Abstract: 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.

Key words: meandering channel, base-level fall, riverbed evolution, armoring layer, elevation frequency distribution of bed surface