Abstract: Neck cutoff is a sudden change phenomenon occurring in the morphological evolution of meandering rivers, but its processes and mechanisms have not yet formed a unified consensus. To reproduce its occurrence and processes, the method of testing a natural model in a laboratory flume is used in this work to study its behaviors under constant and variable flow conditions–the channel evolution before and after cutting off, the formation of the new cutoff channel, and the cutoff threshold. Experimental results show that whether the river discharge varies or not, the main factor that causes the cut off is bank scour upstream and downstream of the neck. The well-developed model meandering rivers in the flume are regularly featured with a sinuosity range of 6-7, a width ratio (neck width to river channel width) of about 0.43, and a channel bed of non-sticky sand. Under the conditions of flow rates per unit width of 0.0039-0.0069 m2/s and a longitudinal riverbed slope of 1‰, neck cutoff can be triggered after long time scour on the river bank. Just before cutting off, the local sinuosity of successive river bay is increased, and the planar channel shape rotates downstream; after cutting off, the new cutoff channel experiences three stages of time evolution–longitudinal scour, horizontal broadening, and relatively slow development.

Key words: meandering river, neck cutoff, variable discharge, bank scour, water level difference