12
S.D.King
Coastal Defence Schemes
There is a distinction between coastal defence and coast protection, yet most schemes
do, in effect, serve to prevent both flooding and erosion. Nevertheless, there is one
fundamental difference that should always be remembered. Whereas alleviating
flooding can be permanent, “erosion is an on-going process that can normally only be
delayed” (Penning-Rowsell, 1992, p. 10). Defence mechanisms may increase erosion
rates, but they are not usually the sole cause.
Increased erosion rates can be a serious problem, and the poor design of
defence structures is often to blame. “By tradition engineers have relied on interposing
static structures between the sea and the shore” (Carter, 1988, p. 443) to prevent
flooding and erosion . Concrete walls tend to be durable, immovable and capable of
withstanding massive pressures. This type of defence is intended to be long-lived and
should be carefully engineered to specific site factors. These are often termed hard
defences, which armour the coast “and introduce materials alien [to it]” (Moutzouris,
1995, p. 154). Although hard defences undoubtedly protect the land behind them from
flooding and erosion, they can cause significant problems elsewhere. Sea walls are
static and conflict with dynamic beach changes (Carter, 1988). Wakelin argues that
“hard defences of all kinds carry within themselves the seeds of their own destruction”
(Wakelin, 1989, p. 140). Their greater reflectivity of wave energy, even where stepped,
causes greater turbulence under wave action, encouraging erosion of beach material on
the foreshore. Not only does this damage the beach, but erosion can also expose the toe
pilings and foundations of the wall. Solid sea walls also impede land-sea sediment
exchanges, preventing the sea from winning material that would normally be
transported further up the coast. Groynes “are wall like structures inserted perpendicular
to the beach to arrest material drifting alongshore” (Carter, 1988, p. 452). Groynes are
often seen as a panacea for all erosion problems, yet while they are useful under some
circumstances, they can be “positively harmful” (Wakelin, 1989, p. 141) in others.
There are two functional types of groyne:
1)
2)
3)
The anchor groyne.
The terminal groyne.
Rock revetment
The anchor groyne is designed to stabilise a length of shoreline by capturing a
proportion of the material drifting along the shore, or by retaining a quantity of
introduced fill. Once the material is caught, the design should allow for bypassing,
either over or around the groyne, so as to maintain drift. Problems arise where
bypassing does not occur, and sediment is prevented from reaching other parts of the
coast. All groynes can act like small jetties, effectively stopping long-shore transport
and enhancing erosion further down the coast. To date there appears to be little
favourable evidence to support the use of groynes; “the inescapable truth that groynes
are a static element within an essentially dynamic environment, mitigates against
optimising their effectiveness” (Carter, 1988, p. 453).
Rock revetments are large boulders, placed at the shoreline as a barrier to the
softer material behind. They have advantages in that they are not an alien material to the
S.D.King
Coastal Defence Schemes
There is a distinction between coastal defence and coast protection, yet most schemes
do, in effect, serve to prevent both flooding and erosion. Nevertheless, there is one
fundamental difference that should always be remembered. Whereas alleviating
flooding can be permanent, “erosion is an on-going process that can normally only be
delayed” (Penning-Rowsell, 1992, p. 10). Defence mechanisms may increase erosion
rates, but they are not usually the sole cause.
Increased erosion rates can be a serious problem, and the poor design of
defence structures is often to blame. “By tradition engineers have relied on interposing
static structures between the sea and the shore” (Carter, 1988, p. 443) to prevent
flooding and erosion . Concrete walls tend to be durable, immovable and capable of
withstanding massive pressures. This type of defence is intended to be long-lived and
should be carefully engineered to specific site factors. These are often termed hard
defences, which armour the coast “and introduce materials alien [to it]” (Moutzouris,
1995, p. 154). Although hard defences undoubtedly protect the land behind them from
flooding and erosion, they can cause significant problems elsewhere. Sea walls are
static and conflict with dynamic beach changes (Carter, 1988). Wakelin argues that
“hard defences of all kinds carry within themselves the seeds of their own destruction”
(Wakelin, 1989, p. 140). Their greater reflectivity of wave energy, even where stepped,
causes greater turbulence under wave action, encouraging erosion of beach material on
the foreshore. Not only does this damage the beach, but erosion can also expose the toe
pilings and foundations of the wall. Solid sea walls also impede land-sea sediment
exchanges, preventing the sea from winning material that would normally be
transported further up the coast. Groynes “are wall like structures inserted perpendicular
to the beach to arrest material drifting alongshore” (Carter, 1988, p. 452). Groynes are
often seen as a panacea for all erosion problems, yet while they are useful under some
circumstances, they can be “positively harmful” (Wakelin, 1989, p. 141) in others.
There are two functional types of groyne:
1)
2)
3)
The anchor groyne.
The terminal groyne.
Rock revetment
The anchor groyne is designed to stabilise a length of shoreline by capturing a
proportion of the material drifting along the shore, or by retaining a quantity of
introduced fill. Once the material is caught, the design should allow for bypassing,
either over or around the groyne, so as to maintain drift. Problems arise where
bypassing does not occur, and sediment is prevented from reaching other parts of the
coast. All groynes can act like small jetties, effectively stopping long-shore transport
and enhancing erosion further down the coast. To date there appears to be little
favourable evidence to support the use of groynes; “the inescapable truth that groynes
are a static element within an essentially dynamic environment, mitigates against
optimising their effectiveness” (Carter, 1988, p. 453).
Rock revetments are large boulders, placed at the shoreline as a barrier to the
softer material behind. They have advantages in that they are not an alien material to the