportedly cre-ated some 50 such washovers. The material in thewashover fan is also lost from the beach system.On the south shore barrier islands or spits, the over-washes can reach the bay. However, studies look-ing at the impact of storms and the characteristicsof the resultant washover fans indicate this rarelyhappens, except occasionally on the eastern barri-ers which tend to be lower in elevation. Washoverfans do help to increase or maintain the elevationof the barrier island behind the dunes, often buryingswales and marshes but providing habitat for shore-birds and other organisms and providing a place fornew dunes to form in a more northerly location.During very extreme events, overwash channelscan grow and deepen, eventually forming a breach,or opening in the barrier island or spit, that al-lows water to flow between the bay and the ocean.Breaches are more frequently formed by hurricanesbecause they tend to have higher storm tides thannor’easters. The 1938 hurricane reportedly openednine breaches in the barriers west of Moriches Inlet.Sand moving along the coast usually fills most ofthese breaches naturally, often during or soon afterthe storm. However, larger breaches can remainopen and grow larger for long periods. Breachesthat stay open and that are maintained by normaltidal currents become inlets. Both Moriches andShinnecock Inlets started out as breaches createdby storms that were then kept open artificially fornavigation (Figure 10).Inlets and breaches have a tremendous impact onthe way sand moves around the coast, which, inturn, exerts a major influence on the behavior of theadjacent shorelines. Currents running through thebreaks in the barriers can transport large quanti-ties of sand landward into the bays and seawardinto the deeper waters of the ocean. This materialusually ends up in large underwater shoals or barsin the bay and in the ocean adjacent to the inlet thatare created by the flood and ebb tides, respectively.The shoals on the bay side are known as flood tidalshoals or deltas; ocean shoals are known as ebbFigure 9. Washover fans and breaches caused by the 1938 hurricane inthe Westhampton area. (Photo: U.S.Army Corps of Engineers 1958)Figure 10. Shinnecock Inlet today and as it looked shortly after itopened during the 1938 hurricane (inset), before it was stabilized.(1938 Photo: U.S. Army Corps of Engineers Beach Erosion Board,Recent Photo: Atlantic Coast of New York Monitoring Program,http://dune.seagrant.sunysb.edu/nycoast)
10Figure 11. Marsh growing on sediment deposited in the bay by a historicalinlet that opened and then closed in the 1800s. (Photo: Atlantic Coast ofNewYork Monitoring Program, http://dune.seagrant.sunysb.edu/nycoast)tidal shoals or deltas. The amount of sand found inthe tidal deltas on the south shore far exceeds thevolume of sand moved by the overwash processes.Inlets are a far more important mechanism formoving material in a cross shore direction (that is,perpendicular to the shoreline, rather than parallel tothe shoreline) than overwash. Some of the marshesfound on the bayside of the barrier islands are actu-ally built on the flood tidal deltas of historical inletsthat opened and closed over the last several hun-dred years (Figure 11).Long-Term Shoreline ChangesAlthough major storms are relatively short in durationand do not occur very frequently, they play a majorrole in shaping how the coast looks and behavesover time. The immediate impact of a single storm isapparent to everyone, but it is the cumulative effectsof these storms that determine how the shorelinemoves and changes over time scales ranging fromtens to hundreds of years.On these longer time scales, much of the south shoreof Long Island is relatively stable compared to manyother coastal areas. Estimates of shoreline changeover the last 100 years or so show that much of theshore has been eroding at average rates of approxi-mately one to two feet per year (Figure 12). However,these rates vary widely along the coast. Some areaswere actually stable or even moving seaward over thesame time span. Averaged erosion rates have to beused with caution, however. For much of the shore,the long-term changes occurring along the coast aretoo small to accurately determine with the data andmeasurement techniques presently available. Part ofthe problem in making these measurements is thatthe beach (and shoreline) can move back and forthhundreds of feet on a yearlybasis in response to thewaves, as described earlier.Yearly fluctuations can be aslarge, or even larger, than themovement we would expectto see due to longer-termerosion or accretion trends.These large yearly changesmake it very difficult to detectlong-term shoreline changerates unless the changesare very large. The highestshoreline erosion rates andaccretion rates, which mayexceed five feet per year,are both usually found nearstabilized inlets and otherman-made structures andare the result of interruptionsin the natural movement ofsand along the coast.(For more information,see section on LongshoreSediment Transport.)Figure 12. Long-term average shoreline change rates for the area between Jones Inlet and Montauk Point.These rates are calculated by comparing the position of historical shoreline positions dating back to 1873to more recent shorelines. Most of the shore is eroding but some areas have been stable or even accretingduring this period.(Data fromTaney 1961 and Leatherman and Allen 1985)
13,000 YBP20,000 YBPPresent8,000 YBPIceLand WaterDunesFlood tidal shoalNew inlet formsMainlandBayBarrier islandSea level 1Flood tidal shoalInlet closesOverwash increasesisland elevationInlet closes Overwash increasesisland elevationDunes formNew inlet formsSea level 2Sea level 111A Look at the PastShoreline changes over time frames spanning de-cades to centuries vary considerably ranging fromerosion to accretion depending on where you are onthe south shore. However, if one considers longerperiods of thousands of years, all of Long Island’sshorelines have moved landward in response to ris-ing sea level. Twenty thousand years ago, glacierscovered the land and stored a significant amountof the planet’s water. With all this water locked upin the glaciers, sea level was some 450 feet lowerthan it is today and our ocean coastline was morethan 80 miles south of its present position(Figure 13).As the climate became warmer and ice in the gla-ciers melted, water poured back into the ocean andsea level rose. The shoreline started migratingHistorical Changes — Sea Level Rise and Barrier Island MigrationFigure 13. Over geologic time, the shoreline has retreated landwardover the last 18-20,000 years as the glaciers melted and sea level rose.YBP =Years Before Present.(Illustration by Loriann Cody)landward, moving north up the gently sloping conti-nental shelf. The rate of sea level rise during thistime was not constant. Sea level rise was veryrapid between 20,000 and about 8,000 years agoand then slowed down to a rate of about three feetevery 1,000 years. The origins of the south shorebarrier islands are not fully understood but theymay have formed when this slowing of sea level riseoccurred. There is evidence that barrier islandsexisted at a location about a mile offshore in waterabout 50 feet deep.Figure 14. Simplified schematic of barrier island migration on the southshore in response to sea level rise. Inlets transport sand to the bays inthe form of flood tidal shoals which provides the platform that allows theisland to move landward. Overwash processes then raise the elevationof the island. This migration is a slow process occurring over periods ofhundreds to thousands of years. (Illustration by John Norton)
