Morphodynamical Studies of Performance of Flood-control Levee Systems

近年来，江河流域的城市化和极端气象事件的增加趋势加剧了日本的洪涝灾害。然而，必须承认，让每条河段都有高堤，即使在过高的洪水位下也不会发生溢流，这是不现实的。本研究的目的是解决如何在洪水易发的低洼地区将极端洪水的后果降至最低的问题。一项有效的战略可能是通过相关的地理信息学促进对与水有关的灾害风险的防备。大多数现有的纸质形态地图都是几十年前出版的，因此在某种程度上与GIS和INTERNET隔绝。此外，从这些现有的危险图中无法读取相关的流体动力学过程的线索。考虑到这些方面，目前的研究项目以相互互动的方式重点关注以下主题。(1)洪水相关地形的高分辨率成像…更多的图形特征；从形态动力学的角度对2004年丸山河决堤事件进行了详细的研究。实地研究确定了一系列由溢流和溃决引起的沉积特征，并利用数字光电经纬仪系统对相关的地形变化进行了量化。此外，还对30年前的纸质灾害图的有用性和局限性进行了严格审查，该地图描绘了有关河流流域易发生洪水的形态特征。(2)制定了液化/流态化泥沙流动三维分析规范；这项工作的目的是建立一个分析框架，通过该框架将洪水诱发的沉积特征与可操作的水动力过程联系起来。对液化泥沙重力流的二维分析代码（LIQSEDFLOW, Sassa et al., 2003）扩展为全三维版本，并应用于上文所述的2004年决堤案例。(3)通过地球物理/沉积学调查识别地下洪水相关特征；在近100多年来洪水灾害频繁发生的乌济河漫滩进行了地球物理和沉积学联合调查。已识别的地下特征包括古河道、埋藏沙洲和含泥屑的事件砂层。例如，该方法可用于评估旧通道填充物的质量。少

The urbanization in river basins and increased trend of extreme meteorological events have recently intensified flood disasters in Japan. Admittedly, however, it is unrealistic to make every river segments sided by high levees in such a way that no overflow can occur even under excessively high flood stage. The purpose of this research is to address the issue as to how the consequences of extreme flooding can be minimized in given flood-prone low-lying areas. An effective strategy may be to promote preparedness for water-related hazard risks through relevant geoinformatics. Most of the existing morphological maps in paper format were published decades ago, thus being somehow isolated from GIS and the INTERNET. Also, no or little clues of related hydrodynamical processes can be read off from such existing hazard maps. With these aspects in mind, the present research project focused on the following themes in a mutually interactive fashion.(1)High-resolution Imaging of Flood-related Topo … More graphical Features ; The 2004 levee-breaching case with the Maruyama River was investigated in detail from a morphodynamical viewpoint. The field study identified a range of sedimentary features that were induced by the overflow and outburst and enabled the associated topographical changes to be quantified by making use of a digital photo-theodolite system. Also, critical examination was made of the usefulness and limitations of a thirty-year-old hazard map in paper format that depicted flood-prone morphological features in the river basin concerned.(2)Development of Three-dimensional Analysis Code for Liquefied/Fluidized Sediment Flows ; The aim of this undertaking was to develop an analysis framework by which to relate flood-induced sedimentary features to the hydrodynamical processes operational. A two-dimensional analysis code (LIQSEDFLOW by Sassa et al., 2003) for liquefied sediment gravity flows was extended to a fully three-dimensional version, with an application to the 2004 levee-breaching case stated above.(3)Identifying Subsurface Flood-related Features through Geophysical/Sedimentological Surveys ; Combined geophysical/sedimentological surveys have been performed in the Uji River floodplain where flood disasters frequently took place even in the past 100 years or so. The subsurface features identified include old channels, buried sand bars and event sand layers containing mud clasts. The methodology may be of use in assessing the quality of old-channel fills, for instance. Less