Estimating Bottom Water Dissolved Oxygen in the Mississippi River Gulf Outlet and Gulf Intracoastal Waterway Resulting from Proposed Structures

Abstract : This study examined the impacts on bottom dissolved oxygen (DO) within canal reaches comprising the Gulf Intracoastal Waterway (GIWW), the Mississippi River Gulf Outlet (MRGO), and the Inner Harbor Navigation Channel (IHNC) resulting from various alternatives for proposed water control structures located within this system. Due to time constraints, an analytical model of reduced form was used for the analysis. Bottom water DO for each canal study reach was predicted using a steady state, fully mixed, single reactor model. August conditions were imposed for the assessment. Monthly average, bottom flushing flow rates were obtained from hourly bottom velocities output from a three-dimensional (3D) hydrodynamic and salinity model that was applied to the system during a separate study. Likewise, monthly average surface and bottom salinities were also obtained from the 3D model output. The salinity data were used to estimate vertical eddy diffusivities, which were used in the model for DO exchange between surface and bottom water. The hydrodynamic information was provided for each study reach and for each alternative scenario. The model indicated that low DO conditions should be expected within the system for the structural alternatives being considered. The model showed that several reaches for several scenarios will have DO less than the standard of 4.0 mg/L. The IHNC reach may present a special concern since bottom DO was predicted to be 0.0 mg/L for three alternatives that included structures placed within the GIWW.