To calculate an average P 



Is- 



a value of P 



Zs 



should be calculated for 



each combination of wave height and angle available. These values can then 



be used to find the average P 



Us- 



It is incorrect to compute an average wave 



Is' 



height and an average wave angle and then use them to find an average P 



Computing P„ using LEO current data is described in Walton (1980). 



4. Method 4 . 



The SPM empirically relates the gross longshore_transport rate in cubic 

 yards per year to the mean annual breaker height, Hi in feet, by the equation 



9 



yd'^ 



yr 



= 2 X 10- 



^d^ 



ff=^/yr 



H 



ft^ 



C9) 



The gross rate, as defined earlier in equation (1), is the total amount of 

 sand moving in both directions. If the net transport, , is needed, this 

 method is unsuitable. 



IV. SU]\f4ARY OF GUIDANCE 



An engineer confronted with the problem of estimating the longshore trans- 

 port rate at a project site must first survey all available data. Following 

 the selection guide in Figure 1, the appropriate SPM method should be used if 

 the engineer has (a) a rate from a nearby site (method 1); (b) historical 

 changes in topography (method 2); (c) wave height and direction (method 3); 

 and (d) wave height data (method 4). 



If enough data, time, and money are available to use more than one method, 

 the engineer can, and is encouraged to, estimate the transport rate using all 

 methods possible. The most preferred method (see Section III) should be used 

 for the design; the other methods should be used as checks. For example, if 

 method 3 is used to find the net transport rate, Q , then method 4 can also 

 be used to find the gross transport rate, Q^. This will allow the engineer 

 a check to make sure Q,^ does not exceed Q^. 



A combination of methods can also be used if not enough data are available 

 for any one method to be used completely. Jarrett (1977) presents an example 

 of using methods 2 dJid 3 to estimate Q^ and then checking the results with 

 Q^ found using method 4. 



15 



