Experimental Study On Chute Cutoff Of Meandering River Under The Condition Of Overbank Flow

Natural cutoff is a critical geomorphic process and inevitable event in the long-term evolution of meandering rivers,which significantly reduces the sinuosity and morphological complexity of meander bends.Neck cutoff and chute cutoff are two types of natural cutoff,which have different trigger conditions,process and influence on water flow and river bed.At present,there is still a lack of experimental results on morphodynamic processes of chute cutoff.Therefore,the analysis of the evolution process of chute cutoff under different water flow and vegetation conditions is of great significance to understand the triggering conditions,mechanical mode and mechanism.The generalized movable bed flume experiment was carried out with the partial curved reach of the Haodou River,a tributary of Zequ River in the Source Region of the Yellow River as the prototype,aiming at the floodplain with/without grass vegetation.In this study,two schemes(RUN 1-RUN9 and RUN 10-RUN 14)were designed to study a total of 14 working conditions,with corresponding flow variation ranges of 1.5-4.5 L/s and 2.0-5.5 L/s,respectively.By studying the chute cutoff process under the dual influence of overbank flow and vegetation under the condition of constant flow,we analyzes river bed adjustment and plane morphology change of continuous bends before and after cutoff occurrence,and the direct effect of floodplain flow on chute cutoff of bends and its channel response are further revealed.(i)Chute cutoff is triggered by the overbank flow scouring of the cascade ditch at the neck,and the development position and process of the cascade ditch are different,and vegetation has an inhibitory effect on cutoff.In scheme 1(no vegetation),neck width increased with the increase of flow discharge,with a maximum increase of 107%,and the maximum erosion distance showed a linear increase trend,with a maximum increase of 39%.In scheme 2(with vegetation),under the condition of flow rate of 3.5-4.5 L/s,the neck width showed a linear narrowing with the fastest decreasing rate of 18.7 mm/h in the initial stage of the experiment,while the width showed a linear increase with the fastest widening rate of 129 mm/h under the condition of flow rate of 5.5 L/s.(ⅱ)The variation of water level at the neck of the cutoff section reflects the corresponding topographic change of river bed.Under the same discharge,the water depth of meandering channel with vegetation is larger than that without vegetation because of the flow resistance increased by vegetation effect.In the whole experiment process,the changes of water level on each section at the neck can be summarized as:the water level values of even sections show sinusoidal changes,while the water level values of odd sections show cosine changes,and it fits well with the sine(cosine)function model in Origin software.(ⅲ)There are different degrees of lateral shift in the position of the river centerline under various working conditions in the river bend.In the initial stage of the experiment,the lateral migration rate of the river centerline in each region reached 144 mm/h at the fastest.In scheme 1,under the condition of flow rate of 1.5-3.0 L/s,the scour area of the downstream estuary was larger than that of the upstream estuary under the condition of flow rate of 3.5-4.5 L/s.In scheme 2,under the condition of flow rate of 3.5 L/s,the scour area at each bend shows a linear trend,and the scouring area at the bottom of the "Ω" shaped bend is greater than that at the apex of the bend under the condition of flow rate of 4.0-5.5 L/s.