Galvin (1972) showed that when field values of longshore transport 

 rate are plotted against mean annual breaker height from the same locality, 

 a curve 



Q = 2 X 10= H^ , (4-44) 



forms an envelope above almost all known pairs of (Q, Hj,) , as shown in 

 Figure 4-40. Here, as before, Q is given in units of cubic yards per 

 year; Hjy in feet. 



Figure 4-40 includes all known (Q, Hj,) pairs for which both Q and 

 H2j are based on at least 1 year of data, and for which Q is considered 

 to be the gross longshore transport rate, Q^, defined by Equation 4-21. 

 Since all other known (Q, H^,) pairs plot below the line given by Equation 

 4-41, the line provides an upper limit on the estimate of longshore trans- 

 port rate. From the defining equations for Q^ and Q^, any line that 

 forms an upper limit to longshore transport rate must be the gross trans- 

 port rate, since the quantities Qrt> Q£t> ^^^ Qn» ^^ defined in Section 

 4.531, are always less than or equal to Q^^. 



In Equation 4-44, wave height is the only independent variable, and 

 the physical explanation assumes that waves are the predominant cause of 

 transport. (Galvin, 1972.) Therefore, where tide-induced currents or 

 other processes contribute significantly to longshore transport. Equation 

 4-44 would not be the appropriate approximation. The corrections due to 

 currents may either add or subtract from the estimate of Equation 4-44, 

 depending on whether currents act with or against prevailing wave-induced 

 transport. 



4,535 Method 4 Example (Empirical Prediction of Gross Longshore Transport 

 Rate. Near the site of the problem outlined in Section 4.533, it is de- 

 sired 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 12 feet of water at the end 

 of a pier. (See Columns (1) and (2) of Table 4-12.) Heights are avail- 

 able 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 4-10 for the problem of 

 Section 4.533. That is, the frequency associated with each H„ in Table 

 4-12 is the sum of the frequencies of the shoreward Hq on the correspond- 

 ing line of Table 4-10.) 



The breaker height, H^j, in the empirical Equation 4-44 is related 

 to the gage height, Hg, by a shoaling-coefficient ratio, (Ks)^/(Kg)^, 

 where (Kg)^, is the shoaling coefficient (Equation 2-44), (H/H^ in 



4-108 



