| Almost all natural river flows are unsteady especially in food seasons. The riverbed experiences drastic transformation and sometimes dyke breaching occurs during the passage of a sharp ascending or descending flood. The study of the unsteady flows together with its influence on sediment transportation is of great significance both in theory and in engineering. The characters of unsteady flows as well as the corresponding bed-load transport have been studied systematically based on experimental evidence. The main results are: 1. A flume experimental system for studying unsteady flows is established. It has the ability to generate unsteady flow, to control and monitor water stage and discharge, and to measure velocity distribution and bed-load transport simultaneously. The system could produce stable and repeatable unsteady flows, which provides convenient circumstances for experiments on the kinetic rules, turbulent characters and sediment transport of unsteady flows in open channels. 2. Various methods to determine the friction velocity u* in steady uniform flows are analyzed and then compared based on experimental data. The quantity of u* directly affects many parameters such as the Karman constant κ, the integral constant B, the wake stress parameter Π, and the suspension index Z. 3. Various methods to obtain the mean and the fluctuating values of velocities in unsteady open-channel flows are analyzed and then compared. It's shown that it is unconvincing to draw a conclusion based on data within only one flow circle. The ensemble-average method proves reliable while others fail in dealing with data of high-frequency, fast-changing situations. The sampling duration of multiple waves should be larger than 1000 s to ensure accuracy. 4. Expressions of streamwise deformation of open channel flood streams are obtained based on experimental data. Preliminary analysis is proposed of the sharp tiding and tardy edd-tiding rules of the flood stream. The expressions representing the mutual relationships between current velocity~water stage and flux~occuring time of the maximum water stage are also obtained from the experimental data. Each item in Saint-Venant equation is compared using the experimental data. It shows that the continuous equation and the dynamic equation can not be satisfied when the stream condition varies intensely. 5. Based on the Saint-Venant equation and with some simplifications, expressions are deduced for the variation of unsteady discharge with water stage. The loop phenomenon in the rating curves of open channel flow is analyzed. The comparative loop size is affected by the unsteady parameter Γ. 6. The phases of stream velocity and turbulent intensities at various water depths basically keep synchro. Log-law is still feasible to describe the distribution of current velocities in unsteady open channel flows. The Reynold stress in the outer current field distributes linearly as in the steady flow. The variation tendency of turbulent fluctuation along the water depth is also similar to that of the steady flow. 7. The probability density in both streamwise and vertical directions are approximately in normal distribution, except in the near surface area where the water is shallow. In addition, the majority of longitudinal current velocity tends to positive derivation, whereas the vertical velocity is on the contrary. 8. For weakly sediment discharge in the present experiments, Einstein's equation is more accurate than the Meyer-Peter's equation. Due to the time lags between the friction velocity and the bed-load transport, the relation of sediment transport intensity Φand the flow parameter ψis no longer unique in unsteady flows. The long-term average sediment transport in unsteady flow is much more than the steady conditions.
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