beach fill is subject to uncertainties: the source of the dredged sand
often contains significant but variable quantifies of finer matenals that
are soon lost to the littoral zone; the surveys of both the borrow area
and the replenished area are subject to uncertainty because sédiment transport occurs during the dredging activities; and in practice only limited
efforts are made to obtain estimâtes of the size distribution of fill
placed on the beach. Thus, the resulting estimate of the quantity of
suitable fill placed on the beach is uncertain. More frequent sampling
and surveys could help identify this significant element in many sédiment budgets.
4.73 SINKS FOR LITTORAL MATERIALS
4,731 Inlets and Lagoons. Barrier islands are interrupted locally by
inlets which may be kept open by tidal flow. A part of the sédiment moved
alongshore by wave action is moved into these inlets by tidal flow. Once
inside the inlet, the sédiment may deposit where it cannot be moved seaward by the ebb flow. (Brown, 1928.) The middleground shoals common to
many inlets are such depositional features. Such déposition may be reduced
when the ebb currents are stronger than the flood currents. (Johnson,
1956.)
It is évident from aerial photography (e.g., of Drum Inlet, N.C.,
Fig. 4-45) that inlets do trap significant quantifies of sand. Caldwell’s
(1966) estimate of the sand budget for New Jersey, calculâtes that 23
percent of the local gross longshore transport is trapped by the seven
inlets in Southern New Jersey, or about 250,000 cubic yards per year for
each inlet. In a study of the south shore of Long Island, McCormick (1971)
estimated from the growth of the floodtide delta of Shinnecock Inlet (shown
by aerial photos taken in 1955 and 1969) that this inlet trapped 60,000
cubic yards per year. This amounts to about 20 percent of the net longshore transport (Taney, 1961a, p. 46), and probably less than 10 percent
of the gross transport. (Shinnecock Inlet is a relatively small inlet.)
It appears that the rate at which an inlet traps sédiment is higher immediately after the inlet opens than it is later in its history.
4.732 Overwash. On low barrier islands, sand may be removed from the
beach and dune area by overwashing during storms. Such rates may average
locally up to 1 cubic yard per year per foot. Data presented by Pierce
(1969) suggest that for over half of the shoreline between Cape Hatteras
and Cape Lookout, North Carolina, the short term loss due to overwash was
0.6 cubic yard per year per foot of beach front. Figure 4-46 is an aerial
view of overwash in the région studied by Pierce (1969). Overwash does
not occur on ail barrier islands, but if it does, it may function as a
source for the beach on the lagoon side.
Il733 P-a-^k_sh°re and Dune Storage. Sand can be temporarily withdrawn
from transport in the littoral zone as backshore deposits and dune areas
along the shore. Depending on the frequency of severe storms, such sand
may reraam in storage for intervals ranging from months to years. Backshore déposition can occur in hours or days by the action of waves after
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often contains significant but variable quantifies of finer matenals that
are soon lost to the littoral zone; the surveys of both the borrow area
and the replenished area are subject to uncertainty because sédiment transport occurs during the dredging activities; and in practice only limited
efforts are made to obtain estimâtes of the size distribution of fill
placed on the beach. Thus, the resulting estimate of the quantity of
suitable fill placed on the beach is uncertain. More frequent sampling
and surveys could help identify this significant element in many sédiment budgets.
4.73 SINKS FOR LITTORAL MATERIALS
4,731 Inlets and Lagoons. Barrier islands are interrupted locally by
inlets which may be kept open by tidal flow. A part of the sédiment moved
alongshore by wave action is moved into these inlets by tidal flow. Once
inside the inlet, the sédiment may deposit where it cannot be moved seaward by the ebb flow. (Brown, 1928.) The middleground shoals common to
many inlets are such depositional features. Such déposition may be reduced
when the ebb currents are stronger than the flood currents. (Johnson,
1956.)
It is évident from aerial photography (e.g., of Drum Inlet, N.C.,
Fig. 4-45) that inlets do trap significant quantifies of sand. Caldwell’s
(1966) estimate of the sand budget for New Jersey, calculâtes that 23
percent of the local gross longshore transport is trapped by the seven
inlets in Southern New Jersey, or about 250,000 cubic yards per year for
each inlet. In a study of the south shore of Long Island, McCormick (1971)
estimated from the growth of the floodtide delta of Shinnecock Inlet (shown
by aerial photos taken in 1955 and 1969) that this inlet trapped 60,000
cubic yards per year. This amounts to about 20 percent of the net longshore transport (Taney, 1961a, p. 46), and probably less than 10 percent
of the gross transport. (Shinnecock Inlet is a relatively small inlet.)
It appears that the rate at which an inlet traps sédiment is higher immediately after the inlet opens than it is later in its history.
4.732 Overwash. On low barrier islands, sand may be removed from the
beach and dune area by overwashing during storms. Such rates may average
locally up to 1 cubic yard per year per foot. Data presented by Pierce
(1969) suggest that for over half of the shoreline between Cape Hatteras
and Cape Lookout, North Carolina, the short term loss due to overwash was
0.6 cubic yard per year per foot of beach front. Figure 4-46 is an aerial
view of overwash in the région studied by Pierce (1969). Overwash does
not occur on ail barrier islands, but if it does, it may function as a
source for the beach on the lagoon side.
Il733 P-a-^k_sh°re and Dune Storage. Sand can be temporarily withdrawn
from transport in the littoral zone as backshore deposits and dune areas
along the shore. Depending on the frequency of severe storms, such sand
may reraam in storage for intervals ranging from months to years. Backshore déposition can occur in hours or days by the action of waves after
4-124