Morphodynamical Studies of Performance of Flood-control Levee Systems

近年来，流域城市化和极端气象事件增多的趋势加剧了日本的洪灾。然而，诚然，让每条河段都站在高堤旁边，即使在过高的洪水水位下也不会发生溢流，这是不现实的。这项研究的目的是解决如何在特定的易发洪水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准备。现有的纸质形态地图大多是在几十年前发布的，因此以某种方式与地理信息系统和互联网隔绝。此外，从这种现有的危险图中也无法读出或很少有相关流体动力学过程的线索。考虑到这些方面，本研究项目以一种相互互动的方式集中于以下主题：(1)与洪水相关的地形…的高分辨率成像更多的图形特征；从地貌动力学的角度详细调查了2004年丸山河决堤案件。实地研究确定了溢流和暴发引起的一系列沉积特征，并使相关的地形变化能够通过使用数字摄影经纬仪系统来量化。此外，还认真审查了已有30年历史的纸质危险地图的用处和局限性，该地图描绘了有关河流流域容易发生洪水的地貌特征。(2)开发液化/流态化泥沙流动三维分析代码；这项工作的目的是建立一个分析框架，据此将洪水引起的沉积特征与可操作的水动力过程联系起来。液化沉积物重力流二维分析程序(Sassa等人的LIQSEDFLOW，2003)已扩展为全三维版本，并应用于上述2004年的决堤案例。(3)通过地球物理/沉积学调查确定与地下洪水有关的特征；在过去100年左右洪水灾害频繁发生的乌吉河洪泛区进行了地球物理/沉积学联合调查。识别出的地下特征包括老河道、埋藏的沙坝和含有泥质碎屑的事件砂层。例如，该方法可用于评估旧通道填充的质量。较少

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