Impacts of Stepped-Falling Base Level on the Flow Motion and Riverbed Evolution in a Sinuous Channel

Natural rivers are basically sinuous. Many natural events and human activities can trigger sudden base-level fall in a natural river. To discuss the impacts of base-level falls on the flow motions and riverbed evolutions in sinuous channels, a generalized sinuous river model with a stepped-falling base level was built, and the cross-sectional instantaneous velocities, cross-sectional instantaneous water levels and terrain elevations were measured. The experimental results showed that for a small base-level fall of -0.50 <= Delta z(br) <0.00, the absolute lateral water surface slopes in the arc segments increase with falling base level, and the cross-sectional circulation current strengths in the downstream regions neighboring to the apex-sectional outer banks do not exhibit obvious changes. For a large base-level fall of -1.00 <= Delta z(br) <= -0.75, the riverbed erosional extents in the straight-segment mainstream belts are small, those in the downstream regions neighboring to the apex-sectional outer banks are large, and the cross-sectional circulation current strengths in the downstream regions neighboring to the apex-sectional outer banks are greatly influenced by the lateral bed surface morphologies. When the channel bottom is unexposed, the raw elevation probability density within a complete wavelength of the sinuous channel is bimodal, but the detrended elevation probability density is unimodal and positively skewed. In any region of the sinuous channel, the mean roughness and the mean influencing scale of the particles around the bed surface increase with falling base level, and those in the stream-wise orientation are apparently greater than the counterparts in the span-wise orientation.