storms. Dune deposits require longer to form - months or years - because
wind transport usually moves material at a lesser rate than wave transport
If the immédiate beach area is the control volume of interest, and budget
calculations are made based on data taken just after a severe storm, allow
ance should be made in budget calculations for sand that will be stored in
berms through natural wave action. (See Table 4-5.)
4.734 Offshore Slopes. The offshore area is potentially an important
sink for littoral material. Transport to the offshore is favored by:
storm waves which stir up sand, particularly when onshore winds create
a seaward retum flow; turbulent mixing along the sédiment concentration
gradient which exists between the sediment-water mixture of the surf zo»e
and the clear water offshore; and the slight offshore component of gravity
which acts on both the individual sédiment particles and on the sedimentwater mixture.
It is often assumed that the sédiment sorting loss that commonly
reduces the volume of newly placed beach fi 11 is lost to the offshore
slopes. (Corps of Engineers, Wilmington District, 1970; and Watts,
1956.) A major loss to the offshore zone occurs where spits build into
deep water in the longshore direction. Sandy Hook, New Jersey, is an
example. (See Figure 4-47.) It has been suggested (Bruun and Gerritsen,
1959) that ebb flows from inlets may sometimes cause a loss of sand by
jetting sédiment seaward into the offshore zone.
The calculation of quantifies lost to the offshore zone is difficult,
since it requires extensive, accurate, and costly surveys. Some data on
offshore changes can be obtained by studies of sand level changes on rods
imbedded in the sea floor (Inman and Rusnak, 1956), but without extending
the survey beyond the boundary of the moving sand bed, it is difficult
to détermine net changes.
4.735 Submarine Canyons. Probably the most frequently mentioned sinks
for littoral materials are submarine canyons. Shepard (1963) and Shepard
and Dill (1966) provide extensive description and discussion of the origin
of submarine canyons. The relative importance of submarine canyons in
sédiment budgets is still largely unknown.
Of 93 canyons tabulated by Shepard and Dill (1966), 34 appear to be
receiving sédiment from the coast, either by longshore transport or by
transport from river mouths. Submarine canyons are thought to be especially important as sinks off Southern California. Herron and Harris
(1966, p. 654) suggest that Mugu Canyon, California, traps about 1 million cubic yards per year of the local littoral drift.
The exact mechanism of transport into these canyons is not clear,
even for the La Jolla Canyon (California) which is stated to be the most
extensively studied submarine feature in the world. (Shepard and Buff
ington, 1968.) Once inside the canyons, the sédiment travels down the
floors of the heads of the canyons, and is permanently lost to the littoral zone.
4-127
wind transport usually moves material at a lesser rate than wave transport
If the immédiate beach area is the control volume of interest, and budget
calculations are made based on data taken just after a severe storm, allow
ance should be made in budget calculations for sand that will be stored in
berms through natural wave action. (See Table 4-5.)
4.734 Offshore Slopes. The offshore area is potentially an important
sink for littoral material. Transport to the offshore is favored by:
storm waves which stir up sand, particularly when onshore winds create
a seaward retum flow; turbulent mixing along the sédiment concentration
gradient which exists between the sediment-water mixture of the surf zo»e
and the clear water offshore; and the slight offshore component of gravity
which acts on both the individual sédiment particles and on the sedimentwater mixture.
It is often assumed that the sédiment sorting loss that commonly
reduces the volume of newly placed beach fi 11 is lost to the offshore
slopes. (Corps of Engineers, Wilmington District, 1970; and Watts,
1956.) A major loss to the offshore zone occurs where spits build into
deep water in the longshore direction. Sandy Hook, New Jersey, is an
example. (See Figure 4-47.) It has been suggested (Bruun and Gerritsen,
1959) that ebb flows from inlets may sometimes cause a loss of sand by
jetting sédiment seaward into the offshore zone.
The calculation of quantifies lost to the offshore zone is difficult,
since it requires extensive, accurate, and costly surveys. Some data on
offshore changes can be obtained by studies of sand level changes on rods
imbedded in the sea floor (Inman and Rusnak, 1956), but without extending
the survey beyond the boundary of the moving sand bed, it is difficult
to détermine net changes.
4.735 Submarine Canyons. Probably the most frequently mentioned sinks
for littoral materials are submarine canyons. Shepard (1963) and Shepard
and Dill (1966) provide extensive description and discussion of the origin
of submarine canyons. The relative importance of submarine canyons in
sédiment budgets is still largely unknown.
Of 93 canyons tabulated by Shepard and Dill (1966), 34 appear to be
receiving sédiment from the coast, either by longshore transport or by
transport from river mouths. Submarine canyons are thought to be especially important as sinks off Southern California. Herron and Harris
(1966, p. 654) suggest that Mugu Canyon, California, traps about 1 million cubic yards per year of the local littoral drift.
The exact mechanism of transport into these canyons is not clear,
even for the La Jolla Canyon (California) which is stated to be the most
extensively studied submarine feature in the world. (Shepard and Buff
ington, 1968.) Once inside the canyons, the sédiment travels down the
floors of the heads of the canyons, and is permanently lost to the littoral zone.
4-127