Optimization For Water-Sediment Operation Of Reservoir And The Sediment-induced Hazard In River Systems In The Lower Yellow River

Mitigating sediment hazards in river systems is one of the key objectives of reservoir construction in silt-laden rivers.With the increasing impact of climate change and human activities,the runoff and its sediment carriage of rivers continue to change;the change in runoff and sediment situation will directly affect water use,hydroelectric power generation,navigation,river ecology and the operation of water conservancy projects.Therefore,it is of great importance to study the water-sediment optimal scheduling of reservoirs under different inflow and sediment scenarios in order to reduce the loss of sediment disaster,flood control,increase the social-ecological benefits of reservoir and restoration of river ecology.The Yellow River,the mother river of China,has historically been plagued by sediment hazards due to the unbalanced water-sediment relationship,with its downstream river-course known as"the hanging river".In recent years,soil and water conservation measures have been widely implemented in the middle river basin of the Yellow River,and soil erosion on the Loess Plateau has been effectively controlled,resulting in a significant decrease in both runoff and sediment inflow of the lower reaches of the Yellow River.The study of sediment hazards in the Lower Yellow River under the changing environment and the water-sediment optimal scheduling on this basis can be beneficial for the scientific management and sustainable development of the Lower Yellow River.Therefore,this study took the Lower Yellow River as the study area,analysed the historical runoff and sediment situation and sediment hazards in the river system,and carried out reservoir water-sediment optimisation scheduling.The main research work and conclusions of this study are shown as follows:(1)The runoff and sediment of the Lower Yellow River rivers were analysed by the MK test and the runoff-sediment double accumulation curve method,and the water-sediment series were divided into high runoff-sediment period of 1966 to 1979,medium runoff-sediment period of 1980to 1999 and low runoff-sediment period of 2000 to 2017.A new stochastic model model,namely MTSQM-EC,is proposed,which is constructed based on the multi-scale quantile mapping method and is able to improve the performance of the series at multiple time scales simultaneously and coupled with the Empirical Copula reordering method.The model is able to accurately reproduce the multi-timescale distribution properties,inter-series correlations and stochasticity of the measured series.The above stochastic simulation model was used to generate stochastic simulation sequences for three different runoff-sediment periods.(2)The cumulative effects of chain sediment hazards in river systems were analysed,a system of sediment hazard thresholds for river systems was constructed,and the response of reservoir,river and estuary sediment hazards to runoff and sediment was modelled respectively.The sediment hazard of reservoirs is mainly reflected in the loss of reservoir function due to siltation,which occurs when the reservoir sediment-trapping capacity is full or the reservoir water quality is below the design standard;the sediment hazard of river courses is mainly reflected in the reduction of river overflow capacity,main channel overflow capacity and main channel instability oscillation,which occurs when the overlevee flow is lower than the river bank flood control standard,the main channel overflow capacity is lower than the threshold of bankfull flow,and the main river channel oscillation is greater than the given threshold,respectively.When the critical flow level of the estuarine channel exceeds the threshold level for diversion,the estuarine channel diversion disaster occurs;when the estuarine delta area is reducing in a long period,the estuarine delta erosion disaster occurs.Based on the previous research,a statistical model of reservoir sand discharge capacity was constructed with incoming runoff,storage out ratio and flood duration as the independent variables,which fitted well in Xiaolangdi Reservoir;a model of bankfull flow and estuarine water level change based on the lagged response principle was constructed with runoff and incoming sand coefficient in flood seasons as the independent variables.The former model fitted well on the changes in the bankfull flow of Huayuankou,Gaocun,Sunkou and Lijin sections,while the latter can basically fit the changes in the water level of the Xihekou accurately.This study also constructed a statistical model of river siltation based on a generalised linear model which considers the effect of riverbed sand coarsening,which fitted well for the variation of river siltation in the Lower Yellow River;and constructed a model of the variation of the delta area of the estuary with the accumulated incoming sand in the estuary as the independent variable,which is in good agreement with the actual measurement results.(3)The runoff-sediment stochastic simulation series generated by the MTSQM-EC stochastic simulation model was used as the incoming runoff-sediment sequence of the reservoir.A reservoir water-sediment optimal scheduling model with reservoir siltation,river siltation,estuary siltation and power generation as the objective functions is established by combining the reservoir discharge capacity model,river channel siltation model and estuary water level change model,the NSGA-Ⅲ algorithm was used to solve the model to obtain the Pareto solution set,from which four extreme solutions(maximum power generation,minimum reservoir siltation,minimum river siltation and minimum risk of estuary diversion)and one compromise solution are selected and analysed.The results show that there is a significant competition of the reservoir siltation target to the river siltation target and the river diversion target,and a positive correlation between power generation and reservoir siltation target.Among the key sections of the river downstream,the Sunkou section has the fastest shrinkage rate of the bankfull flow and is more prone to sedimentation disasters.(4)According to the reservoir water-sediment optimization results,runoff-sediment scenario1(high runoff-sediment scenario)will bring about a greater siltation pressure on the river system,with a unit cubic metre of reservoir siltation being able to reduce siltation in the Lower Yellow River channel by 0.44-0.59 tonnes;the bankfull flow in the Sunkou and Gaocun will drop below the disaster threshold of 4000m~3/s,and the estuarine channel is at great risk of diversion,would reach the estuarine diversion threshold within 3-6 years.Runoff-sediment 2(medium runoff-sediment scenario)has a slightly lower siltation pressure on the river system,with a reduction of0.31-0.48 tonnes of siltation per unit cubic metre of reservoir siltation in the Lower Yellow River channel.The bankfull flow at the Huayuankou,Gaocun and Sunkou sections would fall below the disaster threshold within 30 years;the estuarine channels are at a greater risk of diversion,reaching the disaster threshold for estuarine diversion within 9-14 years.Runoff-sediment 3(low runoff-sediment scenario)has very low siltation pressure on the river system,with only 0.24-0.28tonnes of silt per cubic metre of reservoir siltation in the Lower Yellow River channel,and the bankfull flow keeps higher than the disaster threshold.The other scenarios have a slight increase in estuarine delta area;the estuarine water levels would not break the diversion threshold and the risk of estuarine diversion is low.