shoreline, the tombolo's apex can be shifted downdrift. The tombolo's or 

 salient's equilibrium position is dependent upon the strength of the predomi- 

 nant longshore current and the length of the structure. If there are signifi- 

 cant seasonal variations in the predominant wave direction, the equilibrium 

 position of the salient may readjust accordingly. 

 Wave height 



32. The average, extremes, and seasonality of the wave height control 

 the energy available for sediment transport. One of the greatest challenges 

 in designing a detached breakwater system is not in determining the extreme 

 wave conditions but in determining the average range of conditions which con- 

 trol the planform stability. Wave height also affects the pattern of dif- 

 fracted wave crests. In shallow water, wave celerity is given by linear wave 

 theory: 



C = 



where 



C = wave celerity 



g = gravity acceleration constant 



d = water depth 

 This relationship predicts that for water of constant depth, the crestline of 

 the diffracted waves will be circular (Figure 16a). In this case the entire 

 wave crest moves at a uniform and constant speed. This model is not quite 

 accurate for very shallow water, where wave amplitude affects the wave speed 

 (Weishar and Byrne 1978) and therefore affects diffraction characteristics. 

 Wave celerity in very shallow water may be more accurately expressed as 



C = /g(d + H) 



In very shallow water, wave celerity decreases along the diffracted wave crest 

 in relation to the decrease in wave height. The result is a distortion of the 

 diffracted wave train from the circular pattern shown in Figure 16a to a 

 series of arcs of decreasing radius (Figure 16b) . The significance of this 

 process may be seen in those shallow-water situations which enable the undif- 

 fracted portion of the wave adjacent to the structure to reach the shore 

 before the waves diffracted around the breakwater ends intersect. This 



32 



