Calculation Of Flow Resistance Over Main Channel And Floodplain And Flood Routing Process Modelling In The Lower Yellow River

The Lower Yellow River(LYR)is characterized by complicated river morphology and flow and sediment processes,which show that the law of river resistance change is very complex and the movable bed resistance in main channel and floodplain resistance are not easy to determine.Flood routing modelling is an important research method for flood prediction in the main channel and floodplains of the LYR.In this paper,a typical reach of LYR are taken as was taken as an object of study.Various methods were adopted to calculate the movable resistance in the main channel and typical vegetation resistance and simulate the process of flood routing,including field data analysis,theoretical analysis,laboratory experiments and numerical simulation.Firstly,formulae were presented to calculate the movable resistance in the main channel and typical vegetation roughness on the floodplain area;and one-dimensional(1-D)and two-dimensional(2-D)morphodynamic models were respectively employed to simulate the routing of hyperconcentrated floods and overbank floods.The research results can not only enrich the basic theory of sediment motion mechanics and river simulation,but also provide technical support for the management of LYR.(i)Based on the combination of observed data analysis and theoretical analysis,formulae were presented to calculate the movable resistance in the main channel,which account for the characteristics bed form and flow and sediment transport in the LYR.Firstly,the effects of flow intensity,sediment concentration and relative roughness on the movable bed resistance were analyzed,and the accuracies of the typical movable bed roughness formulae were assessed and analyzed;then flow energy state partition was carried out by establishing the relation between Froude number(Fr)and the dimensionless parameter[log(h J/D₅₀)],and a power function relationship between the movable bed resistance coefficient and Fr and the relative water depth h/D₅₀ was presented.The movable bed resistance formulae were separately validated and verified by the 686 and 2288 sets of observed data in the LYR.The results show that the formula has a higher accuracy to estimate the roughness coefficient,and the correlation coefficient is higher than 0.8.(ii)Taking the typical vegetation in the LYR as an object of study,the flume experiments of muddy water with vegetation under different flow and sediment conditions were carried out,and the characteristics of longitudinal flow velocity distribution in the vertical direction and vegetation resistance were analyzed.The results show that when the vegetation is not submerged,the longitudinal flow velocity increases first and then decreases with an increase of relative water depth;When vegetation is submerged,the longitudinal flow velocity is S-shape distributed,which means the longitudinal flow velociy increases first,and then decreases,and then increases with an increase of relative water depth.The drag coefficient C_D decreases with an increase of Re,while the Darcy-Weissbach resistance coefficient f decreases first and then keeps stable with an increase of Re.Furthermore,the formula for calculating vegetation roughness n_b was presented by means of the multivariable correlation analysis,which is a power function relationship between n_b and Froude number Fr and the relative roughness H_v/H.(iii)A 1-D coupled morphodynamic model was developed,which can predict flood routing in the braided reach of LYR with complex cross-sectional geometry.In the model,the improved equations for hydrodynamics account for the effects of sediment concentration and bed evolution,which are coupled with the equations of non-equilibrium sediment transport and bed evolution.The model was then validated by the measurements of several hyperconcentrated flood events,and close agreement was obtained between the model predictions and the observed data.And the effects of different space increment and distribution modes on the modelling results was analyzed.The results show that the model predictions for hyperconcentrated floods based on the coupled solution were in better agreement with the observed data;Different space increments can influence the predicative accuracy of the proposed model.The effects on the simulated discharges and water levels were relatively lower,compared with the effects on the concentrations and channel evolution volumes.It was found that a suitable cross-sectional distance of less than 3 km should be adopted when simulating hyperconcentrated floods;The effects of different distribution modes on the simulated water levels are relatively lower,as compared with the effect on the discharges,concentrations and channel evolution volumes,especially for the reach undergoing severe deposition,and the results using the uniform allocation mode can agree better with measurements than other allocation modes.The new movable bed resistance formula was applied to the 1-D mathematical model for simulating the2004 hyperconcentrated floods,and close agreement was obtained between the model predictions and the observed data.(iv)Taking a typical braided reach from Jiahetan to Gaocun in LYR as an object of study,the sketched flume experiment and numerical simulation were both adopted to study the routing of farm dike-break induced overbank floods and the effects were investigated of different floodplain roughness degrees on water level,flow velocity and arrival times of wavefront routing.The results show that after the farm dike burst,the diffusion flood wave rapidly routed over the floodplain,which causes the surface-negative wave in the main channel and the surface-positive wave on the floodplain,and with the influence of transversal slope on the floodplains,the flood wave rapidly propagates in the transversal direction first,and then routes downstream along the dike.The floodplain roughness degrees have no significant effects on the breach discharge and overbank flood routing,but have a great influence on the arrival times of wavefront routing.The arrival times of wavefront routing on the floodplain increase gradually with an increase in floodplain roughness degrees,which decrease gradually with an increase in inflow discharge.(v)A 2-D hydrodynamic model using a TVD finite volume method and unstructured triangular mesh was presented for prediction of the routing of overbank floods over Lankao and Dongming floodplains.This model was verified against the previously reported experimental data of dam-break flows in a laboratory flume,and a favourable agreement was achieved between the model predictions and measurements.And the verified model was employed to simulate overbank flood events on Lankao and Dongming floodplains of LYR,and examined in detail the effects of mesh scale,local village and vegetation region roughness on the predictions.The results show that smaller mesh scales cannot achieve better results,but reasonable scales should be those that conform to the density of bed elevation points measured;Predictions of flood inundation by using center-hollowed village regions and different village roughness values are significantly different;The vegetable resistance has less effects on the water depth and significant impact on the flow velocity,and the thickness of bed deposition in vegetation area increase slightly under the condition of vegetation resistance.