Vertical positions of single node wells shown for

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: e computed potential net discharge exceeds the value of Qfrcmx. In MNW1 restarting was lagged one time step. 26 Revised Multi-Node Well (MNW2) Package for MODFLOW Ground-Water Flow Model Figure 26. Variations in nodal flux at the ninth time step (at about 65 days, when Qnet = 0.0) between the multi-node well and aquifer for modified Reilly problem with three additional single-node pumping wells nearby. Vertical positions of single-node wells shown for comparison. Positive values of flux indicate outflow from well and recharge to aquifer. Figure 27. Plot showing relation between computed net discharge (Qnet) from a multi-node well and computed head in the well (hWELL) for the case in which the well is subject to a constraint in which the limiting head (hlim) is set at -7.5 feet, additional nearby pumping wells cause additional drawdown in the multi-node well, and the minimum allowable pumping rate is set at -2,000 cubic feet per day. Average of heads in 12 aquifer nodes ( ) linked to the multi-node well are shown for comparison. Model Features and Processes 27 Figure 28. Plot showing relation between computed net discharge (Qnet) from a multi-node well and computed head in the well (hWELL) for case in which a second 150-day stress period, during which the three additional single-node pumping wells are shut off to allow water-level recovery, is added to the case previously described for figure 27. In this modified case, the minimum allowable pumping rate (Qfrcmn) is set at -1,000 cubic feet per day (ft3/day), and the threshold to restart pumping (Qfrcmx) is set at -1,500 ft3/day. Average of heads in 12 aquifer nodes ( ) linked to the multi-node well are shown for comparison. Selected time steps (TS) and stress periods (SP) discussed in the text are labeled. Flowing Wells: Special Application of Constraints on Pumping Rate The use of constraints can also be applied to simulate discharge from a free-flowing well (or a flowing artesian well). In nonpumped open boreholes, if the head in the well is above a limiting or controlling elevation (such as the elevation of the land surface or the top of the casing), then water should discharge from the aquifer and flow out of the borehole. The flow rate should be proportional to the head difference between the well and the control elevation. Thus, the discharge would decrease, and eventually cease, if the head in the aquifer at or near the well declines over time. To simulate a flowing well with the MNW2 Package, the user should set hlim equal to the controlling elevation and specify the desired discharge rate (Qdes) at an artificially very high value. With these constraints in place, the model will automatically compute the flow rate discharging from the well as a function of the head difference between hWELL and hlim, as long as the head in the well is above the controlling elevation. If hWELL drops below the controlling elevation, then the net discharge from the well will drop to zero, although intraborehole flow will still be allowed. Furthermore, if at a later time the head in the well rises to a level above hlim, then the well will begin to flow again at the land surface. To illustrate this capability of the constraint option within the MNW2 Package, a problem based on another variant of the Reilly test problem was used. A single-node nonpumping well was placed in layer 35, row 30, column 270 of the grid and a limiting head set at hlim = 0.0 ft. The node of this cell is located on the plane of symmetry (see fig. 4) at a depth of 172.5 ft below the top of the model, about 125 ft from the impermeable downgradient boundary, and about 9,875 ft from the upgradient boundary. This well was assumed to have a well radius of 0.1333 ft, a skin radius of 1.795 ft, and a skin hydraulic conductivity of 12.5 ft/day. After an initial equilibrium state is established, the same three single-node pumping wells as described above (for the problem with results described in figure 28) were active for a 150-day stress period and then all three were turned off for a subsequent 300-day stress period. The 150-day stress period was simulated using 15 time steps, and the 300-day stress used 25 time steps; a time-step multiplier of 1.1 was applied in both stress periods. 28 Revised Multi-Node Well (MNW2) Package for MODFLOW Ground-Water Flow Model The original upgradient multi-node well is not present in this simulation. The results of this 450-day transient simulation show that initially the head in the aquifer at the depth of the nonpumping single-node well is about 0.62 ft (fig. 29) and greater than the limiting head, which might represent the land surface elevation or the top of an open casing. Therefore, at this time, the well is free flowing at a rate of about -94 ft3/day. The head in the well is computed to lie at the limiting head (hWELL = hlim = 0.0 ft). Because of the drawdown caused by the discharge from this free-flowing well plus that from the three upgradient single-node pumping wells, both the head and the discharge contin...
View Full Document

This document was uploaded on 01/20/2014.

Ask a homework question - tutors are online