34 



over some time (for instance seasonally), the shoreline will begin to shift in 

 response to the new approach direction. 



The two primary forms of wave data for breakwater design are hindcast 

 data and wave gauge data. Wave height statistics to determine design 

 conditions will generally be based on hindcast data since a relatively long 

 record is needed for data extrapolation. The Wave Information Study (WIS) 

 conducted by the USACE has developed hindcast data for all three ocean 

 coasts and the Great Lakes (Jensen 1983; Hubertz et al. 1993; Jensen et al. 

 1992). EM 1 1 10-2-1414 and EM 1 1 10-2-1502 provide extensive lists and 

 contacts on ways to obtain meteorological and oceanographic data, as well as 

 sources of WIS data and information. 



Longshore sand transport rates 



Longshore transport of littoral material is the most significant process 

 determining beach planform response to breakwaters. Transport rates are 

 needed to determine what type of planform will develop, sediment budget 

 calculations, beach fill requirements, and potential effects of a project on 

 downdrift beaches. 



Longshore transport is typically described in terms of annual net and gross 

 transport rates (Shore Protection Manual 1984, EM 1110-2-1617, EM 1110-2- 

 1502). To an observer looking seaward, transport can be to the right Q R or to 

 the left Q L , with Q R being a positive quantity and Q L assigned a negative 

 value. The annual net transport rate is the net amount of sediment moving 

 past a point on the beach in a year with direction considered and can be 

 computed as: 



Q N - Q R + Q L C 1 ) 



The annual gross transport rate is the total amount of sediment moving past a 

 point, regardless of direction, defined as: 



Qg=\Qr\ + \Ql\ ® 



It is possible that Q N and Q G could have substantially different magnitudes, 

 i.e., a large gross transport may exist for a project area, but net transport 

 could be close to zero. The net transport rate is often used to examine 

 erosion rates on adjacent beaches at breakwater or other coastal structures. 



Estimates of left, right, net, and gross transport rates can be calculated 

 from wave data that include wave heights, periods, and directions. Usually, 

 determination of the net and gross transport rates will be adequate; however, a 

 time series of transport rates can be calculated if a wave time series is 

 available. The Shore Protection Manual (1984) suggests four ways of 

 computing longshore transport rates at a project site. Method 1 involves 

 adopting a transport rate from a nearby site; Method 2 entails calculation of 



Chapter 2 Functional Design Guidance 



