1969, in excess of 20 feet MSL over many miles of the open Gulf Coast, 

 with a peak value of 24 feet MSL near Pass Christian, Mississippi. High 

 water levels in excess of 12 feet MSL on the open coast and 20 feet within 

 bays were recorded along the Texas coast as the result of Hurricane Carla, 

 September, 1961. Water levels above 13 feet MSL were recorded in the 

 Florida Keys during Hurricane Donna, 1960. 



Accumulation of data over many years in some areas, such as regions 

 near the North Sea, has led to relatively accurate empirical techniques 

 of storm surge prediction for some locations. However, these empirical 

 methods are not applicable to other locations. In general, not enough 

 storm surge observations are available in the United States to make accu- 

 rate predictions of storm surge. Therefore, it has been general practice 

 to use hypothetical design storms, and to estimate the storm-induced surge 

 by physical or mathematical models. Mathematical models are usually used 

 for predicting storm surge, since it is difficult to represent some of the 

 storm surge generating processes (such as the direct wind effects and 

 Coriolis effects) in physical laboratory models. 



a. Hydrodynamic Equations . Equations that describe the storm surge 

 generation processes are the continuity equation expressing conservation 

 of mass and the equations of motion expressing Newton's second law. The 

 derivations are not presented here; references are cited below. The equa- 

 tions of motion and continuity given here represent a simplification of 

 the more complete equations. A more simplified form is obtained by verti- 

 cally integrating all governing equations and then expressing everything 

 in terms of either the mean horizontal current velocity or volume trans- 

 port. Vertically integrated equations are generally preferred in storm- 

 surge calculations since interest is centered in the free surface motion 

 and mean horizontal flow. Integration of the equations for the storm 

 surge problem are given by Haurwitz (1951), Welander (1961), Fortak (1962), 

 Platzman (1963), Reid (1964), and Harris (1967). 



The equations given here are obtained by assuming: 



(1) vertical accelerations are negligible, 



(2) curvature of the earth and effects of surface 



waves can be ignored, 



(3) the fluid is inviscid, and 



(4) the bottom is fixed and impermeable. 



The notation and the coordinate scheme employed are shown schematic- 

 ally in Figure 3-43. D is the total water depth at time t, and is 

 given by D = d + S, where d is the undisturbed water depth and S is 

 the height of the free surface above or below the undisturbed depth re- 

 sulting from the surge. The Cartesian coordinate axes, x and y, are in 

 the horizontal plane at the undisturbed water level and the z axis is 

 directed positively upward. The x axis is taken normal to the shoreline 

 (positive in the shoreward direction), and the y axis is taken alongshore 

 (positive to the left when facing the shoreline from the sea). 



3-92 



