Environmental Engineering Reference
In-Depth Information
in 1947, and on the Hwang Ho delta in China after the river mouth was diverted to
another part of the coast during a flood in 1852.
Beach erosion near river mouths can follow the dredging of sand from river
channels, which occurred on the River Rhine during the Second World War, or the
reduction of fluvial sediment supply by soil conservation works in the hinterland,
as exemplified by the rivers draining to the Gulf of Taranto in southern Italy. The
same effect has been produced where long-continued soil erosion in river catch-
ments has removed unconsolidated surface sediment, exposing extensive areas of
bare rock, as in Turkey and Greece, where consequent reductions in fluvial sedi-
ment yield have resulted in beach erosion.
2.3 Reduction of Sediment Supply from the Sea Floor
On many coasts beaches were deposited and prograded when sand was swept
in from the sea floor by wave action during and since the Late Quaternary
(Pleistocene and Holocene) marine transgression. As sea level rose across the
continental shelf waves collected sediment that had previously been deposited by
rivers or wind action and sediment from weathered rock outcrops and carried it
shoreward, and as the Late Quaternary marine transgression came to an end con-
tinuing shoreward drift from shoals prograded these beaches, often forming suc-
cessive backing beach ridges and parallel dunes (Fig. 2.5 a-d).
On many of these coasts shoreward drift has come to an end, with the re-
shaping of the sea floor profile to a transverse concave profile across which wave
action no longer moves sediment on to the shore. If there is no compensating
input of sediment from other sources (such as cliff erosion or supply from rivers)
beach progradation stops, and with continued input of wave energy the trans-
verse nearshore profile migrates landward, so that the beaches are eroded. This
explains why many beaches that prograded earlier in Holocene times are now
being cut back by erosion, continuing wave action driving the transverse con-
cave landward (Fig. 2.5 d). The onset of erosion comes at different times in dif-
ferent places because the development of the concave profile and the cessation
of shoreward drift has occurred at various times on various beaches, and has not
yet been attained on coasts where there is still shoreward drift from nearshore
shoals.
The sequence portrayed in Fig. 2.6 is illustrated on the Ninety Mile Beach
in south-eastern Australia, which borders a sandy coast that formerly prograded
by accretion of sand supplied from the adjacent floor of Bass Strait, and is now
being cut back by marine erosion, except in a sector of continuing accretion along-
side breakwaters at Lakes Entrance (Fig. 2.6 ). There is still plenty of sand in the
nearshore area, but the transverse profile has become smooth and concave, and
there is no longer shoreward drift of sand to this beach, except at the SW end,
sheltered by the granitic upland of Wilsons Promontory, where there are still
nearshore sand shoals.
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