6. Inferences from Profile Shape Alone . 



a. Preliminary Check . Prior to any detailed evaluation of the variables 

 required as input to the model, a preliminary examination of the profile shape 

 near the presumed closure depth, d, will indicate how reliable these evalua- 

 tions need to be. If the profile shape changes abruptly near this depth, then 

 the choice of d may strongly affect the resulting prediction, depending on 

 whether the exact value chosen is above or below the break in slope (see Fig. 

 21, a). If, however, the bottom is planar and sloping at the right angle, the 

 ratio of Z/X and therefore the predicted retreat will be unaffected by vari- 

 ations in d over a wide range. In such cases the exact value of d used to 

 evaluate equation (2) will be unimportant (Fig. 21, b). 



0. UNMATCHED SLOPES 



a, » a, 



Z, 2^ 



and fhe choice tietwecn d, and d2 is criticol 



So 



Contemploted 

 pinch-out depths 



b. UNIFORM SLOPE 



So -r- = 



zZ, zZa 



and exoct choice of d is of no consequence 

 if d, < d < d. 



Contempluled 

 pinch -out depths 



Figure 21. Importance of offshore slope. In case a the prediction 

 of retreat will be much more sensitive to the correct 

 selection of a pinch-out depth than in case b. 



b. Is the Prediction Conservative? The evaluation of d on the basis of 

 wave data may result in an estimate of closure which the engineer feels is 

 either too low or too high. Yet he may have no specific evidence on which to 

 base another choice. The engineer should determine if the suspected error 

 strengthens or weakens arguments based on the sediment balance model. Exam- 

 ination of profile shape resolves the possibilities as shown in Figure 22. 



41 