12Barrier Island MigrationThese barrier islands retreated or migrated north-ward as the ocean continued rising. There is somedebate about how the barriers actually moved.Some research suggests that the barriers slowlydrowned in place and then “jumped” or “skipped”landward to a new position coinciding with the newposition of the shoreline. More recent studies indi-cate the islands move in a more continuous processwhere sand is transported across the island fromthe ocean to the bay, allowing the island to migratelandward. There are three primary ways that sandcan be transported across a barrier island: inletformation, overwash processes and eolian (or wind)transport. On Long Island’s south shore, the inletsare actually far more important than either over-washes or the wind in terms of moving sand land-ward and driving barrier migration. The flood tidalshoals created by historical inlets provide the plat-form that allows the island to maintain itself whilemoving landward over time in response to rising sealevel (Figure 14). Regardless of the actual mecha-nisms by which the barriers move in response to therise in sea level, they have moved landward over thehistorical time frame of thousands of years.However, the rate at which the barriers migratevaries along the south shore when one considersshorter time scales on the order of centuries. Geo-logic evidence indicates that the central portion ofFire Island between Ocean Beach and Watch Hill hasnot migrated for the last 750 to 1,300 years. Thissection of the island has experienced erosion on theocean and bay shorelines, but the position of theisland has remained in the same location. Interest-ingly, there is no evidence of historic inlets in thisarea over the last several centuries (Figure 15). Thestable location and absence of historic inlets in thisarea suggest that barrier migration may not be acontinuous process over timescales of a thousandyears or less. Further to the east, the barriers aremore mobile and one can find evidence of barrierisland rollover processes such as old flood shoalsin the bay that were associated with inlets that haveopened and closed naturally over the last severalhundred years.Sea Level Rise and the FutureAlong the New York coast, sea level is not only ris-ing, the land is also slowly sinking, or subsiding dueto geologic processes. The rise in the water level inrelation to the land surface due to the sinking of theland and the raising of the sea is known as relativesea level rise. In our area, the average rate of rela-tive sea level has been about a tenth of an inch peryear, or about one foot per century. As can be seenin Figure 16, there are considerable monthly, yearlyand decadal fluctuations in the elevation of the water.Short-term changes in sea level caused by stormsare much larger than those associated with the long-term trends. Daily tides change sea level by two tofive feet and storms with return periods of 30 yearscan raise water levels four to six feet above normalelevations in just a few hours.It is not known exactly how much of the erosion wesee on the south shore is directly attributable to theslow rise of relative sea level. Calculations basedon measurements of beach changes going back tothe 1950s show that the sea level increase mightaccount for less than one foot per year of erosionand even this may be an overestimate. Studies alsoshow that the changes a beach may go through in aFigure 15. Locations of historical inlets along the south shore dating back to the 1700s. (Data fromTaney 1961 and Leatherman and Allen 1985)
13single month can be over 200 times more than thatexpected from relative sea level rise alone. In termsof our most severe erosion problems, long-term sealevel rise is of secondary importance compared toother factors acting on shorter, decadal time scales.Long-term relative sea level rise is important, how-ever, in that it ultimately controls the position of theshoreline. An increasing sea level means we willbe faced with erosion problems for the foreseeablefuture. There is a growing consensus that humanactivities are contributing to global warming, whichin turn can increase the rate at which the oceanswill rise. While there is considerable uncertaintyregarding the magnitude and timing of this increase,the most likely scenarios indicate the rate of sealevel rise may double over the next 100 years. In 50years this could result in water levels that are a foothigher than present (as compared to half of a foot ifthe present rate of rise did not change).From a planning perspective of 30 to 50 years, thebiggest impact of an increased rate of relative sealevel rise will be the submergence of the flat, lowlying areas around the bays on the south shore.Communities in these areas could be subject toincreased flooding. Coastal wetlands may also beaffected by long-term sea level rise. Salt marshes,one of the most productive ecosystems on earth,are very sensitive to the position of sea level. Fine-grained material deposited in the marshes raisesFigure 16. Monthly mean sea level measured by a tide gauge in NewYork City. Sea level has been rising at a rate of aboutone foot per century in this area. (Data from:NOAA NOS BatteryTide Gauge,http://tidesandcurrents.noaa.gov)the surface, keeping it in the same relative positionto a rising sea surface. If sea level rises faster thanthe sediments can be supplied, marshes could beflooded and replaced by open water. If depositionand sea level rise are in balance, some marshesmay be able to migrate landward if there is room forthem to retreat. Retreat will probably not be pos-sible if the slope of the land behind the marsh is toosteep or the path is blocked by structures such asroads, seawalls, or houses.On time scales of hundreds to thousands of years,increased sea level rise could accelerate the migra-tion of barriers landward or even lead to their dis-appearance altogether if the rise is very fast. Theprojected increases in sea level could make sectionsof the ocean coast more vulnerable to erosion overtime. However, over planning time frames of 30 to50 years, even increased sea level rise would notsignificantly change the actual observed rates ofshoreline change in those areas experiencing themost severe erosion. On these time scales, sealevel rise is of secondary importance compared toother factors in controlling what happens on thecoast. The frequency and intensity of the storms,discussed above, and the supply of sand in thesystem available for building the beaches play a farbigger role in shaping the coast. In most cases, ourmost severe erosion problems are caused by disrup-tions in the transport of sand, due to either naturalprocesses or human activities.
