1967; Keulegan, 1951; and van Dom, 1953.) These surface currents drift
in the direction of the wind at a speed equal to 2 to 3 percent of the
wind speed. In hurricanes, winds generate surface currents of 2 to 8
feet per second. Such wind-induced surface currents toward the shore
cause significant bottom retum flows which may transport sédiment seaward; similarly, strong offshore winds can resuit in an offshore surface
current, and an onshore bottom current which can aid in transporting
sédiment landward.
4.125 Géologie Factors. The geology of a Coastal région affects the
supply of sédiment on the beaches and the total Coastal morphology. Thus,
geology détermines the initial conditions for littoral processes, but géologie factors are not usually active processes affecting Coastal engineering.
One exception is the rate of change of sea level with respect to
land which may be great enough to influence design, and should be examined if project life is 50 years or more. On U.S. coasts, typical rates
of sea level rise average about 1 to 2 millimeters per year, but changes
range from -13 to +9 millimeters per year. (Hicks, 1972.)
(Plus means
a rise in sea level with respect to local land level.)
4.126 Other Factors. Other principal factors affecting littoral processes
are the works of man and activities of organisms native to the particular
littoral zone. In engineering design, the effects on littoral processes
of construction activities, the resulting structures, and structure maintenance must be considered. This considération is particularly necessary
for a project that may alter the sand budget of the area, such as jetty
or groin construction. In addition biological activity may be important
in producing carbonate sands, in reef development, or (through végétation)
in trapping sand on dunes.
4.13 CHANGES IN THE LITTORAL ZONE
Because most of the wave energy is dissipated in the littoral zone,
this zone is where beach changes are most rapid. These changes may be
errQ
e t0 seasonal changes in wave conditions and to occurrence
ntermittent storms separated by intervals of low waves, or long-term
due to an overall imbalance between the added and erodéd sand. Short-term
profile fFtaP4aTint/n the temPorary redistribution of sand across the
manent shift*n/tk
^g"*®1111 changes are apparent in the more ùearly permanent shift of the shorelrne. (See Figures 4-3, 4-4, and 4-5.)
thoseMSoTineF?°nal
changes across a profile, such as
cally and from
c
tn>ically on the order of a few feet vertiextremT'storœs or V °
horizo"tally. (See Table 4-1.) Only during
usual changes écc« ov" a shj^p^d^ SUPPly “ restricted’ d°
Table
°n southerT1 California beaches are shown in
1- (ShePar“. 1950.) These data show greater changes on the beach
4-6
in the direction of the wind at a speed equal to 2 to 3 percent of the
wind speed. In hurricanes, winds generate surface currents of 2 to 8
feet per second. Such wind-induced surface currents toward the shore
cause significant bottom retum flows which may transport sédiment seaward; similarly, strong offshore winds can resuit in an offshore surface
current, and an onshore bottom current which can aid in transporting
sédiment landward.
4.125 Géologie Factors. The geology of a Coastal région affects the
supply of sédiment on the beaches and the total Coastal morphology. Thus,
geology détermines the initial conditions for littoral processes, but géologie factors are not usually active processes affecting Coastal engineering.
One exception is the rate of change of sea level with respect to
land which may be great enough to influence design, and should be examined if project life is 50 years or more. On U.S. coasts, typical rates
of sea level rise average about 1 to 2 millimeters per year, but changes
range from -13 to +9 millimeters per year. (Hicks, 1972.)
(Plus means
a rise in sea level with respect to local land level.)
4.126 Other Factors. Other principal factors affecting littoral processes
are the works of man and activities of organisms native to the particular
littoral zone. In engineering design, the effects on littoral processes
of construction activities, the resulting structures, and structure maintenance must be considered. This considération is particularly necessary
for a project that may alter the sand budget of the area, such as jetty
or groin construction. In addition biological activity may be important
in producing carbonate sands, in reef development, or (through végétation)
in trapping sand on dunes.
4.13 CHANGES IN THE LITTORAL ZONE
Because most of the wave energy is dissipated in the littoral zone,
this zone is where beach changes are most rapid. These changes may be
errQ
e t0 seasonal changes in wave conditions and to occurrence
ntermittent storms separated by intervals of low waves, or long-term
due to an overall imbalance between the added and erodéd sand. Short-term
profile fFtaP4aTint/n the temPorary redistribution of sand across the
manent shift*n/tk
^g"*®1111 changes are apparent in the more ùearly permanent shift of the shorelrne. (See Figures 4-3, 4-4, and 4-5.)
thoseMSoTineF?°nal
changes across a profile, such as
cally and from
c
tn>ically on the order of a few feet vertiextremT'storœs or V °
horizo"tally. (See Table 4-1.) Only during
usual changes écc« ov" a shj^p^d^ SUPPly “ restricted’ d°
Table
°n southerT1 California beaches are shown in
1- (ShePar“. 1950.) These data show greater changes on the beach
4-6