Environmental Engineering Reference
In-Depth Information
growth. Without natural sand replenishment the beach has been reduced to very
fine sand by attrition, and has become low and flat: it is now firm enough to land
an aircraft, drive a bus or car, or ride a bicycle. As sediment calibre is reduced
the lowered beach has been further eroded as the increasingly fine sediment is
removed by winnowing, either landward into backshore dunes or seaward to bars
and sea floor deposits.
2.12 A Rise in the Beach Water Table
A wet sandy beach is eroded more rapidly by wave action than a dry one because
wet sand is more coherent, and erodes like a soft sandstone, whereas dry sand is
disturbed but not removed by wave swash. Field studies conducted by Grant (
1984
)
demonstrated the considerable impact of beach groundwater level on swash sediment
transport. Seawater infiltration under a low water table was found to enhance onshore
sediment transport, whereas groundwater exfiltration under high water table pro-
moted offshore sediment transport. On Stanwell Park Beach, near Sydney, Australia,
beach erosion increased with rises in the level of the beach water table during wet
weather, due to the ponding or diversion of river or lagoon outlets, or to increased
river or groundwater discharge following land use changes in the hinterland (Bryant
1985
). This process has led to the practice of beach dewatering (artificially lowering
the beach water table) for combating beach erosion, typically utilising drainage sys-
tems (Turner and Leatherman
1997
, Loannidis and Karambas
2007
).
2.13 Removal of Beach Sediment by Runoff
During periods of heavy rainfall beach erosion can result from runoff, particu-
larly where water flows down a backing cliff or steep slope and beach sediment
is swept into the sea. Examples of this are seen during the wet summer season in
NW Australia, notably near Cape Leveque, where runoff cuts gullies across the
beach and builds fans of sediment into the sea. Beach sediment removal by runoff
after rainfall occurs at the mouth of a storm water outfall sited within a beach. On
Collaroy-Narrabeen Beach, north of Sydney, Australia (Fig.
2.14
) beach sediment
has been washed away at a number of storm water outfalls. At Mission Bay and
Kohimarama on the east coast of Auckland, New Zealand, sand was sluiced from
beaches by storm water from outlets until these were extended seaward, beyond
the foot of the beach (Papps and Priestley
2005
).
The effects of runoff are stronger on sandy beaches, especially if they are
already wet, than on gravel where runoff disappears more quickly by percola-
tion. Increased runoff is often due to urbanisation and the construction of roads
and other sealed surfaces from which water runs off quickly, instead of percolating
into the subsoil, as it did before these structures were built.
