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This study aims to clarify the mechanism of riverine levee breach and propose a new 6 numerical model for that phenomenon. We performed large-scale experiments of overtopping breach 7 using an experimental flume located on the floodway of an actual river channel. By taking advantage 8 of the scale of the flume, we monitored the levee breach process with state-of-the-art observation 9 devices under highly precise hydraulic conditions. We performed four test cases with variations of 10 inflow rate, levee material and levee shape, and monitored the levee breach quantitatively using 11 acceleration sensors installed in the levee body. From the results of the experiments, we categorized 12 the breach process into four stages, focusing on the breach progress and hydraulic characteristics. 13We determined that the correlation between the breached volume and the hydraulic quantities of 14 velocity, water level and Shields number can be expressed by an equation similar to that for bed load 15 transport. Finally, we proposed a two-dimensional numerical model by integrating the experimental 16 results into geomechanics, and we obtained a fine reproduction result.

2正会員 工博 (独)土木研究所寒地土木研究所(〒062-8602 札幌市豊平区平岸1条3丁目) 3正会員 Ph.D (独)土木研究所寒地土木研究所(〒062-8602 札幌市豊平区平岸1条3丁目) 3非会員 工学 国土交通省北海道開発局帯広開発建設部(〒080-8585 帯広市西4条南8丁目) Authors did a movable bed and steady flow experiment at the Chiyoda Experimental Flume, a large scale flume located in the Chiyoda New Waterway of Tokachi River, and observed bed wave figure on the local bed materials. In this study, the steepness of the sand waves observed was first estimated using the Yalin-Karahan method, and found that the observed was only slightly less than the estimated value. Next, the normalized bed shear stress and normalized grain shear stress calculated from the observed values were compared, and were found to have similar tendencies as seen in the results of Kishi and Kuroki based on the hydraulic radius separation method. The bed shear stress was also estimated by calculating grain stress and form drag directly from sand wave forms with reference to the method of Kikkawa and Ishikawa etc., and the estimated values were found to be slightly smaller than the observed values.

The authors made erodible bed experiments under steady flow condition at the Chiyoda Experimental Flume, a large-scale facility constructed on the floodplain of the Tokachi River, and observed sand waves on the bed of the flume. In this study, the characteristics of the sand waves are examined along the longitudinal survey lines and confirmed to be dunes. Next, the authors estimated Manning's roughness coefficients from the observed hydraulic values and assumed that the rise of the coefficients attributed to the sand wave development. Finally, vertical flow distribution on the sand waves are examined, and observed velocity distribution on the crest of waves found to be explained by the logarithmic distribution theory.

Vegetation in rivers causes serious problems such as increases in flood risk due to water level rise during flood. Salicaceae trees are dominant on flood plains and sand bars of rivers in cold and snowy Hokkaido. Therefore, it is crucially important to inhibit the growth of salicaceae trees for river management. While there is a problem that salicaceae trees are flourishing in the Satsunai River, one plant species in the family salicaceae, chosenia arbutifolia, is losing its habitat. Chosenia arbutifolia is registered in the red data book of Japan. Hokkaido Development Bureau, the management authority, is performing field experiments to sweep away young salicaceae, and to form fluvial bars covered with gravel by artificial flash flood with the use of snow melting water released from Satsunaigawa Dam. Authors studied the habitat situations of young salicaceae and chosenia arbutifolia before and after an artificial flash flood, and relations between hydraulic conditions and habitat of the plants.