negative for setdown) occuring at a point for random wave conditions. Figure 



3-52 shows sample values of setup for a plane l-on-30 laboratory slope. Setup 



occurs where the water depth-to-deepwater wave height ratio d/H' ranges from 



0.9 to 1.1 where H' is the deepwater significant wave height, measured as 



H . The amount of setup landward of this point increases as deepwater wave 

 m 



steepness decreases for a given value of deepwater wave height (Fig. 3-52). 

 The increasing setup with decreasing steepness occurs in part because lower 

 steepness waves are associated with decreased wave energy dissipation due to 

 breaking; therefore, more wave energy is available to be converted into poten- 

 tial energy associated with wave setup. Note that at the initial Stillwater 

 level intercept (d = 0) the setup is estimated to be on the order of twice the 

 value at d/H' = 0.5 . 



Setup observed on a l-on-30 laboratory slope was used to calibrate a numerical 

 procedure, and resulting predicted values of random (sea) wave setup are 

 illustrated in Figure 3-53 for d/H' = 0.5 . Note that beach slope is 

 predicted to have a small influence on setup for random (sea) wave conditions. 



A simplified numerical procedure for predicting wave setup on a plane beach 

 has been developed by Goda (1975). 



6. Storm Surge and Wind Setup . 



a. General . Reliable estimates of water level changes under storm 

 conditions are essential for the planning and design of coastal engineering 

 works. Determination of design water elevations during storms is a complex 

 problem involving interaction between wind and water, differences in 

 atmospheric pressure, and effects caused by other mechanisms unrelated to the 

 storm. Winds are responsible for the largest changes in water level when 

 considering only the storm surge generating processes. A wind blowing over a 

 body of water exerts a horizontal force on the water surface and induces a 

 surface current in the general direction of the wind. The force of wind on 

 the water is partly due to inequalities of air pressures on the windward side 

 of gravity waves and partly due to shearing stresses at the water surface. 

 Horizontal currents induced by the wind are impeded in shallow water areas, 

 thus causing the water level to rise downwind while at the windward side the 

 water level falls. The term storm surge is used to indicate departure from 

 normal water level due to the action of storms. The term wind setup is often 

 used to indicate rises in lakes, reservoirs, and smaller bodies of water. A 

 fall of water level below the normal level at the upwind side of a basin is 

 generally referred to as setdown. 



Severe storms may produce surges in excess of 8 meters (26 feet) on the 

 open coast and even higher in bays and estuaries. Generally, setups in lakes 

 and reservoirs are smaller, and setdown in these enclosed basins is about 

 equivalent to the setup. Setdown in open oceans is insignificant because the 

 volume of water required to produce the setup along the shallow regions of the 

 coast is small compared to the volume of water in the ocean. However, setdown 

 may be appreciable when a storm traverses a relatively narrow landmass such as 

 southern Florida and moves offshore. High offshore winds in this case can 

 cause the water level to drop as much as 1 meter or more. 



3-107 



