Environmental Engineering Reference
In-Depth Information
Fig. 4.21
Variation in net
longshore drift with aspect:
A
balanced longshore drift
when the wave resultant
is orthogonal to the shore,
B
net northward drift on a
west-facing beach and
C
net
eastward drift on a south-
facing beach when the wave
resultant arrives from the SW.
© Geostudies
will result in losses past headlands or breakwaters. In these circumstances, coast-
line orientation in relation to wave climate determines whether an emplaced beach
will remain in position, or lose sediment in one direction or the other (Fig.
4.21
),
so that there is a contrast between predominantly northward drift on Hampton
Beach (facing south-west) and predominantly south-eastward drift on Quiet Corner
Beach (which has a more southerly aspect) a few kilometres away (Bird
1991
).
4.3.3 Bypassing
Bypassing is the passage of sediment past an obstacle such as a tidal inlet or
breakwater to replenish a beach downdrift. It occurs naturally, but can also be
developed artificially as a means of downdrift beach replenishment. It has been
used at several places worldwide (Boswood and Murray
2001
), but has proved
particularly popular on the coasts of the United States and Australia. An early
example of beach renourishment using bypassing as a means of artificially restor-
ing the longshore drift regime was at South Lake Worth Inlet in Pam Beach
County, Florida. After this tidal inlet was stabilised by breakwater construction in
the 1930s to improve navigation the nearby beaches soon showed updrift accre-
tion and downdrift erosion. When groynes failed to control the downdrift erosion
a sand bypassing scheme was introduced in 1935, taking about 48,000 m
3
of sand
per year from the accreting southern beach round to renourish the depleted north-
ern beach. This was the longest continuously operating fixed sand by-passing plant
in the world (Finkl and Walker
2004
). Similar projects have been used at Santa
