Environmental Engineering Reference
In-Depth Information
Selection of grain size in sediment used for beach renourishment should also
take account of wave energy conditions. This was illustrated on the often stormy
shores of the Gulf of Georgia near Vancouver, Canada. Sand was introduced in an
attempt to form a beach that would stop the erosion of cliffs cut in unconsolidated
glacial deposits, but it was soon dispersed by storm waves. A second project, using
cobbles, was more successful in establishing a beach to protect the cliffs (Downie
and Saaitink
1983
). In general, coarser sediment is needed to stabilise a beach in
a high wave energy environment, but fine sand may be retained on a low wave
energy shore. At Nunn's Beach, adjacent to Portland Harbour, Victoria, Australia,
fine sand that had accumulated south of the harbour breakwater was dredged and
trucked round to widen the beach in 1990. In this sheltered environment the ren-
ourished beach of fine sand has persisted.
Beach renourishment using sediment larger than the natural beach sediment
(often termed 'beach coarsening') can be carried out, with the deliberate aim of
creating a beach more resistant to erosion. Beach coarsening is normally restricted
to beaches that have been severely eroded and are failing to give an adequate level
of backshore protection (CIRIA
2010
).
The durability of a beach may depend on the shape of the sand or pebbles used,
angular material being less readily transported and lost than well-rounded mate-
rial. In the 1960s an experiment was conducted in St Lucie County, Florida, when
1,000 tons of imported oolitic sand from the Bahama Banks and a similar quantity
of local beach sand were placed in rows across the shore. These were redistributed
by wave action during high tides, and it was found that the more angular oolitic
sand was less mobile than the well-rounded native sand, and therefore more likely
to persist on a renourished beach (Cunningham
1966
).
It is necessary also to take account of rates of weathering of beach sediment,
which can reduce grain size. On Delray Beach in Florida it was found that organic
sand dredged from the nearshore zone was brittle, and when placed on the beach it
disintegrated rapidly under wave agitation to finer sediment that was quickly dis-
persed. The specific gravity of introduced sediment is another relevant factor. In
Tauranga Harbour, New Zealand, a renourished beach contained pumice, but this
very light sediment was soon washed away, indicating that wave action selectively
removes lighter as well as finer sediment (De Lange and Healy
1990
).
Where gravel dredged from the sea floor contains sand and silt its use in beach
renourishment can result in the formation of an excessively compact and imper-
meable beach capping, which may develop a firm cliff near the high tide line.
Examples of such scarping have been documented from renourished beaches at
Whitstable and Hayling Island in south-east England and Lodmoor near Weymouth
in Dorset. It is regarded as a hazard to beach users, and a cause of reflective scour
by waves on the lower beach during storms (McFarland et al.
1994
).
Suitability of material available for beach fill was a problem at Aberystwyth,
where in 1963 1,530 m
3
of waste from a quarry on Constitution Hill, to the north,
was dumped on the shore to improve the beach at Victoria Terrace. It contained a
high proportion of laminated and soft shale, which disintegrated and was quickly
dispersed (So
1974
).
