There is a tradeoff between flood control and reservoir storage in the dam operational management. Minimum reserved water in the dam is required for storage to be utilized for water supply particularly in dry season, and might also needed for hydropower operational. Meanwhile, the high precipitation that occasionally reach it peaks is demand an excessive capacity release to prevent the flood. Thus, the optimal dam operational is essential both for flood control and meet requisite storage.
A reservoir generally has its own manual for the operation, which holds predefined rules. These rules are usually presented in the form of graphs or tables that guide the release of the reservoir system, based on the current storage level, hydro meteorological conditions, and the time of the year[2]. This manual, however, still has a chance to be improved since the optimization modeling could be examine more detail in real-time based, especially for extreme cases.
Hsu and Wei (2007) has developed a reservoir real time (RES-RT) model that effectively reduce the peak flow and meet the target storage in Shihmen reservoir, Taiwan. RES-RT consist of three sub-models which are (a) Quantitative precipitation forecast (QPF), developed to predict rainfall, (b) stream flow prediction model (RTRL, developed to predict reservoir inflow, and (c) reservoir operation optimization, developed to forecast the optimal reservoir release hydrograph. The model examining three typhoon cases, typhoon Bilis, Nari, and Aere, which had shown decent results that reduce peak flow 60.8%, 30.6%, and 19.7% respectively, while give the optimum reservoir storage. The constraints in this model are respecting into three time intervals, known as "three-flood-stages". These stages are (1) stage prior to flood arrival, in which water releases are to reserve enough reservoir capacity for the upcoming flood, (2) stage preceding peak flow, in which flood water release are for disaster mitigation, and (3) stage after peak flow, in which releases are to regulate the storage at the end of the flood for future water use.
Two time intervals also could be applied as boundary condition for optimization modeling for dam operational. The gap volume during the first stage is expected to be reduced by releasing water from the upper dams, while during the second stage simulated flood peak is expected to be reduced by the partial or total gates' closure[3]. This two-flood-stages was used in current research in Tone river basin, Japan. The research that operate geomorphological based hydrological model (GBHM) for the simulation model and shuffled complex evolution (SCE) algorithm for reservoir operation analysis, also give a good result. The flood peak, of which one of the highest flood event in the last decade, was reduced successfully by 21% compared to simulated model without optimization. In this research, the weather radar products was utilized for the hydrological model to simulate the discharge within the river network, then the simulated inflows were input to the dam storage function[3]. Consider its fast calculation, this method could be also implemented for real-time model.
The utilization of particular software for optimization modeling for dam operational is also recognized. The operation of MIKE 11 for simulation model and SCE algorithm for reservoir optimization are implemented by AUTOCAL software for modeling of the Hoa Binh reservoir, Vietnam. The first step objective of this study is to minimize the downstream flood peak and maximized the hydropower potential, which is manifested by the reservoir level during the flood season. The second step is to maximize the hydropower generation during the flood season and minimize the deviation from the reservoir target level at the beginning of dry season[2]. Had an input data both from synthetic and observed extreme flood since 1960's in Vietnam, this research allow us to evaluate present regulation that implemented for flood control and hydropower generation. As the result, under the flood control optimal solution the reservoir can protect the downstream river from flooding and keep the average maximum water level in Hanoi smaller than that under present strategy, though the value not so significant regarding the low quality water condition in the downstream river dikes. For a longer term of dam operational modeling, this methodology is considered could be applied.
Reference:
1. Hsu, N., and Wei, C. (2007), A multipurpose reservoir real-time operation model for flood control during typhoon invasion, Journal of Hydrology 336, 282-293
2. Ngo, L., Madsen, H., and Rosbejrg, D. (2007), Simulation and optimization modeling approach for operation of the Hoa Binh reservoir, Vietnam, Journal of Hydrology 336, 269-281
3. Valeriano, O. C. S. , Koike, T. , Yang, K., and Yang, D. , (2010), Optimal dam operation during flood season using a distributed hydrological model and Heuristic Algorithm, Journal of Hydrologic Engineering
