Chapter 11 Large-Diameter Wells Stephanie Fulton February 27, 2014

Large-Diameter Wells Difference from other methods Two methods Well storage previously assumed negligible Must be taken into account When is “large” diameter large? Two methods Fully penetrating well in a confined aquifer Partially penetrating well in an unconfined anisotropic aquifer

Papadopulos’s (1967) Curve Fitting Method Assumptions Confined aquifer Unsteady-state flow Fully penetrating large-diameter well so storage cannot be neglected

Papadopulos’s (1967) Curve Fitting Method (cont) Similar to other methods (Theis equation) except for the well function Well function F(u,α, r/rew) accounts for the size of the well

Papadopulos Type Curves For 1/u and α = (10-1, 10-2, 10-3), select a value for r/rew using look-up tables in Annex 11.1 α is a function of well radius and storativity For long pumping times, F(u,α, r/rew) can be approximated with the Theis equation well function W(u) (Equation 3.5)

Remarks Early drawdown data yields unreliable results Data curve can be readily matched with more than one type curve but estimated S values differ by an order of magnitude Transmissivity (KD) is less sensitive to the choice of type curve Large-diameter wells are often partially penetrating, in which case another solution is needed. Drawdown reaches a max when t > DS/2K Drawdown can be estimated using an equation analogous to Equation 10.7:

Boulton-Streltsova’s Curve Fitting Method Unconfined, unsteady-state flow Homogeneous, anisotropic, uniform thickness Partially penetrating large-diameter well Well diameter is not small so well storage cannot be neglected SY/SA > 10

Boulton-Streltsova’s Curve Fitting Method (cont) Type A curves Early-time drawdown Boulton and Streltsova (1976) developed a well function describing the first segment of the S-curve typical of unsteady-state flow in an unconfined aquifer

Streltsova Type Curves

Boulton-Streltsova’s Curve Fitting Method (cont) Type B curves Late-time drawdown Curves result from Streltsova’s equation for a small diameter, partially penetrating well in an unconfined aquifer Applicable for long pumping times when the effect of well storage is negligible Modifed form of the Dagan solution (1967):