R = Ah 



1 



(5-14) 



The volume of the pyramid Y is one-third of the product of the area B 

 and the vertical distance between B and point c, represented by h2. 



Y = 



Bh-, 



(5-15) 



The sum of the volumes R and Y will then give a simplified straight-line 

 approximation of the volume of the updrift fillet. Since A = B, the total 

 fillet volume can be simplified to 



R + Y = A 



(5-16) 



Dividing this volume by the part of the longshore transport rate assumed 

 intercepted by the groin provides the time period it takes the fillet to form. 

 Areas A, B, C, and D may be determined by standard geometrical formulas 

 or by use of a planimeter. A similar procedure calculates the volume of sand 

 loss due to downdrift-side erosion, as shown in Figure 5-23. Areas A' and 

 B' are equal and represent horizontal triangles at the berm crest and the MLW 

 elevation, respectively. The erosional volumes R' and Y' are calculated 

 as before and are added to give the total volume lost due to erosion. 



R' + Y' = A' |h', + -^ 



(5-17) 



Where h' is the vertical distance from A' 

 distance from B' to the point c. 



to B' and 



^2 



the vertical 



4 . Filling Groins . 



The importance of minimizing downdrift erosion after construction of a 

 groin or groin system cannot be overemphasized. Unless the natural longshore 

 transport is of sufficient magnitude to quickly fill the updrift side of the 

 updrift groin and the groin compartments or unless erosion of the downdrift 

 area is inconsequential, artificial filling will be necessary. Paragraph 8 of 

 this section will further discuss groin filling with respect to the order of 

 groin construction. 



5 . Permeable Groins . 



Permeability allows part of the longshore drift to pass through the groin 

 and induces sand deposi-tion on both sides of the groin. This in turn reduces 

 the abrupt offset in shore alinement found at impermeable groins. Many types 

 of permeable groins have been employed. The degree of permeability above the 

 ground line affects the pattern and the amount of deposition. Insufficient 

 empirical data have been compiled to establish quantitative relationships 

 between littoral forces, permeability, and shore response. Until such data 



5-52 



