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few degrees, caused large-scale coastline retreat. Three sets of sedimentological information
supported this conclusion. First, the spit failed by reduction in supply to its distal end that was
not matched by reductions in supply at the proximal end. This was inconsistent with drift inter-
ruption at Rosslare Harbour. Second, the relative immaturity of the bay planform is evidenced
by observations of erosion and breaching before construction at Rosslare. The bay had not
reached equilibrium with the ambient wave field. Third, a gross estimate of longshore trans-
port supply from the eroding cliffs at Rosslare is approximately equal to the accumulation
rates recorded south of the harbour and therefore there is not a major reduction in longshore
sediment supply.
Orford (1988) also studied changes in nearshore bathymetry recorded on historic charts, and
changes in the estuary of Wexford Harbour. In the harbour, a large area of intertidal land had
been reclaimed in the mid-nineteenth century (Case Fig. 8.2d). The effect was to reduce the tidal
prism of the estuary. This, in turn altered the balance of tidal and wave power at the inlet and
caused large-scale sediment reorganization as the positions and strengths of the main flood- and
ebb-directed currents shifted. These ultimately caused the demise of the spit and the changes in
planform at Rosslare.
Relevant reading
Orford, J.D. (1988) Alternative interpretations of man-induced shoreline changes in Rosslare Bay, southeast
Ireland.
Transactions of the Institute of British Geographers
13
, 65-78.
or to improve navigation. There is often an asso-
ciated undesirable sedimentary consequence.
designs), all of which seek (with variable success)
to lessen their undesirable effects.
Jetties also extend normal or oblique to the
shore. Their purpose is to stabilize inlets and to
prevent inlet migration. In so doing, they inter-
rupt the longshore supply of sediment and cause
the ebb-tidal delta to be destroyed and/or shift
position. A range of ancillary activities usually
accompany jetty construction. These include
channel dredging and artificial bypassing of
sediment around the inlet.
Sea walls are shore-parallel structures placed
on a coast to prevent landward movement of the
shoreline or to act as flood defences. Many sea-
walls were built with the purpose of providing
promenades next to the sea. Various materials,
slope profiles and slope angles are used to absorb
wave energy. The sedimentary effects of seawalls
(Fig. 8.16) include:
1
beach lowering - where wave energy is
reflected from the wall and sediment is stripped
away from the beach surface in front of the
structure;
8.5.1.1 Hard engineering
Groynes are shore-normal or oblique structures
that intercept the longshore drift (Fig. 8.15). They
are frequently constructed in areas deprived of
their sediment supply by other human interven-
tions. They can be successful in trapping sediment
where longshore sediment transport is naturally
present (Komar 1998). At Cape May, New Jersey,
construction of groynes to intercept longshore
drift led to recession of the down-drift coast by
over 800 m (Pilkey & Dixon 1996). There have
been attempts to lessen the impact of groynes on
down-drift coasts. For example, the vertical pro-
files of some groynes have been adjusted as well
as their porosity in order to enable sediment
bypassing when the desired beach profile has
been achieved. A variety of different designs have
been used, including changing the angle to the
shore, and varying planforms (e.g. hammer-head
