99 



in which A is a dimensional profile constant depending primarily on 

 sediment size but secondarily on wave climate. Dean (1977) found that 

 Eq. 7.6 is consistent with uniform wave energy dissipation per unit volume. 

 The quantities A and D* are related by 



24 ^* 2/3 



pg y g K 



in which p is the mass density of water, g is the gravitional constant and 

 K is the ratio of spilling breaking wave height to water depth (/c = 0.8). 



All models of beach profile response described earlier require the 

 identification of a limiting depth of motion h* in Eq. 7.3 and h^ and 

 h]-, in Eq. 7.4. Hallermeier (1981) has proposed an approximate method for 

 predicting this depth, h*, based on average annual significant deep 

 water wave height, Hg , and period Tg and sediment size D, 



h* = (Hg - 0.3a) Tg (g/5000D)°-^ (7.8) 



in which a is the standard deviation of the significant wave height. 



The models presented heretofore invoke the concept of a limiting depth 

 of motion, a depth seaward of which conditions are static or at least there 

 is no substantial exchange of sediment with the more active shoreface. 

 This assumption seems innocent and quite natural, yet the consequences are 

 very substantial. If no interchange with the shelf profile occurs, erosion 

 is the only possible shoreline response to sea level rise (i.e., there can 

 be no shoreward transport contributions from the continental shelf) . There 

 is evidence that shoreward sediment transport is a major contributor to 

 shoreline stability in many areas. The erosion along the south shore of 

 Long Island and at Montauk Point is clearly too small to provide the 



