Table 4-13 indicates the importance of rare high waves in determining the 

 longshore transport rate. In the example, shoreward-moving 4.0-meter waves 

 occur only 0.5 percent of the time, but they account for 12 percent of the 

 gross longshore transport rate (see Table 4-13). 



Any calculation of longshore transport rate is an estimate of potential 

 longshore transport rate. If sand on the beach is limited in quantity, then 

 calculated rates may indicate more sand transport than there is sand avail- 

 able. Similarly, if sand is abundant but the shore is covered with ice for 2 

 months of the year, then calculated transport rates must be adjusted accord- 

 ingly. 



The procedure used in this example problem is approximate and limited by the 

 data available. Equation (4-54), and the other approximations listed in Table 

 4-13, can be refined if better data are available. An extensive discussion of 

 this type of calculations is given by Walton (1972). 



Although this example is based on shipboard visual observations of the 



SSMO type, the same approach can be followed with deepwater data from other 



sources, if the joint distribution of height and direction is known. At this 



level of approximation, the wave period has little effect on the calculation, 



and the need for it is bypassed as long as the shoaling coefficient (or 



1/2 

 breaker height index) reasonably satisfies the relation (K ) =1.14 (see 



assumption 2d, Table 4-11). For waves on sandy coasts, ^this relation is 



reasonably satisfied (e.g., Bigelow and Edmondson, 1947, Table 33; Goda, 1970, 



Fig. 7). 



e. Empirical Prediction of Gross Longshore Transport Rate (Method 4) . 

 Longshore transport rate depends partly on breaker height, since as breaker 

 height increases, more energy is delivered to the surf zone. At the same 

 time, as breaker height increases, breaker position moves offshore widening 

 the surf zone and increasing the cross-section area through which sediment 

 moves. 



Calvin (1972b) showed that when field values of longshore transport rate 

 are plotted agains mean annual breaker height from the same locality, a curve 



Q = 1.646 X 10^ H? (4-56a) 



h 



Q = 2 X 10^ H? (4-56b) 



h 



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

 4-40. In equation (4-56a), Q is given in cubic meters per year and H, is 

 in meters; in equation (4-56b) Q is given in units of cubic yards per year; 



and H, in feet. 



b 



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

 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 (4-31). Since all 

 other known (Q, H,) pairs plot below the line given by equation (4-56), the 

 line provides an upper limit on the estimate of longshore transport rate. 

 From the defining equations for Q^ and Q , any line that forms an upper 

 limit to longshore transport rate must be the gross transport rate, since the 



4-104 



