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Unformatted text preview: ts associated node of
the finitedifference grid. For example, when drilling a well, it
is not unusual that the driller will use a larger diameter drill bit
and casing for the upper part of the hole and then reduce the
bit size and borehole diameter at greater depths. Thus, depending on the relative elevations of the tops and bottoms of model
layers, it is possible that the diameter (and radius) of the open
interval of a well may change within the thickness interval
represented by a particular model layer (fig. 41A). It is also
possible that a well may have been constructed with multiple
screens and that a well may have different skin properties for
different intervals. If the lengths of the screens are short relative to the thickness of a model layer representing an aquifer, then it is possible that more than one screen of a long well
may lie within a single model layer (fig. 41B).
Well A (fig. 41) fully penetrates all three model layers
into which the aquifer is discretized. However, in this example
the larger diameter screen penetrates the upper 40 percent of
model layer 2, and the smaller diameter screen penetrates the
deeper 60 percent of this cell. In cases like this, the MNW2
Package will compute a celltowell conductance value for
each section of screen (or open interval) that has unique properties and then compute a lengthweighted average conductance value for that node as (37) where CWCn is the effective celltowell conductance for the
node, CWCi is the calculated celltowell conductance for
the ith section of screen, k is the number of sections of screen
(open intervals) in the cell, and Li is the length of each section
of screen. The average effective well radius would be calculated in the same manner.
If the specifications of top and bottom elevations of
well screens indicate that there are multiple well screens
within a single model layer (fig. 41B), then the model will
compute the celltowell conductance for each interval and
then compute a lengthweighted average conductance value
for that node using equation 37. Conductances are normally
computed under an assumption that the screen is open to the
full thickness of the model layer in which it is located. In the
case where the sum of the lengths of the multiple well screens
within the cell is less than the thickness of the cell (such as
Well B in figure 41), the effective conductance of the combined length of well screen must be adjusted for the reduced
total length relative to the thickness of the cell, and the effects
of partial penetration on heads should also be considered. That
is, if the sum of the lengths of the well screens in Well B for
example, is 40 ft and the thickness of the cell is 100 ft, then
the celltowell conductance would only be 40 percent of that
for a fully penetrating length of well screen. Therefore, in such
cases, equation 37 above must be modified to account for the
effect on celltowell conductance of having a shorter total
length of screen than the thickness of the cell as (38) where bn is the thickness of the cell. In the case where celltowell conductance values for each screen interval are explicitly specified by the user (under the option in dataset 2b of
LOSSTYPE=SPECIFYcwc), it is assumed that the specified
value of conductance is already appropriate for the actual Model Features and Processes 37 Figure 38. Schematic crosssectional view (looking west) of part of a MODFLOW grid containing a multinode well that changes direction and
is connected in sequence from node “j,i,k” to node “j,i +1,k+2.” The casing is assumed to be closed off to both cells in model layer k +1 through
which the well passes. Each node of the well consists of two segments. A borehole is always assumed to extend vertically from its first node to
the top of the model grid.
Figure 39. Schematic crosssectional view through a
MODFLOW grid showing a
mostly horizontal multinode
well open to columns 2 through
5 in layer 5 and column 1 in
layer 4 and assumed to extend
vertically from the first node to
the top of the model to compute
total borehole length. The well
changes direction at nodes 1
and 2, and the two segments
associated with these two
nodes are of unequal lengths;
Lw1 and Lw2 are labeled for node
2. Modified from Konikow and
Hornberger (2006b.) Figure 40. Schematic cross
section of an unconfined aquifer
represented by three model
layers, showing the relation of
the open and closed intervals
(Lw and Lc, respectively) to
the position of the water table
within unconfined model layer
1. A, at the initial time (t 0), the
water table coincides with the
top of the vertical well screen
segment and Lw equals the
sum of the lengths of the two
segments; B, at a later time (t 1),
the water table has declined and
part of the well screen is dry, so
Lc has increased and Lw has
decreased compared to t0. 38 Revised MultiNode Well (MNW2) Package for MODFLOW GroundWater Flow Model length of the screen, and no further adjustments are made for
its length relative to the layer thickness.
To compute the additional drawdown due to partial
penetrati...
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 Winter '14

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