WAVE DAMPING EFFECT OF SUBMERGED DIKE

Makoto Nakamura, Hidehiko Shiraishi, Yasuo Sasaki

Abstract


Up to present, submerged dikes were mainly used as detatched breakwaters for artificial nourishment of beach or a front dikes to protect a rear main dike. However, the application of submerged dike to artificial creation of a fishing-ground has been recently taken into consideration with great interest. The following four points are considered as the characteristics of a submerged dike. (1) Wave pressure and wave force acting on a submerged dike is smaller than those on other type breakwater because its crown is below the still water level. (2) Submerged dikes damp comparatively great waves, while comparatively small waves freely pass over submerged dikes. This is the most suitable condition for the fishing-ground. Namely, slightly wavy surface of water in the fishing-groud is rather desirable from a view-point of fish ecology. (3) Submerged dikes are usually constructed at comparativel low cost because their heights are lower and wave pressure operating on them is smaller in comparison with other type breakwater. (4) Since the water depth on the crown of submerged dikes is the most important facter that determines the effect of submerged dikes against wave action, the wave damping effect of submerged dike varies according to chang of tidal level. As for study on submerged dikes, many researchers have already published the results of their investigations, such as the theoretical studies by Lamb, Jeffreys, Dean, Johnson and Fuchs, the investigation of trapezoidal and triangular underwater barriers by the B.E.B. (U.S.A.), that of rectangular barriers by Morison, that of horizontal barrier by Hein and that of submerged cylinder by Ursell. Moreover, Johnson has recently reexamined a submerged dike, and in Japan the report by Hosoi and Tominaga was already published. The causes of wave damping due to a submerged dike can be considered that it absorbs some of the incident wave energy by causing the waves to break. Some of the remaining energy is dissipated by reflection and friction on the crown of the submerged dike and some transmitted shoreward. In the published reports as previously mentioned, the transmission coefficient has been obtained mainly by the investigations of wave reflection due to the submerged dike. The theoretical analysis of breaking phenomena is very difficult because of its complication. However, the wave damping effect due to submerged dike is made remarkable by causing the wave to break on the submerged dike. From this point of view, this report mainly deals with the wave damping effect due to breaking on tbe submerged dike and offers the experimental data for the practical use.

Keywords


wave damping; submerged dike; dike

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