Multiobjective hedging rules for flood water conservation

Abstract

Flood water conservation can be beneficial for water uses especially in areas with water stress but also can pose additional flood risk. The potential of flood water conservation is affected by many factors, especially decision makers' preference for water conservation and reservoir inflow forecast uncertainty. This paper discusses the individual and joint effects of these two factors on the trade‐off between flood control and water conservation, using a multiobjective, two‐stage reservoir optimal operation model. It is shown that hedging between current water conservation and future flood control exists only when forecast uncertainty or decision makers' preference is within a certain range, beyond which, hedging is trivial and the multiobjective optimization problem is reduced to a single objective problem with either flood control or water conservation. Different types of hedging rules are identified with different levels of flood water conservation preference, forecast uncertainties, acceptable flood risk, and reservoir storage capacity. Critical values of decision preference (represented by a weight) and inflow forecast uncertainty (represented by standard deviation) are identified. These inform reservoir managers with a feasible range of their preference to water conservation and thresholds of forecast uncertainty, specifying possible water conservation within the thresholds. The analysis also provides inputs for setting up an optimization model by providing the range of objective weights and the choice of hedging rule types. A case study is conducted to illustrate the concepts and analyses.

Reservoir Operation Policy based on Joint Hedging Rules

“…A good hedging rule can effectively reduce a very high-percentage single period shortage [29]. However, unnecessary hedging increases more frequent small shortages, and thus decreases the reliability of water supply [30]. Just as in (d, xd) case, the number of days on SOP whose shortage ratio is smaller than 40% is much smaller than that on HR and JHR.…”

A Forecast-Skill-Based Dynamic Pre-Storm Level Control for Reservoir Flood-Control Operation

“…In practice, ε t s is usually assumed to follow a priori Gaussian, Student's t, or gamma distribution. For the sake of simplicity, the non-Gaussian distributed forecast errors may be transformed and mapped onto Gaussian space by applying the normal quantile transform (NQT) method [41,42], with known transformed variance, σ ε 2 , that is, ε t s ∼ N 0, σ t s 2 , following Ding et al [23,24] and Zhao et al [25].…”

“…Zhao et al [21] derived a flood hedging principle within the flood event, in which it is assumed that both initial reservoir level and final level are set to the designed FLWL. Following that, Hui et al [22] developed an optimization model for pre-storm flood hedging releases considering engineering uncertainty; Ding et al [23,24] established a real-time DC-FLWL optimization model balancing the water conservation loss for the non-flood season use and flood risk for downstream damage. An implied consensus in all these flood control studies is that no excess volume of water beyond the FLWL is allowed to be stored when inflow is relatively large.…”

Floodwater Utilization Based on Reservoir Pre-Release Strategy Considering the Worst-Case Scenario

“…Hui et al [23] considered the uncertainty of flood forecasting as well as engineering uncertainty and determined the minimum risk to derive the optimal condition for releases where the current marginal damage from the hedging release equaled the future expected marginal damage from storm releases. Based on the analysis of the uncertainty of inflow forecast and the accepted flood control risk, Ding et al [24,25] established a two-stage optimal operation model that proposed two objectives: the conservation benefits in the current stage and flood risk in a later stage. They derived hedging rules from the analytical framework and systematically analyzed the effects of the preferences of decision makers for water utilization and reservoir inflow forecast uncertainty on the DC-FLWL; then, they proposed availability criterion and a subsequent method for applying forecast information to DC-FLWL.…”