equal to the circular segment h. The extent of probable beach recession 

 must be taken into account in establishing the length of the horizontal 

 shore section of groin and in estimating the minimum width of beach that 

 may be built by the groin system. As a guide to the spacing of groins, 

 the following genevdl TuZe is suggested: The spacing between groins 

 should equal two to three times the groin length from the beim orest to 

 the seaward end, 



5„665 Length of Groin . To determine the horizontal shore and inter- 

 mediate sloped section shoreline position adjacent to a groin, it is 

 necessary to predict the ultimate stabilized beach profile on each side 

 of the groin. Total length, including the horizontal outer (seaward) 

 section, is based on projected position of the breaking zone for normal 

 waves. The steps involved for a typical groin are: 



(a) Determine the original beach profile in the vicinity of the 

 groin, 



(b) Determine the direction of longshore transport. (See Section 

 4.5, Littoral Transport.) 



(c) Determine the shape of the accretion fillet by the shape of 

 the average impounded fillet over a sufficient period of time at an exist- 

 ing structure where the shore has similar orientation and exposure. If 

 no such structure is available, an estimate may be made by plotting a re- 

 fraction diagram for the mean wave condition, i.e., the wave condition 

 which would produce the greatest rate of longshore transport, and drawing 

 the shoreline or berm crest normal to the orthogonals. 



(d) Determine the minimum beach width desired updrift of the groin. 

 This may be a width desired to provide adequate recreational area; adequate 

 protection of the backshore area; or with a groin system, adequate width 



of beach at the next groin updrift to prevent flanking of this groin by 

 wave action. The last condition is shown at point m on Figure 5-9, if 

 line m n represents the berm crest of the beach. 



(e) The position and alignment of the desired beach relative to 

 the groin under study is indicated by the line m n, Figure 5-9, the line 

 being constructed approximately to the orthogonals based on mean wave con- 

 ditions from m to n. 



(f) Apply the distance a n from Figure 5-9 to Figure 5-11; this 

 distance plus enough length landward of c to prevent flanking, will 

 represent the length of the horizontal shore section. 



(g) The slope of the ground line from the crest of the berm sea- 

 ward to about the mean low water line will depend on the gradation of the 

 beach material and the character of the wave action. This section of groin, 

 the intermediate sloped section. Figure 5-11, is usually designed parallel 

 to the original beach profile. The ground line will assume the slope of 

 the groin section n p or a steeper slope if the material trapped is 



5-38 



