Environmental Engineering Reference
In-Depth Information
After storm surge flooding overtopped Chesil Beach at Chiswell near Portland
in the 1970s the beach crest was raised by bulldozing up shingle, and then sta-
bilised with a capping of gabion mattresses (caged stones). Where beaches lose
sediment by overwashing during storms it can be bulldozed back from the inner
slope to the beach face. This has been successful on the shingle barrier beach north
of Timaru, on the Canterbury coast, South Island, New Zealand, where sediment
washed over into Washdyke Lagoon by storm waves was retrieved in this way
(Kirk and Weaver
1985
).
A different kind of backpassing may be necessary where wind action carries
sand to the back of the beach. At Harrison County, Mississippi sand blown from
the renourished beach by occasional strong wind action during hurricanes piles up
against the wall to the rear, and has to be periodically taken back by trucking to
restore the beach profile.
Many seaside resorts improve their beaches for summer holiday use by sweep-
ing back sand and shingle dumped on their esplanades by winter storms. Some,
such as Weymouth in southern England, take care to maintain a clean, flat sandy
beach as an attraction for children and a venue for sand castle competitions each
summer. Each winter, sand is washed and blown round the shore of Weymouth
Bay to accumulate as low dunes in front of the sea wall at the southern end of the
esplanade, and in spring this is collected and redistributed across the beach.
4.3.7 Overfill
It is generally necessary to add more beach material than is necessary to restore a
beach to its natural dimensions, in order to allow for expected subsequent losses
onshore, offshore or alongshore. It is common practice to use overfill ratios, as
recommended by the Coastal Engineering Manual (USACE
2002
) to compensate
for the rapid removal of finer material, by simply adding more.
James (
1975
) defined the Overfill Ratio as the volume of sediment necessary to
restore a beach similar to the natural beach, allowing for losses of sediment until the
grain size had been sorted to match the natural distribution. The Renourishment Factor
is the ratio of the rate of erosion of beach fill material to the preceding rate of natural
beach erosion, indicating the frequency of replenishment necessary to maintain a sta-
ble beach volume. Overfill is also necessary to anticipate losses due to spilling of sedi-
ment out of the renourished area, around a terminal groyne or bordering headland or
terminal groyne. Renourished Dutch beaches are usually overfilled, with overfill vol-
umes typically varying between 10 and 40 % (Hanson et al.
2002
; Verhargen 1992).
4.3.8 Shore Profile Renourishment
Most beach renourishment projects form a beach terrace, which is then re-shaped
by waves and currents towards a natural concave profile, often with sand bars
