Decision Support System for Optimally Managing Water Resources to Meet Multiple Objectives in the Savannah River Basin

Managers of large river basins face conflicting demands for water resources such as wildlife habitat, water supply, wastewater assimilative capacity, flood control, hydroelectricity, and recreation. The Savannah River Basin, for example, has experienced three major droughts since 2000 that resulted in record low water levels in its reservoirs, impacting dependent economies for years. The Savannah River estuary contains two municipal water intakes and the ecologically sensitive freshwater tidal marshes of the Savannah National Wildlife Refuge. The Port of Savannah is the fourth busiest in the United States, and modifications to the harbor to expand ship traffic since the 1970s have caused saltwater to migrate upstream, reducing the freshwater marsh’s acreage more than 50 percent. A planned deepening of the harbor includes flow-alteration features to minimize further migration of salinity, whose effectiveness will only be known after all construction is completed. One of the challenges of large basin management is the optimization of water use through ongoing regional economic development, droughts, and climate change. This paper describes a model of the Savannah River Basin designed to continuously optimize regulated flow to meet prioritized objectives set by resource managers and stakeholders. The model was developed from historical data using machine learning, making it more accurate and adaptable to changing conditions than traditional models. The model is coupled to an optimization routine that computes the daily flow needed to most efficiently meet the water-resource management objectives. The model and optimization routine are packaged in a decision support system that makes it easy for managers and stakeholders to use. Simulation results show that flow can be regulated to substantially reduce salinity intrusions in the Savannah National Wildlife Refuge, while conserving more water in the reservoirs. A method for using the model to assess the effectiveness of the flow-alteration features after the deepening also is demonstrated. INTRODUCTION The Savannah River Basin (Basin; Figure 1a) is a prototypical large basin whose water-resource managers face conflicting demands, such as wildlife habitat, water supply, wastewater assimilative capacity, flood control, hydroelectricity, and recreation. In the upper Basin, the U.S. Army Corps of Engineers (USACE) controls three large reservoirs Lake Hartwell, Richard B. Russell Lake (Lake Russell), and J. Strom Thurmond Lake (Lake Thurmond). Lake Russell has comparatively little storage, leaving Lakes Hartwell and Thurmond to provide most of the regulated flow to the coast. Since 2000 the upper Basin has experienced three major droughts, resulting in record and near-record low reservoir water-level elevations that impacted dependent economies by reducing tourism and real estate values (Allen et al., 2010; USACE, 2014). The Savannah River estuary (estuary; Figure 1b) contains two municipal water intakes and the ecologically sensitive freshwater tidal marshes of the Savannah National Wildlife Refuge (Refuge). The interaction of regulated streamflow, tides, and weather produces salinity intrusions more than 25 miles upstream at U.S. Geological Survey (USGS) gage 02198840. The gage is located where the river intersects Interstate 95 (I95), and near the City of Savannah’s municipal freshwater intake on Abercorn Creek. The Port of Savannah is the fourth busiest in the United States, and modifications to the harbor to enable more ship traffic have caused saltwater to migrate upstream, reducing the Refuge’s freshwater marsh’s acreage more than 50 percent since the 1970s (Conrads et al., 2006). A currently planned deepening of the harbor includes flow-alteration features to minimize further salinity migration; however, the estuary’s complex hydrology and the extensive scope of all the construction make the final outcome uncertain. For example, a tide gate installed during the 1970s in the estuary’s Back River to reduce shoaling unintentionally increased salinities and decreased dissolved-oxygen levels in the habitat of a large striped bass population. A consequent Water Policy and Planning Track Journal of South Carolina Water Resources Volume 2, Issue 1, Page 16-23, 2015