Environmental Engineering Reference
In-Depth Information
carried away by longshore drift, mentioned above, and is important in beach pro-
file renourishment, discussed below.
Backpassing is a possibility where wave energy is low, especially if there is a
wide intertidal area, as on the southern shores of Port Phillip Bay in south-east
Australia, where a long, gently curving sandy beach, much used by summer holi-
daymakers, extends from Rosebud to Rye. It is fronted by multiple parallel sand
bars that run out seaward across a shallow intertidal area up to 500 m wide, and
move to and fro in response to alternations of obliquely-arriving wave action. In
the 1950s erosion of the beach prompted the building of sea walls and groynes,
but depletion continued, and in 1963 it was decided to renourish one sector by
bulldozing sand in from the nearshore sand bars at low tide. This was successful,
and during the next 20 years several sectors of the beach were built up and wid-
ened in this way. Between 3,000 and 5,500 m
3
of nearshore sand were delivered
to the beach annually, and parts of the nearshore area deepened by up to 30 cm as
the bulldozer scooped sand shoreward. However, there was an ensuing problem of
dense seagrass infestation in the areas deepened by bulldozing, and the beach and
nearshore area had to be restored by renourishment. This was achieved in 1985
by dumping a series of artificial transverse bars of fine sand 5 m wide and 120 m
long, spaced at 100 m intervals, which were widened and moved to and fro by
wave action until they buried and destroyed the nearshore seagrass beds. Within
2 years the artificial transverse bars had been re-shaped by wave action, and their
lateral migration had reduced the seagrass area to a few small patches amid the
distributed sand (Parry and Collett
1985
).
Several beaches on the coast of England and Wales have been modified by the
combing of shingle down the beach by plunging waves during storms. This can be
countered by bulldozing gravel up to the back of the shore to raise the upper beach
profile in front of a sea wall or eroding cliff. This has been effective at Dunwich,
East Anglia (Fig.
4.25
), as a short-term procedure, where the aim is to increase
upper beach protection of soft cliffs cut in glacial drift. Where storm downcomb-
ing becomes frequent it may be necessary to add more sediment in order to ren-
ourish a higher and wider beach that is more protective.
Fig. 4.25
Apron of shingle
bulldozed to form an upper
beach at Dunwich on the
Suffolk coast. © Geostudies
