material normally increases to establish a steeper than normal slope, 

 the residual accreted material is probably, by selective processes, the 

 coarser fraction of the material that was in transport. 



Wlien the accreted slope reaches ultimate steepness for the coarser 

 fraction of available material, impoundment stops, and all littoral drift 

 passes the groin. If the groin is so high that no material passes over 

 it, all transport must be in depths beyond the end of the groin. Because 

 of the nature of transporting currents, the material in transit does not 

 move directly shoreward after passing the groin, and transport character- 

 istics do not become normal for some distance on the downdrift side of the 

 groin„ Thus, a system of groins too closely spaced would divert sediment 

 offshore rather than create a widened beach. 



The accretion fillet on the updrift side of the groin creates a de- 

 parture from normal shore alignment, tending toward a stable alignment 

 perpendicular to the resultant of wave attack. The impounding capacity 

 of the groin thus depends on the stability slope and stability alignment 

 of the accretion fillet. These in turn depend upon characteristics of 

 the littoral material and the direction of wave attack. 



Figure 5-8 shows the general configuration of the shoreline expected 

 for a system of two or more groins. It assumes a well-established net 

 longshore transport in one direction. 



5.66 DIMENSIONS OF GROINS 



Groin dimensions depend on wave forces to be withstood, the type 

 of groin, and the construction materials used. The length, profile, 

 spacing of groins in a system, direction of wave approach, and rate of 

 longshore transport are important functional considerations. 



The length of a groin is determined by the distance to depths off- 

 shore where normal storm waves break, and by how much sand is to be 

 trapped. The groin should be long enough to interrupt enough material 

 to create the desired stabilization of the shoreline or accretion of 

 new beach areas. Damage to downdrift shores must be considered in 

 determining the groin length. For functional design purposes, a groin 

 may be considered in three sections: (a) horizontal shore section, 

 (b) intermediate sloped section, and (c) outer section. 



5,661 Horizontal Shore Section . This section extends far enough land- 

 ward from the desired location of the crest of berm to anchor the groin 

 and prevent flanking. The height of the shore section depends on the 

 degree to which it is desirable for sand to overtop the groin and replen- 

 ish the downdrift beach. The minimum height for a groin is the height of 

 the desired berm, which is usually the height of maximum high water, plus 

 the height of normal wave uprush. Economic justification for building a 

 groin higher than this is doubtful except for terminal groins. With stone 

 groins, a height about 1 foot above the minimum is sometimes used to re- 

 duce passage of sand between large cap stones. The maximum height of a 



5-34 



