Flood Limited Water Level Research Articles

The impact of reservoirs with seasonal flood limit water level on the frequency of downstream floods

Reservoirs alter the flood propagation process. Considering that many reservoirs have different flood limit water levels and flood control storage capacities during different flood seasons, we propose a modified reservoir index. In this study, we construct four types of non-stationary statistical models, taking time t, antecedent rainfall P, and the modified Reservoir Index MRI as candidate explanatory variables. Then we compare the simulation effects of different models. The equivalent reliability method and the extreme value distribution method considering the flood staging are proposed to calculate the flood design values at different return periods for downstream sites of the reservoir. The calculation results at the Huangzhuang Hydrological Station in the lower Hanjiang River in China show that the flood occurrence time in the upper Hanjiang River and its corresponding reservoir seasonal flood limit level and flood control storage capacity directly affect the flood in the downstream river. The non-stationary model incorporating P and MRI as explanatory variables achieves the best fit for the flood. Compared with the existing reservoir indices, MRI can better describe the impact of the upstream reservoir group on the flood sequence. The calculation results of the equivalent reliability method and the extreme value distribution method are comparable. The impact of the staged setting of flood limit water levels should be fully considered. The results of the equal reliability method and the extreme value distribution method can provide a reference for the design of flood control projects.

Optimal allocation of flood prevention storage and dynamic operation of water levels to increase cascade reservoir hydropower generation

The existing trade-off between flood control and water conservation measured by flood limited water level (FLWL) during flood season hinders multi-reservoir system from realizing comprehensive benefits. To overcome this barrier, we reasonably allocated the complementary flood prevention storage based on the aggregation-decomposition technique, theoretically deduced the mathematical formula of power generation variation simplified from the cascade reservoir operation model, and dynamically operated reservoir water levels considering the inflow forecasting information. The Wudongde (WDD) ∼ Baihetan (BHT) ∼ Xiluodu (XLD) ∼ Xiangjiaba (XJB) cascade reservoirs in the downstream Jinsha River in China is selected as the case study. Results demonstrate that: (1) The derived power generation function monotonically increases only with the growth of the pre-discharge water volume. (2) The reduced proportion of the flood prevention storage of the WDD (or XJB) is favorable to boost the hydroelectric benefits of the WDD ∼ BHT (or XLD ∼ XJB) cascade reservoirs. Optimal allocation of flood prevention storage can increase 1.933 billion kW∙h power generation annually (+2.15 %) for the WDD ∼ BHT ∼ XLD ∼ XJB cascade reservoirs. (3) Further cooperated with dynamic operation of water levels, the WDD ∼ BHT ∼ XLD ∼ XJB cascade reservoirs can totally generate 5.925 billion kW∙h more hydropower annually (+6.60 %) using effective 5-day lead-time inflow forecasting information, while maintaining the design flood prevention standards.

The Influence of Time Domain on Flood Season Segmentation by the Fisher Optimal Partition Method

Setting the staged flood limit water level (FLWL) through flood season staging is an important means of fully utilizing reservoir flood resources. The widely-used Fisher optimal partition method requires a certain time domain as the basic unit in determining the optimal staging of a flood season. Currently, 5 and 10 days matching the month and solar terms are usually used as the time unit. This study aimed to analyze the influence of other time-domain units (7 and 15 days) that meet the relevant requirements on the staging results and to provide a scientific basis for the selection of time-domain units in flood season staging. The rationality of the staging scheme was tested using the improved Cunderlik method, and the influence of specific basic units in the Fisher optimal partition method on the staging results was evaluated. The highest relative superiority of 0.9876 was found for 5 d, indicating that this is a suitable time-domain unit. The optimal staging result was determined as 20 June for the first segmentation point and 20 August for the second. A comparison of the staged FLWL with a single fixed FLWL showed that the water level was raised by 1.56 m in the pre-flood season, 0.65 m in the main flood season, and 1.37 m in the post-flood season. Water storage increased by 12.79 million m3 during the flood season, effectively alleviating the mismatch between water supply and storage.

