Disaster prevention, environment, and utilization on sandy beach as shore protection facilities

自从人工沙滩发生下沉事故以来，日本基于故障保险概念建造了带有砾石滤层的人工海滩。如图1所示，暴风雨天气后，许多类型的砾石有时会从海底排出，随后沙滩可能会变成含有砾石的混合沙滩。因此，它们的安全性或稳定性的降低是主要的担忧。但目前还没有设计方法可用于设置砾石滤层的人工海滩。本研究的目的是提出设置砾石滤层的最佳条件；通过水槽试验来评估设置砾石滤层的深度和长度，并提出一种估算设置砾石滤层的坡面上爬高度的数值方法。得到了以下结论：(1)滤波层…更多的Er能促进泥沙淤积，减少泥沙侵蚀。(2)滤层不会阻碍海滩剖面的稳定，即近岸沙洲的发育或岸线附近水深的突然变化。(3)尽管海滩剖面的可逆性强烈依赖于潮位，但通过空间谱分析结果发现，滤层将促进可逆性。(4)由于海滩剖面的变化，滤层中的砾石不会暴露在河床上。(5)滤层内流速约为水域流速的10%，而两者的波位相变化基本相同。(6)滤层渗透性的增加并不一定会导致爬升高度的降低。较少

Artificial beaches with a gravel filter layer have been constructed based on the fail-safe concept in Japan since a sinking accident of artificial sand beaches has occurred. As shown in Fig.1, many types of gravel are sometimes discharged from the sea bottom after stormy weather, and subsequently the sand beach could change into the mixed sand beach with gravel. Therefore, a decrease of safety or stability on them is major concern. But no design methods are currently available for artificial beaches with a gravel filter layer. The objective of this study is to propose the optimum condition for a gravel filter layer ; assess the installation depth and length through experiments in a water flume, also develop a numerical approach to estimate run up height over the slope with a gravel filter layer.Focusing on invertibility of beach profiles, safety of coastal beaches with a filter layer was studied experimentally and theoretically. The following conclusions were obtained :(1) A filter lay … More er could promote the deposition of sediment and reduce the erosion of sediment. By setting the onshore edge of a filter layer in the position of shoreline at H.W.L., the above-mentioned process would be more promoted.(2) The filter layer does not have the tendency to obstruct the stability of beach profile i.e. the development of an offshore bar, or the sudden changes of water depth near the shoreline.(3) Though invertibility of beach profiles depends strongly on the tidal level, it was found through analytical results of the spatial spectrum that a filter layer would promote invertibility.(4) The gravels in a filter layer are not exposed to the bed due to changes of beach profiles. Even if exposed, the erosion of beach profiles could not be promoted easily.(5) The flow velocity in a filter layer was about 10% of that in the water region, while the wave phase variations of both are nearly same.(6) An increase in permeability of a filter layer does not necessarily cause a decrease of the runup height. Less