Simulation-Optimization Framework to Support Sustainable Watershed Development by Mimicking the Predevelopment Flow Regime

Abstract

The modification of land and water resources for human use alters the natural hydrologic flow regime of a downstream receiving body of water. The natural flow regime is essential for sustaining biotic structure and equilibrium within the ecosystem. Best management practices mitigate the increased storm water runoff due to increased imperviousness and are typically designed and located within a watershed to match peak and minimum flows for a small set of targeted design storms. Ecosystems are, however, affected by all the characteristics of a long-term flow regime, including the magnitude, duration, frequency, and timing of flows. A more environmentally sustainable approach for watershed development is presented based on the minimization of differences in the characteristics of the flow regime between predevelopment and postdevelopment conditions. The indicator of hydrologic alteration (IHA) is a set of 33 hydrologic indices that characterize a flow regime and, coupled with the range of variability approach (RVA), can be used to evaluate a development strategy for its alteration of the long-term hydrologic flow regime. This paper presents a methodology to identify watershed management strategies that will have a minimal impact on the flow regime and downstream ecosystems. This methodology utilizes a metric that evaluates development strategies based on an IHA/RVA analysis implemented within a simulation-optimization framework. Continuous simulation of urban runoff for different land use strategies is enabled through the use of the storm water management model, and the resulting long-term hydrograph is analyzed using IHA/RVA. Development is allocated within subcatchments to maintain a predefined minimum level of total development while minimizing the hydrologic alteration. A hybrid optimization approach based on genetic algorithm and Nelder-Meade approaches is used to identify optimal land use allocation. Further analysis is conducted to identify alternative development patterns that allocate impervious development maximally differently among subcatchments while achieving similarly low alteration in the hydrologic flow regime.