Browsing by Subject "Flow and transport modeling"

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Flow and transport modeling in large river networks
(2014-08) Tavakoly Zadeh, Ahmad A.; Maidment, David R.; Yang, Zong-liang
The work presented in this dissertation discusses large scale flow and transport in river networks and investigates advantages and disadvantages of grid-based and vector-based river networks. This research uses the Mississippi River basin as a continental-case study and the Guadalupe and San Antonio rivers and Seine basin in France as regional-case studies. The first component of this research presents an extension of regional river flow modeling to the continental scale by using high resolution river data from NHDPlus dataset. This research discovers obstacles of flow computations for river a network with hundreds of thousands river segments in continental scales. An upscaling process is developed based on the vector-based river network to decrease the computational effort, and to reduce input file size. This research identifies drainage area as a key factor in the flow simulation, especially in a wet climate. The second component of this research presents an enhanced GIS framework for a steady-state riverine nitrogen transport modeling in the San Antonio and Guadalupe river network. Results show that the GIS framework can be applied to represent a spatial distribution of flow and total nitrogen in a large river network with thousands of connected river segment. However, time features of the GIS environment limit its applicability to large scale time-varied modeling. The third component shows a modeling regional flow and transport with consideration of stream-aquifer interactions at a regional scale at high resolution. The STICS- Eau-Dyssée combined system is implemented for entire seine basin to compute daily nitrate flux in the Seine grid river network. Results show that river-aquifer exchange has a significant impact on river flow and transport modeling in larger river networks.
Surface water-groundwater exchanges under conditions of daily river stage fluctuations : implications for fluid, solute, and heat dynamics in dam regulated river cooridors
(2020-05-08) Ferencz, Stephen Bruce; Cardenas, Meinhard Bayani, 1977-; Neilson, Bethany T; Rempe, Daniella; Bennett, Philip
The damming of rivers has resulted in multitude of environmental impacts to river ecosystems. The focus of this dissertation is to assess how dams used for hydroelectric power generation influence surface water-groundwater exchange. The exchange of surface water with groundwater has been demonstrated to be an important mechanism regulating nutrient dynamics in rivers. Hydroelectric dams are often “hydropeaked,” which is when discharge from the dam is increased/decreased in accordance with daily patterns of electricity demand. This creates high frequency river stage fluctuations that enhance the connectivity between rivers and groundwater. The central theme explored throughout this dissertation is how hydropeaking effects the movement of water, dissolved solutes, and heat between rivers and their sediments. Much of the research presented in this dissertation uses numerical models to explore how properties of the river (stage, chemistry, temperature) and subsurface (hydraulic conductivity, groundwater levels) control exchanges of fluid, heat, and solutes, and characterizing how those fluxes alter the chemistry and temperature of sediments near the river. Two of the studies are entirely modeling-based, focusing on the lateral exchanges of fluid and solutes between rivers and their riparian aquifers caused by hydropeaking. The results provide insight into how dam release and subsurface properties influence surface water-groundwater exchange and also the distance downstream from a dam that exchanges occur over. Other key findings relate to how hydraulic conductivity and groundwater flow conditions of the riparian aquifer control fluid and solute exchange, how far river-sourced solutes propagate into the riparian zone, and factors influencing aerobic respiration in the banks of hydropeaked rivers. In another study, field observations of riverbed temperatures from a hydropeaked river are combined with flow and heat transport modeling. The results demonstrate that hydropeaking releases can strongly influence the temperature of the riverbed near the river bank, but not over the majority of the channel. A final study uses temperature observations from hundreds of rivers across the United States to characterize the mean daily temperature and daily temperature range in different ecoregions. The results from this assessment demonstrate how the thermal regimes of rivers near dams have been altered.