14Sand — A Valuable ResourceThe south shore is composed of material left bythe glaciers that has been reworked by waves andcurrents to form the coastline we see today. Com-pared to many other coastal areas, the south shorehas a relatively abundant supply of sand for buildingbeaches. The condition of the beaches and positionof the shoreline is the result of a balance betweenthe sand lost from an area and new sand broughtinto the area. Where this balance is positive,beaches can build up and the shoreline can actuallymove seaward. If more sand is leaving than arriv-ing, the shoreline erodes. For this reason, the waysand moves around in the system and the amountsmoved are very important. This “sediment trans-port” is very complex and not well quantified onthe south shore. Even though precise amountsof sand and exact pathways of movement are notknown at this time, some general patterns andtrends are recognized.Longshore Sediment Transport:Not Quite a “River of Sand”As already described, waves hitting the shore canmove sand landward or seaward in a cross shoredirection. Waves approaching the shore at an anglealso create currents which carry sand parallel to thecoastline in the surf zone. This movement of sandis called longshore sediment transport (the sandmoving in the surf zone is also referred to as long-shore or littoral drift). Longshore transport has oftenbeen described as a “river of sand” picking up anddepositing material on the beach as it moves alongthe shoreline. This analogy is somewhat misleadingfor the south shore, however. While a river usuallyflows in one direction, the longshore transport canbe to the east or the west depending on the direc-tion of the waves and even where you are on theshoreline (Figure 17).The amount of sand moved depends on the size andfrequency of the waves. Bigger waves move muchmore sand, which means that storms, with theirlarge waves, are very important in controlling thedistribution of sand along the shore. The size of thewaves responsible for moving most of the sedimenton the south shore is controlled by three variables:the speed of the wind over the water, the distancethe wind blows over water (called the fetch), and thelength of time the wind blows. The fetch of windsblowing towards the east is limited by the presenceof New Jersey. This limits the size of the waveswhich carry sand east along the New York Atlanticshore. The fetch for winds blowing towards the westis virtually unlimited. As a result, the waves drivinglongshore transport to the west are generally stron-ger than the waves moving sand east. Althoughsand is moved in both directions, more sand tendsto be moved to the west resulting in a net transportof sand from east to west in most years. The rate atwhich sand moves along the coast is usually mea-sured in units of cubic yards per year. To envision acubic yard, think of a volume of sand about the sizeof a typical clothes washing machine.The net longshore transport rate of sand variesalong the south shore (Figure 18). While there is agood deal of uncertainty regarding the exact num-bers, estimates indicate the rate of transport isFigure 17. The direction and magnitude of sand transport alongthe shoreline varies depending on wave conditions as evidenced bythe pattern of erosion and accretion around these structures in EastHampton. The top photo shows transport to the east (right) withaccretion on the west (left) side and erosion on east side. The bot-tom photo shows the opposite pattern at the same location at a dif-ferent time. (Photos: Atlantic Coast of New York Monitoring Program,http://dune.seagrant.sunysb.edu/nycoast)
15approximately 100,000 to 300,000 cubic yards peryear to the west in the eastern end of Long Island.The rate increases to as much as 600,000 cubicyards to the west at Fire Island Inlet and then de-creases to about 450,000 cubic yards nearer NewYork City. Even given the uncertainties associatedwith the estimates, there are obviously substantialquantities of sand moving along the coast. Thismovement of sediment along the coast can have amajor impact on what happens to the shoreline inan area. To give you an idea of how important it canbe, the longshore transport of sand actually allowedthe western end of Fire Island to grow or accretemore than four miles between 1825 and 1940 whena jetty was constructed to slow this westward migra-tion of the island and stabilize the inlet. The originalFire Island Lighthouse was constructed in 1828,at what was then the western end of Fire Island,to guide ships through an inlet that existed thereat that time. The current structure, constructedin 1857 just to the east of the original light, nowsits well east of the new position of the inlet, whichmoved west as the island grew more than 150 feetper year with sand supplied by longshore transport.Where does all this sand come from? For a longtime, people thought the sand transported along thecoast came from erosion of the bluffs at Montauk,but studies of the composition and erosion ratesof these features indicate bluff erosion alone can’tsupply all of the material we see in the system.Some of the sand actually comes from the erosionof the mainland and barrier beaches themselves.More recent studies suggest that a significantportion of the material in the longshore transportsystem may come from offshore deposits of sand.The relative contributions of these three sources isnot known.The longshore transport of sand ties the south shoretogether as a system. Although we do not knowprecisely how much sand is flowing along the shoreor exactly where it is flowing at any given time, wedo know this flow of sand is critical to maintainingthe shoreline. Actions taken in one area can affectadjacent areas. We also know that many of ourmost troublesome erosion problems are the result ofdisruptions of this flow either by natural processesor human activities.Figure 18. Sand moves in both directions along the shore,but generally more sand moves to the west thanto the east resulting in a net westerly transport. There is considerable uncertainty in the values shown heredue to variability in the rates of movement at different times and places and difficulties associated with try-ing to measure the amount of sand moving along the coast.
16Stabilized InletsInlets exert a dominant influence on the behavior ofthe shoreline by interrupting the natural longshoretransport of sand along the coast and capturing sed-iment that might otherwise reach adjacent beaches.The stabilized inlets are especially important. Jet-ties (the long stone structures built at a right angleto the shoreline to fix the navigation channel inplace) trap sand moving along the beach, causingthe beach on the updrift side (usually the east sideon the south shore) to extend seaward (Figure 19).However, the trapping of sand on the beach by theeastern jetty is a very minor impact compared to theproblems caused by the formation of shoals associ-ated with the inlets.When the tide is flooding or rising, the inlets allowsand to be swept into the bay and deposited whereit forms the flood tidal shoals landward of the inlet.During outgoing, or ebbing, tides, currents createdby the water flowing out of the bays push sand off-shore, depositing it in the ocean where it forms ebbtidal deltas. The ebb tidal deltas are less visiblethan the flood tidal deltas because they are sub-merged, but these ebb tidal deltas are more impor-tant in terms of their impact on the shoreline be-cause of their sheer size. They are much larger thanthe flood shoals in the bays. For instance, the ebbtidal delta at Shinnecock Inlet is estimated to holdTidal Inlets — An Important Part of the Systemaround 8 million cubic yards of material (Figure 20)compared to around 0.5 million for the flood tidaldelta. Although very difficult to measure, estimatesof the size of the ebb shoals range from about 4million cubic yards for Moriches Inlet to over 40million cubic yards for Fire Island Inlet. Imagine amound of sand the size of 40 million washingmachines under water!Given the size of the inlets and their related shoals,it is easy to see how they can have a major impacton the shoreline. However, these featuresare actually very complex systems and thefull range and magnitude of their impactsare still not entirely understood. What