Fundamental study on the effects of the keystone height and structure on sediment deposition in a high‐gradient channel

Abstract

AbstractStep–pool structures are among the most important structures in riverbed morphology that can contribute to the stabilization of riverbeds and riverbanks, energy dissipation during floods, and head reduction function. Two concepts have been suggested regarding step–pool formation: the development of antidunes (antidune concept) and keystone concept. The latter suggests that a large boulder (keystone) incidentally left in a river channel can trap pebbles and build a step–pool structure. The ultimate goal of this study is to reveal the step–pool formation process and establish a design method based on the keystone concept. This paper describes the experimental results of the effects of the keystone arrangement on sediment deposition. Step–pool formation based on the keystone concept has previously been focused on the process of sediment deposition a short distance from a keystone. However, this study reveals that the obstruction of sediment supply due to the decreased flow velocity in the main river course is an important factor. When the arrangement interval of keystones is small, the amount of sediment supply downstream decreases because the flow velocity at the main flow channel between the keystones does not increase. In contrast, the flow velocity at the main river course increases between the keystones, and supplied sediment is transported to the most downstream section if the keystone arrangement interval is large enough to join the flow from the bank side of the keystones.