4.526 Slope of the Foreshore . The foreshore is the steepest part of the 

 beach profile. The equilibrium slope of the foreshore is a useful design 

 parameter, since this slope, along with the berm elevation, determines 

 minimum beach width. 



The slope of the foreshore tends to increase as the grain size in- 

 creases. (U.S. Army, Beach Erosion Board, 1933; Bascom, 1951; and King, 

 1972, p. 324.) This relationship between size and slope is modified by- 

 exposure to different wave conditions (Bascom, 1951; and Johnson, 1956); 

 by specific gravity of beach materials (Nayak, 1970; and Dubois, 1972); 

 by porosity and permeability of beach material (Savage, 1958); and prob- 

 ably by the tidal range at the beach. Analysis by King (1972, p. 330) 

 suggests that slope depends dominantly on sand size, and also signifi- 

 cantly on an unspecified measure of wave energy. 



Figure 4-32 shows trends relating slope of the foreshore to grain 

 size along the Florida Panhandle, New Jersey-North Carolina, and the 

 U.S. Pacific coasts. Trends shown on the figure are simplifications of 

 actual data, which are plotted in Figure 4-33. The trends show that, 

 for constant sand size, slope of the foreshore usually has a low value 

 on Pacific beaches, intermediate value on Atlantic beaches, and high 

 value on Gulf beache*:. 



This variation in foreshore slope from one region to another appears 

 to be related to the mean nearshore wave heights. (See Figures 4-10, 4-11, 

 and Table 4-4.) The gentler slopes occur on coasts with higher waves. An 

 increase in slope with decrease in wave activity is illustrated by data 

 from Half Moon Bay (Bascom, 1951), and is indicated by the results of King 

 (1972, p. 332). 



The inverse relation between slope and wave height is partly caused 

 by the relative frequency of steep or high eroding waves which produce 

 gentle foreshore slopes and low accretionary post-storm waves which pro- 

 duce steeper beaches. (See Figures 4-1, 4-26, and 4-2 7.) 



The relation between foreshore slope and grain size shows greater 

 scatter in the laboratory than in the field. However, the tendency for 

 slope of the foreshore to increase with decreasing mean wave height is 

 supported by laboratory data of Rector (1954, Table 1). In this labora- 

 tory data, there is an even stronger inverse relation between deepwater 

 steepness, Wq/Lo, and slope of the foreshore than between Wq a^d the 

 slope. 



To summarize the results on foreshore slope for design purposes, the 

 following statements are supported by available data: 



(a) Slope of the foreshore on open sand beaches depends principally 

 on grain size, and (to a lesser extend) on nearshore wave height. 



(b) Slope of the foreshore tends to increase with increasing median 

 grain size, but there is significant scatter in the data. 



4-85 



