Environmental Engineering Reference
In-Depth Information
In the decade following 1975 10 million m
3
of sand was pumped ashore along
17 km of the coast to form a beach over 150 m wide. The aim was to restore the
beach and provide a formation that would protect the coastline.
In the Netherlands, a region of subsiding coastal land, the response to the grad-
ual rise of sea level over many centuries has been to counter-attack by building
and enlarging sea walls (dykes) to keep out the sea. Extensive areas of land are
now below high tide level. Some of the sea walls enclose tidal marshlands and
shallow sea areas for land reclamation. Large sea walls now dominate long sectors
of the Dutch coastline, and in places beaches have been added on their seaward
side, as at Neuharlingersiel, East Friesland (Jelgersma
1975
). Where beaches and
dunes still exist the aim of Dutch engineers has been to maintain the coast by ren-
ourishing the nearshore zone as well as the beach, and providing sufficient sand
to maintain a beach profile that prevents erosion of the dunes behind the beach
(Roelvink
1989
). It has been calculated that between 6 and 10 million m
3
of sedi-
ment fill will be required annually to maintain the existing coastline as subsidence
continues (Louisse and Kuik
1990
). Where necessary the coast will be built for-
ward to reduce impacts of future erosion, bearing in mind the probability of an
accelerating sea level rise in the next few decades (Pluijm
1990
).
If a sufficiently wide beach is formed, wind and wave action will then shape
a shore profile that includes backshore dunes as well as nearshore sand bars. At
Noosa in Queensland, Australia, beach erosion followed diversion of the Noosa
River outlet (Coughlan
1989
). Sand sprayed by the rainbow technique on to the
shore in Granite Bay drifted round to renourish the main beach at Noosa, where it
was retained by a groyne. In due course it prograded to form a wide area of bare
sandy beach, the landward part of which was shaped by wind action into back-
shore dunes, now stabilised by the planting of trees and shrubs.
4.3.9 Part-Renourishments
Beach renourishments take the form of partial fills, either as a layer of sediment
placed on top of an existing beach or as sediment placed under an existing beach.
For example in the latter case, filled geotextile bags have been used in the United
Kingdom, placed under existing beach material to raise the berm and provide an
increased level of stability. Although not technically classed as beach renourish-
ment, beach sediment is added to the beach. The use of geotextile bags as opposed
to loose sediment provides a more durable feature.
More common is the addition of renourishment sediment placed on top of the
existing beach material. At seaside resorts the losses of sandy sediment during
winter storms may leave a gravelly beach, and it is then necessary to bring back
sand to restore the beach for recreational use. At Whitburn Bay on the Durham
coast in north-east England the resort beach is improved each spring by dumping
sand over the gravelly shore (Fig.
4.26
). This is termed a veneer, as when sand is
deposited over much coarser (boulders coral rocks) or finer (silt and silty sand)
