interpreted as bounding the seaward edge of sédiment moved by waves in significant quantifies. This band of finer material suggests that there is
little exchange between littoral and shelf sands in most places.
c. Sand Budget Balancing. Onshore sand transport has been suggested
as a source of sand for several Coastal localities that lack other obvious
sand sources. (Shepard, 1963, pp. 176-177; and Pierce, 1969.) For example,
Pierce suggests that the offshore must supply 440,000 cubic yards per year
of sand to the Southern segment of the Outer Banks of North Carolina because other known sources do not balance the budget.
d. Transport in Nearshore Zone. Although theoretical, experimental,
and field data show that waves move sand some of the time over most of the
Continental Shelf, most of the data suggest that sand from the shelf is not
a significant contributor to the sédiment budget of the littoral zone.
Sand transport from the nearshore zone is more likely. Surveys show
that sand in the nearshore zone does move, although it is difficult to measure direction of motion. The presence of shore-parallel contours along most
open shores is evidence that the waves actively mold the sand bottom in the
nearshore zone. However, the time scale of transport in the nearshore zone
may be relatively slow.
In tests at Santa Barbara, California, and at Atlantic City and Long
Branch, New Jersey, dredged sands were dumped offshore in depths ranging
from 15 to 40 feet, but no measurable onshore migration of the sand resulted for times of about 1 year. (Hall and Herron, 1950.) Radioactive
tracers hâve shown that gravel moves slowly landward in 30 feet of water
at a rate of about 0.5 cubic yard per year per foot of beach.
(Crickmore
and Waters, 1972.)
At shallower depths in the nearshore zone, onshore sand transport
following storms is well documented. Transport of sédiment suspended
over ripples by the mass transport velocity is more than adéquate to
retum sand eroded from the beach or to transport sand eroded from the
nearshore bottom to the beach.
e* Summary on Seaward Limit. The deepest shore-parallel contour
appears to be a usable estimate of the maximum depth where significant
sand transport can be expected. This depth varies from 15 feet to perhaps 60 feet or more along U.S. coasts. This choice may be modified for
spécifie wave conditions using Figure 4-23 to find the depth where maximum wave-induced velocity first exceeds 0.5 foot per second. If this
less,than the depth of the deepest shore-parallel contour, it
s ou
e used as the seaward limit for the given wave conditions.
4•524 Beach Erosion and Recovery.
j a* jLeac^ Erosion. Beach profiles change frequently in response to
winds, waves, and tides. Profiles are also affected by events in the longshore direction that influence the longshore transport of sand. The most
4-70
little exchange between littoral and shelf sands in most places.
c. Sand Budget Balancing. Onshore sand transport has been suggested
as a source of sand for several Coastal localities that lack other obvious
sand sources. (Shepard, 1963, pp. 176-177; and Pierce, 1969.) For example,
Pierce suggests that the offshore must supply 440,000 cubic yards per year
of sand to the Southern segment of the Outer Banks of North Carolina because other known sources do not balance the budget.
d. Transport in Nearshore Zone. Although theoretical, experimental,
and field data show that waves move sand some of the time over most of the
Continental Shelf, most of the data suggest that sand from the shelf is not
a significant contributor to the sédiment budget of the littoral zone.
Sand transport from the nearshore zone is more likely. Surveys show
that sand in the nearshore zone does move, although it is difficult to measure direction of motion. The presence of shore-parallel contours along most
open shores is evidence that the waves actively mold the sand bottom in the
nearshore zone. However, the time scale of transport in the nearshore zone
may be relatively slow.
In tests at Santa Barbara, California, and at Atlantic City and Long
Branch, New Jersey, dredged sands were dumped offshore in depths ranging
from 15 to 40 feet, but no measurable onshore migration of the sand resulted for times of about 1 year. (Hall and Herron, 1950.) Radioactive
tracers hâve shown that gravel moves slowly landward in 30 feet of water
at a rate of about 0.5 cubic yard per year per foot of beach.
(Crickmore
and Waters, 1972.)
At shallower depths in the nearshore zone, onshore sand transport
following storms is well documented. Transport of sédiment suspended
over ripples by the mass transport velocity is more than adéquate to
retum sand eroded from the beach or to transport sand eroded from the
nearshore bottom to the beach.
e* Summary on Seaward Limit. The deepest shore-parallel contour
appears to be a usable estimate of the maximum depth where significant
sand transport can be expected. This depth varies from 15 feet to perhaps 60 feet or more along U.S. coasts. This choice may be modified for
spécifie wave conditions using Figure 4-23 to find the depth where maximum wave-induced velocity first exceeds 0.5 foot per second. If this
less,than the depth of the deepest shore-parallel contour, it
s ou
e used as the seaward limit for the given wave conditions.
4•524 Beach Erosion and Recovery.
j a* jLeac^ Erosion. Beach profiles change frequently in response to
winds, waves, and tides. Profiles are also affected by events in the longshore direction that influence the longshore transport of sand. The most
4-70