Surface and Subsurface Flow and Contaminant Transport Modeling in Lower Altamaha Watershed
Aral, Mustafa Mehmet
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Large scale modeling has long been an important challenge for the hydrologist. Numerous models have been developed to analyze possible flow patterns over a watershed in response to some precipitation event. Although most of these models used empirically-based lumped parameter formulations by neglecting or oversimplifying the underlying physical processes, they have served their purpose and provided basic data without going through a detailed analysis. In recent years, the trend has switched to a more fundamental understanding of the processes affecting the overall response of the watershed. In this regard, hydrologic modelers have directed their focus on physically-based distributed parameter models that are based on rigorous mathematical formulations of physics laws defining the flow of water over a watershed. In this study, a hybrid surface/subsurface flow and transport model is developed that blends the powerful distributed parameter models with relatively simpler lumped parameter models. The proposed hybrid model solves the channel flow and saturated groundwater flow domains in continuous time using fully distributed physics-based formulations. This system is supported with the overland flow and unsaturated groundwater flow that uses lumped parameter descriptions in discrete time. This hybrid formulation decreases the computational requirements associated with overland and unsaturated zone domains in a large scale continuous watershed modeling task but still allows a representative description of the watershed flow processes. In the proposed model, a one-dimensional channel flow model is dynamically coupled with a two-dimensional vertically-averaged groundwater flow model along the river bed. As an alternative to the commonly applied iterative solution technique, a so-called simultaneous solution procedure is developed to provide a better understanding to the couple flow problem. This new methodology is based on the principle of solving the two flow domains within a single matrix structure in a simultaneous manner. The method eliminates the iterative scheme that is otherwise required to obtain the convergence of the solution and provides a faster solution. In addition to the flow model, a coupled contaminant transport model is also developed to simulate the migration of contaminants between surface and subsurface domains. Based on its flow counterpart, the contaminant transport model dynamically couples a one-dimensional channel transport model with a two-dimensional vertically-averaged groundwater transport model. The coupling is performed at the river bed interface via advective and dispersive transport mechanisms. A modified extension of the proposed simultaneous solution procedure is also implemented to solve the coupled contaminant transport problem. The dynamic coupling provides the much needed understanding for the continuity of contaminants in strongly interacting surface/subsurface systems such as a river and an unconfined aquifer. The coupled flow and transport models are finally applied to the lower Altamaha watershed in southern Georgia. The flow model is used to perform simulations of hydrologic and hydraulic conditions along the river and in the dynamically linked surfacial aquifer. The model predicted the flood patterns including the magnitude of peaks and their arrival times with sufficient accuracy. Under the given flow conditions, the transport model is then implemented to test alternative contaminant transport patterns both in the river and within the aquifer.