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This report presents the development of a numerical model simulating water flow, contaminant transport, and sediment transport in watershed systems. The model is composed of two modules: flow and transport. Three options are provided in modeling the flow module in river/stream network and overland regime: the kinematic wave approach, diffusion wave approach, and dynamic wave approach. The kinematic and diffusion wave approaches are known to be numerically robust in terms of numerical convergency and stability, i.e., they can generate convergent and stable simulations over a wide range of ground surface slopes in the entire watershed. The question is the accuracy of these simulations. The kinematic wave approach usually produces accurate solutions only over the region of steep slopes. The diffusion wave approach normally gives accurate solutions over the region of mild to steep slopes. However, neither approach has the ability to yield accurate solutions over the region of small slopes, in which the inertial forces are no longer negligible compared with the gravitational forces. The kinematic wave approach cannot even address the problems of backwater effects. On the other hand, a dynamic wave approach, having included all forces, can theoretically have the potential to generate accurate simulations over all ranges of slopes in a watershed. A total of eight groups of example problems were given in this report to demonstrate the capability of this model. Continuing work is underway to incorporate a three-dimensional subsurface flow and chemical transport model into this watershed model. The Richards' equation and advection-dispersion reactive chemical transport equations will form the basis to simulate the subsurface flow and chemical transport module in saturated-unsaturated media.
This report presents the development of a numerical model simulating water flow, contaminant transport, and sediment transport in watershed systems. The model is composed of two modules: flow and transport. Three options are provided in modeling the flow module in river/stream network and overland regime: the kinematic wave approach, diffusion wave approach, and dynamic wave approach. The kinematic and diffusion wave approaches are known to be numerically robust in terms of numerical convergency and stability, i.e., they can generate convergent and stable simulations over a wide range of ground surface slopes in the entire watershed. The question is the accuracy of these simulations. The kinematic wave approach usually produces accurate solutions only over the region of steep slopes. The diffusion wave approach normally gives accurate solutions over the region of mild to steep slopes. However, neither approach has the ability to yield accurate solutions over the region of small slopes, in which the inertial forces are no longer negligible compared with the gravitational forces. The kinematic wave approach cannot even address the problems of backwater effects. On the other hand, a dynamic wave approach, having included all forces, can theoretically have the potential to generate accurate simulations over all ranges of slopes in a watershed. A total of eight groups of example problems were given in this report to demonstrate the capability of this model. Continuing work is underway to incorporate a three-dimensional subsurface flow and chemical transport model into this watershed model. The Richards' equation and advection-dispersion reactive chemical transport equations will form the basis to simulate the subsurface flow and chemical transport module in saturated-unsaturated media.
This report presents the development of a numerical model simulating water flow and contaminant and sediment transport in watershed systems of one-dimensional river/stream network, two-dimensional overland regime, and three-dimensional sub surface media. The model is composed of two modules: flow and transport. Three options are provided in modeling the flow module in river/stream network and overland regime: the kinematic wave approach, diffusion wave approach, and dynamic wave approach. The kinematic and diffusion wave approaches are known to be numerically robust in terms of numerical convergency and stability; i.e., they can generate convergent and stable simulations over a wide range of ground surface slopes in the entire watershed. The question is the accuracy of these simulations. The kinematic wave approach usually produces accurate solutions only over the region of steep slopes. The diffusion wave approach normally gives accurate solutions over the region of mild to steep slopes. However, neither approach has the ability to yield accurate solutions over the region of small slopes, in which the inertial forces are no longer negligible compared to the gravitational forces. The kinematic wave approach cannot address the problems of backwater effects. On the other hand, a dynamic wave approach, having included all forces, can theoretically have the potential to generate accurate simulations over all ranges of slopes in a watershed. The subsurface flow is described by Richard's equation where water flow through saturated-unsaturated porous media is accounted for.
