EXTENSION OF 

 THE LINE 

 THROUGH 

 d, and d. 



r- Predicted losses will be 



closure depth Is 

 .ith J 

 R RlSmr, L Predicted losses will be 



closure depth Is 



r Predicted gains will be too small If the estluu 

 I closure, depth is too small. 



'- Predicted gains »111 be too large <f the estlna 



too large If the estlmtted 

 depth Is too large. 



closure depth Is too large. 



all if the estimated 



Predicted gains will be too large If the estimated 



closure depth is too small. 



Predicted gains will be too small If the estimated 



closure depth Is too large. 



Figure 22. Diagram for determining if a suspected error weakens or 



strengthens arguments based on the sediment balance prediction. 



If a line connecting two comtemplated closure depths extends below the average 

 height of the dunes throughout the section of shore under study, then over- 

 estimating the closure depth causes equation (1) to overpredict the response; 

 underestimating the closure causes equation (1) to underpredict the response. 

 But, if the line extends above the dune height, then overestimating the 

 closure underpredicts response and underestimating the closure overpredicts 

 response. If the extended line intersects the dune crest, the prediction 

 remains unaffected by the error in d. These relationsips will apply 

 regardless of whether the predicted response is a retreat or an advance of the 

 shore. 



c. Ideal Long-Term Development . The longshore variability in slope near 

 a depth of d is another item to check. If all profiles have the same off- 

 shore slope, a, and if it is assumed that long-terra recession unearthed 

 deposits similar to the modern substrate and the wave climate has been sta- 

 tionary, then tan a = Z/X; i.e. , a is not only the actual bottom slope, but 

 also Hick's (1972) effective angle of shore response. Profile migration in 

 response to rising water levels under these conditions would ideally leave a 

 slope below its trailing edge which could serve as a clue to past recession. 

 If the shore formerly supplied a greater volume of littoral material per unit 

 of recession (because dunes or bluffs were higher or contained a larger per- 

 centage of suitable littoral material), then the slope beyond the trailing 

 edge would tend to be convex. An increasing supply of sand would tend to 

 produce a concave slope as erosion provides more and more sediment for each 

 unit of recession. 



42 



