黄河下游水沙演进与洪水风险评估耦合模型

A coupled model for flow-sediment transport and flood risk assessment in the lower Yellow River

黄河泥沙主要来源于中游,而水沙灾害在下游最为严重,导致黄河下游河道治理与滩区发展矛盾十分突出。为此,构建了黄河下游水沙演进与洪水风险评估的耦合模型,包括长河段水沙演进的一维模型与局部河段二维水沙演进及洪水风险评估模型。对一维模型,嵌入改进后的动床阻力与水流挟沙力计算公式;对二维模型,采用无结构三角网格,能精确反映主槽及滩区的不规则边界,同时采用有限体积法求解水沙耦合控制方程,并嵌入滩区洪水风险评估模块。采用2017年低含沙洪水过程与2004年高含沙洪水过程分别对不同模块进行了验证。最后采用该耦合模型计算了1958型极端洪水过程中长河段及重点河段的水沙输移过程,并开展了不同治理模式下的滩区洪水风险评估。计算结果表明:1)在当前黄河下游河床边界条件下,洪峰沿程衰减较快。其中夹河滩至高村河段几乎全部漫滩,漫滩历时较长,平均为140 h,漫溢系数平均为1.5,是下游漫滩最严重的河段;2)在现状边界条件下,群众及农作物等受淹对象处于高风险区(风险等级大于0.85)的面积均最大。在防护堤模式下,群众处于高风险区的面积很小,占比仅为23%,但农作物处于高风险的面积较大。在三滩分治模式下,房屋及农作物处于高风险区面积均最小,高滩上仅7.3 km2区域中的人群将处于高洪水风险中。

Sediment of the Yellow River mainly comes from the middle reaches,but the severest flood and sediment hazards commonly occur in the lower Yellow River(LYR),which results in the contradiction between river management and floodplain development.In this paper,a coupled model is established for flow-sediment transport and flood risk assessment in the LYR,including a coupled 1D flow‐sediment evolution model for a long channel and a coupled 2D model of flow-sediment transport and flood risk assessment for a local reach.The improved formulas for bed roughness and sediment transport capacity were embedded in the 1D model.Unstructured triangular meshes were used to reflect the irregular boundaries of the main channel and the floodplain area in the 2D model.The finite volume method was adopted in the numerical solution of the module of flow-sediment transport,and additionally the flood risk assessment module was also embedded in the 2D model.The different models were validated using the measurements during the 2017 low sediment-laden flood event and the 2004 high sediment flood event.Finally,the coupled models were adopted to simulate the water and sediment transport processes of the long river and the key reaches during an extreme large flood event in 1958,and further evaluate the flood risks of the floodplain under different management modes.Results indicated that:(i)the flood peak discharge would rapidly reduce along the channel under the current riverbed boundary conditions of the lower Yellow River.The reach between Jiahetan and Gaocun was the most vulnerable area,with an average inundation time of 140 h and an average overbank flow coefficient of 1.5;(ii)the area of flooded objects such as people and crops in the highly vulnerable zones(risk level greater than 0.85)was the largest in the current condition without any management.The highly vulnerable area for people was the smallest,which accounted for 23% of the total area of floodplain,but the area of crops at high risk was large under the mode of defense levee.Under the mode of floodplain zoning treatment,the highly vulnerable areas of buildings and crops were the smallest,and the people in a small area of only 7.3 km2 in the high floodplain were in high flood risk.