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 élévation, détermines
minimum beach width.
The slope of the foreshore tends to increase as the grain size increases.
(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 spécifie gravity of beach materials (Nayak, 1970; and Dubois, 1972);
by porosity and permeability of beach material (Savage, 1958); and proÿably by the tidal range at the beach. Analysis by King (1972, p. 330)
suggests that slope dépends dominantly on sand size, and also significantly 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 beaches.
This variation in foreshore slope from one région 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 produce steeper beaches.
(See Figures 4-1, 4-26, and 4-27.)
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 laboratory data, there is an even stronger inverse relation between deepwater
steepness, H^/L^, and slope of the foreshore than between H slope.
To sununarize the results on foreshore slope for design purposes, the
following statements are supported by available data:
on
(a)
grain
Slope of the foreshore on open sand beaches dépends principally
size, and (to a lesser extend) on nearshore wave height.
(b) Slope of the foreshore tends to increase with increasing médian
grain size, but there is significant scatter m t e a a.
4-85
beach profile. The equilibrium slope of the foreshore is a useful design
parameter, since this slope, along with the berm élévation, détermines
minimum beach width.
The slope of the foreshore tends to increase as the grain size increases.
(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 spécifie gravity of beach materials (Nayak, 1970; and Dubois, 1972);
by porosity and permeability of beach material (Savage, 1958); and proÿably by the tidal range at the beach. Analysis by King (1972, p. 330)
suggests that slope dépends dominantly on sand size, and also significantly 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 beaches.
This variation in foreshore slope from one région 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 produce steeper beaches.
(See Figures 4-1, 4-26, and 4-27.)
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 laboratory data, there is an even stronger inverse relation between deepwater
steepness, H^/L^, and slope of the foreshore than between H slope.
To sununarize the results on foreshore slope for design purposes, the
following statements are supported by available data:
on
(a)
grain
Slope of the foreshore on open sand beaches dépends principally
size, and (to a lesser extend) on nearshore wave height.
(b) Slope of the foreshore tends to increase with increasing médian
grain size, but there is significant scatter m t e a a.
4-85