qn the erosion regime before it. In other words, the beach seaward of the 

 wall would erode in the same manner as if the wall were not there. Since 

 the determination can only be very approximate, rules of thumb may be 

 adopted. 



127. Consider a beach as shown in Figure 58 where the line MCEB 

 represents an average profile. It is desired to place a seawall at 

 point A as shown. From prior records, either the loss of beach width per 

 year, or the annual loss of material over an area which includes the 

 profile is knovv'n. In the latter case, the annual quantitative loss may 

 be converted to an annual loss of beach width by the rule of thumb 

 "loss of 1 cubic yard of beach material is equivalent to loss of 1 square 

 foot of beach area at the berm," 



128. In general, beach slopes are fairly steep shoreward of a depth 

 of O.to 10 feet, and fairly flat seavrard of that depth. Analysis of 

 beach profiles on eroding beaches would indicate that it may reasonably 

 be assumed that the beach seaward of a depth of 30 feet will remain 

 essentially unchanged, that the point of slope break will remain at about 

 the same elevation, and that the profile of the beach shcsreward of the 

 point of break in slope will remain essentially unchanged, accordingly 

 the ultimate depth at the wall may be estiiiBted as follows: 



72 



