River improvement techniques for mitigating river bed degradation and channel width reduction in the sandy Hii River where sediment transport occurs at normal times

Takahisa Gotoh,Shoji Fukuoka,A Paquier,N Rivière

doi:10.1051/e3sconf/20184002033

Abstract

In the sandy Hii River, a large amount of sediment yield from upper river basin had brought developments of braided channels covered with sand waves. In the braided channels, sediment materials on the river beds are capable to move in normal discharge conditions. In recent years, however, the sediment yield decreases due to constructions of check dams and ground sills in the upper river basin. Thus, the river beds downstream of the ground sill have gradually degraded and the main channel widths have been narrowed with the progressing bed degradation. Firstly, we clarified that the effects of non-equilibrium sediment transports around the ground sill during normal discharge conditions on the bed degradation and the channel width reduction by using annual observed data and numerical simulations for bed variations. In addition, we provided the river improvement techniques for mitigating bed degradation and channel width reduction by improving state of non-equilibrium sediment transports passing through the ground sill.

Highlights

The Hii River is located at Shimane prefecture in Japan and flows into the Sea of Japan through the Lake Sinji and the Lake Naka-umi
It means that the river bed degradation has progressed due to effects of non-equilibrium sediment transports passing across the ground sill during the normal discharge conditions and small scale floods
We investigated mechanism of the bed degradation and the main channel width reduction in the sandy Hii River by using annual observed data

Summary

Introduction

The Hii River is located at Shimane prefecture in Japan and flows into the Sea of Japan through the Lake Sinji and the Lake Naka-umi (see Figure 1). The upper basin of the Hii. River had yielded a large amount of sediment composed of fine sand materials. River had yielded a large amount of sediment composed of fine sand materials It had caused great aggradation of the river bed in the lower Hii River until 1960s. In recent years, the sediment yield from the upper river basin has decreased due to constructing a series of check dams and ground sills since 1960s It has caused rapidly degradation of the river bed from the downstream of the ground sill in the lower Hii. River. We attempt to provide the idea of river improvement techniques for mitigating bed degradation and channel width narrowing in sandy braided rivers where sediment supplies from upper river basins are almost zero at normal discharge conditions

Mechanism of river bed degradation and channel width narrowing

The idea of river improvement technique

Calculation method and conditions

Calculation results

Conclusions