Monday, April 29, 2013: 9:00 a.m.
Regency West 6 (Hyatt Regency San Antonio)
One typical approach to parameterizing a MODFLOW groundwater model is to assign model layers to stratigraphic units and vary the thickness and elevation of the model layers accordingly. This approach is well suited to systems with simple horizontal layer stratigraphy (e.g., large-scale alluvial-fill basins) but is less well suited to cases involving more complex small-scale stratigraphic and topographic settings. For example, systems with units that dip or pinch out, variable thickness beds, or incised topography are difficult to represent with grids whose layers strictly follow unit boundaries, as those grids inevitably require certain cells to have near-zero thickness, resulting in numerical instability. Use of head-dependent flux boundary conditions with these variable-thickness layers can cause further instability and, in some cases, prevent model closure. In this study we examine the use of model grids with uniform thickness layers and weighted cell properties for representing these complex topo-stratigraphic systems. Grid-independent parameterization followed by interpolation of properties onto the grid allows for arbitrarily complex parameter zonation. For cells above the land surface we assign synthetic hydraulic properties that approximate a porous medium with negligible resistance to flow (i.e., high hydraulic conductivity [“K”]) and zero skeletal volume (i.e., unity storage coefficient). Use of such properties allows those cells to approximate free-surface water bodies when wetted. This “high-K” approach is first examined under several simplified hypothetical steady-state scenarios, including 2-D vertical slice flow through a hillslope, 3-D convergent flow towards a closed basin, and 3-D convergent flow towards a stream. Next, a real world example involving inclined stratigraphic units, hilly terrain, and several large surface water bodies is presented. While the high-K approach lacks the robustness of fully-coupled saturated/ unsaturated/surface water models, it is easy to implement with the widely-used and accepted MODFLOW code and allows approximation of certain surface water/groundwater systems with complex geometries.
| Christopher S. Heppner, Ph.D. , Erler & Kalinowski, Inc. | |
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Christopher S. Heppner is Associate Hydrogeologist with Erler & Kalinowski Inc. in Burlingame, California. He received his Ph.D. in Hydrogeology from Stanford University. His work has included groundwater flow and solute transport modeling in support of hazardous waste site investigations and remedial action efficacy evaluations; aquifer test planning, performance, and analysis; watershed hydrologic response and sediment transport modeling; research into unsaturated flow and recharge processes; and development and application of hydrologic data analysis tools. Heppner’s previous positions include graduate teaching and research assistantships at Stanford, and Hydrologist in the Unsaturated Zone Flow research group at the USGS in Menlo Park. |
| Vera H. Nelson, P.E. , Erler & Kalinowski, Inc. | |
| Ms. Nelson is a registered civil engineer with more than twenty-six years of professional consulting experience performing hydrogeologic studies and remedial investigations, managing implementation of soil and groundwater remedial actions, and assisting with environmental aspects of industrial property redevelopment. She has performed extensive aquifer testing and analytical and numerical modeling to evaluate subsurface chemical transport and to design extraction and other groundwater remedial systems. She has performed water supply assessments and water resources evaluations for municipalities and developers of non-contaminated properties. She has also developed human-health risk assessments, engineering cost estimates, and risk management plans for remediation of industrial properties. | |