This report presents the development of a numerical model simulating water flow and contaminant and sediment transport in watershed systems of one-dimensional river/stream network, two-dimensional overland regime, and three-dimensional sub surface media. The model is composed of two modules: flow and transport. Three options are provided in modeling the flow module in river/stream network and overland regime: the kinematic wave approach, diffusion wave approach, and dynamic wave approach. The kinematic and diffusion wave approaches are known to be numerically robust in terms of numerical convergency and stability; i.e., they can generate convergent and stable simulations over a wide range of ground surface slopes in the entire watershed. The question is the accuracy of these simulations. The kinematic wave approach usually produces accurate solutions only over the region of steep slopes. The diffusion wave approach normally gives accurate solutions over the region of mild to steep slopes. However, neither approach has the ability to yield accurate solutions over the region of small slopes, in which the inertial forces are no longer negligible compared to the gravitational forces. The kinematic wave approach cannot address the problems of backwater effects. On the other hand, a dynamic wave approach, having included all forces, can theoretically have the potential to generate accurate simulations over all ranges of slopes in a watershed. The subsurface flow is described by Richard's equation where water flow through saturated-unsaturated porous media is accounted for.
Hydro-system within a watershed includes many environmental processes, such as rainfall, runoff, groundwater flow, infiltration, evapotranspiration, recharge, upland erosion, sediment transport, and contaminant transport. In order to investigate these processes and evaluate their effects on water environments, numerical models have been recognized as an increasingly efficient and effective tool. Due to the natural intrinsic connections between surface and subsurface waters, modeling of flow, upland soil erosion, and contaminant transport should be considered as an integrated system. This dissertation has developed a physically-based integrated numerical model for flow, sediment, and contaminant transport in the surface-subsurface system. In this model, the surface flow is calculated using a depth-averaged 2-D diffusion wave model, and the variably saturated subsurface flow is computed using the 3-D mixed-form Richards equation. Interactions between surface and subsurface flows are considered using the continuity conditions of the pressure head and exchange flux at the ground surface. A general form of the surface flow equation based on the diffusion wave approximation is developed, which is intrinsically coupled with the variably saturated subsurface flow equation. The upland soil erosion and transport model employs the concept of nonequilibrium that considers both erosion and deposition. The model simulates nonuniform total-load sediment transport, with detachments from rainsplash and/or hydraulic erosion driven by overland flow. Contaminant transports in both surface and subsurface domains are described using advection-diffusion equations. The model considers the sediment sorption and desorption of the contaminant, as well as contaminant exchanges between surface and subsurface due to infiltration, diffusion, and bed change. The integrated numerical model is evaluated by simulating several published laboratory- and field-scale experiments. It is further applied to compute flow discharge, sediment and pesticide concentration during storm events in the Deep Hollow Lake watershed, Mississippi. The sensitivity analysis of the model is also performed using different values for several model parameters. The results have shown that the integrated model framework is capable of simulating the rainfall-runoff related hydrological processes in natural surface-subsurface systems.
This dissertation presents the design of an integrated watershed model, WASH123D version 3.0, a first principle, physics-based watershed-scale model of integrated hydrology/hydraulics and water quality transport. This numerical model is comprised of three modules: (1) a one-dimensional (1-D) simulation module that is capable of simulating separated and coupled fluid flow, sediment transport and reaction-based water quality transport in river/stream/canal networks and through control structures; (2) a two-dimensional (2-D) simulation module, capable of simulating separated and coupled fluid flow, sediment transport, and reactive biogeochemical transport and transformation in two-dimensional overland flow systems; and (3) a three-dimensional (3-D) simulation module, capable of simulating separated and coupled fluid flow and reactive geochemical transport and transformation in three-dimensional variably saturated subsurface systems. The Saint Venant equation and its simplified versions, diffusion wave and kinematic wave forms, are employed for surface fluid flow simulations and the modified Richards equation is applied for subsurface flow simulation. The reaction-based advection-dispersion equation is used as the governing equation for water quality transport. Several physically and mathematically based numerical options are provided to solve these governing equations for different application purposes. The surface-subsurface water interactions are considered in the flow module and simulated on the basis of continuity of interface. In the transport simulations, fast/equilibrium reactions are decoupled from slow/kinetic reactions by the decomposition of reaction networks; this enables robust numerical integrations of the governing equation. Kinetic variables are adopted as primary dependent variables rather than biogeochemical species to reduce the number of transport equations and simplify the reaction terms. In each time step, hydrologic/hydraulic variables are solved in the flow module; kinetic variables are then solved in the transport module. This is followed by solving the reactive chemical system node by node to yield concentrations of all species. Application examples are presented to demonstrate the design capability of the model. This model may be of interest to environmental scientists, engineers and decision makers as a comprehensive assessment tool to reliably predict the fluid flow as well as sediment and contaminant transport on watershed scales so as to evaluate the efficacy and impact of alternative watershed management and remediation techniques prior to incurring expense in the field.
