Causes of Beach Erosion - Beach Renourishment - page 16

topography of the oceans. Coastal submergence has been widespread, and may
provide at least a partial explanation for the modern prevalence of beach ero-
sion (Bird 1996 ), although it is often cited as the primary cause of beach erosion
(Douglas et al. 2000 ).
2.6 Increased Wave Energy
As has been noted, beach erosion occurs where wave energy increases (i.e.
larger and higher waves approach the shore) because nearshore water is deep-
ened by coastal submergence, due to a rise in sea level or subsidence of coastal
land. Nearshore water can also be deepened where a shoal or reef is removed,
either by natural erosion or by dredging, or where nearshore seagrass mead-
ows disappear, so that sediment that had been retained is dispersed. On the
coast at Benacre Ness, Suffolk, United Kingdom, a sector of beach that had
been protected from wave attack by a nearshore shoal began to erode as the
shoal moved away alongshore, while in Botany Bay, Australia, beach erosion
accelerated at Brighton-le-Sands after the bay floor was dredged to provide
sediment for the extension of a runway at Sydney International Airport. In the
1930s deepening of nearshore water following the disappearance of seagrasses
(which had retained sediment in the sea floor shoals) led to beach erosion on
the shores of Danish islands such as Kyholm (Christiansen et al. 1981 ). On the
Arctic coast of Russia increased beach erosion has been attributed to larger
waves arriving as the result of nearshore deepening due to downwarping of the
adjacent sea floor.
Some beaches that had been stable or prograding began to erode as the result of
an increase in the frequency and severity of storms in coastal waters. A series of
storms in quick succession is particularly destructive because the second and sub-
sequent events occur on beaches already reduced to a concave eroded profile. An
example of this has been documented from Estonia, where the climate has become
stormier during the past few decades, with sea level more frequently raised by
storm surges, so that coastline erosion became more rapid and more extensive,
notably on the west coast of Saaremaa Island (Orviku et al. 2003 ).
2.7 Losses of Beach Sediment Alongshore
Beaches are depleted when sand or gravel are carried away by longshore drift (due
to the arrival of waves at an angle to the shore), unless these losses are compen-
sated by the arrival of more sediment from updrift. Beach erosion will occur if the
losses downdrift exceed the supply from updrift, for example where the source of
sediment updrift is a cliff that has been stabilised, or a river where fluvial sediment
yield has diminished. Some beaches develop lobes of sand or gravel that migrate