Overview
The Perdido and Wolf Bay system in Alabama, USA, is an estuarine system linking freshwater from the Perdido and Wolf Bay watersheds with tidal saltwater from the Gulf of Mexico through Perdido Pass, Dolphin Pass, and the Gulf Intracoastal Waterway. A three-dimensional hydrodynamic model using the Environmental Fluid Dynamics Code (EFDC) was developed to analyze the complex and dynamic flow, salinity, and temperature distributions in the system.
Model Setup
The external driving forces for the model include river discharges from natural and urban watersheds, atmospheric winds, and astronomical tidal elevations at the open boundaries. Simulated water surface elevation, temperature, and salinity were compared against field data at several observation stations in 2008 and 2009 with good agreement (R² = 0.92 between measured and modeled water surface elevations). The calibrated model was used to examine system responses to high, mean, and low inflows and to sea level rise under climate change, applying the concept of the age of water to understand pollutant transport.
Key Findings
The age of water reveals dynamic and complex interactions between tides from the Gulf of Mexico and inflows from the streams. It is less than 20 days under 2-year high inflows and up to 160 days under 7Q10 low inflows. Under mean inflow conditions, the age of the tracer released from Wolf Bay is 50–70 days in the lower Perdido Bay and larger in the upper Perdido Bay, indicating strong tide–inflow interaction that results in recirculation of flow and pollutants. The age of water is projected to increase by up to 60 days under estimated sea level rise scenarios.