isknown is that inlets disrupt the natural flowof sand along the shore and can have atremendous impact on the adjacentbeaches. The vast amount of materialstored in associated shoals is essentiallylost from the nearshore beach system.Cut off from the natural supply of sand, thebeaches immediately downdrift (west) of theinlets experience greatly accelerated ero-sion. While this erosion helps restore theflow of sand along the shore by replacingmaterial trapped by the inlet, it also causesrapid shoreline recession adjacent to theinlet on the downdrift side. As a result,the inlets on the south shore exhibit aFigure 19. Inlets can have a major impact on adjacent areas as indicated by the his-torical shoreline positions near Shinnecock Inlet. After the inlet opened in 1938, theeastern or updrift shoreline (to the right) moved seaward while the western shorelineeroded and moved landward. (From: Atlantic Coast of NewYork Monitoring Program,http://dune.seagrant.sunysb.edu/nycoast)Figure 20. Shinnecock Inlet has trapped approximately 8 million cubicyards of sand from the longshore system in the ebb tidal shoal locatedseaward of the inlet. This representation of the shoal was constructedfrom high resolution surveys of the seafloor. (Survey data by R. Flood,GIS integration by B.Batten)
17characteristic pattern of shoreline accretion on theeast and erosion on the west seen in Figure 19.Based on long-term shoreline changes, the impactof each of the individual inlets appears to becomemore substantial to the west probably because thesize of the inlets increases as does the magnitudeof the longshore transport of sand. Measuredrecession rates of over 20 feet per year have beenobserved on the beaches downdrift of some of thewestern inlets (Figure 12).The large ebb tidal deltas also interact with theocean currents and waves. In some cases, theseinteractions change local conditions around theinlets dramatically. Ebb tidal deltas can change thedirection of sand transport by altering the directionof the incoming ocean waves. For example, theebb tidal delta off of Fire Island actually bends thewaves coming from the southeast. Waves strikingthe coast west of the inlet actually push sand eastinto the inlet setting up a net longshore transportto the east (opposite of the net westerly movementfor the south shore as a whole). The “reversal”of sediment transport results in a situation wheresand is moving both to the east and to the westat some point west of the inlet. Areas where thesand is being lost in both directions are known asnodal points and have very high erosion rates. Oneof these nodal points is thought to be near GilgoBeach, west of Fire Island Inlet.Presently, it is not known how long stabilized inletscontinue to affect adjacent areas after they areopened or how far along the coast these effectsextend. Generally, most experts believe the influ-ence of inlets on shoreline change rates shoulddecrease with time from the formation of the inletsand with distance from the inlet. However, deter-mining where and when the influence of the inlet isovershadowed by the other factors causing shore-line erosion is extremely difficult. Ebb tidal deltasshould eventually arrive at an “equilibrium” statewhere they reach their maximum capacity and stopgrowing. They no longer trap all the sand movingalong the coast and allow some or all of the mate-rial to naturally “bypass” the inlet. Unfortunately,there are no universally accepted criteria for de-termining when an inlet has actually reached thistheoretical equilibrium state. It is also not knownwhether all of the sand “bypassing” the inlet actuallymakes it to beach on the other side, as it would ifthe inlet were not present.Smaller inlets, like Shinnecock and Moriches Inlets,should reach this equilibrium state more quicklythan the larger inlets to the west. Based on obser-vations of the configuration of the ebb tidal deltasand the behavior of the adjacent shorelines, it ap-pears that both inlets are bypassing sand to someextent. However, detailed surveys of ShinnecockInlet, which opened in 1938, showed the ebb tidalshoal trapped significant amounts of sand (on theorder of hundreds of thousands of cubic yards peryear) especially in deeper waters between the years2000 and 2002. This suggests that the inlet isnot bypassing all the sand and is still disruptingthe longshore sediment transport. Shinnecock andMoriches Inlets are probably bypassing some sand,but, at this time, no one can say with certainty whatportion of the total amount of sand moving along thecoast is actually able to flow across the inlets andback onto the beaches to the west. As a result, itis not possible to accurately assess how much ofan impact the inlets are having on the shorelines inthese areas.The effects of inlets can be moderated by initiatingartificial “bypassing” programs where material ismechanically moved across the inlet to restore thenatural longshore sediment transport. But deter-mining how much sand should be moved, where itshould be moved and when it should be moved isnot a trivial task. In the past, dredging projects atthe inlets were designed solely for navigation pur-poses with safety and cost the primary concerns.In some cases, sand dredged out of the channelswas actually disposed of offshore and lost from thebeach because it was cheaper than placing it on thedowndrift areas. The only inlet on the south shorethat has had a regularly scheduled bypassing pro-gram is Fire Island Inlet. There, over 800,000 cubicyards are dredged from the inlet every two years,with most of this material being placed on the down-drift beaches of Jones Island.
18Breaches and New InletsAs we have seen, storms, particularly hurricanes,have periodically carved new inlets and breachesthrough the south shore barriers. Historically, theseinlets have been concentrated in the eastern por-tion of the barrier system (Figure 15). Inlets play animportant role in barrier island migration by transfer-ring sediment to the back side of the barrier, allow-ing the barrier to move landward and providing aplatform for marsh creation if the conditions allow.However, an inlet must be open for decades to trans-port enough sand to the back side of the island toprovide the platform necessary for barrier migration.Short-lived inlets or breaches that are only openfor less than a year or two are not as important interms of barrier island rollover or marsh creationbecause they do not move enough sand to the backbay. They are, however, a concern from a manage-ment perspective because they can cause significantchanges in the bay and mainland areas, as wellas along the ocean shore. A number of potentialimpacts associated with new inlets or breaches havebeen identified.New inlets or breaches can result in increased tidalranges and storm water level elevations in the baysunder certain conditions. This, in turn, can causeincreased flooding and erosion on bay shorelines.Measurements taken when the Little Pike’s Inlet(Figure 21) opened in Westhampton during the 1992nor’easter showed the tidal range (the difference inelevation between low tide and high tide) in MorichesBay increased by 30 percent, from 2.0 to 2.6 feet.There were also reports of increased flooding on themainland shoreline of the bay. Dredging of new chan-nels in Moriches Inlet in 1958 and 1968 increasedthe tidal range by about 0.3 feet which also repre-sented an increase of about 30 percent of the tidalrange at that time. Studies indicate the effect of newinlets would be greater in smaller bays, like Moriches,than in the larger bays, for the same size opening.It is unlikely an inlet the size of Little Pike’s Inlet inGreat South Bay would have affected the tidal rangeto the same extent.New inlets can also cause changes in the physicaland environmental characteristics, such as salinity,temperature, circulation and shoaling patterns inthe bays behind the barriers. These changes can,in turn, affect biological resources, including finfish,shellfish and plants. In some cases, certain resourc-es may benefit while others are adversely affected.For instance, a breach may help increase flushingand improve water quality by letting more ocean waterinto the bay, but it may also allow more predators ofshellfish to invade the bay.Inlets and breaches disrupt the longshore flow ofsand on the ocean beaches leading to increasederosion. At the same time, they can supply the bayshoreline with sand. New inlets would also divertsome of the tidal flow from existing stabilized inlets,which could cause the channels to fill in more rapidlyand adversely affect navigation.It is clear that inlets and breaches cancause substantial physical and environ-mental changes in the back bays andthese changes could affect some of theimportant biological resources in theseareas. Some these changes may berelatively small, or actually have benefi-cial impacts. Others may have significantimpacts on traditional uses of the southshore bays and mainland coast. There areresearch efforts underway to identify and,to the extent possible, quantify the impactsof new inlets on the physical characteris-tics and biological resources of the baysbut, presently, we do not have the informa-tion necessary to accurately predict thechanges that might occur.Figure 21. The Westhampton barrier breached during the December 1992 northeaststorm forming Little Pike’s Inlet in Moriches Bay. (Photo: First Coastal Corp)
