Author(s): Youichi Yasuda

Linked Author(s): Youichi Yasuda

Keywords: Stacked boulders; Energy dissipator; Migration route; Multiple Flows; Surface jet flow

Abstract: The low drop structure is obstacle for the migration of multi-aquatic animals during normal stages. Also, a local scouring is formed behind drop structure during flood stages, and it is easy to form river degradation. The roughness slope with stacked boulders is effective for both migration of multi-aquatic animals and energy dissipator. The roughness slope is recommended to be smaller than 1/8 slope. Also, the size of stacked boulder at long side is larger than 0.6 m in prototype. For the protection of river bed, the concrete block with hollow care type is installed behind the downstream end of roughness slope with stacked boulders. A parabola section in stacked boulders makes multiple flows near water side on stacked boulders. The top width of water surface changes with discharge. If the transverse slope becomes milder as 1/8, 1/10 slopes, a possibility of migration route can be kept for a wide range of discharge. In the transition from supercritical to subcritical flows behind roughness slope, a main flow concentrates at a center part of roughness slope with stacked boulders and a surface jet flow is formed. In this case, it is easy for multi-aquatic animals to find the migration route. During flood stages, as the roughness slope with stacked boulders is smaller than 1/8 slope, approaching Froude number is smaller than 2.5, the surface jet flow is always formed even if the tailwater elevation is varied. As the main flow lifts to water surface for the formation of the surface jet flow, the bed velocity can be reduced less than the velocity for sediment transport. If the main flow transits from supercritical to subcritical flows, it is easy to lift to the water surface because of the installation of protection block behind the roughness slope with stacked boulders. From view point of the construction for the roughness slope with a parabola section, the construction of stepped channel with sill is recommended, and it is easy to construct stacked boulders at each step. These results reveal from the experimental investigation by using 1/10 scale model installed in rectangular channel with 0.8 m wide, 15 m long, and 0.6 m height. Velocity and water surface measurements support the efficiency of roughness slope with stacked boulders as an energy dissipator. The structure was applied to drop structure for Sabo works in Oomu River, Yamanashi prefecture. The efficiency of the structure can be verified in field. This paper presents characteristics of the roughness slope with stacked boulders on basis of experimental and field works.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022642

Year: 2022