183. Offshore Breakwaters in Series . - It is not necessary to build 

 offshore breakwaters ss a single unit, A series of relatively short 

 structures will have the same t^eneral effect as a single one, but the 

 efficiency of the series as a sand trap will be decreased] a condition 

 which is sometimes desirable. The tendency for a tom^olo to form will be 

 decreased. Fi^,are 70 is an aerial photograph taken in 19U9 oi the, 

 brealcwater set off Winthrop Beach, Massachusetts construe ted in 1931-1S''33. 

 The characteristic convex accretion in the breakvjater lee is evident, as 

 is also the erosion zone downdrift. Note that here shoals forraed from 

 the breakfjaters extending landward, indicating that these breal<:i;aters are 

 in the littoral movement band. 



IGI4.. Height of Offshore Breakwater . - For the purpose of height 

 determination, offshore breal^'aters can be considered in two categories; 

 (1) submerged breakwaters, and (2) exposed breakviaters, 



185. Submerged Breakwaters , - Submerged breakwaters (also known as 

 submerged sills or submerged barriers) are those with thei|" tops below 

 the low water line. These may be used to retain beach fill or to reduce 

 the wave forces by causing the waves to breako VJhere usfd only to retain 

 beach material, the top of the structure should be in line^ with the design 

 slope of the beach fill. In theory (Tratman 19U0), where 'a structure is 

 designed to reduce the wave force, the sweep of the waves is broken by 

 the obstruction, so that their destructive force is dissipated or expend- 

 ed in the semi-enclosed stretch of water betx^'een the barrier and the shore. 

 In addition, the return currents flowing back aJ.ong the bottom strike the 

 inner side of the submerged barrier and are compelled to ri,se vertically. 

 This upvrard flow assists in breaking up the flow of water shoreward over 

 the barrier, 



186. Model studies of the effects of submerged structures on wave 

 heights and wave energy were made by the. Beach Erosion Board and published 

 in 19U0, Other model studies were made by the University of California 



at Berkeley. In general, both studies indicated thst an underwater 

 structure parallel to a shore line would decrease wave height and wave 

 energy on the shore. A vertical wall was found to be the most effective 

 structure for decreasing landward wave action, with the effectiveness 

 increasing with the width of the v.'all. However, the difference in 

 effectiveness of variously shaped structures was sufficiently slight that 

 construction costs and difficulties probably would govern the choice of 

 structure. 



167. The extent to v/hich wave action was reduced was found to be 

 a function of the distance between the top of the structure and the sur-- 

 face of the water or, to use a dimensionless parameter, of the r atio of 

 the depth of water over the structure to the depth of iirater at the site. 

 The average effects of submerged structures on wave height and w^ve 

 energy in terms of relative depth, found in these model studies, is shotjn 

 on Figures 71 and 72. Sy making use of these curves, rough approximations 

 of wave action and sand moving capabilities behind a submerged breakwater 



9U 