Research on Flood Risk Control Methods and Reservoir Flood Control Operation Oriented towards Floodwater Utilization

Since improving floodwater utilization may increase flood risk, flood risk control methods for trade-offs between these factors have research value. This study presented a flood risk control method oriented towards floodwater utilization which considers multiple main flood risk factors. The proposed method not only achieves the boundaries of the flood limited water level (FLWL) under various acceptable risks but also dynamically controls the water level to enhance floodwater utilization. A case study conducted on the Danjiangkou reservoir yielded the following results: (1) The proposed method provides FLWL dynamic control boundaries under various acceptable risks. (2) The proposed method reveals the potential to raise the FLWL, with a possibility to raise it by 1.00 m above the present FLWL under the absence of flood risk. (3) The available flood resources in both the wet and dry seasons increase, on average, by 0.83 and 0.81 billion m3, and the flood risk remains within the acceptable range after raising the FLWL by 1.00 m, which contributes to enhancing floodwater utilization.

Dynamic Control of Flood Limited Water Levels for Parallel Reservoirs by Considering Forecast Period Uncertainty

The objective of this study is to achieve the dynamic optimization of the flood limited water level (FLWL) in parallel reservoirs, using Luhun Reservoir and Guxian Reservoir as case studies. The innovation lies in establishing a dynamic control optimization model for the FLWL of parallel reservoirs, considering the uncertainty in the forecasting period of the flood forecast due to the varying locations of the rainstorm center from upstream to downstream. To commence, the Fisher optimal segmentation method is employed for flood season staging to determine the staged FLWL of each reservoir. Subsequently, considering the uncertainty in the foresight period, the upper range of the dynamic FLWL is determined through the improved pre-discharge capacity constraint method and Monte Carlo simulation. Finally, a multi-objective optimization model is established to determine the optimal dynamic FLWL control operation scheme for parallel reservoirs, utilizing the Non-Dominated Sorting Genetic Algorithm II (NSGA-II). This model takes into account both downstream flood control requirements and the water supply benefits of the parallel reservoirs. Through the optimization of the scheme, the water supply of the parallel reservoirs can be augmented by 15,347.6 m3 during the flood season. This optimization effectively achieves a harmonious balance between flood control and water supply, holding significant implications for mitigating drought risks amid changing conditions.

Theory and practice of basin-wide floodwater utilization: typical implementing measures in China

Floodwater utilization (FU) helps alleviate water shortages in regions with uneven rainfall. This study focuses on basin-wide FU, considering diverse water infrastructure and the delicate balance between flood risks and water scarcity. FU measures including optimizing reservoir flood limited water level (FLWL), extending the use of flood detention areas, and compensatory use of channel capacities, were reviewed and exemplified by real-world applications in China. The proposed procedure for implementing FU involves assessing FU potential, selecting appropriate utilization methods, and conducting a comprehensive evaluation of the associated risks and benefits. Using the Nansi Lake basin in China, as a synthesis example, the current-level FU potential and FU rate are estimated to be 1.47 km3/yr and 65%. This FU value exceeds the theoretical potential, indicating the severe crowding out of ecological water uses. Multi-stage FLWL through raising the post-flood FLWL from 34.20 m to 34.35 m could increase the reservoir (the upper Nansi Lake) ability to refill by 19.1%. Appropriate control of the sluices of interconnected rivers could conserve up to 0.135 km3 of floodwater from a single flood event without increasing flood risk.

Optimizing the Flood Limit Water Level of Reservoirs in Sediment-Laden Rivers under Changing Water and Sediment Conditions: A Case Study of the Xiaolangdi Reservoir

Optimizing the flood limit water level (FLWL) of reservoirs in sediment-laden rivers under changing water and sediment conditions is an important research topic that could improve comprehensive utilization benefits. Because reservoir operation has multiple objectives in sediment-laden rivers, this study established a water–sediment mathematical model, a comprehensive benefit evaluation model, and an evaluation index system. Taking the Xiaolangdi Reservoir of the Yellow River as an example, the operation mode of the FLWL under changing water and sediment conditions was studied. Under the scenarios of incoming sediment amounts of 300–800 million tons, when using the operation mode of gradually raising the FLWL, the sediment retention period was 4–13 years longer; the lower average annual siltation of the downstream channel and minimum bank-full discharge of the downstream channel after 50 years was larger by 150–260 m3/s than the operation mode of raising the FLWL at one time. However, with enhanced benefits of sediment blocking and siltation reduction, other benefits such as water resources supply, hydropower generation, and ecological improvement are reduced. The average annual number of days that do not meet the downstream water resources supply requirements, irrigation, and ecological improvement was increased by 0.64–2.16 days, and 91–197 million kW·h reduced average annual hydropower generation. The critical amount of incoming sediment was 350 million for conversion between the two FLWL operation modes, and it will increase to 450 million tons if the incoming runoff of the Yellow River increases by 20%. After constructing the Guxian Reservoir in the middle of the Yellow River, the critical amount of incoming sediment will increase to 600 million tons. This study is of great significance for improving the utilization efficiency of water resources and promoting the socio-economic development of river basins.

