Basic Dune Physical Characteristics - Sea...

New Jersey’s Beach and Dune Characteristics

Dunes in New Jersey are mainly confined to a narrowstrip of land between the beach berm and uplanddevelopment. While there are a few locations in whichthe sand supply and beach width are adequate fordunes to reach their full form (Island Beach State Park,for example) most dunes are limited in height andwidth by the sand available from the beach berm areaand the infrastructure landward of the beach. As aresult, on most New Jersey beaches only a single line ofdunes exists.

e shape of the beach on any given day is the resultof a complex set of interactions between the air, theland, and the sea. Some of the more important factorsinclude the:

roughout much of New Jersey and the Mid-Atlantic, present-day dune heights reach 8 to 15 feetabove the flat part of the beach known as the berm(Hamer et al. 1992; Psuty and Ofiara 2002). isheight has been found to be related to the limit of theability of the wind to move sand higher on the duneand deposit it on the back or lee side of the dune(Hamer et al. 1992). Dunes have the potential underideal conditions to grow in height up to 4 feet per year,although growth rates of less than 2 feet per year aremore typical.

In a typical beach configuration, known as a beachprofile (Figure 2), the dune is located along the back

part of the sandy beach. e line of dunes closest tothe water is called the foredune or primary dune. eseprimary dunes are particularly susceptible to erosion,overwash, and breaching during extreme storms. Duneslocated landward of the foredune are numberedconsecutively and are referred to as the secondary dune,the tertiary dune, and so on. Along developed coasts,the natural dunes are sometimes enlarged withartificially placed sand or replaced altogether with astructure such as a seawall, bulkhead, or revetment.

Seaward of the dune is the beach berm, which is theflat, dry section of the beach that is normally used byrecreational beach users and by several species ofwildlife and plants. Some of these include rare,threatened, and endangered species. Beyond the bermis a sloped area known as the foreshore which leads intothe water. is foreshore is exposed to constant waveaction and generally shifts shape and slope betweenseasons and storm events in response to changes inwave action.

e beach profile continues under water. e areaclosest to the dry sand is called the nearshore. Like theforeshore, the nearshore is constantly affected by theenergy of the breaking waves. e nearshore generallytakes one of two shapes. During calm periods, thebeach slopes off uniformly into deep water andgenerally takes on a characteristic shape known as anequilibrium beach profile (Dean 1977). During stormyperiods, however, sand eroded from the dry beachcollects in nearshore sand bars which form near thepoint at which the majority of the waves are breaking.e part of the beach between the breakpoint and thearea that is constantly dry is sometimes referred to asthe surfzone. e area beyond the breakers is generallyreferred to as the offshore portion of the beach profile.

4 Dune Manual

• wave heights and water levels experienced atsite during the recent past

• present wave and water level conditions• characteristics of the sand• underlying geologic conditions at the site• presence of any artificial structures.

Basic Dune Physical Characteristics

Dunes

Berms

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High tide

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Figure 2. Example of a typical beach profile.

Dunes are constantly changing in response to bothshort- and long-term environmental processes at agiven location. Dunes are sometimes referred to asreservoirs of sand. eir size fluctuates in response tothe amount of available sand on the beach berm, thestrength and duration of the wind, and the storm surgeand wave action. Dunes can either be erosional ordepositional depending on the interaction betweenthese processes. Sand loss from wind- and wave-generated erosion can impact the dune directly orindirectly by decreasing the supply of sand available tobuild the dune. During large storms, high water levelsmay allow waves to erode the base of the dune andcreate a vertical cut or scarp (Figure 5A, B). Typically,the sand eroded from the dune gets swept into thesurfzone, where it becomes a part of the active littoral

transport system until it is deposited offshore in a baror on a downdrift beach.

Winds play an important role as well, with offshorewinds blowing sand from the dune onto the beach,and onshore winds blowing the sand back into thedune. Vegetation plays a critical role both in trappingsand blown towards the dune and in limiting the sandblown off the dune. ese dynamic environmentalconditions cause dunes to shift between erosive andaccretional phases. On many beaches, the dune shiftslandward during the winter in response to erosiveconditions and seaward in the summer in response toaccretional conditions (Figure 3). On a stable coast,the forces building the dune are balanced out by theforces acting to erode the dune.

Figure 3. Dune migration due to sand movement from wind.

