Environmental Engineering Reference
In-Depth Information
them by large waves. Sea walls also decay by physical, chemical and biological
weathering, a process that can be rapid on certain sandstones and limestones.
Construction of sea walls and similar structures on a particular sector of coast
to protect a building or a seaside resort is usually followed by continuing recession
on adjacent sectors, so that in due course the protected area becomes a promon-
tory. This has happened at the seaside resort of Mundesley, on the East Anglian
coast, which now protrudes between retreating cliffs of soft glacial drift, and at
Bray on the dune coast of north-eastern France. Eventually the flanks of such
promontories have to be stabilised artificially, and in due course the protected area
could become an island.
Another response to beach erosion has been to introduce structures designed to
retain a beach that protects a coastline from strong wave action. A breakwater built
out from the coastline can intercept longshore drift in order to form a higher and
wider beach sector updrift for some distance along the shore. The prograded sector
becomes triangular and extends towards the outer end of the breakwater.
Multiple groynes (groyne fields), built of timber, masonry, sheet metal, boul-
ders or concrete, have been inserted on some coasts, especially at seaside resorts
such as Eastbourne in Sussex (Fig.
3.2
), with the aim of retaining longshore drift
and so protecting the coastline. Beach sand and gravel are intercepted in the inter-
vening compartments, and accumulate until sediment spills over or round each
groyne. Sometimes a larger terminal groyne is built at the downdrift end (Fig.
3.3
).
As sediment drifting along the shore is trapped by the groynes the supply to
downdrift beaches is reduced, and erosion is thus transferred along the coast.
There is then a temptation to extend the groyne field: there are sectors of the coast-
line of England and Wales that now have multiple groynes for several miles.
Groynes have been successful in retaining beaches on some coasts, particu-
larly where wave energy is generally low (Fig.
3.4
), but storm waves may break
in such a way as to withdraw sand or shingle seaward from beach compartments
to the nearshore sea floor. Some of the withdrawn beach sediment may be returned
during subsequent periods of calmer weather, but it is possible that longshore
drift will carry it away along the nearshore zone, leaving the beach compartment
between the groynes depleted (Fig.
3.5
).
Fig. 3.2
Groynes defining
beach compartments at
Eastbourne on the Sussex
coast, England. © Geostudies
