changes may reduce the value of P^g when deepwater wave height statis- 

 tics are used as a starting point for computing P^g. (Walton, 1972; 

 Bretschneider and Reid, 1954; and Bretschneider, 1954.) 



The Equations 4-35 through 4-38 in Table 4-8 are related to the Equa- 

 tion for Eq previously recommended for use with this method (Caldwell, 

 1956, Equations 5 and 6; or the equations in Figure 2-22, page 175, CERC 

 Technical Report No. 4, 1966 edition) by a constant 



E^ = (8.64 X 10'*)P£^ (4-39) 



where E^ is in units of foot-pounds per foot per day and P^g is in 

 units of foot-pounds per foot per second. 



The term in parenthesis for Equation 4-32 in Table 4-7 is identical 

 to the longshore force of Longuet-Higgins (1970a). This longshore force 

 also correlates well with the longshore transport rate. 



The relation between Q and P^g in Figures 4-36 and 4-37 can be 

 approximated by 



Q = (7.5X103)Pj^ (4-40) 



Equation 4-40 tends to overestimate Q at the higher values of P^g 

 for the plotted field data, but it falls below the estimated rates com- 

 puted from the data of Johnson (1952). (See Das, 1972, Figure 6.) The 

 value of 7.5 x 10^ in Equation 4-40 is approximately twice the equiva- 

 lent value from the design curve of CERC Technical Report No. 4, 1966 

 edition, and is about 5 percent greater than the value estimated by Komar 

 and Inman (1970). 



Judgement is required in applying Equation 4-40. Although the data 

 in Figures 4-36 and 4-37 appear to follow a smooth trend, the log- log 

 scale compresses the data scatter. For example, the average difference 

 between the plotted points from field data and the prediction given by 

 Equation 4-40 is at least 28 percent of the value of prediction (average 

 difference derived by Das (1972) is 42 percent). In addition, some in- 

 complete measurements suggest transport rates ranging from tv;o orders of 

 magnitude below the line (Thornton, 1969) to one order of magnitude above 

 the line, (Johnson, 1952.) These additional data are plotted by Das 

 (1972). 



As an aid to computation. Figures 4-38 and 4-39 give lines of con- 

 stant Q based on Equation 4-40 and Equations 4-35 and 4-36 for Pis 

 given in Table 4-8. To use Figures 4-38 and 4-39 to obtain the longshore 



4-101 