Instability of Embankment Slopes Due to Overflow and Drawdown

Establishing a geotechnical rationale for an embankment overflow is challenging. The occurrence of embankment overflow is deemed unlikely since the initial highest water level of the embankment is assumed to be fixed as a designed flood water level or a seasonal flood-limited water level when calculating the rainfall amount in the hydrological stability analysis. However, the possibility of overtopping can be assessed by employing copula functions. This approach takes into account the return frequency of overflow and the fluctuation of probable water levels due to climate change rather than relying on the fixed highest water levels of an embankment. This study examined the effect of tensile cracks leading to the shallow failure of embankment slopes during drawdown and placed an emphasis on the vulnerability of embankment slopes due to extreme rainfall events. Throughout the comprehensive numerical simulations, it was found that the critical period for embankment slope collapse was immediately after drawdown for the upstream slope and during the initial stages of overflow for the downstream slope.

Flood Risk Quantification, Transmission, and Propagation Analysis for Flood Water Utilization of Parallel Reservoirs

Joint operation of the reservoir system can greatly improve flood water utilization and alleviate the contradiction between water supply and water demand. But yet, the flood risk brought by forecast uncertainties transfers between the reservoirs and propagates from upstream to downstream, which is crucial for flood water utilization but is rarely investigated. To address this issue, this study proposes a FLWL optimal control framework, which consists of three modules: the FLWL upper bound derivation module, the flood risk analysis module, and the optimal FLWL decision-making module. The first module is to derive the upper bound relationship of the flood limited water level (FLWL), i.e., the specific substitution relationship of storage, for the parallel reservoirs coupling the capacity-constrained pre-release method and the aggregation-decomposition method. In the second module, the method to calculate the flood risk of the reservoirs and downstream caused by rainfall and runoff forecast uncertainties based on the total probability formula is proposed. In the third module, the transmission and propagation of flood risks in the parallel reservoir system under different rainfall grades are quantified, based on which the optimal FLWLs are determined. The implication in the Qinghe-Chaihe parallel reservoir system located in Northeast China shows that FLWL in Qinghe Reservoir and Chaihe Reservoir can be raised to 129.5 m from 127 m and 108 m from 104 m, respectively. Controlling the FLWL at any point in the safety domain can improve both flood water utilization and flood control benefits. However, the flood risk nonlinearly transfers between the parallel reservoirs and nonlinearly propagates from the upstream to the downstream. Based on this relationship, the optimal FLWLs of the reservoirs under different decision preferences are recommended.

A Joint Dispatch Operation Method of Hydropower and Photovoltaic: Based on the Two-Stage Hedging Model

The randomness and volatility of large-scale clean energy output represented by wind power and photovoltaic lead to difficulties in grid connection. The problems of abandoned wind, light, and water become increasingly prominent. The adjustment capacity of traditional thermal power is limited and it is difficult to ensure the consumption of high proportion clean energy. On this basis, the marginal benefit hedging rule in economics is introduced into the hydropower and photovoltaic joint operation system in this paper. A two-stage spatio-temporal hedging strategy is designed to solve the spatio-temporal conflict problem in the hydropower and photovoltaic joint system. The multi-objective joint dispatching model of hydropower and photovoltaic system considering system benefits, risks, and stability is established, which can be solved by a MOEA/D-GABS algorithm with selection strategy. The joint system scheduling schemes under different schemes are analyzed by case. The results demonstrate that, compared with the traditional multi-objective decision-making scheme, the flood control risk in each period of the reservoir in the proposed method is controlled to be no more than 1.63 × 10−3 (the flood control standard corresponding to the 50-year flood control risk is 0.006); the flood limit water level of the reservoir is increased from 583.00 m to 583.70 m, which improves the benefit of the reservoir; and the water utilization rate is effectively improved. On the other hand, compared with the traditional scheme, the proposed method reduces the peak valley difference of the combined system by 50.67% and 59.68% in typical sunny and cloudy scenarios, respectively, which greatly reduces the uncertainty of photovoltaic output, and the stability of the combined system is improved. It is shown that the proposed method can be used to guide the economic dispatch of a complementary system with hydropower as the regulating energy.