The complex interaction between flow, vegetation and sediment drives the never settled changes of riverine system. Vegetation intercepts rainfall, adds resistance to surface flow, and facilitates infiltration. The magnitude and timing of flood flow are closely related to the watershed vegetation coverage. In the meantime, flood flow can transport a large amount of sediment resulting in bank erosion, channel degradation, and channel pattern change. As climate changes, future flood frequency will change with more intense rainfalls. However, the quantitative simulation of flood flow in vegetated channel and the influence of climate change on flood frequency, especially for the arid and semi-arid Southwest, remain challenges to engineers and scientists. Therefore, this research consists of two main parts: simulate unsteady flow and sediment transport in vegetated channel network, and quantify the impacts of climate change on flood frequency. A one-dimensional model for simulating flood routing and sediment transport over mobile alluvium in a vegetated channel network was developed. The modified St. Venant equations together with the governing equations for suspended sediment and bed load transport were solved simultaneously to obtain flow properties and sediment transport rate. The Godunov-type finite volume method is employed to discretize the governing equations. Then, the Exner equation was solved for bed elevation change. Since sediment transport is non-equilibrium when bed is degrading or aggrading, a recovery coefficient for suspended sediment and an adaptation length for bed load transport were used to quantify the differences between equilibrium and non-equilibrium sediment transport rate. The influence of vegetation on floodplain and main channel was accounted for by adjusting resistance terms in the momentum equations for flow field. A procedure to separate the grain resistance from the total resistance was proposed and implemented to calculate sediment transport rate. The model was tested by a flume experiment case and an unprecedented flood event occurred in the Santa Cruz River, Tucson, Arizona, in July 2006. Simulated results of flow discharge and bed elevation changes showed satisfactory agreements with the measurements. The impacts of vegetation density on sediment transport and significance of non-equilibrium sediment transport model were accounted for by the model. The two-dimensional surface flow model, called CHRE2D, was improved by considering the vegetation influence and then applied to Santa Cruz River Watershed (SCRW) in the Southern Arizona. The parameters in the CHRE2D model were calibrated by using the rainfall event in July 15th, 1999. Hourly precipitation data from a Regional Climate Model (RCM) called Weather Research and Forecasting model (WRF), for three periods, 1990-2000, 2031-2040 and 2071-2079, were used to quantify the impact of climate change on the magnitude and frequency of flood for the Santa Cruz River Watershed (SCRW) in the Southern Arizona. Precipitation outputs from RCM-WRF model were bias-corrected using observed gridded precipitation data for three periods before directly used in the watershed model. The watershed model was calibrated using the rainfall event in July 15th, 1999. The calibrated watershed model was applied to SCRW to simulate surface flow routing for the selected three periods. Simulated annual and daily maximum discharges are analyzed to obtain future flood frequency curves. Results indicate that flood discharges for different return periods are increased: the discharges of 100-year and 200-year return period are increased by 3,000 and 5,000 cfs, respectively.
Many U.S. Department of Defense installations contain soil, sediment, surface water, and groundwater environments contaminated with explosives and energetics. Modeling as part of watershed management to meet water quality goals is not new but most current models were developed and tested two decades ago. Watershed models are largely confined to lumped and semi-distributed surface water simulation. Models that reflect hydrologic and aquatic impacts from military conditions are rare. Watershed models that enable diagnostic, predictive, and operational applications in conjunction with monitoring and data collection programs are virtually non-existent across the board and are urgently needed within the scientific and modeling communities. To meet this need a physically based, distributed source Contaminant Transport, Transformation and Fate (CTT & F) sub-module was developed by the U.S. Army Engineer Research and Development Center, to simulate point and non-point sources across a watershed. The model operates on a cell by cell basis. Soil types, land uses, and other hydrologic characteristics can be varied spatially among cells. Potential chemicals are routed through the cells from the watershed divide to the outlet. The distributed, process-oriented structure of the model can be used for identifying critical source areas of non-point sources within the watershed, and can give insight on both the persistence and fate of explosives. The expectation is that a model of this type is able to quantify transport and transformation of multiple contaminants and can facilitate the assessment of the fate of distributed sources and lead to better management of the watershed environment for military installations. CTT & F can be linked to any distributed hydrologic model, assuming the hydrologic model provides the required flow and sediment transport fluxes. The ability of the model to simulate explosives in watersheds is demonstrated by a test case study in the laboratory environment.