MODFLOW Model of Groundwater Flow in Sloping Surficial Sand and Gravel
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Purpose and Scope of MODFLOW Webpage
The purpose of this nontechnical webpage is to present a MODFLOW model of groundwater flowing downslope in surficial sand and gravel over impervious bedrock. This type of groundwater system is of interest because of the common occurrence of contaminants moving downslope from sources on sloping ground toward streams and associated alluvial aquifers. Modeling groundwater flow in such systems is relatively difficult because the saturated zone between the water table and bedrock is often thin. This condition tends to cause failure of the MODFLOW modules that solve the system of equations describing the flow. The tendency to fail is due to repetitive drying and wetting of portions of the model during the iterative solution process. Instability is produced in the MODFLOW solver modules. Nevertheless, a successful model may be developed by experimentation with input variables that control the behavior of the solvers.
This page uses nontechnical language plus some technical language that is explained in the text . It also contains endnotes that provide technical details for readers who are interested. MODFLOW is a finite-difference groundwater modeling computer program maintained by the United States Geological Survey (USGS). A finite-difference model divides the groundwater system into rectangular cells, applies an equation that balances inflow, outflow, and change in water content for each cell, and solves the equations simultaneously to get hydraulic head in each cell. Hydraulic head at a point in a groundwater system may be thought of as the water level in a well if it got its water at that point. Groundwater moves in the direction of the head gradient, from higher to lower head. Thus MODFLOW can provide the rate and direction of groundwater movement.
MODFLOW Model
Figure 1 shows the topography of the area simulated in the MODFLOW model and the grid used in the finite-difference computations. The topography is shown as the elevation of the top of each grid cell. This is a sloping one-layer model. The layer represents the surficial sand and gravel. The thickness of the surficial layer is shown in Figure 2. This is also the height of the model cells. The water table is within this layer and the saturated part of the sand and gravel varies in thickness, as shown in Figure 3. Model cells west of the stream are inactive because all of the groundwater enters the stream rather than passing below it. There is no need to extend the active part of the model across the stream.