Reservoir Flood Season Segmentation and Risk–benefit Cooperative Decision of Staged Flood Limited Water Level

Reservoir Flood Season Segmentation and Risk–benefit Cooperative Decision of Staged Flood Limited Water Level

Risk Analysis of Dynamic Water Level Setting of Reservoir in Flood Season Based on Multi-index

Long-term operation of the reservoir at the flood limit water level (FLWL) in the flood season is not conducive to the exertion of the comprehensive benefits of the reservoir, especially in the flood free period, causing a certain waste of resources. In order to make full use of water resources and avoid the risk of flooding due to extreme events, this paper proposes a dynamic water level decision-making model in flood free period, which considers the utmost of resources, effective response measures for possible catastrophic floods and the credible decision method, thus improving the operational benefit of hydropower station in the flood season and reducing flood control risk. The proposed model includes four modules. Firstly, historical data and the fuzzy statistical test method are used to divide the flood season into multiple stages. Secondly, the maximum inflow process in the effective forecast period of flood forecast in each stage is selected, and according to this inflow process, the operating limited water level (OLWL) in the flood free period of each stage is determined based on the reservoir discharge capacity, then the operating water level range and discharge ratio are discretized. Thirdly, multi-order Monte Carlo Markov Chain (MCMC) and Monte Carlo method are used to calculate the risk rate, and the scheme set is established by the three variables of power generation benefit, discharge ratio and risk rate. Finally, the weighted Topsis method, considering subjective and objective weighting method, is applied to determine the best scheme. This method has been verified in Youjiang reservoir in Yujiang River Basin. The main conclusions are as follows: (1) Combined with the discharge in the forecast period, the reservoir operating water level has a certain raising space in the flood free period. (2) The multi-order MCMC method effectively reflects the relationship between adjacent periods of runoff forecast error, and its simulation process is closer to the reality. (3) The Topsis method, which combines AHP with entropy weight method, can fully consider the characteristics of power generation efficiency, flow ratio and risk rate, and can effectively select satisfactory schemes from non-inferior schemes. This method provides a new idea for determining the operating water level of the reservoir in the flood season.

Dynamic Control of Water Level in Flood Limited Reservoir Based on Intelligent Calculation

Determining the flood-limited water level is the focus of the contradiction between flood control and profit-making. The dynamic control of the flood-limited water level of the reservoir is an effective way to ensure flood control safety and realize flood resource utilization. Based on the analysis of the availability of forecast information, this paper conducts a study on the dynamic control of the flood-limited water level in Panjiakou Reservoir. The dynamic control area of Panjiakou Reservoir is determined by the “predischarge capacity constraint method.” On the premise of satisfying the flood control safety of the reservoir, the “prestorage and predischarge method” is adopted according to the real-time forecast information, the water conditions, work conditions, disaster conditions, etc. to develop a dynamic control plan for the flood-limited water level in the main flood season of the reservoir. Finally, based on the improved first-order second moment method (AFOSM), the dispatch risk model of the dynamic control of the flood-limited water level of the reservoir is constructed, and the risk analysis of the flood forecasting system is carried out. The results show that (1) the flood forecast accuracy of Panjiakou Reservoir conforms to the standard of the B-level forecasting scheme, and the results can be applied under the condition of considering the forecast error; (2) it is suggested that the maximum water level of the Panjiakou Reservoir in the main flood season is dynamically controlled to be 218 m. In the case of water supply guarantee rate P = 75 % , compared with the current situation of the flood-limited water level of 216 m, the water supply is increased by 40 million m3, and the flow of electricity is increased by 30 million. The flood control risk rate under the rain leakage error is lower than 0.1, which is the minimum risk level, which is acceptable. However, when the net rain leakage error is greater than 25 mm, there may be insufficient leakage or excessive leakage, which will cause inconvenience to downstream flood control.

