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Unformatted text preview: on effects, the model assumes that the cell contains
a single composite well screen (or open interval) having a
length equal to the sum of the lengths of all sections of screen,
an effective well radius equal to the lengthweighted average
radius, and an effective celltowell conductance as described
above. The composite well screen is generally assumed to
be centered vertically halfway between the top of the highest individual section of well screen and the bottom of the
deepest individual section of well screen (fig. 41). However,
if the uppermost active layer is unconfined (or convertible)
and contains a partially penetrating composite well screen, it is positioned in the cell so that the bottom of the composite
screen is aligned vertically with the bottom of the lowermost
screened interval within the cell (fig. 42). This assures that the
well will continue to be connected to the cell if the calculated
watertable elevation is above the bottom of the deepest part
of the actual screen. Similarly, if the uppermost active layer
is unconfined (or convertible) and contains a partially penetrating well screen defined by a partial penetration fraction
for a node of a multinode well, then the composite screen
is positioned in the cell so that the bottom of the screen is
aligned vertically with the bottom of the cell. Note also that in
these cases, the userdefined partial penetration fraction and
the initial saturated thickness of the cell are used to calculate
the length of the screen. This length remains constant with Figure 41. Diagrammatic cross section showing wells with complex (nonuniform) well screens or open intervals within a model layer. Well A
has uniform open intervals in model layers 1 and 3, but sections with two different well radii in model layer 2. Well B has two short well screens
with different characteristics within a single model layer, which in MNW2 would automatically be replaced by an equivalent composite well
screen that is partially penetrating (Well C). Figure 42. Schematic cross
section of an unconfined aquifer
showing a multinode well and
the relation of multiple screens
in the uppermost model layer A,
to the length and position of the
reconstructed single composite
screen in model layer 1. B, the
position and length of the part of
screen 2 in model layer 2 remain
fixed. Model Features and Processes
time and provides the basis for adjusting the partial penetration fraction if the water table changes position over time in a
transient flow simulation.
Given the effective celltowell conductance, the effective
value of the combined wellloss terms in equation 15 can be
computed as
(39)
An updated value for the celltowell conductance that
includes the effects of partial penetration can subsequently be
calculated using equation 25.
MODFLOW allows the simulation to include a quasithreedimensional representation of a groundwater system;
typically, in this mode, a confining unit is not represented
explicitly, but instead is represented with an adjusted vertical
conductance value connecting the overlying layer with the
underlying layer. If a multinode well passes through such a
nonsimulated confining unit, MNW2 assumes that blank casing exists within that interval. Flow Routing in Borehole
Flow magnitude, flow velocity, and solute concentration
can vary within a long borehole in which inflows and outflows
vary along the well screen or open interval. Borehole flow
meter surveys and sampling devices (for example, see Izbicki
and others, 1999; Paillet, 2001) can collect such data in the
field. If the model could calculate these variables, it would
provide another type of dataset against which observations
could be compared and the reliability of the model assessed
or demonstrated. Therefore, the capability to route flow and
solute through a multinode well has been added. Because
MNW2 does not compute head losses within a borehole, flow
is routed using a relatively simple massbalance approach
similar to that used in the Streamflow Routing Package to
route flow downstream from one stream reach to the next (Prudic, 1989; Prudic and others, 2004). MODFLOW calculates
the flux between the aquifer and the well at every node. After
the numerical solution is obtained for a given time step, the
flow from one well node to the next is determined from the net
of the exchange with the aquifer and inflow from the previous
node.
The MNW1 Package inherently assumes that if a pump
is present in a long borehole, the pump intake is located above
the first node of the multinode well. The MNW2 Package
adds the flexibility of specifying a pump intake (or discharge
point for an injection well) location at any depth or associated with any single node of the multinode well. This will not
affect the calculated head in the well, but will, of course, affect
how flow (and solute, if the GWT process is active) is routed
through the borehole. Therefore, the net discharge from a well
at the location of a pump intake represents another element
that must be accounted for in the massbalance approach...
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This document was uploaded on 01/20/2014.
 Winter '14

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