Studies on optimum design and conservation of artifical reefs

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The aim is to make clear the optimum design and conservation method to keep the man-made structures (e.g.artificial reefs) set in the shallow sea area from overturning and burying thorough the hydraulic model experiments. In particular the hydraulic problems concerning to this study are the wave control and tractive force which induces the sediment transport. Their hydraulic characteristics are revealed experimentally in this research(1) Ohashi and Kainou constructed the fixed bed model of the pocket beaches, which are commonly seen at the islands in the Seto Inland Sea, in the coastal wave test basin (20m*15m). The model is scaled down to one-fortieth of the prototype. Mainly to control the waves by setting the permeable offshore breakwater, which can function as fish reef, the experiments with the regular waves were conducted. From the experimental results, it was clear the effect of the improvement, was recognized only around the center of the breakwater. Because the existences of t … More he fishing port and the small promontory at both sides of the pocket beach affect largely the wave condition in it. Therefore, further improvement were examined. Considering the complex wave characteristics (e.g.three dimensional diffracted wave and the distribution of overtopping rate, and etc.), the optimal length and opening width of breakwater were finally decided.(2) Ohashi and Fujihara experimented the decay of wave height by setting the four types of reefs (step type and bar type with 30cm, 50cm, 70cm of crown width) in the irregular wave test tank. The experiments under two different energy levels with 7-9 cases of water level (2cm interval) were conducted and wave heights were measured at 8 points. The Optimal hydraulic conditions of each type of reefs as wave control structures were investigated from the experimental results (wave reflection ratio, significant wave height, maximum energy, total energy and set-up value).These results are useful not only for a defending measure when stormy, but also for a utilizing measure when calm from the point of leisure. Less