Multi-objective risk analysis for flood control operation of a complex reservoir system under multiple time-space correlated uncertainties

Many uncertainties are involved in flood control operation of a multi-reservoir system and result in risks for the system. Risk analysis for reservoir flood control operation is essential for decision making. Traditional flood control risk analysis only takes flood forecast error as the main risk source, ignoring the influence of dynamic control of flood-limited water level on reservoir initial water level for flood regulation. In this study, the spatial correlation of reservoir initial water level errors and the spatiotemporal correlation of flood forecast errors are identified via copula function. A risk analysis model considering both upstream and downstream is established for multi-objective flood control operation of a complex reservoir system, coupling multi-dimensional uncertainties of reservoir initial water levels, flood forecast errors, reservoir capacity curve errors, reservoir discharge curve errors and river flood routing errors. The impact of multiple risk sources with spatiotemporal correlations on reservoir flood control operation is then evaluated and the competitive tradeoff between flood control risks in upstream and downstream analyzed. The model is applied to a mixed four-reservoir system in Pi River Basin in China. Results indicated that 1) there is a strong correlation between the reservoir initial water level errors in space and also between the flood forecast errors in space and time, which can be effectively described by copula function, and ignoring the correlations will underestimate the flood control risk; 2) the two objective values of upstream and downstream show a clear competitive relationship, and the solution that prefers one objective has low risk marginal benefit, thus choosing a compromise solution can balance risks; 3) coupling the initial water level uncertainty reduces the flood control risk caused by the flood forecast errors to a certain extent, but also lead to the increase of extreme risk loss.

Study on the Staged Operation of a Multi-Purpose Reservoir in Flood Season and Its Effect Evaluation

A reasonable analysis of flood season staging is significant to the management of floods and the alleviation of water shortage. For this paper, the case of the Chengbi River Reservoir in China was selected for study. Based on fractal theory, the flood season is divided into several sub-seasons by using four indexes (multi-year average daily rainfall, multi-year maximum rainfall, multi-year average daily runoff, and multi-year maximum daily runoff) in this study. Also the Benefit-Risk theory is applied to evaluate the effects of staged dispatching. The results show that the flood season of the Chengbi River basin should be divided into the pre-flood season (13 April–6 June), the main flood season (7 June–9 September) and the post-flood season (10 September–31 October). After adjusting the flood limit water level for sub-season and benefit assessment, the probability of exceedance after reservoir flood season operation increases by 0.13×10-5, the average annual expected risk is 0.2264 million RMB, and the average annual benefit increases by 0.88–1.62 million RMB. The benefits obtained far outweigh the risks, indicating the importance of staging the flood season.

Combined Dispatching of Hydropower and Wind Power Based on the Hedging Theory

In order to improve the utilization rate of water resources in the flood season of the reservoir effectively and promote wind power consumption, this paper proposes an optimization model for the combined dispatching of wind power and hydropower based on the hedging theory. First, the conflicting relationship between the water storage benefits of hydropower stations, flood control risks, and the joint output of hydropower and wind power in joint dispatching is studied. The introduction of hedging theory divides the combined dispatching of wind power and hydropower into a two-stage dispatching problem including the decision-making stage and the remaining stage; Second, considering the uncertainty of water forecasting and wind power forecasting, a multi-objective optimal dispatching model of hydropower and wind power based on hedging theory is constructed. This model aims to minimize flood control risks, maximize water storage benefits, and minimize wind power and hydropower combined power output volatility. Finally, the non-dominated sorting genetic algorithm (NSGA2) is used to solve the specific examples. The results show that the model built in the article controls the flood control risk at each time period not to be higher than 1.63 × 10−3 (the flood control standard corresponding to the flood control risk in 50 years is 0.006). Additionally, the water level of the reservoir increased from the flood limit water level (583.00 m) to 583.70 m. It greatly increases the water storage capacity and effectively improves the utilization rate of water resources. At the same time, the optimized scheduling scheme reduced the peak-valley difference of joint output from 125.00 MW to 35.66 MW, and the peak-valley difference was greatly reduced. It effectively improves the volatility of wind power. The validity of the model is verified, and the obtained scheme can provide decision-making for the joint dispatch scheme of hydropower and wind power.

