Geoscience Reference
In-Depth Information
(a)
Direction of natural beach sand transport
Dune
line
(b)
Groynes trap sand,
erosion accelerated
Former
shoreline
Dune
line
(c)
Progressive need for additional groynes
Remnants
of early
groyne
Groyne
Former
shoreline
Dune
line
Fig. 8.15
Typical impacts and temporal pattern of
groyne construction. (After Pilkey et al. 1998.)
2
edge erosion - where localized turbulence at
the end of the wall causes severe erosion;
3
potential grain-size changes - natural sedi-
ment sorting can be significantly altered by the
emplacement of a sea wall;
4
separation of dune and beach systems - where
a seawall removes any dune or back-beach
material from the active profile and precludes
incorporation of such sediment into the profile
during storms - this also causes loss of sediment
supply from the beach to adjacent dunes;
5
new littoral currents and sediment transport
may be generated by the presence of sea walls,
which in turn create additional problems for
sites further along the coast, and may threaten
the stability of the seawall itself.
Seawalls are commonly used to protect infra-
structure from long-term coastal retreat. In many
cases they are also used inappropriately to com-
bat seasonal erosion. More often than not this
induces irreversible changes on the coast. Perhaps
the greatest impact of seawalls is in fixing the
landward boundary of the coastal sediment sys-
tem. Under a rising sea-level (see 8.7) landward
migration of beaches is precluded. Thus beaches
backed by seawalls become narrower and steeper,
often disappearing altogether.
Offshore breakwaters are designed principally
to modify wave patterns and to produce sedi-
mentary effects on the shoreline. They absorb
wave energy before waves reach the shore.
This effect reduces onshore - offshore sediment
transport (especially during storms) but can, in
certain circumstances, reduce littoral drift and
create cuspate accumulations or tombolos as
a result of wave refraction effects. Attempts to
simulate natural log-spiral forms of headland-
embayment cells using artificial headlands
have been made using both attached and off-
shore headlands (Silvester 1976). Tombolos
and salients have also been produced through
installation of offshore structures (French 2001)
.
As with all engineering activities in the coastal
zone, unexpected coastal events can cause
undesired morphodynamic effects. Under storm
conditions, for example, the water layer above
the crest will no longer intercept energy from
incoming waves. The height of the breakwater
poses serious problems because if too low, in-
adequate protection will ensue, and if too high,
