Environmental Engineering Reference
In-Depth Information
Management of the Adelaide beaches in South Australia also used monitor-
ing of changes in beach width and profile to determine where and how much sand
replenishment was needed (Fotheringham and Goodwins
1990
). Instrumental sur-
veys have been carried out since 1975 on a series of beach profiles spaced approxi-
mately 750 m apart, with closer monitoring where necessary at 50 m intervals. The
results were presented in the form of maps that shade areas with surface gains or
losses of between 0.2 and 1.0 m, and more than 1.0 m. More recently the data have
been processed using Geographical Information Systems to produce coloured con-
tour maps of the beaches, from which patterns of gain and loss can be identified,
and areas of developing deficit replenished by dumping sand (Noyce
1993
).
On the shores of the Great Lakes in North America monitoring has shown that
there have been changes on renourished beaches accompanying irregular oscilla-
tions in water level of up to 5 m over periods of several years. When lake lev-
els rise beach erosion occurs, and when they fall there is progradation. In 1974
175,000 m
3
of beach fill was emplaced on the shore of Michigan City, Lake
Michigan, and a further 61,000 m
3
in 1981. The emplaced beach profile was soon
modified by wave action, becoming relatively stable in relation to the variable lake
levels (Jansen
1985
). Renourished beach profiles thus adapt to hydrodynamic vari-
ations, often with a time lag of several weeks or months (Thompson
1987
). The
United States Army Corps of Engineers has since renourished beaches at several
other sites on the Lake Michigan coast, using coarse sand dredged from the lake
floor below the 5 m contour. Most of these beaches have remained in position even
during phases of high lake level (Macintosh and Anglin
1988
).
4.6 Assessment of Beach Performance
Changes on renourished beaches are often rapid, and there is disappointment and
criticism when an emplaced beach quickly diminishes. A completed beach renour-
ishment project typically lasts between 3 and 10 years depending on the site, pro-
ject plan, and number and intensity of storms (Weggel
1995
): this is referred to as
beach durability. The time between placement and loss of 50 % of the renourish-
ment volume has been termed the half-life (Leonard et al.
1990b
) and can be used
as a measure of beach durability when assessing the performance of a beach ren-
ourishment (Elko et al.
2005
; USACE
2002
).
There has been much discussion of beach renourishment performance, particularly
on the coasts of the United States. Before the 1950s beach renourishment projects on
the Atlantic coast were intuitive, without much planning or design, and there seemed
to be an assumption that the sandy beaches between New York and Miami were all
more or less the same. Subsequently more attention was given to scientific research,
acknowledging that there are variations in beach morphology, aspect, and nearshore
conditions as well as contrasts related to the location and dynamics of tidal inlets.
Most Atlantic coast beaches are of sand washed in from the sea floor, but there are
some fluvially-fed sectors and in the north some areas of cliff-derived beach sediment.
