wave energy, the beach adjusts its profile to facilitate the dissipation of 

 the additional energy. This is most frequently done by the seaward transport 

 of beach material to an area where the bottom water velocities are suffi- 

 ciently reduced to cause sediment deposition. Eventually enough material is 

 deposited to form an offshore bar which causes the waves to break farther 

 seaward, widening the surf zone over which the remaining energy must be dis- 

 sipated. Tides compound the dynamic beach response by constantly changing the 

 elevation at which the water intersects the shore and by providing tidal 

 currents. Thus, the beach is always adjusting to changes in both wave energy 

 and water level. 



Natural protective dunes are formed by winds blowing onshore over the 

 foreshore and berm, transporting sand landward from the beach (see Figs. 1-6 

 and 1-7). Grass and sometimes bushes and trees grow on the dunes, and the 

 dunes become a natural levee against sea attack. Dunes provide a reservoir of 

 beach sand which in turn provides the final natural protection line against 

 wave attack. 



2. Beach Response to Storms . 



The subtle changes in the beach which occur during normal conditions are 

 nearly imperceptible to the untrained observer, but the beach's defense 

 mechanisms become obvious when storms attack. Storms do not occur often, but 

 their effects are often devastating in terms of shoreline erosion. 



During storms, strong winds generate high, steep waves. In addition, these 

 winds often create a storm surge which raises the water level and exposes to 

 wave attack higher parts of the beach not ordinarily vulnerable to waves. The 

 storm surge allows the large waves to pass over the offshore bar formation 

 without breaking. When the waves finally break, the remaining width of the 

 surf zone is not sufficient to dissipate the increased energy contained in the 

 storm waves. The remaining energy is spent in erosion of the beach, berm, and 

 sometimes dunes which are now exposed to wave attack by virtue of the storm 

 surge. The eroded material is carried offshore in large quantities where it 

 is deposited on the nearshore bottom to form an offshore bar. This bar 

 eventually grows large enough to break the incoming waves farther offshore, 

 forcing the waves to spend their energy in the surf zone. This process is 

 illustrated in Figure 1-8. 



Beach berms are built naturally by waves to about the highest elevation 

 reached by normal storm waves. When storm waves erode the berm and carry the 

 sand offshore, the protective value of the berm is reduced and large waves can 

 overtop the beach. The width of the berm at the time of a storm is thus an 

 important factor in the amount of upland damage a storm can inflict. 



In severe storms, such as hurricanes, the higher water levels resulting 

 from storm surges allow waves to erode parts of a dune. It is not unusual 

 for 18- to 30-meter-wide (60- to 100- foot) dunes to disappear in a few 

 hours. Storm surges are especially damaging if they occur concurrently with 

 high astronomical tides. 



1-10 



