As described in Part 1 of this series, Drain/Fracture elements are useful internal boundaries for easily modeling a variety of hydrologic features including collector trenches, interconnected ponds, and transmissive fracture or fault zones.
This example will demonstrate the use of a drain/fracture line element to connect a series of ponds in such a way that water flows freely between the ponds and keeps them hydraulically linked regardless of additions or withdrawals from one of the ponds.
A pond in direct communication with the aquifer
In this example, a series of three ponds are in direct
communication with the aquifer, and each pond is connected by a small stream. Because
the ponds are in direct communication with the aquifer, groundwater is able to
easily flow through the ponds as if they were a high hydraulic conductivity
unit within the aquifer. And because the three ponds are linked by streams, the
water level within all three ponds is nearly identical. This simple model uses
five drain elements with high conductance values to represent the three ponds
and two connecting streams (Fig 1). The Drain elements conduct water along
their length effectively causing the three ponds to have roughly equal surface
elevations (Fig 2).
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| Fig 1. Three ponds (represented by circular drain elements) connected by short stream segments (represented by linear drain elements). |
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Fig 2. Groundwater elevations in the vicinity of the ponds
and connecting stream. Note that the easy flow among the ponds transmitted by
the stream drain segments maintains the elevation of water in the ponds, and
elevation of the adjacent groundwater in the aquifer, very close to 90 ft msl. |
Increasing or decreasing the conductance of the drain
elements can be used to control the elevations of the ponds relative to each
other. A higher conductance will cause the three pond surface elevations to be
closer together, while using a lower conductance will cause the three pond surface
elevations to be further apart. If we increase the conductance of the model
shown in the previous figure by a factor of 10, we can see that the 90-foot
contour extends around the first pond, rather than cutting between the first
and second ponds (Fig 3).
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| Fig 3. Increasing drain element conductance in the model enhances the hydraulic connection among the ponds, and all elevations more closely approximate 90 ft msl. |
Similarly, decreasing the conductance of the original model by a factor of 100 causes additional contours above and below the 90-foot contour to separate the ponds, resulting in a larger hydraulic gradient between the three ponds.
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Fig 4. Decreasing drain element conductance in the model
reduces the hydraulic connection between the ponds and the aquifer, and from
pond to pond, causing a significant decreasing head condition through the pond
system from left to right. |
In the above examples, the conductances of all five drains used to represent the ponds and connecting streams were changed uniformly, but each drain segment can be adjusted individually to better represent observed conditions.
Summary
Drain/fracture elements can easily represent ponds and networks of ponds that are in direct communication with the aquifer and hydraulically linked to each other. Using a high conductance for the drain/fracture elements (on the order 1 Million to 10 Million ft2/d) will cause ponds to have approximately equal surface elevations. Ponds at different surface elevations can be manual matched by adjusting the conductance of some or all of the drain/fracture elements to match observed surface water elevations. Representing ponds and streams using drain/fracture elements assumes that the surface water bodies are in direct communication with the aquifer and are neither extracting nor supplying water to the aquifer system. If the ponds are in-fact a source or sink in the aquifer system, spatially variable area source/sinks should be used for separate pond domains and river line boundaries should be used for the streams.
Tips and Tricks
- For larger ponds, adding one or more drain lines within the interior of the pond will help to maintain heads across the entire pond.
- To represent water extraction from within one of the ponds, a well or constant flux line element can be placed within the pond at a specified extraction rate.