19Figure 22. The most commonly used erosion control structures on the south shore are shore perpendicular structures like the groin on left and shoreparallel structures like the bulkheads on right. (Bulkhead photo: First Coastal Corp)As would be expected in an area as densely popu-lated as the New York City and Long Island region,human activity in the coastal zone is substantial andcan have a significant impact on the shoreline. Inaddition to activities related to the stabilization anddredging of the inlets previously discussed, humanresponses to erosion and flooding problems prob-ably have the greatest potential for affecting coastalprocesses and the beach. These responses includestructural measures, such as groins and seawalls,as well as “soft” erosion control responses that of-ten involve the placement or rearrangement of sandon the shoreline.Structural ResponsesErosion control structures commonly used on thesouth shore of Long Island can be divided into twocategories: “shore perpendicular” structures and“shore parallel” structures (Figure 22). As thenames imply, the shore perpendicular structuresare built at a ninety degree angle to the trend ofthe shore and they extend across the beach to-ward the water. Groins and jetties are examples ofthese structures. “Shore parallel” structures arebuilt in line with the shoreline, usually landward ofthe beach. These structures include bulkheads,seawalls and rock revetments. Because they havethe potential to cause considerable damage if usedImpacts of Human Responses to Shore Erosionimproperly or in the wrong place, erosion controlstructures require permits from state and local juris-dictions as well as federal permits if they are placedbelow the spring high waterline.Shore Perpendicular Structures: Althoughmany people use the terms interchangeably, groinsand jetties are not really the same thing. Groins arelong, thin structures that extend from the dune tothe water. They can be made of rock, steel, woodor concrete. Ideally, they are used in conjunctionwith sand fill projects and are designed to slowdown the rate at which sand placed on the beach isremoved by the longshore currents. The structuresthemselves do not provide any protection. Rather,the beach they create by trapping or holding thesand provides the protection for the landward area.Groins do disrupt the natural transport of sand alongthe beach and, if they are not designed and builtproperly, can cause problems.Jetties, on the other hand, look like groins but arefound only at inlets. Their primary function is to holda navigation channel in one place and prevent it fromfilling in with sand. Jetties also trap sand movingalong the shore. Since they are usually much longerthan groins, jetties can have a much larger impact.
20Figure 23. Groins interrupt the natural flow of sand and can increase erosion in adjacent areas (left). However,these structures can be designed to slowdown erosion and minimize adverse impacts in certain situations.The 49 groins constructed in the 1920s in Long Beach (right) have helped maintain arecreational beach that protects the developed upland.Because of the net east to west flow of sand alongthe south shore, jetties and groins usually tend totrap material on the east side. As with the inletsdiscussed earlier, these structures interfere with thelongshore transport of sand and can cause severeerosion problems on the shores to the west of thestructures. The magnitude of the impact increasesas the length and height of the structure and therate of longshore transport increase. To help mini-mize adverse impacts of these structures, sandshould be placed on the east or updrift side of thestructure to create a protective beach. This helpsminimize the disruption of the flow of sand alongthe coast (but does not necessarily eliminate all theimpacts). The severely eroded area west of the 15groins at Westhampton that eventually breachedduring the 1992 December nor’easter is a graphicexample of the impact groin projects can have whennot properly constructed (Figure 23 and Figure21). The compartments between the groins werenot filled with sand as they should have been. Thestructures trapped an estimated five million cubicyards of sand that was naturally moving along theshore, depriving the beach to the west of the mate-rial it should have received.In certain situations, however, these structurescan help maintain a recreational beach and provideupland protection. There are 69 major groins andjetties along the south shore. The 48 groins at LongBeach, built in the 1920s, have helped slow downerosion and preserve the beach in front of this heav-ily-developed area for over 80 years (Figure 23).Shore Parallel or Armoring Structures:The other type of erosion control device found onLong Island is the shore parallel structure. This cat-egory includes bulkheads, seawalls and revetments.These structures can be made of different materialsincluding rock, wood, concrete, and sand-filled bags,but they all function in the same way. They are builtparallel to the shore, usually behind the beach.Since they function by hardening or armoring theupland, they are often called shore armoring struc-tures. They are not designed to protect the beach.Armoring structures built to protect individual privateproperties probably have minimal impact on thebehavior of the shoreline over very long time scales(geologic time) because of their limited area of cover-age and relatively short functional lifetime (usuallyless than 50 years). However, they may cause sub-stantial short-term, localized impacts on the beach ifused improperly or in the wrong place. The potentialfor adverse impacts depends primarily on the condi-tions at the site, especially longer-term shorelinetrends in the area, as well as on the design andlocation of the structure on the beach. Multi-decadal
21Figure 24. In stable areas with an adequate sand supply, studies over the last thirty years have shown shore parallel structures like this rock revetmentcan provide erosion protection during severe storms without adversely affecting natural beach building processes. (Arrows indicate the same houses inthe two photos for reference.) (2007 Photo: M.Slattery)studies on Long Island have shown that at certainsites these structures can provide protection for theupland during storms without adversely affectingnatural beach building processes (Figure 24).Typically, these are areas experiencing episodicdamage from storms but that have a shoreline thatis stable or accreting on decadal time scales and anadequate supply of sand in the longshore system.In these areas, the structures are often completelycovered with sand during calm periods. They areexposed during severe storms, preventing erosionof the upland and then covered again as the beachrebuilds naturally after the storm.On the other hand, in areas experiencing chronicshoreline recession and a deficit of sand, wherethese structures are frequently proposed, armoringthe shoreline can adversely affect the beach andadjacent areas unless other measures are alsotaken to mitigate their impacts (Figure 25). Thesemeasures might include bringing in additional sandto make up for the sand impounded or retainedby the structure. Where you have rapid shorelineretreat, shore armoring structures usually lead toa narrowing or loss of the beach, not because thestructures increase erosion but because they pre-vent the beach from migrating landward. In extremecases, the structures may end up being surroundedby water as the shoreline recedes on either side.These structures eventually fail because they are notdesigned to handle the forces found in the surf zone.Before failure, they can block the transport of sandFigure 25. In areas experiencing chronic recession, shore parallelstructures like this bulkhead can prevent the landward migration ofthe shoreline eventually resulting in the loss of the dry beach. (Photo:H.Bokuniewicz)along the shore, essentially acting as groins andcausing increased erosion in downdrift areas.“Soft” ResponsesTo overcome some of the disadvantages and negativeimpacts associated with the structural erosion controlmeasures, so-called “soft” erosion control responsesare gaining increasing popularity primarily becausethey are considered more environmentally benign.For the purposes of this primer on coastal processes,these soft solutions are defined as activities that in-
