if the user indicates that partial penetration

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Unformatted text preview: imation, the application of the correction is optional, and the user must specify whether or not the heads or water levels in a multi-node well should be corrected according to the algorithm described in equation 25. (The effects of partial penetration are included in MNW2 calculations if the user specifies a value of input variable PPFLAG greater than 0 in input item 2b.) If the user indicates that partial penetration corrections should be made, and the open intervals of a vertical well are specified in terms of the elevations of the tops and bottoms of one or more open (or screened) intervals in a long well, then MNW2 will calculate the length of open interval associated with each node of the multi-node well and automatically Figure 17. Plot showing sensitivity of total intraborehole flow (Q) to variations in the penetration fraction for the uppermost node of the multi-node well in the Reilly problem. 19 calculate the penetration fraction at all nodes of the well. If the user indicates that partial penetration corrections should be made, but nodes are specified in terms of model layers, then the user must also explicitly define the penetration fraction associated with every node of that multi-node well (using input variable PP in item 2d-2 of the MNW2 input). Seepage Face In the MNW2 Package, it is normally assumed that flow between the well and each cell associated with the well is driven by the head difference between the well and the respective cell. In an unconfined aquifer, however, it is possible that the water level in the well can be computed to lie some distance below the water table. Chenaf and Chapuis (2007) note that when a well is pumping from an unconfined aquifer, “…the water table usually does not join the water level in the well. There is a seepage face inside the well, which is a key element in evaluating the well performance.” Thus, in a numerical model, if the computed water level in a well in an unconfined aquifer drops in elevation over time, then its position may fall into the model layer underlying the cell containing the water table (fig. 19). This creates a seepage face and a disconnect in the hydraulic continuity between the saturated zone in the cell and the water in the well. When the computed water level in the well falls below the bottom elevation of a cell, the normal assumption in the model that the flow is driven by the hydraulic gradient—defined by the difference between the water-table elevation and the water level in the well—would be an overestimation of the driving force because of the loss of continuous saturation between the water in the upper cell and the water in the well. Under such conditions, a seepage face can be surmised to occur in the upper part of the well screen—at least in the cell containing the water table. When such a seepage face is detected by the model, flow in that cell from the saturated zone into the multi-node well is simulated as a general-head boundary, and the flow is Figure 18. Plot showing flux into or out of each node of the borehole for penetration fractions of the top node of the well of 0.0, 0.5, and 1.0 in the Reilly test problem. Negative values represent flow from the aquifer into the borehole and positive values represent flow out of the borehole and into the aquifer. 20 Revised Multi-Node Well (MNW2) Package for MODFLOW Ground-Water Flow Model computed on the basis of the cell-to-well conductance (based on the saturated thickness of the cell containing the water table) and the hydraulic gradient as defined by the difference between water-table elevation in that grid cell and the elevation of the bottom of the cell. Figure 19 also shows that a small seepage face develops in model layer 2 in that hypothetical example. However, the model does not treat this seepage face any differently than normal because the cell is fully saturated and the water level in the well is above the bottom elevation of the cell, so the governing hydraulic gradient would be the same over the entire thickness of the cell. If both the water level in the well and the head in the grid cell fall below the bottom elevation of the cell, then the model will assume that hydraulic continuity is lost. Therefore, no flow is allowed between the aquifer and that particular node of the multi-node well under those conditions. The effect of computing the presence of a seepage face is illustrated with an example based on the Reilly test problem. To generate a seepage face condition, the Reilly problem as described above was modified to convert it to a transient flow problem with a pumping rate (Q = -10,000 ft3/day) sufficient to generate drawdown in the multi-node well so that the water level in the well would decline to a level below the bottom elevation of the first cell connected to the multi-node well. Additionally, the hydraulic conductivity of the skin was set equal to one-tenth of that in the base case (KSKIN = 12.5 ft/day). The modified parameters for this te...
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This document was uploaded on 01/20/2014.

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