Groins 51-60 is presented in Figure 12. Inspection of aerial photography taken at 

 earlier dates did not reveal significant changes in shoreline position compared 

 with the October 1 997 photographs. 



Figure 12. Groin field between Structures 51-60 (11 October 1997) 



To quantify groin performance within the evaluated region, groin length, 

 longshore groin spacing, and shoreline position within groin cells were measured. 

 In a review of groin functioning. Kraus, Hanson, and Blomgren (1994) found that 

 the ratio of groin length to the width between them was recommended in the 

 literature to lie between 2 and 4. Kraus et al. also found through numerical 

 modeling with GENESIS that groin permeability and bypassing play central roles 

 in groin sand-retention capability. Bypassing, in turn, depends on the 

 representative wave height and depth at the end of a groin, which is related to 

 groin length and the beach-profile shape. 



For the empirical analysis, a baseline typically colocated with the seawall 

 was established between groins. This baseline served as a point of reference for 

 measuring groin length (distance from baseline to waterline at tip of structure) 

 and shoreline position within the groin cells. Shoreline position was measured at 

 five locations within each groin cell, at each structure, and at three equidistantly 

 spaced locations between structures. Two empirical measures, fill ratio and 

 minimum (landward) shoreline position, were selected as criteria to evaluate 

 groin performance. 



Estimates of the relative quantity of sand impounded within a groin com- 

 partment provide a measure of the effectiveness of adjacent groins. The fill ratio 

 is defined as the ratio of dry beach area within a groin compartment to the area 

 that would result if the groin compartment were completely filled with a dry, 

 straight beach between groin tips, as illustrated in Figure 13. The fill ratio is 

 given by 



F = 



^'"^^./n {'A y, + V: ^y.+y.+Yi ys ) 



Area,, 



2(1, +1:) 



(1) 



Chapter 3 Functional Design 



13 