22volve adding sand to the system or artificially enhanc-ing the dunes. Other non-structural alternatives suchas relocating structures, requiring special buildingcodes for structures in hazard zones and minimizingdevelopment in these zones are also often describedas “soft” responses. These are management alterna-tives with limited impact on coastal processes, andtherefore are not discussed here.Beach Nourishment: The most popular soft re-sponse to erosion is beach nourishment or replen-ishment which involves placing sand on the shore tobuild up the beach, which in turn provides protectionfor the upland area (Figure 26). New York has a longhistory of beach nourishment. In fact, the first beachnourishment project in the United States actually tookplace in Coney Island in 1923 when some 2.5 millioncubic yards of sand were added to the shoreline. Theobjective of this project was not to protect the upland,but to create a wider beach for recreational purposes.Since the 1920s, Long Island beaches have beennourished with an estimated 128 million cubic yardsof sand in various projects.The main advantages of beach nourishment as an ero-sion management option are that it can create (or main-tain) a recreational beach and that it is viewed as moreenvironmentally compatible than some of the structuraloptions because it involves adding sand to the beach.Nourishment doesn’t really affect the processes causingerosion. Rather, it simply moves the shoreline seaward.Eventually, the shore will return to its pre-project positionif more sand is not added as the beach erodes. Sinceit is not permanent, beach nourishment is consideredsomewhat reversible compared to structural alternatives.By the same token, beach nourishment requires along-term commitment to maintain the project as wellas an abundant source of sand. To provide adequateprotection, beach nourishment projects must replenishthe whole beach, which, as we have seen, can extendout to a depth of 20 to 30 feet below the surface ofthe water, not just the visible beach. A crude “rule ofthumb” in coastal engineering that can be applied to thesouth shore is that one cubic yard of sand creates ap-proximately one square foot of dry beach. This meansa beach nourishment project would require one cubicyard of sand for every one foot of shoreline to move thewaterline one foot seaward. To create a new 100-footwide beach for a mile stretch of shoreline would requireover 500,000 cubic yards of sand. This sand has to besimilar in grain size (or slightly larger) and compositionto the native sand or the restored beach will erodemore rapidly. The restored beach also has to be replen-ished on a regular basis to replace the sand lost as theresult of the natural background erosion, if continuedprotection is needed.Because of its glacial origins, the area off of Long Is-land’s south shore contains some of the most extensivesand deposits found on the east coast. However, thesupply of sand available for beach nourishment is not in-exhaustible. Some of the deposits may not be availablefor nourishment for environmental reasons and someare too far offshore to access practically with today’sdredging technology. Others may not contain sufficientmaterial of the right size or composition. In somecases, such as the central portion of Fire Island, recentstudies suggest offshore sand may already be feedingthe beaches through natural processes. Using this sandfor nourishment could disrupt the natural transport ofmaterial and accelerate erosion in the future. An im-portant component of any nourishment project is findinga suitable source of sand for the lifetime of the projectthat can be used without adversely affecting other ar-eas. Since Long Island has significant amounts of sand,it may be feasible to maintain some nourishment proj-ects for time periods on the order of decades depend-ing on the size and the scope of the effort. However,offshore sources of sand are finite so these projects arenot sustainable indefinitely. Unfortunately, at the pres-ent time, we do not have the necessary information onthe total volume of offshore sands that may be availablefor nourishment to say how long the projects could becarried into the future. Similarly, our limited knowledgeof how sand moves offshore does not allow us to quanti-Figure 26. Inlet bypassing and beach nourishment project on JonesBeach Island. Sand dredged from Fire Island Inlet (in the background)is piped to the site and deposited on the shoreline to build a beach toprotect the Ocean Parkway. (Photo: American Dredging Company)
23Figure 27. In some areas, development has replaced the naturaldunes. Dunes along many developed shores are artificially created andmaintained.tatively assess the long-term impacts on the shore thatmay be associated with using some of these resourcesfor nourishment now.Oceanfront beach nourishment projects are only practi-cal when implemented on a regional or community scaledue to technical constraints and cost considerations.These projects are usually fairly expensive becauseof the need for periodic maintenance and the largevolumes of sand necessary to provide adequate protec-tion. A properly implemented nourishment project cancost millions of dollars per mile of shoreline depend-ing on the erosion rate, conditions of the shoreline,the level of protection required and the proximity of asuitable supply of sand. In most cases, nourishmentprojects are only economically justified in those areaswhere there is a high level of development or heavy useof the shoreline being protected.Beach nourishment projects intended to protect up-land areas are usually designed to provide a beachand dune system large enough to prevent wave attackand flooding by overwash and, in the case of barriers,by breaching and inlet formation. Since inlets are theprimary mechanisms for transferring sediment landwardalong Long Island’s barrier island systems, nourishmentprojects that cover large areas and are maintained forvery long periods of time could lower the rate of crossshore sand transport and, eventually, affect barrierisland migration. The lack of quantitative informationon the relationship between barrier island migration andthe rate of sand transport across the barrier by newinlets, makes it very difficult to determine exactly how anourishment project might alter long-term barrier migra-tion rates or how long it would take.The time frame being considered is an important factor.Most major beach nourishment projects are usually de-signed to last 50 years or less. In areas where the bar-rier may not be migrating over periods of hundreds tothousands of years and there is no evidence of historicinlet activity, nourishment may have minimal impact onthe cross shore sand transport processes that drivebarrier migration processes over the lifetime of such aproject. However, there may be more of an impact inthose areas where there is evidence of migration, suchas historical inlet formation, occurring on time scalescloser to the design life of the project. In these areas,more detailed information on the amount of sand