Environmental Engineering Reference
In-Depth Information
demands for beach renourishment intensify. On the barrier islands of the Atlantic
coast of the United States local residents now regard truck traffic as acceptable if it
is the only way of maintaining their beaches.
Public demands for beach renourishment are often a response to obvious deple-
tion by storms, and the fear that further losses would ensue. At Long Beach Island,
New Jersey, such demands led to renourishment of a depleted beach in 1979, but
the added sediment had disappeared after 7 years. There is a divergence of long-
shore drift here, sand from the northern part of the beach being washed back into
Barnegat Inlet (from which it had been obtained) while sand from the southern part
moved away southward along the coast (Ashley et al.
1987
). Although narrow, this
renourished beach became relatively stable, except for brief and temporary deple-
tions during stormy periods, and it is now locally regarded as acceptable. Public per-
ception of erosion hazards can be fickle: in the words of Pilkey and Clayton (
1987
)
“community apprehension over a narrow beach softens with time and the absence
of storms”. There was also the feeling that the beach could, if necessary, be restored
again, this time with sand deposition concentrated in the zone of divergence, and
losses northward and southward perhaps delayed by the insertion of groynes.
It has been noted (Sect.
4.3.1
, p. 59 and Sect.
4.4.2
, p. 79) that beach renourishment
should use sediment at least as coarse as that on pre-existing natural beaches, because
finer sediment is quickly lost. Coarse beaches are less attractive to beach users
(Campbell and Beachler
1984
), although more durable shingle beaches are used for
recreation at seaside resorts such as Brighton in southern England, Dieppe in northern
France and Nice on the French Riviera.
4.7 Modelling of Beach Renourishment
More detailed modelling (both physical and numerical) may be needed to refine
the design of a renourishment project, or to investigate how it will alter following
placement.
Laboratory simulation of shore processes to help design beach renourishment
projects has been attempted with scale models such as water tanks in which waves,
tides and currents can be generated and their combined effects assessed. The aim
is to test hypotheses concerning the ways in which these processes cause erosion,
move sediment and promote deposition on the sea floor and along the coast. Such
physical models have limitations because of the difficulty of scaling down mate-
rials and processes without modifying their physical properties (e.g. coherence,
friability, expansion and contraction of sediments; viscosity and surface tension
in water), but they have been useful in exploring potential responses to marine
and nearshore processes (Silvester
1974
). They are also useful in examining the
impacts of structures, as at Borth in Wales (Fig.
4.31
), where the future evolution
of the beach following renourishment was examined in relation to the introduction
of groynes and a submerged breakwater.
Numerical modelling has been much used by engineers as a basis for com-
