quantities Q , Q , and Q , as defined in Section V,3,a are always less 

 than or equal to Q . 



In equation (4-56) vave height is the only independent variable, and the 

 physical explanation assumes that waves are the predominant cause of transport 

 (Galvin, 1972b). Therefore, where tide-induced currents or other processes 

 contribute significantly to longshore transport, equation (4-56) would not be 

 the appropriate approximation. The corrections due to currents may either add 

 or subtract from the estimate of equation (4-56), depending on whether 

 currents act with or against prevailing wind-induced transport. 



f . Method 4 Example (Empirical Prediction of Gross Longshore Transport 

 Rate . Near the site of the problem outlined in Section V,3,d, it is desired 

 to build a small craft harbor. The plans call for an unprotected harbor 

 entrance, and it is required to estimate costs of maintenance dredging in the 

 harbor entrance. The gross transport rate is a first estimate of the 

 maintenance dredging required, since transport from either direction could be 

 trapped in the dredged channel. Wave height statistics were obtained from a 

 wave gage in 3.66 meters (12 feet) of vvater at the end of a pier (see columns 

 (1) and (2) of Table 4-14). Heights are available as empirically determined 

 significant heights (Thompson and Harris, 1972). (To facilitate comparison, 

 the frequencies are identical to the deepwater frequencies of onshore waves in 

 Table 4-12 for the problem of Section V,3,d. That is, the frequency 

 associated with each H^ in Table 4-14 is the sum of the frequencies of the 

 shoreward H on the corresponding line of Table 4-12.) 



The breaker height Hj, in the empirical equation (4-56) is related to the 



gage height H by a shoaling coefficient ratio (K )i^/(K. ) , where (K ), 



is the shoaling coefficient (eq. 2-44), evaluated at the breaker position and 



(K ) is the shoaling coefficient evaluated at the wave gage: 

 s g 



Kg can be evaluated from small-amplitude theory if wave-period information is 

 available from the wave gage statistics. For simplicity, assume shoaling 

 coefficient ratios as listed in column 4 of Table 4-14. Such shoaling 

 coefficient ratios are consistent with the shoaling coefficient of K = 1.3 

 (between deepwater and breaker conditions) assumed in deriving P (Table 

 4-10), and with the fact that waves on the inland sea are usually steep, 

 locally generated waves. 



Column 5 of the table is the product fH (Kg)^/(Kg) . The sum (0.531 

 meter ) of entries in this column is assumecr equivalent to the average of 

 visually observed breaker heights. Substituting this value in equation 

 (4-54), the estimated gross longshore transport rate is 464,000 cubic meters 

 per year. It is instructive to compare this value with the value of 683,000 

 cubic meters per year obtained from the deepwater example (see Table 4-13). 

 The two estimates are not expected to be the same, since the same wave statis- 

 tics have been used for deep water in the first problem and for a 3.66-meter 

 depth in the second problem. However, the numerical values do not differ 

 greatly. It should be noted that the empirical estimate just obtained is 



4- 105 



