An integrated real-time optimal flood and drought control operation model of a multi-purpose two-reservoir system : the Waiau River system

Abstract

The need to improve the operational performance of the Waiau River System, WRS, while avoiding dramatic failure by being risk-averse, became obvious after the 1991/1992 drought when the New Zealand hydro-system and the country energy production system as a whole experienced one of the worst long-duration, extreme hydrological events in its history.

This need is addressed in this study by developing a methodology and model for the real-time, flood and drought operation of the river system. This methodology is based on integrating the solutions of a deterministic risk aversion optimal control problem, using the min-max reservoir control approach, into a fuzzy logic controller. The min-max reservoir control approach's solutions are a range of possible and effective daily storages and releases that can guarantee optimum operation of the system under any hydrologic conditions. Using these solutions as a rule-base, the effective real-time operational policies were derived through the fuzzy logic controller. The integrated fuzzy logic control model using this methodology was tested for both the 1988 flood and the 1991/1992 drought. The results of the test were satisfactory for both extreme hydrological events. In retrospect, if the model had been available and applied prior to those two events, it would have been possible to operate the system more effectively by: enhancing its performance in terms of water releases for environmental and non-environmental uses, and as fuel for energy generation, and yet; Guaranteeing that the system states stayed within their acceptable limits under the 1988 and 1991/92 extreme hydrological circumstances and under any other hydrological circumstances not worse than the historically recorded, or system operator suggested, critical hydrological circumstances.

The results of the study also suggest that contrary to the concept of fixed flood mitigation and fixed water rationing trigger points adopted by the current mode of operating the system, the flood mitigation and water rationing trigger points for effective operation should be allowed to vary throughout the year depending on the day and season. It is a more flexible and improved way of operating the WRS and guaranteeing optimum results.

The other advantage of the proposed approach is that the developed fuzzy logic controller can, besides being an effective real-time operating tool, be used as a training tool helping senior operators to refine their techniques and providing guidance to novice operators.