Experimental Study On Occurrence And Process Of Neck Cutoff In A Small-scale Meandering River

On the long-term evolution of meandering rivers on alluvial plains,deposition of inner bank and erosion of outer bank drive the river bay continuously squat laterally and creep longitudinally.Thus,the free-developed river bay undergoes a cyclical evolution of formation,development and cutoff.When the evolution of the river bay reaches the limit state,that is,the formation of the Ω shape on the plane,the occurrence of natural cutoff becomes a high probability event.Natural cutoff is an indispensable part in the evolution process of meandering river itself.Cutoff is also the abrupt inflection point of the evolution of the river bay,including neck cutoff and chute cutoff.This study only focuses on neck cutoff.Neck cutoff is such a process as follows.When the two adjacent curved tops on the Ω-shaped river are close together,the neck becomes very narrow.Under the continuous flooding or near shore scouring,the neck is scoured away or naturally penetrated into a new river.In this study,the wide-body flume experiments were carried out in the background of the Qigongling continuous bends at the end of the Jingjiang River.The neck cutoff was observed under different water flow and riverside vegetation conditions in the high-sinuosity river.Cutoff process and short-term adjustment of the channel were analyzed to explore the conditions and mechanisms for the occurrence of cutoff.The neck cutoff experiments consisted of three parts,namely experiments under constant flow,experiments based on step flow process and experiments under the condition of vegetation.Combined with numerical simulation,the development of the diversion of new river channel at different stages of neck cutoff was studied.The main results were as follows:(1)The neck cutoff was realized under condition of constant flow in the indoor experiments,and the process and mechanism of the neck cutoff were observed and researched.The type of neck cutoff was the bank scouring-penetrated cutoff.The reason led to neck cutoff was that the river banks of both the upstream and downstream of the neck were continuously scoured by the water and collapsed.Before cutoff,the change of the neck section experienced the river bank collapse stage,the scouring erosion stage and the flow connection stage.Neck cutoff was such a process.The bank slope foot on the upstream and downstream of neck was scoured by flow of both upstream and downstream.So that,the neck width was gradually reduced and width-to-depth ratio of uppstream and downstream sections increased.The water level and elevation of the river bed on both upstream and downstream of neck were gradually raised.When neck width was reduced to zero and the upstream water flowed over the neck,the upstream and downstream were connected and cutoff was achieved.The erosion rate of neck followed the high-low-high variation law.The river evolution took a long time before cutoff,and the neck bank was rapidly eroded near cutoff,erosion rate increased by 3.33-41.4 times and finally cutoff occurred in a short time.(2)Combined the change of stream power per unit length of bend under the condition of step flow and vegetation with the principle of minimum energy consumption of the river,the development of neck cutoff could be analyzed from the perspective of energy consumption.Cutoff was a process in which energy consumption adjusted gradually.When the relative equilibrium state of the high-sinuosity river was broken by external conditions,the channel entered the recession phase away from the relative equilibrium state.The channel was adjusted in the direction of cutoff,and the energy consumption of channel increased.The stream power per unit length was equivalent to the energy consumption.When the stream power per unit length reached a maximum value,the cutoff occurred as a sudden event.According to the experiment,on the high-sinuosity river with the bending coefficient of about 7.0 and bed load movement,the critical condition of neck cutoff caused by the collapse of river bank was that stream power per unit length on the river bay is 0.051-0.110 N·s-1.Compared to the relative equilibrium phase,stream power per unit length increased by 3.6%-110.8%,and bending coefficient increased by 1.2%-18.9%.After cutoff,new river experienced the formation stage and the widening stage,and then entered a new phase of relative equilibrium.Until the external conditions changed,the river wiould enter the recession stage again.and cycled like this.The energy consumption both in the development of cutoff and the new river was at a relatively high level.The energy consumption in the relative equilibrium phase was the smallest,and in the recession phase was gradually increased.For similar water and sediment conditions,the stream power values per unit length values in the river channel were similar when cutoff occurred.(3)The occurrence and process of neck cutoff under the condition of flow step change,the reason led to cutoff and the change of neck were the same as those of neck cutoff under constant flow.The change of flow mainly changed the erosion rate of the neck and the broadening rate of new river,but it did not affect the process of neck change before cutoff and the development process of the new river after cutoff.In the case of low discharge,since the average flow velocity of section was smaller than the starting velocity of sediment,the bank slope of neck did not undergo erosion and the neck width was constant.The erosion that contributed to the occurrence of cutoff occurred only during periods of middle and high flow.(4)The evolution of neck before cutoff under the action of vegetation experienced the river bank collapse stage,the scouring erosion stage,the overhead scouring stage and the flow connection stage.Compared with the experiment without vegetation,the neck evolution process added an additional shallow water overhead scouring stage.The role of vegetation was reflected in the following aspects.The lodging of the fallen vegetation stems on the edge of the river bank protected the bank slope,reducing the bank erosion rate,thereby suppressing the occurrence of cutoff and increasing the time required to achieve cutoff.Vegetation roots and sediments formed a root-soil complex weakening the erosion intensity of the river channel and reducing the width-to-depth ratio of the new river.Riverside vegetation reduced the water surface slope,which in turn increased the single width discharge required for sediment start-up,indirectly increased the flow required to achieve the cutoff.Therefore,the role of vegetation was equivalent to increasing the flow required to achieve the cutoff.(5)The numerical simulation study of the neck cutoff process confirmed that the flow and flow field of the new and old river channels were redistributed after the cutoff.The new river diversion ratio and width-depth ratio increased with the development of the cutoff,and the two had a linear positive correlation.