Principle of equivalency of bed structures and bed load motion

Abstract

Step-pool systems and cobble clusters are structures composed of boulders and cobbles on mountain streambeds rearranged by flood flow to reach high resistance and high bed stability. Both bed structures and bed load motion can protect the riverbed from incision. At each step-pool, the flow energy is transformed into turbulence, and finally into heat. Bed load motion also consumes flow energy and plays a role to protect the bed from erosion. The collision of bed load particles with the bed results in a force, known as dispersive force, which balances the lift force and controls the erosion of bed sediment. Field investigations and field experiments were conducted in the Xiaojiang River basin on the Yunnan-Guizhou plateau of China, where there were incised streams and stable streams with bed load motion or with step-pool systems. This study reveals that for a given stream power, strong bed structures are associated with low or zero bed load transportation; and weak or no bed structures are associated with intensive bed load motion. Experiments showed that for incised streams, the final bed profiles were the same if there was bed load motion or there were bed structures. When key stones that made up the bed structures, for instance the large boulders in steps, were removed, the flow immediately scoured the sediment bed. The bed load transportation sharply increased by 100 times and the median diameter of bed load increased by 2-20 times. Bed structures and bed load motion are mutually replaceable for their effects on flow energy consumption and streambed incision control. This is the principle of equivalency of bed load motion and bed structures. It is due to the principle that there was no bed load motion in the Yalutsangpo Grand Canyon, where a very strong step-pool system had developed, although the bed gradient and shear stress of flow were extremely high. A possible application of this principle for incision control of the downstream reaches of the Three Gorges Dam is also discussed in this paper.