Experimental Study On Streambed Structures And Bed Load Transport In An Incised Stream

The dissertation studies the impacts of streambed structures on the bed load transport and effects of an artificial streambed structure on channel incision control and ecological restoration through two field experiments in the Diaoga River, a tributatry mountain stream of Xiaojiang River in Southwest China.One experiment was conducted to investigate the bed load transport under unsteady flow conditions. The bed load transport rate was measured with a double-box pit trap sampler and the motion of bed load particles were observed by using a video-capturing instrument in a flume buried within the channel bed. The streambed structure was quantified by using a parameter SP, which was measured with a specially-designed instrument.The flow discharge and sediment transport respond well to the rainfall process in mountain stream, showing the feature of"drastic increase and dramatic fall". Bed load transport rate may be thousands times less after the first flood of the year than before under the same flow conditions, and the bed load transport rate versus stream power diagram shows a clockwise looped curve. The difference of incoming sediment also causes distinct discrepancy of total annual bed load between years.The bed load transport and the development degree of streambed structures in mountain streams is closely interrelated with each other. For streams with well-developed streambed structures, bed load transport rate is low, and particles moves mainly in saltation. For streams without streambed structures, both contact load motion and saltation load motion occur; and if the flow intensity and incoming sediment reach a critical high value, laminated load motion may occur and be the main part of bed load transport. Both the bed load transport and streambed structures consume flow energy; they act as resistance to the flow and are inter-restricted. Given a stream power, the bed load transport rate is inversely proportional to the development degree of streambed structures in a logarithmic coordinate system for the same stream reach. Step-pool system is the strongest bed structure in mountain streams on flow energy dissipation and bed incision control. In case of no naturally developed step-pool system, artificial step-pool system may be used. The second experiment was conducted by constructing an artificial step-pool system to study its effects on streambed incision control and aquatic ecology restoration. The experiment has been showing marked positive effects on these aspects. Initiation of bed particles have been controlled and bed load from upstream is partly stored in the experiment sections, therefore the stream bed incision in the experimental reach is effectively controlled, and consequently improves stream bed and bank slope stability. Different substrate, flow velocity and water depth environment alternating upstream steps and in pools creates spatially diversified stream habitats which enhances the development of the aquatic creatures. The habitat diversity index increases from 11 for natural channel to 42 nine months after the artificial step-pool system. The number density of individual benthic invertebrate and taxa richness (number of benthic invertebrate species) increased from 61.5/m2 and 17 to 5217/m2 and 22 respectively, indicating the stream ecology is obviously improved. The results of the field experiment show that, artificial step-pool system is worthwhile for further research, and is positive on restoration of incised mountain streams in China.