IV. Retreating Shorelines
207
Overtopping, viz. water passing over the structure, influences the degree of wave
reflection and consequently the standing wave's extent. The flatter the slope the
greater the influence of roughness. The amount of scour at the toe of a permeable
breakwater is allied withy degree of reflection, while the wave run-up is with the
standard wave developed which itself depends on toe water depth and, if small,
the slope of bed leading to the structure.
Wave transmission seaward can occur with marine structures that have water on
both sides (water-backed). They thus result in reflection, dissipation and partial
energy transmission landward. Permeable breakwaters fall in this class, with
waves of lesser height transmitted. So do submerged ones which allow
overtopping and reduce amplitude of waves.
Composite breakwaters, common in Japan, are rock mounds shaped to be a
foundation for monolithic concrete modules (boxes) floated into position, sunk
and filled with sediment or rock. A similar technique was used on Kislaya Bay
(Russian Federation) to construct the tidal power station.
Pile arrays may be considered "open breakwaters". That are series of closely
spaced piles, in a linear or diagonal arrangement, protecting against onslaught of
waves or as the support of a jetty. They attenuate the impact of waves.
A major project in Japan has been the development of effective permeable
offshore breakwaters at depths between 10 and 20 m. Six types of self sustained
permeable offshore breakwaters have been considered. [Takaaki Uda an Y.
Murai, 1988. Development of a self sustained permeable offshore breakwater.
Proc. Techno-Ocean '88 Symp. (Kobe, Japan), 146-153].
Mobile breakwaters rest on the sea-bed but may be easily removed. In the same
class fall pneumatic and hydraulic breakwaters. The first opposes currents
generated by near-bed released air bubbles which drag water to the surface as
they themselves rise. The flow divides and part goes towards incoming waves.
The latter two systems are more effective against short-period- and deep-waterwaves (J.L. Evans, 1955, Pneumatic and similar breakwaters : Proc. R. Soc.
London, A-231, 457-466) ; J.A. Williams & R.L. Wiegel, 1962, Attenuation of
wind waves by a hydraulic breakwater : Proc. 8th Conf. Coast. Engng, 500-520).
They are energy costly to operate.
Among the group of'portable" breakwaters, one may place the Leonard-Huspeth
membrane wave barrier. The long, sausage-like rubbery membranes, once filled
with sea-water can be used as a rigid temporary breakwater. However, itself
influencing the wave's impact strenght. Proposed in August 1986, it is still
undergoing laboratory testing.
207
Overtopping, viz. water passing over the structure, influences the degree of wave
reflection and consequently the standing wave's extent. The flatter the slope the
greater the influence of roughness. The amount of scour at the toe of a permeable
breakwater is allied withy degree of reflection, while the wave run-up is with the
standard wave developed which itself depends on toe water depth and, if small,
the slope of bed leading to the structure.
Wave transmission seaward can occur with marine structures that have water on
both sides (water-backed). They thus result in reflection, dissipation and partial
energy transmission landward. Permeable breakwaters fall in this class, with
waves of lesser height transmitted. So do submerged ones which allow
overtopping and reduce amplitude of waves.
Composite breakwaters, common in Japan, are rock mounds shaped to be a
foundation for monolithic concrete modules (boxes) floated into position, sunk
and filled with sediment or rock. A similar technique was used on Kislaya Bay
(Russian Federation) to construct the tidal power station.
Pile arrays may be considered "open breakwaters". That are series of closely
spaced piles, in a linear or diagonal arrangement, protecting against onslaught of
waves or as the support of a jetty. They attenuate the impact of waves.
A major project in Japan has been the development of effective permeable
offshore breakwaters at depths between 10 and 20 m. Six types of self sustained
permeable offshore breakwaters have been considered. [Takaaki Uda an Y.
Murai, 1988. Development of a self sustained permeable offshore breakwater.
Proc. Techno-Ocean '88 Symp. (Kobe, Japan), 146-153].
Mobile breakwaters rest on the sea-bed but may be easily removed. In the same
class fall pneumatic and hydraulic breakwaters. The first opposes currents
generated by near-bed released air bubbles which drag water to the surface as
they themselves rise. The flow divides and part goes towards incoming waves.
The latter two systems are more effective against short-period- and deep-waterwaves (J.L. Evans, 1955, Pneumatic and similar breakwaters : Proc. R. Soc.
London, A-231, 457-466) ; J.A. Williams & R.L. Wiegel, 1962, Attenuation of
wind waves by a hydraulic breakwater : Proc. 8th Conf. Coast. Engng, 500-520).
They are energy costly to operate.
Among the group of'portable" breakwaters, one may place the Leonard-Huspeth
membrane wave barrier. The long, sausage-like rubbery membranes, once filled
with sea-water can be used as a rigid temporary breakwater. However, itself
influencing the wave's impact strenght. Proposed in August 1986, it is still
undergoing laboratory testing.