A Holistic Approach to Multi-Scale, Coupled Modeling of Hydrologic Processes, Flow Dynamics, Erosion, and Sediment Transport.

Abstract

Watershed systems supply services and goods to human society. They should be sustainable, maintain natural structure and function, and continue to meet societal needs in the long-term. Numerous efforts investigated the effects of climate change on watershed components. However, comprehensive studies of climate impacts relevant to the scale of human decisions have been extremely limited. One of the goals of this dissertation is to develop a holistic, multi-scale watershed model that describes essential physical processes. A coupling framework between hydrologic processes, hydrodynamics, and soil erosion and sedimentation is developed and presented. A previously existing model describing hydrological processes (tRIBS) has been integrated with a solution of the Saint-Venant shallow water equations (OFM) and the Hairsine-Rose formulation of erosion and deposition processes (HRM). The system of equations is resolved using the finite volume method based on the Roe’s approximate Riemann solver on an unstructured grid. The resultant tRIBS-OFM-HRM model is one of the most comprehensive, process-scale tools required for evaluations of climate signals that propagate through a non-linear hydrological system. The model has been used in several basic science applications. First, it has been applied to address the problem of roughness upscaling for areas covered by partially submerged obstacles, such as vegetated hillslopes. Two approaches, “Equivalent Roughness Surface” and the “Equivalent Friction Slope”, for computing the upscaled Manning roughness coefficient are proposed. Predictive equations with several prognostic variables are developed to quantify the additional resistance caused by partially submerged vegetation. The effects of all independent variables are quantitatively investigated. Second, the coupled model has been used to address a possible mechanism leading to the non-uniqueness of soil erosion. It is attributed to two major conflicting effects: erosion enhancement, due to supply of highly erodible sediment, and erosion impediment, due to formation of a shielding layer that constrains the availability of lighter particles overlain by heavier sediment. Two characteristic time scales, the time to peak and the time to steady-state, are shown to separate three characteristic periods that correspond to flow-limited, source-limited, and steady-state regimes. These time scales are demonstrated to be log-linearly and negatively related to the spatially averaged Shields parameter.