IV. Retreating Shorelines
227
Breakwaters are however expensive structures and the lack of examples of
segmented systems act as a deterrent to funding. Yet they have been in use in
Japan for more than thirty years, generally built rather close to shore20. Others
are currenlty found in Denmark, France, Italy, Israel and Singaporte. In the
United States, several segmented breakwaters have been constructed in locations
spreading from Boston to Virginia, and on the Great Lakes.
Many projects combine sand suppletion with construction of hard defence
structures. This was done in Monaco, Israel and Singapore (fig. 7, 8 and 9,
section 1.3). This is a commonly recognized approach especially where suitable
material is available close-by.
It has often been suggested to counter recession of weak outcrop cliffs by building
and maintaining a broad beach, through the construction of groins that intercept
longshore drifting, and supplemented by beach nourishment on the foreshore or
updrift 2~. When such groins intercept longshore drifting erosion may, however,
be accentuated because sediment is withheld from the downdrift area, as has
happened in the Newhaven area (Sussex, Great Britain). With sea walls, fronting
beaches are rapidly depleted by scour due to the reflection of storm waves from
that wall.
However, in some cases, as for instance in Belgium, depending on predominant
wave direction governing the littoral drift, simple sand suppletion alone can
provide adequate coastal protection (fig. 26). In Westhampton, NY, when sand
addition was stopped but more groins added storms stripped the sand beyond the
groins field in downdrift direction.
3.1.2
Placement of Material
By adding new sand to a beach, but not to the immediate offshore area, the beach
slope may become out of equilibrium with such adjacent underwater area and
steepens shoreface profile. So as to flatten the profile, waves, during storms,
remove sand and the "shoreline" is again rolled back landward. This must be
carefully reckoned with when undertaking an artificial accretion project. The
slope of the "filled beach" should match as far as possible the slope of the original
beach.
Evaluation of the optimum position and method of placement of the nourishment,
sand is required and the number of choices is limited, but their consequences can
vary significantly. Basic choices consist in placing the nourishment material on
the natural dune line or on the dry back zone of the visible beach and into the
swach zone ; or most of the material in the swash zone and the rest into the water
out to the fine weather breaking line ; or still place all the nourishment materials
on the seabed offshore of the fine weather breaking line.
227
Breakwaters are however expensive structures and the lack of examples of
segmented systems act as a deterrent to funding. Yet they have been in use in
Japan for more than thirty years, generally built rather close to shore20. Others
are currenlty found in Denmark, France, Italy, Israel and Singaporte. In the
United States, several segmented breakwaters have been constructed in locations
spreading from Boston to Virginia, and on the Great Lakes.
Many projects combine sand suppletion with construction of hard defence
structures. This was done in Monaco, Israel and Singapore (fig. 7, 8 and 9,
section 1.3). This is a commonly recognized approach especially where suitable
material is available close-by.
It has often been suggested to counter recession of weak outcrop cliffs by building
and maintaining a broad beach, through the construction of groins that intercept
longshore drifting, and supplemented by beach nourishment on the foreshore or
updrift 2~. When such groins intercept longshore drifting erosion may, however,
be accentuated because sediment is withheld from the downdrift area, as has
happened in the Newhaven area (Sussex, Great Britain). With sea walls, fronting
beaches are rapidly depleted by scour due to the reflection of storm waves from
that wall.
However, in some cases, as for instance in Belgium, depending on predominant
wave direction governing the littoral drift, simple sand suppletion alone can
provide adequate coastal protection (fig. 26). In Westhampton, NY, when sand
addition was stopped but more groins added storms stripped the sand beyond the
groins field in downdrift direction.
3.1.2
Placement of Material
By adding new sand to a beach, but not to the immediate offshore area, the beach
slope may become out of equilibrium with such adjacent underwater area and
steepens shoreface profile. So as to flatten the profile, waves, during storms,
remove sand and the "shoreline" is again rolled back landward. This must be
carefully reckoned with when undertaking an artificial accretion project. The
slope of the "filled beach" should match as far as possible the slope of the original
beach.
Evaluation of the optimum position and method of placement of the nourishment,
sand is required and the number of choices is limited, but their consequences can
vary significantly. Basic choices consist in placing the nourishment material on
the natural dune line or on the dry back zone of the visible beach and into the
swach zone ; or most of the material in the swash zone and the rest into the water
out to the fine weather breaking line ; or still place all the nourishment materials
on the seabed offshore of the fine weather breaking line.