oceans, waves and tides create difficulties in establishing a datum. At times, 

 it is impossible to distinguish the effect of long-period swell in creating 

 waves on the sounding record from actual sand waves on the sea floor (Magoon 

 and Sarlin, 1970; Bruno and Gable, 1976). 



Finally, if deep reliable profiles are found, evolution of the closure 

 depth for use in equation (1) requires two surveys separated by an appropriate 

 time period during which profile adjustment actually occurred in response to a 

 known change in water levels. Hallerraeier (1977) demonstrates the dependence 

 of profile closure on local wave conditions. The difference between depths of 

 closure at two sites with identical wave and sediment characteristics, one with 

 a stable mean water level and the other with a recently displaced water level, 

 has not been studied. It seems plausible that storm waves could cause a net 

 profile change where equilibrium had been perturbed by the recent shift in the 

 mean water level, and yet cause only sediment motion and (almost by definition) 

 no net change in bottom elevation where the profile was in equilibrium with a 

 constant water level. If this is the case, real water level changes are 

 essential if repetitive profiles are to reveal a closure depth suitable for 

 testing the Bruun concept. Clearly, many problems plague the determination of 

 the appropriate closure depth and therefore discourage application of Bruun' s 

 concept for predicting future shore retreat. 



d. R. or the Percentage of Sediment Loss (Item b) . Equation (1) can be 

 adjusted to account for any sediment lost from the active profile, but only if 

 the volume losses can be determined. Often they cannot. Loss occurs when 

 there is an uncompensated exchange of sediment beyond the surveyed boundaries. 

 Losses can occur offshore, onshore, or alongshore. On the west coast, subma- 

 rine canyons complicate the determination of offshore losses. On the gulf 

 coast, hurricane processes have moved coarse sediment from as deep as 20 meters 

 onto barrier islands (Hayes, 1967). Return currents after hurricane passage 

 reportedly spread a 1- to 2-centimeter layer of beach sand over homogenous muds 

 8 kilometers from shore; even thicker layers of nearshore silts and muds re- 

 portedly moved much farther gulfward as turbidity currents (Hayes, 1967). 

 Onshore losses are a problem on the east coast. High tides and severe storms 

 transport beach sand to the bay side of barriers at rates ranging from more 

 than 40 cubic meters per meter within individual overwash deposits during 

 single storms to about 1 cubic meter per meter for long stretches of shore 

 yearly (Schwartz, 1975). The engineer must consider the contribution of these 

 or other processes to sediment losses over the period of his study. If QT is 

 found to be the net exchange of sediment in time, T, across the boundaries of 

 a control area with longshore length, Y, then the anticipated retreat should 

 be reduced by QT/YZ: 



zX(R^)«g (^) 



X =— 1^ -_ (4) 



Z YZ 



e. Adequacy of an Equilibrium Model (Item a) . Use of equilibrium assump- 

 tions to model dynamic coastal changes also deserves scrutiny. The idea of an 

 "equilibrium beach profile" has had a long history (e.g., Fenneman, 1902); how- 

 ever, opinions still differ as to exactly what the concept actually entails. 

 By one definition, the profile of equilibrium is the ultimate shape which 

 coastal processes strive to impart to a beach. Of coarse, nature seldom re- 

 mains constant long enough for a strict equilibrium to develop. In the present 



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