Calculation Of Optimal Ecological Flow For Fish Breeding In The Lower Reaches Of Jinsha River And The Benefit Evaluation Of Reservoir Operation

The lower reaches of the Jinsha River is the section with the richest hydropower resources in the Yangtze River.From upstream to downstream,four large hydropower stations,including Wudongde,Baihetan,Xiluodu and Xiangjiaba are planned to be constructed.While cascade hydropower stations exert huge economic benefits,they also changs the hydrological situation of the upstream and downstream rivers,thereby affecting the habitat of aquatic organisms,especially the fish at the top of the food chain.How to optimize the ecological flow to provide better habitat conditions for downstream fish while taking into account flood control and power generation is of great significance.In this paper,combined with the actual project,a study on the optimal scheduling of power generation for the lower Jinsha cascade hydropower stations based on the ecological flow required for fish spawning and reproduction is carried out.First,a variety of calculation methods are used to quantify the ecological flow required by the downstream river;secondly,the analytic hierarchy process was applied to evaluate the effect of the joint ecological operation of Xiluodu and Xiangjiaba reservoir,and the actual outflow and ecological flow process before and after the ecological operation were compared and analyzed;Finally,the optimal operation model of cascade reservoirs considering fish breeding is constructed,and the Hybrid Dynamic Programming Algorithm(DP-POA-DPS A)is used to solve the model,and the optimal operation schemes of cascade reservoirs in the lower Jinsha River considering different ecological flow constraints in different typical years are obtained.The main research contents are as follows:(1)In the aspect of river ecological flow calculation under the influence of water conservancy projects,the four traditional hydrological ecological flow calculation methods recommended in the relevant industry specifications are selected during the non sensitive period,and on this basis,a method is proposed that full considers the year and month of inflow,which is based on FDC method and combined with Tennant method based on the idea of equal difference sequence classification,the minimum value of the optimal ecological flow is calculated firstly,and then based on the different combination of annual and monthly inflow,the Copula function is used to construct the probability weight of monthly flood and drought under different annual inflow,and the optimal ecological flow of each month under different annual inflow is calculated.Taking the cascade hydropower stations in the lower reaches of Jinsha River as an example,the optimal ecological flow values of Xiluodu and Xiangjiaba controlled hydrological stations Pingshan and Wudongde and Baihetan controlled hydrological stations Qiaojia are calculated respectively,which provides better ecological flow constraints for ecological operation.(2)During the sensitive period of rare and endemic fish reproduction(March to June),the ecological flow required for fish spawning was quantified by constructing a physical habitat model.Taking the Yibin River section in the lower reaches of Xiangjiaba as an example,the Myxocyprinus asiaticus is selected as the representative of the spawning adhesive eggs fishes,and the Coreius heterodon is selected as the representative of the spawning pelagic eggs fishes.Based on the Physical Habitat Simulation Model(PHABSIM),the Weighted Usable Area(WUA)of habitat under different discharge of Xiangjiaba reservoir was quantified.The results show that:the weighted usable area of habitat increases first and then decreases slowly with the increase of discharge,it is inferred that the appropriate ecological flow range of Myxocyprinus asiaticus in Yibin river section is 2200-4200 m3/s,the best ecological flow is 3200 m3/s;and the appropriate ecological flow range of Coreius heterodon in Yibin river section is 2800-5300 m3/s,the best ecological flow is 3700 m3/s.(3)In the aspect of benefit evaluation of optimal operation of reservoir,in view of the lack of an authoritative and complete reservoir ecological dispatching evaluation index system,the cascade reservoir ecological dispatching evaluation index system has been further improved.Combining with the relevant specifications and literature,this paper proposes an evaluation index system for the effect of reservoir ecological regulation for fish reproduction suitable for the Jinsha River Basin.Taking the hydrological process joint ecological operation carried out by Xiluodu and Xiangjiaba as an example,the Analytic Hierarchy Process(AHP)is used to evaluate and score the effect of ecological operation in each year,and the results showed that the effect of ecological dispatching in 2018 was the best.At the same time,the discharge before and after the ecological operation of Xiluodu and Xiangjiaba reservoir is analyzed,and the results show that the discharge flow during the ecological regulation period is consistent with the valve range of the ecological flow during the high,normal and low seasons,and meets the optimal ecological flow required for the fish spawning and breeding period,which proves the rationality of ecological flow calculation.(4)In the aspect of improving the habitat of aquatic organisms in the downstream.an optimal operation method of cascade reservoirs is proposed by constructing an ecological operation model,which takes into account the breeding of fish in the drawdown of the Jinsha River.The dispatch model is solved by a hybrid dynamic programming algorithm(DP-POADPSA),and the optimal dispatch method of cascade reservoirs under different ecological flow constraints is obtained.The results show that the annual power generation benefit is the lowest when the ecological flow requirements in the operation specifications of hydropower stations are adopted.The results show that the annual power generation benefit is the lowest when the ecological flow requirements in the operation specifications of hydropower stations are adopted;the scheme based on the improved Copula function probability weighting method,the operation specifications of each hydropower station,and the Physical Habitat Simulation Model can obtain better economic benefits,and when the Xiluodu Reservoir has an ultra-low water level(the water level is 580m lower than the tail water level of the Baihetan Hydropower Station)during the ebb and flow period,the discharge of Baihetan hydropower station can provide a better habitat conditions for fish breeding in the downstream.