Dune Manual 5

Long-Term Dune Evolution

Historically, sea level rise has had the greatest impacton the shape of our coastlines. At the end of the lastglacial period (approximately 20,000 years ago), sealevel was about 450 feet lower than it is today, and theshoreline was located approximately 100 miles seawardof its present-day position (Psuty and Ofiara 2002). Asthe earth’s temperature increased and the glaciers beganto melt, sea level rose rapidly and continued to do sountil about 2,500 years ago, when it slowedsignificantly. Geologists generally agree that the barrierisland shoreline of New Jersey at this time was low andnarrow, with poorly developed beaches and dunes thatwere frequently overwashed during large storms (Psutyand Ofiara 2002). During this time the coastallandscape was extremely dynamic. As the rate of sealevel rise decreased to approximately its current rate,the waves and currents began to move the sedimentsthat were originally flooded during the period of rapidsea level rise onto New Jersey’s barrier islands. e rateof sand transfer was sufficient to accumulate large

quantities of sediment, increasing the width and heightof the barrier islands (Psuty and Ofiara 2002). As thebarrier islands became more stable, vegetation began togrow and coastal dunes began to develop landward ofthe beach berm (Figure 4). Within the last 500 yearsor so, New Jersey’s barrier islands entered a new phasein their evolutionary development as the oncebountiful supply of offshore sand dwindled, causing atransition from a period of relative stability to one ofgradual loss (Psuty and Ofiara 2002). As surges andlarge waves associated with coastal storms began toinundate the beaches on a more regular basis, thedunes began to erode and migrate landward. Morerecently, human alterations to the environment, suchas shoreline armoring, have further reduced the naturalsupply of sediment to the coast. Without a naturalsource of sediment and to compensate for sandremoved from the system during storms, beaches anddunes struggle to survive.

Figure 4. Dune width and height increase over time and migrate in a seaward direction (Rogers and Nash 2003).

Year 1

Year 2

Year 3

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6 Dune Manual

On eroding coastlines, dunes respond by shiftinglandward as the beach erodes. e existence of dunesalong eroding shorelines is an indication that sand isintermittently being transported into the dunes. Onnatural dunes, as sand is lost on the seaward side, it istypically deposited on the crest and landward side of

the dune (Psuty and Ofiara 2002) during overwash.Along developed coastlines, however, there is typicallylimited space for the dunes to shift inland, whichexposes the dunes to increased wave attack andscarping (vertical erosion) that threatens to destroy thedune system (Figures 5A, B).

Figure 5A. Dunes eroded by Hurricane Ida in New Jersey. Figure 5B. Eroded dune base resulting fromstrong wave action during a severe storm.

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Short-Term Evolution

Sediment Deposition for DuneGrowthWind is the main force responsible for theestablishment, growth, and maintenance of coastaldunes. When a strong wind blows across a sandybeach, it picks up grains of sand which then move bysliding, rolling, or even hopping past one another.Lighter grains of sand are capable of being movedmore easily and can travel farther, while the heaviergrains are left behind. Natural and artificialobstructions on the beach cause the wind to slowdown, which results in depositional areas in selectlocations. On a natural beach, vegetation is the mostcommon obstruction, while on developed beaches,snow fencing or sand fencing is frequently installed tomimic the effect of vegetation and accelerate dunegrowth (Figure 1). e rate at which the wind cansupply sand to a dune depends on the strength of the

wind and the availability of a sand source. A beachwith a wide berm typically provides an adequate sandsource and will allow the dune to grow; however, if thebeach is narrow, there may be an insufficient sandsupply, which limits dune growth. On naturallynarrow beaches, beach nourishment projects representan opportunity to encourage dune growth that mightnot occur otherwise.

As a dune grows, it becomes asymmetrical in shape,with a steep seaward face and a mildly slopinglandward tail. As the dune becomes taller and thevegetation becomes denser, the backdune is shelteredfrom the wind. Under these conditions, growthlandward of the crest is reduced, while seaward of thecrest, growth remains steady. On wide, depositionalcoastlines, it is possible for several distinct dunefeatures to develop. If more than one line of dunesexist, the landward line must be established first.

Dune Manual 7

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Figure 6A, B, C. American beachgrass on dunes on Long Beach Island, New Jersey.

Otherwise the line closest to the shoreline will trapsand, preventing it from reaching the original dune.Generally, the seaward-most dune will continue togrow seaward until the toe of the dune reaches thelandward limit of seasonal beach fluctuations. If thetoe of the dune reaches the mean high water line,beach use for both humans as well as rare, threatened,and endangered species can be inhibited. Healthybeach and dune systems require both a dune and afronting beach, which may require periodic beachnourishment projects in areas where natural sedimentsources have been compromised.

