Equation 3-49 provides a conservative estimate for wave setup at the 

 shore. The difference between laboratory data and theory, however, is not 

 likely to exceed the uncertainties of field data. 



************** 



EXAMPLE PROBLEM ************** 



GIVEN : Hh = 20 feet, T = 12 seconds. 

 FIND : Wave setup, Sw 

 SOLUTION : Using Equation 3-49, 



S^ = 0.19 



Sj^ = 0.19 



1- 2.82 



1-2.82 20 



Sj^; = 3.1 feet . Say 3 feet . 

 Equation 3-49 is only applicable to normal beach slopes. 

 ************************************* 



3.86 STORM SURGE AND WIND SETUP 



3.861 General . Reliable estimates of water-level changes under storm 

 conditions are essential for the planning and design of coastal engineer- 

 ing 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 25 feet on the open 

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

 and reservoirs are less, 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. How- 

 ever, setdown may be appreciable when a storm traverses a relatively narrow 



3-82 



