Environmental Engineering Reference
In-Depth Information
Chapter 2
Causes of Beach Erosion
Abstract
Before renourishing an eroded beach it is necessary to know why it has
been eroded and where the sediment has gone: landward, seaward or alongshore.
This chapter deals with the causes of beach erosion, including alterations in pro-
cesses and sediment supply, along with anthropogenic influences.
Before renourishing an eroded beach it is necessary to know why it has been
eroded and where the sediment has gone: landward, seaward or alongshore
(Fig.
2.1
).
Beach erosion is usually marked by the evolution of a concave-upward shore
profile, whereas accreting (prograding) beaches typically have convex-upward
shore profiles. There is sometimes a receding microcliff (an erosional scarp),
a metre or more in height, where an upper convex beach is being undercut as a
lower concave profile becomes established (Fig.
2.2
). Backshore dunes are often
cliffed behind beaches that have been lowered and cut back by erosion, as are
backshore terraces, and in both cases vegetated land surfaces are truncated as ero-
sion proceeds.
Erosion can be temporary, reversed by a following period of accretion, or long-
term, resulting in a net retreat of the coastline (recession). The retreat of the coast-
line can be traced by comparing dated sequences of maps and charts, or air and
ground photographs. Average annual rates of coastline recession are usually small
(a few centimetres each year), but there have been instances of recession rates of
more than 40 m per year, as on beaches fringing rapidly eroding delta shores, or
on beaches fronting relatively less resistant cliffs, such as those comprising glacial
deposits for example.
Beach erosion can be caused by natural or anthropogenic alterations to the sedi-
ment budget (including both the sources and sinks of beach sediment) or the pro-
cesses that work on them. The main causes of beach erosion are as follows:
1. Reduction in sediment supply from eroding cliffs
2. Reduction of fluvial sediment supply to the coast
3. Reduction of sediment supply from the sea floor