Sand Grain Sizee size of sand grains on a beach influences the rateof sand transport and dune growth. On a naturalbeach, a given sample of sand will contain particles ofvarying sizes, each of which respond differently to agiven set of wind and wave conditions. Fine-grainsands will move more easily, remain in motion longer,and therefore travel farther than coarse-grain sands.e result is a tendency for wind and waves to sortbeach sediment. Fine-grain sand is more easilyremoved from the beach face and is commonly foundin dunes and offshore in deeper water. Once the initiallayer of fine sand is removed from the beach, theremaining larger grains armor the ground surface,making it more difficult to transport additional sand.Higher wind speeds or larger waves are then required

to continue the transport process after the fine surfacematerials have been removed. On most beaches, thesand found at the bottom of the foreshore slope, nearthe location of most intense wave action, is typicallythe coarsest sand.

VegetationVegetation is an important part of the evolution ofdunes, as it serves multiple ecological and physicalpurposes. Vegetation slows the wind, causing some ofthe trapped wind-blown sand to settle to the ground.e heavy grains fall out first, as more energy isrequired to keep them suspended. As the wind depositsthe sand, it accumulates around the vegetation,initiating the creation of a new dune or contributingto the growth of an existing one. e accumulatingsand promotes the growth of vegetation uniquelyadapted to dune environments. Both the vegetationand the dune grow in elevation simultaneously as theroots of the vegetation strengthen and help reinforcethe base of the dune. While barriers such as sandfences can recreate the trapping efficiency ofvegetation, they cannot grow with the dunes likevegetation and therefore require maintenance overtime.

e most effective dune systems in both promotinghabitat and providing storm protection are those inwhich the appropriate vegetation is encouraged in each

8 Dune Manual

Figure 7A, B. Multiple species of dune plants that colonize along the landward sideof an established dune.

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zone. ‘Cape’ American beachgrass (Ammophilabreviligulata) is the primary plant species found onforedunes (or pioneer zone) in New Jersey (for moreinformation, see the dune planting section of thisguide) (Figure 6A-C). American beachgrass thrives onwindblown sand deposits and collects sparse nutrientsfrom the incoming sand, stimulating growth andreproduction. American beachgrass establishes quicklyand spreads rapidly, making it a favorite amongcommunities undertaking dune restoration projects.Trapping rates of 2 to 4 feet per season have beenobserved along densely cultivated dunes.

In natural dune systems, as growth progresses, multiplespecies of plants will begin to colonize areas landwardof the foredune. In New Jersey, typical species arerugosa rose (Rugosa rosa), bayberry (Morellapensylvanica), and goldenrod (Solidago sp.) (Figure 7A,B). Species diversity is important for dune stabilizationbecause each plant type has a different capacity toretain sand through its roots and above-groundvegetation. One of the most common mistakescommunities make when undertaking a dunerestoration project is planting a single species (mostoften American beachgrass) everywhere within thedune complex. Within the dune system, dense anddiverse native herbaceous plants in the appropriatezones can provide habitat to pollinators and otherwildlife. In the beach berm zone, the appropriatevegetation is native, herbaceous, and very sparse(<10% cover). More information on the type ofvegetation appropriate for each zone is covered insubsequent chapters; however, dune planting should

always be consistent with a community’s beachmanagement plan and should be coordinated with theENSP to ensure that habitat provisioning is notcompromised.

Dune Manual 9

Sand FencingSand fencing or snow fencing is often used as a meansof accelerating or controlling the growth of dunes.Sand fencing performs a role similar to the stems ofthe vegetation on a natural beach. By slowing the windto a point below that which is required to keep sandgrains entrained, deposition is encouraged. Sandaccumulates around the base of the sand fence, similarto the way snow accumulates around a snow fence incolder climates. Fencings of various materials anddensities have been utilized. However, experimentshave found that the typical wire-bound, wood-slatfencing with a width roughly equal to the spacingbetween slats (50% porosity) traps about as much sandas other, more costly materials (Manohar and Bruun1970) (Figure 8A, B). Due to the relative costeffectiveness of this type of fencing, it is the type mostfrequently used in New Jersey.

In terms of effectiveness, it has been found that sandwill accumulate and eventually reach a level aboutthree-fourths of the exposed height of the sand fence(Hamer et al. 1992), depending on factors such aslimitation of space and sediment supply. In DoverTownship (now Toms River Township), New Jersey, a4-foot-tall sand fence placed in a straight line parallelto the ocean was observed to trap concave sand

deposits 40 feet wide and 3.5 feet high (Herrington2004). Although there are some subtle differences insand-trapping ability, the most important factor is thetotal length of fence in a given area. Studies haveshown that where windblown sand is abundant,installing twice as much fence (one row about 40 feetbehind the first) traps about twice as much sand,regardless of fence alignment (Hamer et al. 1992).

