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Coastal defence index
Shingle forms a natural coastal
defence. However, at many locations the supply of material to the
beach is either finite or limited, with little new sediment being supplied to
the systems. There is therefore a general tendency towards erosion of shingle
beaches and structures. Because of this the approach to beach management in
many location has favoured coastal
defence, including structures and actions designed to prevent erosion,
over-topping and the landward movement of shingle structures. An eroding (State
1) beach is caused by a complex set of interactions, which may include
both 'natural' factors such as:
- depletion of sediment sources (see guidance below);
- changes in shoreline morphology;
- changes in the pattern of waves and storms.
and human actions including:
- excavation of the sand and gravel both offshore and in the nearshore beach
environment;
- construction of harbour breakwaters, groynes and other structures;
- protection of eroding cliffs.
The construction of artificial coastal
defences can have potentially serious implications for coastal vegetated
shingle hence the following guidance is given. In addition it is importance
to recognise that natural shingle beaches perform an effective natural
coastal defence in their own right.
Guidance: Before embarking on any coastal
defence activity on a shingle beach or structure it is important to understand
the geomorphological context within which the action is proposed. There
is a general view that many present day shingle structures are derived
from deposits laid down during the early Holocene
some 7,000 to 10,000 years ago (Pye 2001)
and most systems appear to have a limited or non-existent contemporary
sediment supply. As a result, unless there is significant new
contemporary material being made available either from cliff retreat,
river borne shingles or offshore deposits then it is likely that erosion
may be inevitable. Not only are the human activities identified above
likely to be unsustainable in the medium to long term, but also they may
make the situation worse.
Where coastal defence structures are built then there are ways of limiting
the damage to coastal vegetated shingle. Guidance is given for new
schemes and mitigating damage during emergency
works. |
References
Pye, K.
2001.
The nature and geomorphology of coastal shingle.
In: Ecology & Geomorphology of Coastal Shingle, eds.Packham, J.R., Randall, R.E., Barnes, R.S.K. & Neal, A.Westbury Academic and Scientific Publishing, 2-22.
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