groin to retain all sand reaching the area (a high groin) is the height 

 o£ maximum high water and maximum wave uprush during all but the most 

 severe storms. This section is horizontal or sloped slightly seaward, 

 paralleling the existing beach profile or the desired slope if a wider 

 beach is desired or a new beach is to be built. 



5.662 Intermediate Sloped Section . The intermediate section extends be- 

 tween the shore section and the level outer section. This part should 

 approximately parallel the slope of the foreshore the groin is expected 

 to maintain. The elevation at the lower end of the slope will usually be 

 determined by the construction methods used, the degree to which it is 

 desirable to obstruct the movement of the material, or the requirements 

 of swimmers or boaters. 



5o663 Outer Section . The outer section includes all of the groin extend- 

 ing seaward of the intermediate sloped section. With most types of groins, 

 this section is horizontal at as low an elevation as is consistent with 

 economy of construction and safety, since it will be higher than the de- 

 sign updrift bottom slope in any case. The length of the outer section 

 will depend on the design slope of the updrift beach. 



5.664 Spacing of Groins . The spacing of groins in a continuous system 

 is a function of the length of the groin and the expected alignment of 

 the accretion fillet. The length and spacing must be so correlated that 

 when the groin is filled to capacity, the fillet of material on the up- 

 drift side of each groin will reach to the base of the adjacent updrift 

 groin with a sufficient margin of safety to maintain the minimum beach 

 width desired or to prevent flanking of the updrift groin. Figure 5-9 

 shows the desirable resultant shoreline if groins are properly spaced. 

 The solid line shows the shoreline as it may develop when erosion is at 

 a maximum at the updrift groin. The erosion shown occurs while the up- 

 drift groin is filling. At the time of maximum recession, the solid 

 line is nearly normal to the direction of the resultant of wave approach 

 and the triangle of recession, a^ is approximately equal to the triangle 

 of accretion, b. The dashed line m n shows the stabilized shoreline 

 that will obtain after material passes the updrift groin to fill the 

 area between groins and, in turn, commences to pass the downdrift groin. 

 The fillet of sand between groins tends to become and remain perpendicu- 

 lar to the predominant direction of wave attack. This alignment may be 

 quite stable after equilibrium is reached. However, if there is a marked 

 variation in the direction and intensity of wave attack, either season- 

 ally or as a result of prolonged storms, there will be a corresponding 

 variation in the alignment and slope of the shore between groins. Where 

 there is a periodic reversal in the direction of longshore transport, an 

 area of accretion may form on both sides of a groin as shown in Figure 

 5-10. Between groins, the fillet may actually oscillate from one groin 

 to the other as shown by the dashed lines, or may form a U-shaped beach 

 somewhere in between, depending on the rate of supply of littoral mate- 

 rialo With regular reversals in the direction of longshore transport, 

 the maximum line of recession would probably be somewhat as shown by the 

 solid line, with the triangular area a plus triangular area a about 



5-36 



