Research On The Coordinated Control Model Of Flood Control And Water Conservation In Flood Season

As populations expand and economies grow, demand for water increases and the environmental degradation is intensified. Meanwhile, the climate change leads to more frequent hydrological extreme events, such as heavy precipitation, floods, and droughts. These all intensify the contradiction between water supply and water demand and put forward new challenges for water resources management. With the advanced science and technology, as well as taking advantage of the hydraulic engineering, improve the efficiency of flood water utilization while ensuring the safety of flood control is an effective non-structural measures to alleviate this contradiction. For the flood resources utilization, reservoir is the most important project to retain flood water for non-flood season, and the flood limited water level of the reservoir is the key indicator to compromise between flood control and water conservation for reservoir operation during the flood season. So far, a lot of researches have been done for the dynamic control of flood limited water level, however, some of the key issues need further quantitative research theoretically. Therefore, on the basis of the existing research, this paper focuses on the theoretical research of the competitive mechanism and the conversion relationship between flood control and water conservation during flood season, and establish the theoretical model and method of coordinated control of flood control and water conservation during flood season in order to improve the level of innovation and application. The main contents and results are as follows:(1) A two-stage optimization model is established to compromise the flood control risk and water conservation benefit during flood season, and the operation during flood season is converted to the hedging between water supply deficit at the current stage and the flood control risk in the future. Firstly, the reservoir operation procedure is generalized to a two-stage rolling over system. Secondly, the two-stage optimization model, to maximum the water conservation benefit at the current stage and minimize the flood control risk in the future, is established based on the analysis of the hydrological forecast information at the two stages and the forecast uncertainty in stage 2. And finally, combined the method of mathematical analysis and the economic principle, the economic meaning of the objective functions and different types of constraints is illustrated, and also the optimality condition of the model is derived, that is making the marginal utility of water conservation at the current stage and the marginal utility of flood control in the future as close as possible. In addition, taking advantage of the optimality condition and the economic characteristics of diminishing marginal benefit of the objective functions, an algorithm to solve the model is proposed.(2) The competitive mechanism and the conversion relationship between flood control and water conservation during flood season is explored. Firstly, the definition of the minimum safety margin ?min is given, and effects of forecast uncertainty and downstream flood control standard on ?min are analyzed. And then, the value of ?min can be characterized as large, medium and small, according to the relationship between marginal utility of water conservation and flood control with different levels of forecast uncertainty and flood control standard. Based on this, the competitive and conversion relationship between flood control and water conservation is analyzed, and the scope of the forecast inflow that there exists competitive relationship between two objectives is given quantitatively, and also the optimization decision within or outside the scope is presented, as describle as follows. If the forecast inflow is within the scope, then the optimality excess storage is to equalize the marginal utility of two objectives, otherwise, if the forecast inflow is above the scope, then either the flood control scheduling rule or water supply scheduling rule is followed, and if the forecast inflow is below the scope, then the water level can be raised to the upper bound. At then, the variation of optimal excess storage with forecast inflow, i.e., the hedging rule, for three cases can be derived.(3) The effects of weighting factor of the objectives and forecast uncertainty on the hedging between flood control and water conservation during flood season are analyzed, and method to determine the range of weighting factor that there exists hedging between two objectives and the maximum allowable forecast uncertainty are proposed. The analysis shows that:Firstly, the range of weighting factor that there exists hedging between two objectives decreases with forecast uncertainty. Secondly, the maximum allowable forecast uncertainty is related with downstream flood control standards, the magnitudes of forecast inflow and the weighting factor of the objectives, which is demonstrated as follows. The maximum allowable forecast uncertainty is smaller as forecast inflow or the flood control standards increases. And the effects of weighting factor is related with forecast inflow:when the forecast inflow is smaller than a given value, which is given analytically in the paper, then the maximum allowable forecast uncertainty is determined by the marginal utility principle, and it is larger if the decision maker is more inclined to water conservation benefit; otherwise, the maximum allowable forecast uncertainty is determined by the non-negative constraint of excess storage, that is the forecast uncertainty that flood storage is required to be all vacated and furthermore, it is smaller if the decision maker is more inclined to flood control.(4) The effects of forecast uncertainty at the current stage and the distribution of forecast error in the future are analyzed. The analysis shows that:Firstly, the forecast uncertainty at the current stage decreases the conservation benefits and increases the flood control risk in the future, and intensifies the competitive relationship between two objectives, and then more safety margin is reserved for the forecast uncertainty. Secondly, compared with the identical independent distribution, when the forecast error follows heteroskedastic distribution, there exists hedging between flood control and water conservation as forecast inflow is smaller, and the variation of optimal excess storage with forecast inflow is slower.(5) The Nierji Reservoir is selected as the case study to examine the rationality of the theoretical analysis conclusions. Firstly, the two-stage optimization model to balance flood control and water conservation is established, and the corresponding hedging rule is derived, and also the maximum allowable forecast uncertainty is presented based on the distribution of forecast error. Secondly, combined with the hedging rule, the flood control scheduling rule to evaluate the flood control risk and conservation benefit of the hedging rule. Lastly, the calculation results show that:the hedging rule can increase the water conservation benefits and hydroelectricity and decrease the spill water without lowering the primary flood control standards.Finally, a summary is given, and some problems to be further studied are discussed.