Morphodynamical Studies of Performance of Flood-control Levee Systems

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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