Environmental Engineering Reference
In-Depth Information
Fig. 4.24a
Northward drifting sand on the Adelaide coast trapped by a breakwater near West
Beach is loaded into trucks and carried southward along the beach to Glenelg (Fig.
4.24b
).
© Geostudies
calmer weather) are necessary to determine the depth from which such shoreward
transportation will occur, and therefore the optimum location for dumping sedi-
ment to be washed on to the beach.
The proportion of sand dumped in the nearshore zone that moves up on to the
beach varies with wave conditions. In North Carolina Schwartz and Musialawski
(
1977
) found that up to 75 % of dredged river sediment dumped in the nearshore
zone was washed on to the beach, but if stormy conditions followed the nearshore
dumping little or none of it moved onshore. There is usually also some long-
shore drift, which will determine which sectors of the beach actually receive the
inwashed sediment.
At New River Inlet on the coast of North Carolina, an attempt was made to
renourish an eroding beach that had formerly received sediment from the sea floor
by dumping sand nearshore in the expectation that it would be washed on to the
beach by wave action. 26,750 m
3
of coarse sand dredged from New River was
dumped on the sea floor by split-hull barges, and its movement followed by moni-
toring beach and nearshore profiles. The study showed that the sand deposited in
depths of less than 4 m moved shoreward over the ensuing 13 weeks, whereas
sand deposited at greater depths moved seaward. The sand that moved shoreward
was deflected alongshore by obliquely-arriving waves to beaches down the coast
