Effects of riverbed structure on transport rate of bed load in mountain streams

Riverbed structures, which affect bed load transportation in mountain streams, are structures of boulders and cobbles on mountain streambed rearranged by flood flow to reach high resistance and high bed stability. A parameter Sp was introduced to describe the intensity of a riverbed structure. A specially designed instrument was used to measure the riverbed structure intensity, and a double-box bed load sampler was used to measure the transport rate of bed load. Field measurements on the bed load transportation, riverbed structure and hydraulic features were conducted in 15 tributaries of Xiaojiang River in the upper Yangtze River Basin in China. It has been found that the transport rate of bed load is affected by both the unit stream power and riverbed structure. In rivers or reaches with a similar unit stream power, the higher the riverbed structure intensity (the bigger the Sp), the lower the transport intensity of bed load, while the smaller the Sp, the higher the transport intensity of bed load. When the unit stream power is less than 50 kg/(m&middots) and Sp larger than 0.35, the transport rate of bed load would be 0