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Unformatted text preview: EAS 44600 Groundwater Hydrology Lecture 12: Well Hydraulics 1 Dr. Pengfei Zhang Radial Flow to a Well The exploitation of a groundwater basin leads to water-level declines that limit the yields of the basin. Therefore, one of the primary goals of groundwater resource evaluation is to predict hydraulic-head drawdowns in aquifers under proposed pumping schemes. As mentioned in previous lectures, a pumping cone, or cone of depression , will form in the aquifer around a pumping well as the water level declines. In this lecture we will discuss how to compute the decline in the water level, or drawdown, around a pumping well if we know the hydrologic properties of the aquifer. Groundwater flows to a well as if it moves along the spokes of a wagon wheel toward the hub. In other words, flow toward a well is radial. Since hydraulic-head drawdowns around a well possess radial symmetry in an idealized system, it is advantageous to convert Cartesian coordinates to radial coordinates. In Cartesian coordinates, two-dimensional groundwater flow in a confined aquifer with transmissivity T and storativity S is: t h T S y h x h = + 2 2 2 2 (12-1) Equation 12-1 can be transformed to radial coordinates through the relation, 2 2 y x r + = , and the resulting equation is: t h T S r h r r h = + 1 2 2 (12-2) where h is the hydraulic head, S is storativity, T is the transmissivity, t is time, and r is the radial distance from the pumping well. Flow in a Completely Confined Aquifer As discussed in previous lectures, pumping from a completely confined aquifer will cause drawdowns of the potentiometric surface, and water is released from storage by expansion of water due to decreased fluid pressure and by expulsion of water due to aquifer compaction under increased effective stress. For a confined aquifer, the storativity ( S ) is small (0.005 or less), and pumping affects a relatively large area of the aquifer. Furthermore, if there is no recharge, the area of drawdown of the potentiometric surface will expand indefinitely as pumping continues....
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This note was uploaded on 12/29/2010 for the course EAS 44600 taught by Professor Pengfeizhang during the Spring '10 term at CUNY City.
- Spring '10