Numerical Simulation Of Flow Characteristics And Comparison Of Flow Calculation Methods In Compound Channel

Natural rivers are mostly compound channel,whose section consists of floodplain and main channel.The water mainly flows in the main channel in dry season,but during flood season,the water will overflow the floodplain.The interaction of flow occurs between the floodplain and main channel after the floods over the floodplain,which will change the velocity distribution of the section and the bed shear stress directly,and will lead to great error when calculating the flow of compound channel by means of Manning formula.In addition,the interaction will reduce the flood discharge capacity of the main channel and increases this capacity of the floodplain,which has a great impact on the safety of flood protection projects.Therefore,it is of great significance to study the hydraulic characteristics of the compound channel.Firstly,Reynolds stress model is used to simulate the flow characteristics of compound channel with different cross-section shapes.Then,by analyzing the function characteristics of discharge and water depth in Manning formula and combining with the results of numerical simulation,the causes of errors in Manning formula in calculating the relationship between water level and discharge are discussed.Next,12 calculation methods commonly used at present for the calculation of the discharge are summarized,and the effects of floodplain width,slope coefficient of main channel and water depth on the accuracy of these methods are analyzed.Finally,based on the experimental data of SERC-FCF,the accuracy of these methods in different cross-section channels are compared,and the optimal methods with wide application range,high accuracy and simple calculation are selected.The main conclusions are as follows:(1)The main channel vortices and the floodplain vortices with the opposite direction of motion will occur at the junction of the riverbank-channel when water floods over the floodplain,and the motion intensity of two vortices will increase with the increase of water depth.In addition,the steeper the main channel slope is,the stronger the secondary flow is,and there is a small eddy under the main channel vortices at the side wall of the main channel,which is opposite to the main channel eddy movement.Moreover,the smaller the width of the overbank,the greater the influence of secondary flow on the velocity distribution of this region.(2)The velocity isolines in the main channel are relatively uniform,the maximum velocity is on the free water surface.When approaching the side wall of the main channel,the bottom velocity isolines gradually bend to the free water surface,the isolines at the boundary of the floodplain and channel are inclined to protrude the water surface of the main channel,and the direction is consistent with the secondary flow at the very location.Furthermore,the steeper the slope of the main channel,the greater the curvature of the isolines;the smaller the width of the floodplain,the more uneven the mainstream isolines in this area.(3)From the distribution of transverse motion intensity,longitudinal velocity and vertical average velocity,it can be seen that the flow in the main channel or the overbank is less affected by momentum exchange,but the flow at the its junction is more.(4)The bed shear stress in the main channel is larger than that in the overbank.With the increase of water depth,the shear stress difference between the riverbank and the trough decreases gradually.The steeper the slope of the main channel,the more uniform the distribution of the shear stress in the main channel,the smaller the width of the beach,the distribution is more uneven in the floodplain.(5)When the wet perimeter and hydraulic radius of the channel satisfy the inequality of?R/R+0.6(??/?)(1+??/?)<0,the discharge-depth function in Manning formula decreases monotonously,and the relationship of water leincrease of water depth.The velocity difference between the floodplain and channel makes the vel and discharge decreases unreasonably with the water flow move transversely and forms a secondary flow at its junction,the results by Manning formula are smaller and the error is greatest when the water flow is just overflowing.With the gradual increase of water depth,the momentum exchange intensity increases,the flow tends to uniform flow,and the calculation error decreases gradually.(6)Water depth is the main factor affecting the calculation accuracy of the above methods,followed by beach width,and the slope is the smallest.The accuracy of CES and coordination methods are only affected by depth,and other methods are related to section shape besides depth.(7)CES method has higher accuracy and simpler operation compared with other methods,and its accuracy is not affected by section shape.Therefore,CES method is recommended for calculating the flow capacity of compound channel.