coarser than the original beach material. The length of the outer section 

 p r depends on the amount of littoral drift it is desired to intercept. 

 It should extend deep enough for the new profile p s to intercept the 

 old profile ads within the toe of the groin. 



(h) The final beach profile on the updrift side of the typical 

 groin shown in Figure 5-11 is indicated by the line a n p s. 



Until the groins are filled, the shoreline on the downdrift side of 

 a groin will be different for an intermediate groin in a system than it 

 will for a single groin or for the farthest downdrift groin in a system. 

 If the system is properly planned and constructed, the shorelines would 

 be about the same for the single and downdrift groins. 



Considering first an intermediate groin in a groin system, the maxi- 

 mum shore recession on the downdrift side of the groin would occur before 

 the updrift groins fillo During this time the maximum recession would 

 occur when the shoreline between the intermediate groin and the next down- 

 drift groin has reoriented to a position normal to the predominant wave 

 orthogonal s such that area a = area b in Figure 5-12. 



To determine the profile of maximum recession of the downdrift side 

 of the groin, draw the proposed groin on the original beach profile as 

 in Figure 5-12. From the crest of berm at d, lay off distance / d 

 taken from Figure 5-12. Draw the foreshore from crest of berm / to 

 datum plane (MLW) parallel to the original beach slope, and connect 

 that point of intersection with the original profile at the seaward end 

 of the groin. 



After the line of maximum recession has been reached, as shown by 

 f g on Figure 5-12, the shoreline will begin to advance seaward, main- 

 taining its alignment perpendicular to the net wave orthogonals until 

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

 stabilized shoreline as shown by the line m n in Figure 5-12. 



To determine the stabilized downdrift line, see Figure 5-13. From 

 the crest of berm at d, lay off the distance d m taken from Figure 

 5-12. Draw the foreshore slope from the crest of berm / to datum plane 

 (MLW) parallel to the original beach line, then connect that point of 

 intersection with the original profile at the seaward end of the groin. 



Considering a single groin or the downdrift groin of a system, the 

 maximum recession that could occur may be determined by assuming that the 

 downdrift area loses an amount equal to the full rate of longshore trans- 

 port for the period required for the groin to fill to capacity. It is 

 known that a percentage of the total littoral drift moves seaward of the 

 seaward ends of the groins. It is also known that an additional percent- 

 age of the material moving shoreward of the seaward ends of the groins 

 will bypass the groin before it is completely filled. Accordingly, to 

 approximate the position of the downdrift ground line, it is believed 

 safe to reduce the net longshore transport by some amount depending on 



5-39 



