Relationship between Characteristics of Sediment Transport and Physical Environment in Riffles

Abstract

In order to restore river ecosystem, it is necessary to manage the sediment dynamics in rivers. Therefore, the forming process of microhabitat in a riffle was focused on, and sediment dynamics in it was analyzed.
In this study, the following three analyses were carried out in consideration of hierarchical structure of spatial scales. Firstly, it was analyzed how riffle shape was determined by the condition of a gravel bar that depends on the sediment condition. Next, it was investigated how the distribution of depth and velocity was in each riffle shape. Finally, the relationship between microhabitat condition on riffle bed and hydraulic condition was analyzed.
Consequently, the following characteristics were clarified. Riffles can be classified into three types; Convergence, Long, and Wide type. The shape is determined by the amount of sediment which determines the development of gravel bar. Each riffle has its original bed slope, cross-sectional shape and hydraulic condition. Moreover, thickness of the open work gravel on a riffle bed tends to be thick in a riffle that discharge is large. Therefore, the thickness of the surface layer is largest in a convergence type riffle that has the most diverse hydraulic condition. In terms of micro habitat in a riffle, the area with thick surface layer locates downstream side of a riffle. And also, the surface layer at both sides of riffle is thinner than that on a thalweg. Moreover it was clarified that it takes a few weeks to develop the gravel structure in a riffle after flood.