actu-ally transported and the rate at which it was carriedacross the barriers by historic inlets is needed beforewe can accurately assess how and when beach nourish-ment projects may affect barrier migration.DunesDune CharacteristicsDunes are a common coastal landform along thesouth shore. These features are created when windcarrying sand encounters an obstacle, such as veg-etation or a fence, and slows down causing the wind-borne sand to be deposited. On the south shore, thedominant winds are from the west and northwest sohighest rates of wind (also called eolian) sand trans-port are actually in a west to east direction parallel tothe shore. Much less sand is blown in a cross shoredirection. Based on measurements of sand transporton the south shore, it is estimated that the amountof sand carried landward across the crest of the dunefrom beach is about 0.08 cubic yards of sand per footof dune or less than one cubic yard per year for a 10-foot wide stretch of beach.Dunes vary greatly in size and form depending on siteconditions. In general, the size of the dunes increas-es from west to east on Long Island. In the urbanareas to the west, most of the natural dunes havebeen heavily impacted by human activities. In someareas, they have been entirely removed or replaced bydevelopment along the shoreline (Figure 27). Most ofthe dunes found along these heavily used areas havebeen artificially created or maintained, such as thedune fields on Long Beach in the Town of Hempstead.Further to the east, human manipulation of the duneis still common but there are also places, such as the
24Wilderness Area on Fire Island, where developmentis less dense and natural dunes can still be found.These dunes can take many forms from low scatteredmounds to high continuous ridges (Figure 28).In some areas there are multiple rows of dunes.The seaward dunes adjacent to the beach are calledforedunes or primary dunes. These dunes inter-act with the beach, especially during storms. Thedune landward is known as the secondary dune. Inessence, these dunes are cut off from the beachand are no longer receiving sand. Some of thesesecondary dunes are actually the largest dunes inthe area. It is thought they might have been createdwhen more sand was available for dune building andbecame stranded when the beach accreted and anew foredune formed. The larger secondary dunesare often separated by a well-developed swale thatmay be tens of feet wide.The volume of sand found in even the largest dunesis relatively small compared to the volume of sandmaking up the beach. Dunes usually contain lessthan five to ten percent of the amount of sand foundin the beaches (remember, the true beach extendsoffshore). Because the beach has so much moresand, it actually provides the bulk of protection fromerosion during storms. Nevertheless, foredunes dointeract with the beach and are an important compo-nent of this dynamic system.Dune DynamicsAs we have seen, high water levels during stormsallow waves to attack the dune. Sand in the dunesis removed and redistributed along the beach con-tributing to the building of the bar and the longshoretransport. Essentially the dunes act as a sand stor-age system that can provide material during stormevents. Depending on the size of the dune and theintensity of the storm, high continuous dunes canalso provide a barrier to storm surge and overwash,reducing flooding on the landward side.Natural dune recovery after a storm depends on theseverity of the storm and the resultant topography.If the front of the dune is eroded, or scarped, by thewaves, the vertical face of the scarp eventually driesout and collapses, moving sand and the beach grassto the toe of the dune (Figure 29). Windblown sandfrom the beach collects at the toe of the dune andthe beach grass sends out rhizomes (undergroundstems and roots). This initiates new plant growththat traps and holds sand, allowing the dune to growseaward if the beach is wide enough.Dunes can be completely flattened or overtopped dur-ing a storm (Figure 30). If the washover deposits arenot too deep and the vegetation has not been eroded,new beach grass shoots can emerge and begin thedune building process. Otherwise, dune recoveryFigure 28. Natural dunes can take many forms,from small mounds tohigh continuous ridges.Figure 29. Beach grass slides down the face of an eroded dune,takingroot at the toe and starting the natural recovery of the dune.
25has to start at the landward edge of the washover fanwhere there is vegetation or, in some cases, wherethere is a wrack line (the accumulation of vegetationand other natural debris left at the high waterline) thatcan begin trapping windblown sand. On the southshore, sand can be transported from the inland areatowards the beach on these washover fans becausedominant winds are from the west and north. As thelandward side of the dune becomes vegetated, sandtransport from this direction is slowed down and moresand comes from the beach. In response, the dunetends to grow seaward.The seaward growth of dunes is limited by the widthof the beach and distance from the waterline. Awider beach can provide more windblown sandand protection for the dune from the ocean. Sincedunes are primarily composed of finer sands, theyare very susceptible to damage from even smallwaves. While dunes can provide some protectionfrom episodic storm events, even the largest dunesare not effective in combating long-term or chronicerosion where they are consistently exposed to waveaction. The foredune is dependent on the beach. Ina sense, the dune and beach can be thought of aslinked components that move together in responseto changes in the shoreline position.Natural dune rebuilding processes operate relativelyslowly. Left solely to natural processes, dunes maytake years or even decades to recover after a severestorm. Because of the protection they provide andtheir aesthetic and environmental benefits, main-taining and enhancing dunes are common shorelinemanagement practices.Figure 30. Dunes can be overtopped and flattened during storms by waves andelevated water levels leaving washover deposits. The dunes can rebuild naturally butthis is usually a slow process and complete recovery can take years to decades.Humans and DunesCoastal dunes can be affected by human activityespecially when it prevents the movement or altersthe position of the dunes. The potential impactsof houses on the dunes is of particular concern,but studies looking at dune dynamics on the southshore found that properly built houses that areelevated on piles above the dune height and free ofobstructions underneath do not significantly weak-en the dune’s integrity or its protective capabilities.However, houses built directly on the ground canalter the deposition of windblown sand and, thus,may affect dune building processes. Studies havealso suggested that removing these houses with-out revegetating those areas can create bare sandpatches on the back side of the dune which canpersist for long periods. Since these bare patchesare susceptible to erosion and blowouts from thedominant westerly and northwesterly winds, theyalso have the potential to weaken the dune. Suchcomplex scenarios illustrate the difficulties associ-ated with trying to manage a resource as dynamicand fragile as the dunes. Management actionsmay have unintended consequences that can bestbe identified and rectified through comprehensivemonitoring and research efforts.Dune plantings and fencing: Human activity onthe dunes and programs of dune stabilization mayplay a more important role than elevated structuresin controlling what happens to these features. Mostpeople are aware that dune vegetation, especiallythe beach grass, is very vulnerable to foot traffic.Uncontrolled pedestrian access over the dunes canremove the vegetation and allow winderosion causing low spots that are moresusceptible to overwash. Beach grassspreads by sending rhizomes out under-ground. The rhizomes can extend 20feet from the plant. As we have seen,regrowth from rhizomes is an impor-tant mechanism in dune recovery afterstorms. However, the rhizomes arefragile and can be damaged by vehicletraffic even though they are beneath thesurface. For this reason, beach vehicletraffic should be discouraged within 20feet of the dune vegetation line.
