Environmental Engineering Reference
In-Depth Information
coast of Bournemouth in southern England after a concrete promenade built along
the base of eroding cliffs cut in soft sandstone and gravel (Figs.
2.3
,
2.4
) cut off
the supply of sand and gravel to the beach, which then gradually diminished. The
promenade was built partly for the benefit of seaside holidaymakers, but it was
also intended to halt cliff recession and preserve coastal properties. The coastal
slope was artificially landscaped and planted with vegetation, but by the 1970s
Bournemouth beach was severely depleted, and it was decided that it should be ren-
2.2 Reduction of Fluvial Sediment Supply to the Coast
Beach erosion occurs where beaches that have been supplied with sediment car-
ried down to the coast by rivers are depleted following a reduction in sediment
yield to river mouths as a result of reduced runoff. In Southern California dimin-
ished river flow during droughts resulted in beach erosion, but the beaches were
restored during intervening wet years when the fluvial sediment supply revived
(Orme
1985
). Reduction of fluvial sediment supply commonly results from the
construction of dams to impound water upstream. These intercept fluvial sedi-
ment discharge and so cut off the supply of sand and gravel to beaches at and near
the river mouth. This leads to the onset of erosion on beaches that were formerly
maintained or prograded by the arrival of this fluvial sediment. Erosion develops
more quickly, and becomes more severe, where there is strong longshore drift of
sediment away from the river mouth.
The best known example of such erosion is on the shores of the Nile delta,
where sandy beaches that had been prograding for many centuries as the
result of the delivery of sediment to the mouths of Nile distributaries and its
distribution by longshore drift became depleted after the construction of dams
upstream. Erosion of beaches near the mouths of the Rosetta and Damietta dis-
tributaries started soon after barrage construction began in 1902, and became
much more rapid and extensive after the completion of the Aswan High Dam
in 1964, which impounded Lake Nasser and resulted in large-scale sediment
entrapment. During the next few years beach erosion on parts of the deltaic
coastline attained annual rates of up to 120 m (Sestini
1992
). Some of the sedi-
ment removed from these beaches was carried away eastward by longshore drift
along the coast towards Port Said, but much has been lost offshore (Lotfy and
Frihy
1993
).
Similar beach erosion has occurred on the shores of other deltas following dam
construction upstream: for example on the Rhne delta in France, the Dnieper and
Dniester deltas in the Ukraine, the Citarum delta in Indonesia and the Barron delta
in Australia.
Diversion of a river mouth, either naturally or artificially, halts the fluvial sedi-
ment supply to the coast and leads to erosion of adjacent beaches, as on the shores
of the Cimanuk delta in Java after a distributary changed its course during a flood
