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Unformatted text preview: st are listed in table 3. All other
parameters are identical to the base case for the Reilly problem. The transient simulation was run for a stress period of
150 days, using 12 time steps and a time-step multiplier of 1.3.
The initial heads were defined by the solution generated in a
preliminary steady-state stress period in which the multi-node
well was assigned a net discharge of zero.
This test case was simulated using the described correction for the presence of a seepage face and without the correction (assuming that driving force for flow between the aquifer
and the well is always the head difference between the hn and Table 3. Modified parameters in Reilly problem for evaluating
seepage face calculation in multi-node pumping well.
[ft/d, feet per day; ft, feet; ft3/d, cubic feet per day] Parameter
Specific storage (all layers)
Specific yield (layer 1)
-10,000 ft3/d hWELL). As seen in figure 20, hWELL did not drop below the
bottom elevation of the cell associated with the top node of the
well (-10.0 ft) until the ninth time step (at about 65 days). (Note
that this figure shows changes during the transient stress period,
and not for the preliminary steady-state stress period.) From
then on, there was a small difference in the computed value of
hWELL between the uncorrected and corrected cases, with the
head in the well being somewhat lower when the correction is
made. During this time, the head in the top cell connected to
the well (located in model layer 2) remained above the bottom
elevation of the cell through the 11th time step (at about 114
days) while the computed water level in the well was below
the bottom elevation of that cell—creating the conditions for a
seepage face to develop. During the 12th and final time step, the
head in this cell dropped below the bottom elevation of the cell
(-10.0 ft), thereby disconnecting it from the well. Also during
the final time step, hWELL dropped below the bottom elevation
of the cell in layer 3 (-15.0 ft), thereby creating a seepage face
condition in that cell, which contains the second node of the
multi-node well. If hWELL drops below several layers of active
nodes, then the model will allow the development of multiple
seepage faces and compute adjusted inflows accordingly.
The flow into the top node of the well was more substantially affected by the implementation of the correction (fig. 21)
than were the heads. The inflow from the aquifer into node 1 of
the well (located in model layer 2) decreased slightly over time Figure 19. Schematic cross
section of A, an unconfined
aquifer showing a multi-node
well open to parts of the
uppermost five model layers,
and B, the relation of the
screens (open intervals) to
lower water levels at a later
time (t1), when the water level
in the well has fallen below
the bottom elevation of model
layer 1. Model Features and Processes 21 Figure 20. Computed changes
in head in the multi-node
pumping well (hWELL) and in the
aquifer cell connected to the
uppermost node of the multinode well (hn) for the modified
Reilly problem, both with and
without a correction for the
development of a seepage face
in the uppermost cell. Top node
of well located in model layer 2.
Results of preliminary steadystate stress period represent
initial conditions for the 150day transient stress period
shown in this figure. through the eighth time step as the head in the aquifer and the
water level in the well both declined. However, once the water
level in the well dropped below the bottom elevation of the cell,
the rate of decrease in the inflow was much greater with the correction on, which resulted in a lower flow rate to the well compared with the simulation without the correction. Because the
net discharge from the well is specified, as the inflow decreases
in node 1 there are compensating increases in inflow at the other
nodes of the multi-node well (fig. 22). Once the cell in model
layer 2 goes dry (or when the head in the aquifer falls below
the bottom elevation of the cell if the cell is not convertible, as
in this example), the inflow to the well from that cell ceases. In
this example, this occurs during the last time step (at 150 days,
as seen in fig. 21). Overall, the correction for the development
of a seepage face, as described above, yields changes that are
logically consistent with expectations based on well hydraulics.
Although this example problem shows only small effects of the
seepage face, in other situations, the effects can be much greater.
The calculations related to a seepage face are performed
automatically by MNW2 whenever it detects the presence of
conditions for which a seepage face is expected to occur, as
described above. The model user does not have an option to
control this feature. Constraints on Pumping Rate
The range over which the water level in a well can
potentially change may not be unbounded. For example, in discharging wells, pumping cannot continue if the water level
drops below the depth of the pump settings and screen intakes.
In recharging wells, the water lev...
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This document was uploaded on 01/20/2014.
- Winter '14