An Analytical Risk Analysis Method for Reservoir Flood Control Operation Considering Forecast Information

Risk analysis is vital for reservoir flood control operation considering forecast because forecast uncertainties may create risks of multiple hazard events, including the single-hazard events, the union and intersection of single-hazard events (UHE and IHE). The probability of UHE and IHE are the major concern of decision-makers. This research proposes an analytical flood risk probability calculation framework for single-hazard event, UHE and IHE caused by uncertainties in flood forecasting and takes Dahuofang Reservoir, located in the Hunhe River basin, Northeast China, as a case study. In the framework, the risk is calculated by the integral of the joint probability density function (Pdf) of the risk sources over the hazard domain, which is the collection of the values of risk sources that cause hazards. The determination method of the hazard domain and the Pdf of risk sources and the procedure of risk analysis are elaborated. Results of single-hazard events show that the proposed methodology is of higher precision compared with the risk analysis method based on the law of total probability. Meanwhile, the flood risks of UHE and IHE are calculated for the reservoir operation under different levels of flood limit water level. The study provides a new method of flood risk analysis for flood control reservoir operation.

Flood season partition and flood limit water level determination for cascade reservoirs downstream jinshajiang river

The internal contradiction between flood risk and power generation benefit can be alleviated by studying the flood season and flood limit level of reservoir.It is of great significance to adjust the floodwater storage in flood season. In this paper, based on the normal distribution theory improved fuzzy set analysis method, according to the inflow flood of cascade reservoirs downstream Jinshajiang River, the flood season is divided into stages. The flood limit water level of cascade reservoir is determined by the mapping relationship between subordinate degree and flood control storage capacity.A intertemporal sampling method is proposed to correct the staging results and flood limit water level. The research results show that the end of the pre-flood season of cascade reservoirs downstream Jinshajiang River is July 15, and the beginning of the post-flood season is September 16, and the reasonable flood limit water levels of the four reservoirs from upstream to downstream are 962m, 800m, 575m and 374m respectively. The rationality of the stage flood limit water level is tested by the flood process with different typical flood magnification. The results of the lower stages of the Jinshajiang River and the stage water level of each reservoir can improve the economic benefits such as reservoir power generation and improve the utilization ratio of flood resources in flood season to a certain extent.

Risk analysis for reservoir flood control operation considering two-dimensional uncertainties based on Bayesian network

The uncertainty of initial water level being above the designed flood limited water level and the uncertainty of inflow caused by flood forecast error can cause flood risk in real-time reservoir flood operation. A risk analysis model for reservoir flood regulation under two-dimensional uncertainties based on Bayesian network is proposed. The nodes of Bayesian network are determined and the network structure is established with expert knowledge; the parameter learning is conducted with the training samples obtained from Monte Carlo simulation. Thereafter, through the prior probability inference without posterior information and the posterior probability inference with given posterior information, the variation of flood risk is analyzed under singular-factor uncertainty and two-factors uncertainties. The case study of Xianghongdian Reservoir using a flood of 100 years return period indicates: the risk resulted from inflow uncertainty is greater than that of the uncertainty of initial water level; there is a certain complementarity between the uncertainties of inflow and initial water level, and the combination risk is between the results of two single-factor risk levels. Moreover, Bayesian Network is able to conduct bi-directional inferences and infer the probability distribution of any other node, which has practical value for risk assessment and control of reservoir flood control operation.

Risk analysis of reservoir floodwater utilization coupling meteorological and hydrological uncertainties

Dynamic control of flood-limited water level (FLWL) is an effective method to realize floodwater utilization, which can alleviate the contradiction between flood control and conservation during flood seasons. In this study, a method of identifying upper bound of FLWL is presented simultaneously considering pre-release for water supply in rainless period and pre-release for flood control in the early period of flood. A risk analysis framework considering continuous rainless-day forecast error and flood forecast error is established. An improved truncated Gaussian distribution to describe rainless-day forecast error is proposed, combined with normal distribution describing flood forecast error to derive the distribution of excess storage determined by the method of pre-release based on forecasts. Then risk assessment and decision making are conducted. The Xianghongdian Reservoir is selected for a case study. The results indicate that the allowable excess storage can be determined under a given acceptable risk by using this analytical method. Moreover, the excess storage decreases with the safety margin reserved for the downstream increasing under the same acceptable risk.