IV. Retreating Shorelines
215
The idea has been also put forth by Beavis and Charlier (Int. Seaweed Symp., S~o
Paulo, Brazil, July-Aug. 1986), and others, to use artificial reefs as "natural
breakwaters" (fig. 14). Such algae as Macrocystis pyrifera (kelp) and Laminaria
ssp. appear particularly suited for the purpose.
Wave attenuation could be achieved by large kelp beds. Over 0.8 km (0.5 mi)
wide and several kilometers long, in water 15.25 m (50 fl) deep such beds could
theoreticMly reduce wave height by half as far as 0.8 km (0.5 mi) into the field.
They play a role comparable to that of porous breakwaters ~s°.
A natural-weed screen has been proposed as a means of stabilizing sand losses of
artificially nourished beaches. A short line of cages, containing e.g. Laminaria
sp., could break up rip currents that develop at the head of groins where
longshore drift may carry a bed load of sediments seaward (fig. 14).
As artificial nourishment buries the bottom end of groins, back- beach erosion is
increased. A weed screen could break up currents confluence and favor landward
sand accumulation ; additionally it contains subsequent storm-induced erosion :
grains carried seawards strand on the seaweed barrier leading to formation of a
sandbank in front of the screen. In short the beach nourishment result could be
naturally maintained.
As regards artificial seaweed ~sd, the following remarks are of interest"
The hydrodynamic response of artificial seaweed to surface gravity waves is
modeled analytically in a perturbation expansion. Because commercially
available seaweed is only weakly buoyant, the perturbation expansion involves
the density ratio, Dr/r, as the small parameter. The solution indicates that the
compliant fronds of the artificial seaweed are excited to oscillations which closely
approximate those of the oscillatory water motion. Because of this lack of relative
motion between the seaweed fronds and the water, dissipation of wave energy is
relatively small.
Relative motion between fronds and water is small, as their oscillations'
amplitude nears that of close-to-bottom water motion. The transport capacity
remaining about the same, sand accretion and/or erosion reduction is doubtful.
One could consider lengthening the fronds of artificial seaweed but this would
compound current problems with the mooring system.
However, because sediment transport is a highly non-linear function of the water
velocity, a small retardation of the bottom oscillations induced by artificial
seaweed can lead to a large reduction in sediment transport. The formation of
bars and shoals over the seaweed bed would consequently be expected.
215
The idea has been also put forth by Beavis and Charlier (Int. Seaweed Symp., S~o
Paulo, Brazil, July-Aug. 1986), and others, to use artificial reefs as "natural
breakwaters" (fig. 14). Such algae as Macrocystis pyrifera (kelp) and Laminaria
ssp. appear particularly suited for the purpose.
Wave attenuation could be achieved by large kelp beds. Over 0.8 km (0.5 mi)
wide and several kilometers long, in water 15.25 m (50 fl) deep such beds could
theoreticMly reduce wave height by half as far as 0.8 km (0.5 mi) into the field.
They play a role comparable to that of porous breakwaters ~s°.
A natural-weed screen has been proposed as a means of stabilizing sand losses of
artificially nourished beaches. A short line of cages, containing e.g. Laminaria
sp., could break up rip currents that develop at the head of groins where
longshore drift may carry a bed load of sediments seaward (fig. 14).
As artificial nourishment buries the bottom end of groins, back- beach erosion is
increased. A weed screen could break up currents confluence and favor landward
sand accumulation ; additionally it contains subsequent storm-induced erosion :
grains carried seawards strand on the seaweed barrier leading to formation of a
sandbank in front of the screen. In short the beach nourishment result could be
naturally maintained.
As regards artificial seaweed ~sd, the following remarks are of interest"
The hydrodynamic response of artificial seaweed to surface gravity waves is
modeled analytically in a perturbation expansion. Because commercially
available seaweed is only weakly buoyant, the perturbation expansion involves
the density ratio, Dr/r, as the small parameter. The solution indicates that the
compliant fronds of the artificial seaweed are excited to oscillations which closely
approximate those of the oscillatory water motion. Because of this lack of relative
motion between the seaweed fronds and the water, dissipation of wave energy is
relatively small.
Relative motion between fronds and water is small, as their oscillations'
amplitude nears that of close-to-bottom water motion. The transport capacity
remaining about the same, sand accretion and/or erosion reduction is doubtful.
One could consider lengthening the fronds of artificial seaweed but this would
compound current problems with the mooring system.
However, because sediment transport is a highly non-linear function of the water
velocity, a small retardation of the bottom oscillations induced by artificial
seaweed can lead to a large reduction in sediment transport. The formation of
bars and shoals over the seaweed bed would consequently be expected.