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).
|Fig 1. Three ponds (represented by circular drain elements) connected by short stream segments (represented by linear drain elements).|
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).
|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.
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.
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.
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