162. Length of Groin, Before the total length of a groin and the 

 position of the vshore line adjacent to a groin, can be determined, it 



is necessary to determine the location of the ground line, or stabilized 

 beach profile, on each side of the groin. The two ground lines are 

 determined separately and then combined to determine the relative positions 

 of the shore ].ine. 



163. Determination of the ground line on the updrift side of a groin 

 is the saiiie v^hether there is one .Toin or several in a groin field. The 

 steps involviiid for a typical gicin are« 



groin; 



u Determine the original beach profile in the vicinity of the 



b. Determine the conditions of littoral drift; 



c. Plot a refraction diagram for the mean wave condition: die. ' 

 the wave condition which would produce the predominant direction and net 

 rate of littoral drift; 



d. Determine the minimum width beach desired updrift of the 

 groin. This may be a width desired to provide adequate recreational area, 

 adequate protection to the upland area, or in the case of a groin field, 

 adequate depth of beach at the next eroin updrift to prevent flanking 



of this groin bv wave action. The latter criterion ia depicted at point 

 m on Figure 62, if line m-n represents the barm crest of the beach. 



e. The position and alinement of the beach to the groin under 

 study is indicated by the line m-n. Figure o2 drawn approximately normal 

 to the orthogonals based on mean wave conditions from m_ to nj 



f. Apply the distance c-n from Figure 62 to Figure 6ii and 

 this distance, plus sufficient groin landward of c to prevent flanking, 

 will represent the length of the horizontal shore r.ection;, 



g. The slope of the ground liiie from the crest of the berm 

 seaward to about the mean low water lint will depend upon the gradation 

 of the beach material and the character of the wave action. This section 

 of groin , the intermediate sloped section. Figure 6k, is usually de- 

 signed parallel to the original beach profile. The ground line will 

 assurae the slope of the groin section n-p or, if the material trapped is 

 coarser than the original beach material, v.'ill assume a steeper slope. 

 The length of the outer section p-r depends upon the amount of littoral 

 drift it is ( esired to intercept. It should extai d to sufficient depth 

 so that the new profile p-s^ will intercept the old profile c-d-s within 

 the toe of the groin; 



h. The final gcounri^ine on the updrift side of the typical 

 groin shovm in Figxire 61+ is indicated by the line c-n-p-s. 



85 



