hindcasted, deepwater storm wave data of Resio and Vincent (1976) and actual 

 profile changes measured in Lake Michigan (Hands, 1980). If the engineer 

 feels that the published values seem too deep for a particular site (or per- 

 haps too shallow) but lacks sufficient evidence to justify making a different 

 choice, it would be easy and worthwhile to determine what effect the suspected 

 error would have on the predicted response and on the conclusions that the 

 prediction would support. 



An error in estimating closure will cause errors in both the numerator and 

 denominator of the equation. In some cases, these two errors may reinforce 

 each other; in other cases, they may compensate for one another, or they may 

 even cancel and have no effect on the results or conclusions. Which situation 

 exists can be determined readily if, as previously recommended, a number of 

 shore-normal profiles have been obtained, and the following steps are taken. 



First identify the point where the typical profile reaches the published 

 closure depth, then the point where it reaches the alternately contemplated 

 closure depth. Draw a line through these two points. If the line extends 

 below the average height of the erodible backshore deposits, then overesti- 

 mating closure depth will cause the equation to overpredict the response; 

 underestimating closure will cause the equation to underpredict the response. 

 If the line extends above the backshore deposits, overestimating closure will 

 underpredict response and underestimating will overpredict response. These 

 relationships will apply regardless of whether the response is a retreat or an 

 advance of the shore. The possibilities are indicated in Figure 2. Inter- 

 section of the line with the crest of the backshore deposits means that the 

 choice between the two closure depths will have no effect on the calculated 

 outcome. 



EXTENSION OF 

 THE LINE 

 TKBOUGK 

 d, and d. 



UNDERCUTS DUNES 



OVERSHOOTS DUNES 



WATER RISINr, 



WATER FALLING 



WATER RISING 



I 



{ 



i]\ \f the estlMted 



Predicted losses win be too Urge if the estliuted 

 closure depth Is too large. 



Predicted gains will be too small If the estimated 

 closure depth is too sr,all. 



Predicted gains will be too large if the e$tiwte<l 

 closure depth Is too large. 



\ 



s win be too small If the «t1m 

 depth Is too large. 



^ 



Predicted gains will be too large if the estiwted 



closure depth is too small. 



Predicted gains will be too small if the estiuted 



closure depth is too large. 



Figure 2. Diagram for determining if a suspected error would weaken or 



strengthen arguments based on the sediment balance prediction. 



13 



