This Is AuburnElectronic Theses and Dissertations

Understanding Unsteady Flow and Modeling Water Quality Dynamics in the Coastal Systems under Various Hydrological and Climate Scenarios




Devkota, Janesh

Type of Degree



Civil Engineering


Three dimensional hydrodynamic Environmental Fluid Dynamics Code (EFDC) model was used to simulate flow, temperature, dye, age of tracer, and salinity in the Alabama coastal systems. The study area Perdido Bay and Wolf Bay (PBWB) estuarine system was considered for the study of age of water / tracer, and water and salt flux exchange, and tidal Mobile River was considered to study the possible flow and thermal recirculation of warm water from the discharge canal to the intake canal of a power plant. The PBWB system is a shallow estuarine system connected to the Gulf of Mexico through three open boundaries; Perdido Pass, Dolphin Pass, and the Gulf Intracoastal Waterway (GIWW). Perdido EFDC model was developed for the PBWB system and was calibrated and validated against observed data (water surface elevations, temperatures, and salinity measurements) under measured river inflows, tides, and atmospheric parameters as boundary conditions. Age of a water parcel, an abstract quantity was calculated to understand the pollutant pathway and pollutant distribution in PBWB. Several numerical experiments were designed and performed to study the age of water under different hydrological (inflow) and climate change (sea level rise) conditions. The age of water was less than 20 and 160 days for numerical experiments with dye released from all rivers under high and low flows. For model experiments with dye release from Wolf Bay tributaries and mean inflows, age of water in the lower Perdido Bay was around 50–70 days and age increased at middle and upper Perdido Bay locations. The calibrated Perdido EFDC model using 2008–2009 data was then used to study the water and salt exchange at five selected cross sections in the PBWB system. Eulerian and isohaline decomposition methods were used to investigate the subtidal (low pass filtered) water and salt flux at the cross sections. From the Eulerian decomposition method, it was found that tidal oscillatory transport (FT) was dominant at Perdido Pass and Dolphin Pass and shear dispersive transport (FE) was dominant at the Perdido Pass complex, Wolf-Perdido canal and lower Perdido Bay. Incoming and outgoing flows and salinity classes at all the cross sections were calculated using isohaline methods. In Perdido Pass, the average incoming salinity to Perdido Bay was 33 psu and outgoing salinity to the Gulf of Mexico was 27.7 psu. The multi-linear regression was performed to establish the relationship of incoming and outgoing inflows at the cross sections with river inflows, water surface elevations at open boundaries, and wind stresses. Two three-dimensional EFDC models were developed for tidal Mobile River to study the possible recirculation of warm water from the discharge canal back to the intake canal of a power plant. The domain of the study area was the Mobile River segment from the USGS Bucks gaging station to the downstream towards Mobile Bay at the intersection of I-65 Bridge for model calibration. The calibrated model was extended approximately 13 km upstream of the USGS Bucks station to simulate unsteady flow, dye, and temperature distributions under different upstream inflows and downstream harmonic tides. Velocity profiles and distributions of flow, dye, and temperature at various locations were analyzed. It was found that the recirculation of warm water could only occur under small river inflows (50 m3 s-1 or smaller) when the downstream tides control the flow pattern in Mobile River.