THE EFFECT OF ALLUVIAL FAN RIVERS’ BED MATERIAL SIZE DISTRIBUTION AND SORTING CHARACTERISTICS ON THEIR BED CONFIGURATION

Abstract

 Due to the various of evaluation methods on bed configuration of braided channel in alluvial fan rivers, the common and individual characteristics of bed configuration among alluvial fan rivers are still unclear. Therefore, we evaluated the bed configuration of 5 alluvial fan rivers in Japan, by using 3 indexes for braiding intensity: islands braiding intensity (number of islands per 1km), nodes braiding intensity (the total number of confluences, bifurcations and pools on bars per 1km), and node density of island. We divided channels into 3 sections according to sections of fanhead, mid-fan, and fanbase. We estimated 3 indexes of braiding intensity, measured the grain size of bed materials, bed slope, river width, and calculated the non-dimensional shear stress of representative grain-diameter (dr) under average annual maximum discharge (AAMD) in each section. The result shows that, in alluvial fan rivers, bed slope decreases discontinuously between sections from upstream to downstream, and at the same time bed material size sorting process occurs during the section transition. We classified these sections as s1-① which the channel lag deposits (CLD) consists of boulders, s1-② which the CLD consists of cobbles, and s2-① which doesn’t have CLD materials due to lack of cobble or boulder, and the main content of the bed materials is gravel. Moreover, in s1-②, the slope reduces gradually and cooperates with river width expanding, in order to adjust the non-dimensional shear stress of dr under AAMD near to the critical non-dimensional shear stress. Meantime, islands braiding intensity and nodes braiding intensity tend to be the maximum value among the sections.