Seepage Face Option

One of the traditional problems with many unconfined MODFLOW models is that there is no mechanism to alter the flow conditions if the water table elevation rises above the ground surface.  With MODFLOW 2000, the user is always required to specify the top elevation of layer 1 (which often corresponds to the ground surface elevation) as part of the input.  However, it is largely ignored by MODFLOW during the solution process and the final computed head is allowed to rise well above the ground surface.  Regions where the head is above the ground surface are displayed as "flooded cells" in GMS. 

In many cases, the presence of flooded cells simply means that the computed heads are too high and the model parameters need to be adjusted during the calibration process to bring them down to more reasonable values.  However, in many cases the water table is quite high relative to the ground surface and seepage face conditions exist.  In GMS v5.0, we have implemented a new source/sink type polygon in the Map Module for simulating seepage face conditions.  When this option is assigned to a polygon, all of the cells in the MODFLOW grid that are coincident with the polygon are assigned a drain object where the drain elevation is equal to the top elevation of the cell.  The conductance for the drain cells is computed from a user-defined factor (typically a large value indicating minimal resistance).  During the MODFLOW simulation, when the head is below the top elevation, the seepage face drains have no effect.  If the head rises above the top elevation, the water begins to exit through the drains and the computed head is constrained to be approximately equivalent to the top elevation (assuming a large enough conductance).