Basic Dune Physical Characteristics
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Basic Dune Physical Characteristics New Jersey’s Beach and Dune Characteristics Dunes in New Jersey are mainly confined to a narrow part of the sandy beach. e line of dunes closest to strip of land between the beach berm and upland the water is called the foredune or primary dune . ese development. While there are a few locations in which primary dunes are particularly susceptible to erosion, the sand supply and beach width are adequate for overwash, and breaching during extreme storms. Dunes dunes to reach their full form (Island Beach State Park, located landward of the foredune are numbered for example) most dunes are limited in height and consecutively and are referred to as the secondary dune , width by the sand available from the beach berm area the tertiary dune , and so on. Along developed coasts, and the infrastructure landward of the beach. As a the natural dunes are sometimes enlarged with result, on most New Jersey beaches only a single line of artificially placed sand or replaced altogether with a dunes exists. structure such as a seawall, bulkhead, or revetment. e shape of the beach on any given day is the result Seaward of the dune is the beach berm , which is the of a complex set of interactions between the air, the flat, dry section of the beach that is normally used by land, and the sea. Some of the more important factors recreational beach users and by several species of include the: wildlife and plants. Some of these include rare, threatened, and endangered species. Beyond the berm • wave heights and water levels experienced at is a sloped area known as the foreshore which leads into site during the recent past the water. is foreshore is exposed to constant wave present wave and water level conditions • action and generally shifts shape and slope between characteristics of the sand • seasons and storm events in response to changes in underlying geologic conditions at the site • wave action. • presence of any artificial structures. e beach profile continues under water. e area roughout much of New Jersey and the Mid- closest to the dry sand is called the nearshore . Like the Atlantic, present-day dune heights reach 8 to 15 feet foreshore, the nearshore is constantly affected by the above the flat part of the beach known as the berm energy of the breaking waves. e nearshore generally (Hamer et al. 1992; Psuty and Ofiara 2002). is takes one of two shapes. During calm periods, the height has been found to be related to the limit of the beach slopes off uniformly into deep water and ability of the wind to move sand higher on the dune generally takes on a characteristic shape known as an and deposit it on the back or lee side of the dune equilibrium beach profile (Dean 1977). During stormy (Hamer et al. 1992). Dunes have the potential under periods, however, sand eroded from the dry beach ideal conditions to grow in height up to 4 feet per year, collects in nearshore sand bars which form near the although growth rates of less than 2 feet per year are point at which the majority of the waves are breaking. more typical. e part of the beach between the breakpoint and the area that is constantly dry is sometimes referred to as In a typical beach configuration, known as a beach the surfzone . e area beyond the breakers is generally profile (Figure 2), the dune is located along the back referred to as the offshore portion of the beach profile. 4 Dune Manual B ack- s F hore ore- s hore Nea Dunes (th rsho roug re Berm crest h br eake rs) Offs hore Berms Beach scarp Longshore trough High tide Longshore bars Low tide (dotted line) Figure 2. Example of a typical beach profile. Dunes are constantly changing in response to both transport system until it is deposited offshore in a bar short- and long-term environmental processes at a or on a downdrift beach. given location. Dunes are sometimes referred to as reservoirs of sand. eir size fluctuates in response to Winds play an important role as well, with offshore the amount of available sand on the beach berm, the winds blowing sand from the dune onto the beach, strength and duration of the wind, and the storm surge and onshore winds blowing the sand back into the and wave action. Dunes can either be erosional or dune. Vegetation plays a critical role both in trapping depositional depending on the interaction between sand blown towards the dune and in limiting the sand these processes. Sand loss from wind- and wave- blown off the dune. ese dynamic environmental generated erosion can impact the dune directly or conditions cause dunes to shift between erosive and indirectly by decreasing the supply of sand available to accretional phases. On many beaches, the dune shifts build the dune. During large storms, high water levels landward during the winter in response to erosive may allow waves to erode the base of the dune and conditions and seaward in the summer in response to create a vertical cut or scarp (Figure 5A, B). Typically, accretional conditions (Figure 3). On a stable coast, the sand eroded from the dune gets swept into the the forces building the dune are balanced out by the surfzone, where it becomes a part of the active littoral forces 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 impact quantities of sediment, increasing the width and height on the shape of our coastlines. At the end of the last of the barrier islands (Psuty and Ofiara 2002). As the glacial period (approximately 20,000 years ago), sea barrier islands became more stable, vegetation began to level was about 450 feet lower than it is today, and the grow and coastal dunes began to develop landward of shoreline was located approximately 100 miles seaward the beach berm (Figure 4). Within the last 500 years of its present-day position (Psuty and Ofiara 2002). As or so, New Jersey’s barrier islands entered a new phase the earth’s temperature increased and the glaciers began in their evolutionary development as the once to melt, sea level rose rapidly and continued to do so bountiful supply of offshore sand dwindled, causing a until about 2,500 years ago, when it slowed transition from a period of relative stability to one of significantly. Geologists generally agree that the barrier gradual loss (Psuty and Ofiara 2002). As surges and island shoreline of New Jersey at this time was low and large waves associated with coastal storms began to narrow, with poorly developed beaches and dunes that inundate the beaches on a more regular basis, the were frequently overwashed during large storms (Psuty dunes began to erode and migrate landward. More and Ofiara 2002). During this time the coastal recently, human alterations to the environment, such landscape was extremely dynamic. As the rate of sea as shoreline armoring, have further reduced the natural level rise decreased to approximately its current rate, supply of sediment to the coast. Without a natural the waves and currents began to move the sediments source of sediment and to compensate for sand that were originally flooded during the period of rapid removed from the system during storms, beaches and sea level rise onto New Jersey’s barrier islands. e rate dunes struggle to survive. of sand transfer was sufficient to accumulate large Year 4 Year 3 Year 2 Year 1 Figure 4. 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