section is horizontal with a height determined by the method of con- 

 struction. Otherwise^ the height of this part of the groin also de- 

 pends upon the extent to which it is desirable to withhold sand from the 

 littoral streamo With this as a controlling factor^ the outer section 

 of the groin may be either level or parallel to the bottom^ with a top 

 elevation as low as h feet above the bottomo The length of the outer 

 section will depend upon the slope of the beach and the extent to which 

 it is desired to internipt the littoral drift. 



161. Spacing of Groins . - As a long section of shore line must be 

 protected by more than one groin^ a gx^oup^ or field.c of groins must be 

 used. The spacing of groins is a function of the length of the groin 

 and the wave direction which would cause the net rate and direction of 

 littoral drift. The length and spacing of these groins must be so 

 correlated that when the groin is filled to capacity the fillet of 

 material on the updrift side of each groin will reach to the base of the 

 adjacent updrift groin with sufficient margin of safety to maintain 

 the minimum width beach desired or prevent flanking of the updrift groin. 

 Figure 62 shows the desirable resulting shore line if groins are properly 

 spaced. The solid line shows the shore line as it will become shortly 

 after construction of the groin field, when erosion is a maximum at the 

 toe of the updrift groins. This assumes both groins were constructed 

 simultaneously. The erosion shown occiirs before the updrift groin is 

 full aid material begins to pass around and over it into the area between 

 groins. At the time of maximum recession, the solid line is nearly 

 normal to the direction of net wave approach and the triangle of recession 

 "a" is approximately equal to the triangle of accretion' "b"„ In other 

 words there has been a shifting of material between the groins. The 

 dotted line m-n shows the stabilized shore line which will obtain after 

 material passes the updrift groin to fill the area between groins and in 

 turn commences to pass the downdrift groin. It will be noted that the 

 fillet of sand between groins tends to become and remain peipendicular 

 to the predominent direction of wave attack. This alinement may be quite 

 stable after equilibrium is reached. Howeverj if there is a marked 

 seasonal variation in the direction and intensity of wave attack there 

 will be a corresponding seasonal variation in the alinement and slope of 

 the fillets between groins. In areas where there is a periodic reversal 

 in the direction of littoral drift,, an area of accretion may form on 

 both sides of a groin as shown in Figure 63. Between groins the fillet 

 may actually oscillate from one groin to the other as shown by the 

 dotted lines or may form a U-shaped beach somexfhere in between depending 

 on the rate of supply of littoral material. With regular reversals in 

 littoral drift, the maximum line of recession would probably be somewhat 

 as shown by the solid line, with the triangular area (a) + triangular 

 area (c) approximately equal to the circular segment (b)o 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 minlmiim width of beach that may be built by the groin field. 



83 



