概化设计了一在出口处布设有高度可变的拦沙坎的弯曲河道模型,利用声学多普勒流速仪和数字波高仪测得13个特征断面的瞬时流速与瞬时水位数据,依托激光扫描技术获得试验床面的精确数字高程,以此来探讨侵蚀基准面下降对弯道河床演变的影响。结果发现:粗化层对河床有一定的保护作用,当侵蚀基准面小幅下降而导致流速小幅增大时,粗化层的保护作用可一定程度地抵减流速的增量,减弱甚至避免水流对河床的冲蚀;当侵蚀基准面下降幅度较大时,河床会被全局冲蚀,但各局部的被冲蚀程度并不相同,其中过渡段主流带的被冲蚀程度较小,弯段凹侧主流顶冲区的被冲蚀程度较大;越靠近侵蚀基准面的区域,侵蚀基准面的下降越易增强该区域的水流强度,越易使该区域的河床被冲蚀;在侵蚀基准面下降幅度较小且槽底未见裸露的情况下,连续弯道完整周期的床面高程频率分布总体上接近于正态分布,但若从细节上看其分布具有双峰特征;双峰特征为半周期床面高程频率分布所特有,完整周期床面高程频率分布的双峰特征只是半周期床面高程频率分布双峰特征叠加的结果;当侵蚀基准面下降至槽底时,主流顶冲区的槽底会严重裸露,连续弯道完整周期的床面高程频率分布仍具有双峰特征,但其总体形态已明显偏离正态分布。

A meandering channel model with variable-height sediment barriers at its outlet is designed to research the effect of base-level fall on riverbed evolution in a meandering riverway. The instantaneous flow velocity and water level data at 13 characteristic cross-sections and the digital elevations of bed surfaces were measured by an acoustic Doppler velocimeter and a digital water level altimeter and a handheld laser scanner, respectively. The experimental results showed that the static armoring layer protects the riverbed and resists the increasing flow intensity caused by a small extent of base-level fall. When the base-level falls to a larger extent, the riverbed will be eroded globally. But the erosion extent varies in local areas: the mainstream areas in the straight-line crossover segments are eroded slightly, while the mainstream head-on areas near the concave sides are eroded largely. The closer to the base level, the easier will the base-level fall enhance the flow intensity, and hence the easier the riverbed is to be eroded. When the base-level falls slightly and the flume bottom is not exposed, the elevation frequency distribution of bed surface in a complete meander is close to a normal distribution on the whole; but if we look into the details, the distribution presents a double peak. Double peak is specifically featured by the elevation frequency distribution of bed surface in a half meander; the double peak corresponding to a complete meander is merely a superposition generated by that of two half meanders. When the base level falls to the flume bottom, the flume bottoms near the mainstream head-on areas will be seriously exposed, and the elevation frequency distribution of bed surface in a complete meander also presents a double-peak characteristic, but its overall shape has obviously deviated from a normal distribution.