Risk-Benefit Analysis Of Late-Term Diversion For High Dam Considering Early Water Impoundment

The high dam construction period spans several flood seasons.More than one year of flood control due to the large quantity of dam filling.The continuous improvement of construction technologies and management level of high dams in our country,give more opportunities to impound water earlier during construction compared with the original plan.Therefore,more and more projects chose to close the gates of diversion tunnels in advance to impound water earlier to obtain more generation gains.However,early water impoundment time has a certain impact on the flood control in late-term diversion,even increase the overtopping risk.Because the discharge capacity of diversion structures in lateterm is much lower than that of the early and mid-term,the upstream water level of the dam rises rapidly and the risk increases obviously.In addition,the high dam reservoirs are larger and the project is almost completed during the late-term diversion.Once the flood level exceeds design standard for water retaining the diversion failure will bring incalculable loss of life,as well as the property for unbuilt station itself and downstream urban and rural residents.Hence,early impounding water in construction can bring both considerable benefits and risk costs,which make the decision-making of diversion schemes for late-term construction more complex.Therefore,this paper took the high rockfill dam construction during flood period as the research object,and the early water impoundment was taken as the research premise.Firstly,the hydrological and hydraulic uncertainties were considered to predict the risk probability of the late-term construction diversion.Secondly,early water impoundment processes of schemes were calculated,and the days from initial store water level to the generation water level for the first unit of different schemes were compared,and then the increases of generation for each early impoundment schemes were estimated.Thirdly,combined with risk probability of diversion,the increased generation capacity and impounding durations were used to quantify the cost and benefits of early water impoundment.Finally,the multiobjective decision-making method was applied to the late-term diversion schemes,and the optimal late-term diversion scheme and start time of impoundment were obtained.Therefore,the main contents in this paper are as follows:(1)The diversion risk of late-term construction was calculated.Firstly,the definition of the diversion risk in late-term construction was given,and the late-term diversion risk model was established considering the boundary conditions of water storage and gate close.Secondly,the upstream water level of the dam was simulated considering the uncertainties of discharge capacity and flood.Finally,Monte-Carlo method was used to simulate the risk rate during the late-term construction of early water impoundment.(2)The cost and benefit indexes of late-term diversion schemes for early water impoundment were defined and quantified.Firstly,taking the initial impoundment of LHK hydropower station as the background,the schemes of 15 days and 30 days ahead of schedule were put forward during flood season,and their impoundment process was analyzed.Based on this,the times to reach the generation water level for the first unit of each scheme were deduced and the early generating benefits were estimated.Secondly,the submerged loss caused by overtopping was calculated based on the risk probability of the late-term diversion.Finally,according to the demand for early impoundment water,the cost for catching up was calculated by the production intensity method.(3)The benefit-cost evaluation model of late-term diversion schemes for early water impoundment was established,and the economic evaluation of late-term diversion schemes were carried out by using the economic benefitcost ratio evaluation index;The late-term diversion schemes of early water impoundment was regarded as a multi-objective decision-making of risk loss,rush-up cost and power generation benefit.The Delphi method and entropy method were used to determine the weight of each objective.Based on the concept of relative target schedule of gray target theory,the distance between the index value of each program and the target distance were calculated,and the late-term diversion schemes of early water impoundment were sorted in order to find out the best late-term diversion scheme.