Abstract
The river administrators often have difficulty with cohesive sediment accumulation/transportation and the vegetation management on the river bank faced to the Ariake sea. These phenomena occur a decrease of cross-sectional area of a river and a lowering of function as the flood flow discharge. Especially, the impediment of water flow caused by spreading of Phragmites australis can't be ignored in the maintaining of the river cross-section. Previous studies have shown the effectiveness of a storage pond and a shielding slab for interrupting progress of the underground root. In this study, four proto-type trenches as a new river structure for the river management are proposed and the field surveys were carried out in the Ushizu river of the Rokkaku river basin. As a result, it was indicated that the water storage trenches supposed for the vegetation management worked to prevent the growth of Phragmites australis. At that time, it will be necessary to maintain more than 0.3m in depth. And a small fence with 0.15 m height around the trench can prevent the inflow of fluid mud with the high concentration. In addition, the result of nutrient concentration makes clear that the influence of the water quality in the trenches is insignificant to the river. Finally, it was shown that series of trenches are useful for the vegetation control.
