In many cases, dewatering problems and designs can be adequately solved with simple analytical models. It should be noted that Equation (7.2) will only give adequate results for h when r is greater than The height of the phreatic surface (drawdown curve) at any distance from the well can be estimated as Where Q is the flow rate, k the coefficient of permeability, H the height of original water table (saturated thickness), the height of water table at the well, the radial distance of the zone of drawdown influence, the radius of the well.įigure 7.8 Idealized drawdown in a water table aquifer. In the theoretical or idealized case for a well in a water table aquifer, the discharge rate can be approximated using a simple equation (Equation and geometries as depicted in Figure Three critical parameters of interest are the location of the drawdown curve (identifying the zone where dewatering will take place), the lateral extent of drawdown influence, and the discharge flow rate. According to this method, the hydraulic gradient is equal to the slope of the drawdown curve at any point. A simple method for estimating the hydraulic gradient can be made using the Dupuit-Theim approximation. Second, Darcy’s Law is assumed to be valid such that all flows are considered to be laminar, and that the flow rate is directly proportional to soil permeability and hydraulic gradient. First, the water bearing strata, or is assumed to be horizontal. For the most basic analytical analyses of water drainage to wells, a number of simplifying assumptions are usually made. For more complex situations, numerical models are often used for better accuracy. Depending on the complexity of the aquifer or soil mass from which water is to be drawn, dewatering design can use simplified analytical models, which inherently assume pumping is in equilibrium or a steady state of flow. The combination of these estimates will play a vital role in designing an adequate dewatering system, including placement and spacing of wells or wellpoints. In order to design any dewatering pumping system, one must be able to estimate the total amount of water that needs to be pumped in order to achieve the desired project goals, and the amount of water that can reasonably be expected to be drawn from an individual well. In order to better understand the requirements and capabilities of a dewatering system, one needs to first understand the basics of well hydraulics and soil water storage/discharge capacity. 7.3 Well Hydraulics and Dewatering Design
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