Environmental Engineering Reference
In-Depth Information
Barbara, Ventura, and Channel Islands Harbour in California, both of which had
accretion updrift and erosion downdrift of harbour breakwaters.
There are many techniques of sand bypassing (Bruun and Willekes
1992
;
USACE
1991
; Bruun
1996
; Boswood and Murray
2001
). Hydraulic methods of
both sediment collection and delivery have proven to be one of the most effec-
tive contemporary methods for sand bypassing (Boswood et al.
2005
; Mocke et al.
2005
; Acworth and Lawson
2011
).
At the entrance to the Tweed River on Australia's Gold Coast the Tweed River
Entrance Sand Bypassing Project (TREBP) was established in 2001 to mitigate the
substantial decrease in northward drift intercepted by the Tweed River Entrance
training walls. A permanent sand intake jetty intercepts sand moving northward,
which is then piped under the Tweed River at a rate consistent with the average
natural longshore transport (of the order of 500,000 m
3
), then released on the
beaches north of the Tweed River Entrance. Previous to the TREBP intermittent
sand nourishment was undertaken using offshore sand reserves (Boak et al.
2001
).
The TREBP has been successful in changing the morphology of the beaches north
of the entrance (Castelle et al.
2006
) providing a wider beach, better amenity, a
demonstrated storm buffer and surfing benefits (Boswood et al.
2005
).
As a result of a combination of weir jetty, interior sand trap and dredging, the
Hillsboro Inlet in Broward County, southeast Florida, has been able to bypass
100 % of the estimated net longshore drift since 1965, when this was the first sand
by-passing weir jetty in the world (Finkl
1993
).
An alternative is to carry sediment round past a harbour entrance in trucks,
as at Hvidesande in Denmark, or ferry it past the entrance, as at Port Hueneme,
California. However, some eroding beaches downdrift of breakwaters have been
renourished with sediment from other sources, notably the sea floor, as at Seaford
in England, Timaru in New Zealand and Lagos in Nigeria.
The Lagos coastline has been much modified by the building of breakwaters
to stabilise the harbour entrance. These caused updrift accretion on Lighthouse
Beach and downdrift erosion on Victoria Beach (Usoro
2010
). To counter ero-
sion Victoria Beach was renourished with sand pumped in from the sea floor in
1976, and it is now accepted that this beach will need to be replenished frequently,
perhaps with the aid of a sand bypassing system from the accreting Lighthouse
Beach, west of the breakwaters (Ibe et al.
1991
).
4.3.4 Recycling
Beach sediment carried along the coast by longshore drift can be brought back to
renourish an eroded beach, a process termed recycling (Willis and Price
1975
).
The shingle beach at Rye on the Sussex coast in England is subject to longshore
drift, which has depleted the Cliff End beach at the updrift end. The supply of shin-
gle to this beach has been reduced because recurrent landslides on the Fairlight coast
to the west interrupted longshore drift from Hastings. Stabilisation of the mouth of
