Application of the Ramp With Assembled Boulders to Prototype

In boulder movement area, many gravel control structures were installed, and multiple types of aquatic organisms have inhabited in the area. Generally, fish passages were installed for the upstream migration at each drop structure. But most of structures are obstacles for both upstream and downstream migrations. Recently, the author proposed the ramp with the consecutively assembled boulders, and flow velocity and flow condition on the assembled boulders during floods were investigated experimentally as a scale model, considering the Froude similarity. The experimental results revealed that the flow velocity near boulders was reduced by the formation of the seepage flow inside boulders and that a surface jet flow was formed downstream of the ramp. The most significant point for the application from scale model to prototype is the evaluation of the seepage flow inside boulders. Seepage flow velocity inside the boulders may depend not only on gravity but also on water viscosity. In prototype, five different ramps with the assembled boulders were constructed in accordance with the experimental results. The size of boulders was determined based on the size of boulders transported during big floods. Especially, if the averaged size of boulders is larger than 0.6 m, boulders with 0.1 m to 0.2 m were inserted in assembled large boulders. In this paper, the flow condition in five different ramps with the assembled boulders during normal stage is discussed based on the application from experimental results to prototype. Also, the construction of the assembled boulders is explained.