Long stretches of sand fencing and artificially plantedvegetation used in dune building programs probablyhave more of an impact on dune processes than ei-ther elevated houses or pedestrian traffic. While theamount of windblown sand in the system is not large,these efforts can be extremely efficient at capturingthe sand that is available. When not sited, planned,or implemented properly, dune building projects canresult in a dune that is much closer to the water thanwould be found under natural conditions. Dunes builttoo close to the water will experience more erosiondue to more frequent wave action at the toe. Thesedunes may appear to have a high steep face but theyusually will not have as much sand as a dune placedfurther landward, due to the constant removal ofmaterial. Less sand usually means less protectionduring storms. The high continuous crest of artificialdunes may also interfere with the landward transportof sand and prevent more natural dune formationfurther inland.Beach scraping: Beach scraping is a techniquethat has also been used to build or repair dunes.A thin layer of sand is scraped from the top of theberm and pushed landward in an attempt to restorea dune (Figure 31). These projects are regulatedby the state in terms of when the scraping can takeplace, how much sand can be removed and where itcan be placed. The present regulations allow scrap-ing about two cubic yards of sand per foot of beach.While the effects of beach scraping have not beenrigorously examined on Long Island, limited studiesof this activity elsewhere suggest it has a limited im-pact, either positive or negative, on coastal process-es or protection of the upland area where it occurs.Basically, scraping simply redistributes the sandwithin the system and does not change the amountof sand available for dune and beach building. Thevolume of sand allowed to be moved is very small.Measurements on Fire Island, where many of thebeach scraping projects take place, show the aver-age volume of sand contained in the active beach(out to a depth of 24 feet) is about 925 cubic yardsper foot of beach. This means beach scrapingrearranges only about 0.2 percent of the totalamount of sand on the beach in those areaswhere it is permitted. Projects are limited to60-foot wide lengths of shoreline, further minimizingtheir impacts.Beach scraping probably has minimal adverseeffects on the beach, but, by the same token, it alsoprovides minimal benefits in terms of protectionfor the landward area. The small amount of sandadded to the dune would provide limited protectionagainst even a small storm. If the scraped sand isplaced seaward of the position where the naturaldune would normally form, the resultant feature ismore susceptible to erosion. Equipment operatingwithin 20 feet of the existing vegeta-tion line could also damage beachgrass rhizomes, hindering natural dunerecovery. Because of the drawbacksassociated with these projects, someexperts have suggested efforts mightbe better spent on bringing in beach-quality sand from an outside sourcefor dune building rather than relying onscraping. However, the difference incost between these alternatives couldvary considerably depending on siteaccess and has to be evaluated on acase by case basis.Figure 31. Beach scraping projects remove a thin layer of sand from the beach berm andpush it landward to form a mound. This redistribution of a relatively small amount of sandon the beach probably has minimal impact,either positive or negative,on coastal processesor protection of the upland.26
In Conclusion...Long Island’s south shore ocean coast is a remark-ably diverse and complex place. It is this diversityand complexity that provide the many environmental,recreational and economic benefits the coast has tooffer. This area is also very dynamic and, in manyways, very fragile. The shoreline we value and enjoytoday was created by a variety of forces and process-es operating on time scales ranging from hours tomillennia. The result is a coastline that is naturallychanging all the time. In some cases, human activi-ties have altered or disrupted the natural system,creating some of our most severe erosion problems.Proper management of this important area requiresa solid understanding of the factors affecting a27particular stretch of shoreline, the way the shorelineis actually responding to these factors, and the de-sired uses of the area. It also requires a variety ofstrategies that can be tailored to match the diverseconditions found along the south shore. In someareas, the best management strategy may be to donothing and let the natural processes continue un-impeded. In other areas, some form of interventionmay be warranted. However, care must be takento ensure that efforts to mitigate erosion problemswork in concert with, and not against, natural pro-cesses. Management strategies must be adaptableto changing conditions to ensure future generationscan also enjoy this unique resource.Selected ReferencesThe information presented here was derived from a number of technical articles and reports. Principal sources include:1) Bokuniewicz, H.J. 1998. Monitoring beaches: Conditions at the Village of East Hampton. Shore and Beach 66:12-17.2) Kana, T.W. 1995. A mesoscale sediment budget for Long Island, New York. Marine Geology 126:87-110.3) Leatherman, S.P., and J.R. Allen. 1985. Geomorphic analysis of the south shore barriers of Long Island, New York. National Park Service, Boston, MA Technical Report. 305 pages.4) McCluskey, J.M., K.F. Nordstrom and P.S. Rosen. 1983. An eolian sediment budget for the south shore of Long Island, New York. Final Report to the National Park Service, Boston, MA. 100 pages.5) Psuty, N.P., M. Grace and J.P. Pace. 2005. The coastal geomorphology of Fire Island: A portrait of continuity and change (Fire Island National Seashore science synthesis paper). National Park Service, Boston, MA Technical Report NPS/NER/NRTR-2005/021. 56 pages.6) Schwab, W.C., E.R. Thieler, J.R. Allen, D.S. Foster, B.A. Swift and J.F. Denny. 2000. Influence of inner continental shelf geologic framework on the evolution and behavior of the barrier-island system between Fire Island Inlet and Shinnecock Inlet, Long Island, New York. Journal of Coastal Research 16(2):408-422.7) Taney, N.E. 1961. Geomorphology of the south shore of Long Island, New York: Technical Memorandum 128, U.S. Army Corps of Engineers, Beach Erosion Board, Washington, D.C. 97 pages.8) Tanski, J., H. Bokuniewicz and C. Schlenk [Eds.]. 2001. Impacts of barrier island breaches on selected biological resources of Great South Bay, New York. New York Sea Grant Institute Technical Report NYSGI-01-002. 103 pages.9) U.S. Army Corps of Engineers. 1958. Atlantic Coast of Long Island New York (Fire Island to Montauk Point): U.S. Army Corps of Engineers, New York District, Cooperative Beach Erosion Control and Interim Hurricane Study. 75 pages.10) Williams, S.J., and E.P. Meisburger. 1987. Sand sources for the transgressive barrier coast of Long Island, New York: Evidence for landward transport of shelf sediments. Proceedings of Coastal Sediments ‘87, New Orleans, American Society of Civil Engineers Press, pages 1517-1532.Selected On-Line ResourcesAtlantic Coast of New York Erosion Monitoring Website http://dune.seagrant.sunysb.edu/nycoastFire Island National Seashore http://www.nps.gov/fiisNational Park Service Northeast Region Science Website http://www.nps.gov/nero/science/New York Sea Grant http://www.seagrant.sunysb.edu/U.S. Geological Survey Studies in the New York Bight http://woodshole.er.usgs.gov/project-pages/newyork/index.htmlMetric Conversion FactorsMULTIPLY BY TO OBTAINinch 2.54 centimeterfoot 0.305 meter (m)yard (yd) 0.914 meter (m)mile 1.609 kilometer (km)cubic yards 0.764 cubic meters
New York Sea Grant Extension Program(631) 632-8730