Sand fencing is also commonly used for several otheractivities related to dune building in New Jersey.Several coastal communities deploy sand fencingseasonally to reduce the amount of wind-blown sanddeposited in the street and/or on private propertyduring the winter months. Typically, this material isredistributed on the berm during the spring. Somecommunities also try to use sand fencing to createdesired dune geometries. By strategically placingfencing, dune characteristics such as height, width, anduniformity can be controlled. Although these practicesare not advocated, if they are conducted they shouldbe included in a community’s beach maintenance planand should be coordinated with the NJDEP to ensurethat the beach/dune system (including habitatprovisioning) is not compromised during the process.

Seasonal VariabilityOn stable or eroding coastlines, the seasonalfluctuation in the beach profile limits the seawardextent of the coastal dune system. Pioneer dune grassestypically encroach on the berm during the summer,

advancing the dune seaward. During the winter, thissummer growth is subsequently cut back by stormsthat erode the berm and cut into the dune toe. Wherefully established dune systems exist, the berm willtypically be eroded back to the seaward line of dunevegetation. is seasonal process gives rise to therelatively clear, straight vegetation lines found on manynatural beaches (Rogers and Nash 2003) whichrepresent the landward limit of wave-induced erosionduring the last several storm seasons.

In New Jersey, seasonal weather patterns causedynamic changes in the beach profile. During periodsof high wave energy, the foreshore slope is flattened assand erodes from the berm and moves offshore.Typically, much of the sand is stored relatively close toshore in underwater sandbars. If the storms are severeenough, the erosion may extend all the way across thebeach berm and impact the base of the dunes. iscauses an avalanching processes known as beachscarping, which can result in large vertical cuts in theseaward dune face. During periods of reduced waveenergy, the New Jersey coast is exposed to smaller, lessfrequent storms and long-period ocean swell thattransports sand from offshore back onto the dry beach.During this process, the foreshore region is steepenedand the widest beaches of the season (typically duringthe late summer/early fall) are formed. Overall, theseseasonal fluctuations can add up to 75 to 100 feet ofhorizontal and 2 to 5 feet of vertical change.

10 Dune Manual

Figure 8A, B. Examples of dune fences with sand accumulation.

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Storms and RecoverySmall storms typically only impactthe beach/berm system and leave thedunes intact. e pre-storm beachwidth, elevation, and presence/absence of vegetation will affect theseverity of the erosion experiencedduring a storm. In addition, if thebeach has been left vulnerable byprevious storms, increased erosion is likely. In fact, thecumulative effects of two closely spaced minor stormscan often exceed the impact of a single severe storm.

Dune recession typically only occurs during larger,more infrequent storm events. During these strongstorm events, the berm erodes first, which exposes theforedune to direct wave impact. As the waves erode thebase of the dune, the front face eventually becomesunstable and collapses. During routine storms, most ofthe eroded sand is temporarily stored offshore insandbars that migrate onshore during calm weather.e beach berm can recover to its pre-storm width andelevation in a matter of months. Depending on theextent of the erosion, post-storm dune recovery cantake much longer. While the dune begins to recoveralmost immediately, it typically takes several growingseasons for the beach, dune, and vegetation to returnto pre-storm conditions. In some cases, it has beenfound that full recovery from the most extreme stormscan take up to a decade (Rogers and Nash 2003). Inthe wake of severe storms, emergency beach

nourishments are often utilized as a way to rebuild thebeach and accelerate dune recovery.

BreachesStorms like Sandy that generate extreme water levelsand intense wave action can generate breaches inundersized sand dune systems. Breaches can occureither from the land side or the bay side, depending onthe relative elevation of the water. On undevelopedcoastlines, breaches and overwash are natural processesthat contribute to barrier island migration and refreshimportant habitats. On developed barrier islands,however, breaches represent one of the mostdestructive short-term erosion hazards as swift currentscan create deep channels capable of carrying awayanything in their paths. During Superstorm Sandy,multiple breaches were formed in the Borough ofMantoloking, resulting in extensive damage to thecommunity (Walling 2015). When they occur indeveloped areas such as Mantoloking, breaches aretypically closed manually through sand placement orstructural means; however, when they occur on naturalcoastlines, breaches may be left to evolve naturally.

Dune Manual 11

Pre-storm dune configuration

Post-storm sand from dunerelocated in water

Figure 9. Relocation of sand after a storm.

Basic Dune Physical Characteristics - Sea...

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