New Methodology for Optimization of Freshwater Inflows to Estuaries

Abstract

The objective of this work is to develop a general methodology for determination of the optimal freshwater inflows into bays and estuaries to balance freshwater demands with the harvest of various types of estuarine resources. The methodology is based upon solving a large‐scale nonlinear programming problem formulated in an optimal‐control framework. Constraints of the optimization problem include regression equations for harvest of the various species that express fishery harvest as a function of the quantity of freshwater inflow. The stochastic element of the problem (i.e. the uncertainty associated with the regression equations for harvest) is considered by expressing constraints in a chance‐constrained formulation. A nonlinear programming optimizer is interfaced with a hydrodynamic transport model to implicitly solve the hydrodynamic‐salinity constraint equations for salinity levels. An augmented Lagrangian method is introduced to incorporate the salinity constraints into the objective so that the problem size for the optimizer is significantly reduced. A computer model OPTFLOW has been developed by interfacing a simulator for the hydrodynamic‐salinity (HYD‐SAL) with a nonlinear optimizer (GRG2) to apply the methodology by the present writers in an accompanying companion paper. An efficient approximation scheme is developed for evaluation of the objective function and its reduced gradient to reduce the computational effort dramatically. The new methodology can provide a very useful tool for decision makers to quantitatively analyze various water‐management strategies.