In recent years, disasters caused by typhoons, localized downpours, and other phenomena have occurred frequently, and concern over the risk of large-scale water damage caused by river flooding has risen. Although damage resulting from levee breach is particularly serious, there is a lack of technical data on the mechanism behind that phenomenon. Accordingly, this mechanism needs to be clarified to reduce the risk of flood damage. We examined the relationship between the quantification of the levee breach volume and hydraulic parameters. In the widening stage of levee breach, the breach phenomenon can be described in terms of the traction of soil masses from the levee body as a result of flood flow. It was also found that the relationship between the extent of dimensionless levee collapse and dimensionless traction force around the mouth of the breach can be expressed in a form similar to that of the bed load formula.

This study aims to propose a new numerical model for levee breach based on the results of relating experiments. Authors performed large-scale experiments of overtopping levee breach using the Chiyoda experimental flume located on the floodway of an actual river channel. By taking advantage of the scale of the flume, authors monitored the levee breach processes under highly precise hydraulic conditions, and the results showed a correlation between breached volume and shields number. Then authors applied the experimental results to a conventional two-dimensional shallow water with moving bed model, and found some limitations. Next, authors proposed a revised model based on the conventional model by integrating the experimental results into geomechanics, then obtained a fine reproduction result with regard to the levee breach widening during an actual flood of Yabe River in 2013.

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Tomonori SHIMADA, Daisuke TOBITA, Syunichi MAEDA, Kazuhisa KASHIWAYA and Hideaki YOKOHAMA 1 正会員 土木研究所寒地土木研究所寒地河川チーム(〒062-7602 北海道札幌市豊平区平岸1条3丁目)2 正会員 国土交通省北海道開発局帯広河川事務所(〒089-0536 北海道中川郡幕別町札内西町73-6)For the purpose of mitigating flood damage by overflow from a levee breached, the authors conducted an experiment using the full-scale levee at the Chiyoda Experimental Channel on the Tokachi River. The scale of the channel is 1300m long and 30m wide, accompanying a flood area of 80m width. In the experiment works, a group of blocks were installed on the slope of the levee 10 and 20 m downstream from the artificial breach, and water was let flow from upstream. After the water was let flow, when the breach progressed downstream to the concrete armor blocks, the blocks protected the failure levee. The downstream progress of breach was expected to be arrested by the concrete blocks which were expected to weaken the inundation flow from the channel. The control effects by the concrete blocks were examined.

Although damage resulting from levee breaches is particularly serious, there is a lack of technical data on the mechanism behind the phenomenon. Accordingly, the authors have conducted a series of levee breach experiments which were set up in the Chiyoda experimental flume. In this study, the authors focused on the sedimentation within the flood areas attached next to the experimental levees. The experimental conditions were variety of discharge, levee material, and levee shape. Using the results of the experiments, the authors clarified the relationship between the sediment distributions and the experimental conditions. Then, comparing the volume of breached levee and the accumulated sediment, the origin of the accumulated sediment was examined. The authors also conducted a numerical calculation using Nays2D model and identified the relationship of the plenary velocity distributions and the sediment distributions.