volume change over a specific time period at a known feature such as an inlet 

 or coastal structure; Method 3 depends on the longshore component of energy 

 flux in the surf zone to compute a potential longshore transport rate, also 

 known as the CERC formula; and Method 4 provides an empirical estimate of 

 Q G . These methods are discussed in detail in the Shore Protection Manual 

 (1984), EM 1110-2-1617, and EM 1110-2-1502. Another method commonly 

 used along the Great Lakes and Pacific coast develops a sediment budget 

 based on estimates of inputs including bluff recession and stream sediment 

 contributions. The Shoreline Modeling System (Gravens 1992) includes 

 programs to calculate Q L , Q R , and Q G for a given time series. 



It is important to examine longshore transport variability as part of 

 functional design, since transport rates can fluctuate significantly on a 

 monthly, seasonal, or yearly basis. Beach planform can vary and shift 

 substantially in response to prevailing transport conditions. 



Offshore bathymetry 



Offshore bathymetry is required to aid in determining the distance offshore 

 at which the breakwater will be constructed, wave and current forces which 

 the structure will be subjected to, and quantities of construction materials. 

 Knowledge of offshore bathymetry is also needed to examine wave 

 transformations that may affect the local wave environment at the site. 

 Additionally, bathymetry and beach profiles can provide data to determine the 

 closure depth (the depth beyond which there is no significant sediment 

 transport), if they extend to a sufficient depth and have sufficient vertical and 

 horizontal control to allow comparison of profiles. Depth of closure can also 

 be estimated by reference to a maximum seasonal or annual wave height 

 (Hanson and Kraus 1989b, Hallermeier 1983). 



Bathymetric surveys of the project vicinity during the planning and design 

 stages should be conducted for detailed site data. Less accurate bathymetry 

 information can be acquired from U.S. Geological Survey quadrangle 

 topographic maps and or Naval Hydrographic Office charts; however, 

 bathymetry is continually changing and these sources generally do not 

 maintain the most up-tonlate information. 



Shoreline change 



Shoreline change data are required primarily to determine short- and long- 

 term erosion and accretion rates at a project site, prior to design of a 

 breakwater system. This information is necessary to determine the 

 breakwater's location relative to the post-project shoreline and to estimate the 

 volume of sand that will accumulate behind the breakwater. 



Historical and recent shoreline change data include beach profile surveys, 

 aerial photography, and other records documenting changes in the shoreline 



Chapter 2 Functional Design Guidance 



35 



