Geoscience Reference
In-Depth Information
geomorphological forms (planform and profile)
in response to wave energy. If sediment is re-
moved from nearshore areas, the net effect is
to steepen the nearshore profile, which in turn
alters the relationship between beach morpho-
logy and wave dynamics. In certain cases, off-
shore breakwaters are constructed in front of the
newly nourished beach to help break up incom-
ing wave energy in order to reduce erosional
effects. Scarcity and cost of available sediment
is a major constraint on beach nourishment.
There is an inherent lack of accuracy in pre-
dicting the longevity of nourished beaches
and thus their cost-benefit relationship can be
difficult to assess (Pilkey & Dixon 1996). Beach
nourishment has the effect of widening the dry
part of the beach, which in turn may lead to
enhanced aeolian activity. Engineering history
has shown that there is a strong regional differ-
ence in the durability of nourished beaches along
the eastern coast of the USA. Leonard et al.
(1989) noted that beach durability was prob-
ably related to many factors, most of which
could not be isolated, but that storm frequency
and overall wave energy were probably particu-
larly important.
Other forms of soft engineering involve
the use of ecological elements to deliberately
induce morphological change. Examples include
vegetation planting on foredune systems to
induce additional sediment build-up (Woodhouse
1978) and to provide additional buffering against
storm attack. Similar effects can be achieved by
construction of various forms of wind trap that
encourage aeolian sediment to accumulate. In
Ireland, non-indigenous species (sea buckthorn,
laurel and sycamore) have been planted in sand
dune systems in order to stabilize blowouts
and block access. In certain cases a scenario of
'overmanagement' may develop in coastal sand
dunes. Attempts to stabilize the dune morpho-
logy have sealed the main landward corridors
of sediment transport. The US National Parks
Service artificially built up many foredune sys-
tems through trapping and marram planting on
barrier islands during the 1930s. This resulted
in a deficit in natural sediment and nutrient
throughput within the dunes. In turn, this induced
a massive reduction in washover and blow-over
processes and increased beach erosion on the
seaward margin. By the 1970s there was a major
reappraisal and a decision was made to allow
the barriers to readjust naturally.
8.5.2 Agricultural activities
Domestic animals (sheep, cattle, horses, etc.)
are frequently grazed on coastal dunes. The
impact on the dune surface depends on stocking
densities. Trampling and grazing can result in
vegetation damage. The resulting instability may
cause large-scale wind blow in dune systems. Low
stocking levels can promote vegetation stability
by stimulating vigorous growth via grazing and
the addition of manure. Rabbits were imported
for food into coastal dunes in Ireland from
the twelfth century onward. Rabbit burrowing
subsequently led to the destabilization of sand
dune systems and the dynamics of the rabbit
population became a major factor in dune system
dynamics. The myxomatosis-induced popula-
tion crash in the 1950s led to enhanced dune
stability. The presence of bare sand in dunes has
often been seen as an opportunity for large-scale
afforestation. In Spain and Portugal significant
stretches of coast have been planted with pine
trees, which have resulted in almost total block-
age of inland sediment movement.
8.5.3 Anthropogenic influence on
sediment supply
Human influences on sediment supply can be
substantial. The manner in which humans alter
sediment supply and transport at the coast
includes the following:
1
reduction or increase in sediment supply
through river regulation and land-use change;
2
loss of sand by dune destruction or stabilization;
3
loss of sediment through dredging (navigation
or aggregate extraction);
4
blocking of littoral transport by structures;
5
loss of sediment by stabilization of cliff sources.
In many cases, agricultural practices alter
sediment supply. In central Italy widespread
coastal erosion post-1950 was ascribed to gravel
