l6i|.. The ground line on the downdrift side of a groin will be 

 different for an intermediate groin in a field than it will for a single 

 groin or for the farthest downdrift groin in a fields. If the field is 

 properly planned and constructed, the giound lines would be about the 

 same for the latuer twoo ' ' 



163, Considering first an intermediate groin in a groin field, the 

 maximuin shore recession on the downdrift side of the g;roin would occur 

 before the updrif t groins were full, permitting the entire quantity of 

 littoral material to move past the groins. During this time the maximum 

 recession would occur when the shore line between the intermediate groin 

 and the next downdrift groin has reoriented to a position normal to the 

 net wave orthogonals such that area a e area b in Figure 65» 



l66„ To determine the maximum recession of the downdrift ground line 

 show the proposed groin on the original beach profile as in Figure 66, From 

 the crest of berm at station 0, lay off distance cd t»ken from Figure 65. 

 Draw the foreshore from crest of berm to datum plane parallel to the 

 original beach line and connect that point of intersection with original 

 profile at the seaward end of the groin. 



167. After the positiai of maximum recession has been reached, as shown 

 by c-g on FigTire 65, the shore line will begin to advance seaward main--, 

 taining its alinement normal to the net wave orthogonals until sufficient 

 material flows around or over the downdrift groin to produce a stabilized 

 shore line as shown by the line m-n in Figure 65 . 



168. To determine the stabilized downdrift line, pee Figure 660 P^om 

 the crest of berm at station lay off the distance ^ taken from Figure 

 65. Draw the foreshore from the crest of berm to datu^ plane parallel 



to the original beach line and connect that point of intarsection with 

 the original profile at the seaward end of the groin, 



169. Considering a single groin or the last groin of a field, the 

 maximum recession that could occur would be to assume that the downdrift 

 area loses 'an amount equal to the full annual rate of littoral drift 

 for the period required for the groin to fill to capacityc It is known 

 that some unknown percentage of the total littoral drift moves seaward 



of the seaward ends of the groins. Also, it is knoT«i that some additional 

 percentage of the material moving shoreward of the seaward ends of the 

 groins, will bypass the groin before it is completely filledc According- 

 ly, for approximating the position of the downdrift ground line, it is 

 believed safe to reduce the net littoral drift by some amount depending 

 on the type of groin constructed. Percentage of net littoral drift con- 

 sidered safe for computing downdrift losses due to certain groin types are 

 given as follows: 



a. For high groins extending to a depth of water 10 feet or 

 more, use luO per cent of the total littoral drift; 



87 



