Doctor of Philosophy (PhD)
Civil and Environmental Engineering
A watershed-based modeling framework is developed in this dissertation for simulating temporal and spatial variations in DO in lowland rivers with organic-rich fine-grained sediment. The modeling framework is based on three major contributions/new models, including (1)VART-DO model for improved estimation of reaeration coefficient (K2) in natural streams, (2)VART-DOS model for simulation of temporal variations in DO in response to sediment resuspension, and (3)VART DO-3L model for simulation of spatial variations in DO. A major advantage of VART-DO model is the capability of simulating DO exchange across the water-sediment interface through the hyporheic exchange mechanism in addition to the air-water exchange. Simulation results from VART-DO model revealed that hyporheic exchange can reduce K2 by 30% while longitudinal dispersion increases K2 by 50%. VART-DOS model is developed for simulation of temporal variations in DO particularly due to sediment resuspension effect during high flow. Application results of VART-DOS model to the Amite River in Louisiana showed that 83% of DO consumption in water column in July 1990 was because of sediment resuspension. A novel feature of VART DO-3L model is that a fine-grained stream with the flocculent layer can be vertically modeled with three layers: overlying water column, an advection-dominated storage zone, and a diffusion-dominated storage zone in relatively consolidated stream bed-sediment. While the importance of flocculent layer to instream DO has been widely reported, VART-DO-3L model is the first modeling tool that incorporates the flocculent layer into DO modeling. This is a unique feature of VART-DO-3L model, making it possible for determining both longitudinal and vertical profiles of DO in streams. Results of VART-DO-3L for the Amite River indicated that the DO level decreases longitudinally from 7.9mg/L at the Denham Springs station to 2.89mg/L at the Port Vincent station. Vertically, DO level drops rapidly from overlying water column to the advection-dominated storage zone and further to the diffusive layer. The DO level in the advective layer is about 40% of that in water column. The thickness of the diffusive layer varies between 0-10mm, depending on effective diffusion coefficient. Developed models in this dissertation are also applicable to sandy/gravel rivers.
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Zahraeifard, Vahid, "Modeling temporal and spatial variations in dissolved oxygen in Amite River" (2013). LSU Doctoral Dissertations. 16.