Bed load transport under different streambed conditions - a field experimental study in a mountain stream

Bed load transport in mountain streams is closely linked to streambed structures. Strambed structures are arrangements of boulders and cobbles deposited during extreme floods, in a stable configuration exhibiting high dissipation of flow. Field experiments were carried out in a mountain stream in Yunnan, southwestern China, studying bed load movement on three typical streambeds, i.e., with well developed, partially developed, and no structures. An underwater observation and video-capturing system was designed to observe and measure the movement of bed load particles. The initiation mode, trajectory, velocity, and acceleration of bed load particles under the three conditions were observed and analyzed. Results showed that the bed load movement was highly associated with streambed condition. With well-developed structures, bed load particles moved intermittently through saltation and the bed load transport rate was very low. For partially-developed structures most bed load particles moved through saltation but a portion of sediment moved in sliding and rolling. In the case with no streambed structure (plane bed) contact load motion (sliding and rolling) gradually became dominant. Moreover, laminated load motion occurred and became the main component of bed load transport when the flow discharge and incoming sediment load were very high. Laminated load motion was a special form of bed load motion with an extremely high intensity. Bed load transport and streambed structure both acted to dissipate flow energy and were mutually constraining. High rates of bed load transport occurred in the streams with no or poor bed structures, and low bed load transport was associated with well developed structures. The bed load transport rate was inversely correlated to the degree of streambed development.