1. GETTING STARTED with DUNES
Total Page：16
File Type：pdf, Size：1020Kb
1. GETTING STARTED with DUNES Introduction Sand dunes are important coastal features that provide critical habitat for coastal species and protection for the communities behind them. Dunes can be found along beaches all over the world, and range in size from small piles of sand to the 920 foot tall Mount Tempest found on Moreton Island in Australia. The official dune definition adopted by the State of New Jersey in Title 7 of the New Jersey Administrative Code, is as follows: “...wind or wave deposited or man-made formation of sand (mound or ridge), that lies generally parallel to, and landward of the beach and the foot of the most inland dune slope. “Dune” includes the foredune, secondary or tertiary dune ridges and mounds, and all landward dune ridges and mounds, as well as man-made dunes, where they exist.” (N.J.A.C.7:7E Coastal Zone Management Rules) This report has been written as a part of a comprehensive dune manual being compiled by the New Jersey Sea Grant Consortium. The intent of this portion of the dune manual is to provide basic information on the physical characteristics of dune establishment, growth, and erosion and to highlight some of the important considerations related to dune management. In New Jersey, Superstorm Sandy helped highlight the importance of a healthy beach and dune system to coastal resilience. Coastal dunes are dynamic geologic features that are created naturally as wind-blown sand is deposited preferentially along the back beach (Figure 1). As the sand deposits grow, it is typical for vegetation to colonize the dune. The vegetation helps stabilize the dune and promotes dune growth by trapping additional wind-blown sand. Naturally established dunes often contain multiple layers of ridges and valleys with a variety of sediment, vegetation, and wildlife. Dunes that are man-made often are linear and lack the complexity of natural systems. Figure 1. Coastal sand dunes with established vegetation (landward of the fence) and newly accumulated areas of wind-blown sand (along the fence). The present day configuration of the New Jersey coastline is the result of the complicated interaction between the coastal processes occurring on any given day and the underlying landscape created by geologic processes that took place tens of thousands of years ago. Climate change, sea level rise, waves, currents, coastal storms, and other processes cause sediment movement, erosion, overwash, and deposition, which have created and shape the features of the New Jersey barrier island coastline (Psuty and Ofiara, 2002). Historically, sea level rise has had the greatest impact on the shape of our coastlines. At the end of the last glacial period (approximately 20,000 years ago), sea level was about 450 feet lower than what it is today. At this time in history, the shoreline of New Jersey was located approximately 100 miles seaward of its present day position (Psuty and Ofiara, 2002). As the earth’s temperature rose and the glaciers began to melt, sea level rose rapidly, and continued to do so until about 2,500 years ago, when it slowed down markedly. Coastal geologists generally agree that the barrier island shoreline of New Jersey at this time was a low, narrow sand ribbon with poorly developed beaches and dunes that were frequently overwashed and extremely dynamic (Psuty and Ofiara, 2002). As the rate of sea level rise decreased to approximately its current rate, the waves and currents then began to transfer the sediment that was originally flooded by the rapid sea level rise onto what are now New Jersey barrier islands. The rate of sand transfer was sufficient to accumulate large quantities of sediment, increasing the width and height of the barrier islands (Psuty and Ofiara, 2002). Increased stability of the islands allowed vegetation to grow and coastal dunes began to develop landward of the beach berm (see Figure 2 below). Approximately 500 years ago, the sediment supply in the shallow waters offshore of New Jersey’s barrier islands was exhausted, causing the transition from a period of relative stability, to one of gradual loss (Psuty and Ofiara, 2002). In most locations, surges and large waves associated with coastal storms began to inundate the beaches, and the dunes began to erode and migrate landward. More recently, human alterations to the environment such as the construction of dams and reservoirs, has 2 | P a g e further reduced the natural supply of sediment to the coast. Without a source of sediment to maintain the natural balance, beaches and dunes struggle to survive. Figure 2. Dune width and height increase over time and migrate in a seaward direction (Rogers and Nash 2003). Modern Day Shoreline Configuration The primary factors that affect the present-day shape of a beach profile are: the wave heights experienced at the site during the past week or so (sometimes called antecedent conditions); the water level, including astronomical tide conditions and recent storm surges; the present wave conditions; the characteristics of the sand (predominantly the grain size); the underlying geologic conditions at the site; the presence of any man-made structures. A typical beach configuration known as a beach profile is depicted in Figure 3. The dune is typically configured along the back of the beach. The first line of dunes located closest to the water is called the foredune. Dunes located landward of the foredune are numbered consecutively, and are referred to as secondary dunes, tertiary dunes, and so on. Along developed coasts, the natural dunes are sometimes enhanced with artificially placed sand, or replaced altogether with a man-made structure such as a seawall, bulkhead, or revetment. Just seaward of the dune is the beach berm, which is the dry, flat section that is normally used by recreational beach goers. Following the berm is the foreshore area, which has a much steeper slope and leads into the water. This section of the beach profile is exposed to constant wave action, and generally shifts shape and slope between seasons and storm events in response to the changes in wave action. During periods of high wave energy, such as during storms or throughout the winter, the foreshore slope is flattened, as sand erodes and moves offshore, building up the sandbars. This is why beaches in New Jersey tend to be the narrowest in the spring, after the winter storms have had an opportunity to shape the beach. During periods of reduced wave energy, such as during the summer months, the sand moves from the bars, back 3 | P a g e toward the shoreline. This steepens the foreshore region, and generally results in the widest beaches of the season during the late summer/early fall. The underwater part of the beach profile generally takes one of two shapes. During calm periods, the beach slopes off uniformly into deep water and generally takes on a characteristic shape known as an equilibrium beach profile (Dean, 1977). During stormy periods, the sand eroded from the berm collects in sand bars which form near the breakpoint, and enhance wave breaking. The portion of the beach between the breakpoint and the maximum limit of the wave uprush is frequently referred to as the surfzone. Figure 3. Example of a typical beach profile (Beach profile: Beach profile figure). 4 | P a g e .