Estimating Streambed Hydraulic Conductivity Using Mini-Piezometers with Drilled-Holes Screens

Simple mini-piezometers built by drilling holes in pipe are commonly used in hyporheic research as a time- and cost-effective way of estimating streambed hydraulic conductivity K. Both common methods to compute the shape factor F (by Hvorslev and Bouwer & Rice - BR), allowing in turn for K estimation from Darcy's law, only require knowing the discharge, head, screen length, and well radius. However, these were derived for production wells, assuming fully opened slotted screens, so that actual screen features do not affect estimation (except for length). If drilled holes are used instead, their number, size, and spatial arrangement should affect how energy is dissipated as water flows in the medium. Moreover, the literature assumes that Darcy's law holds for the flow throughout the sediment, which is not the case directly below the holes. A detailed search could not retrieve any studies for mini-piezometers, experimentally assessing (i) F for injection experiments, (ii) the suitability of drilled-holes screens, or (iii) the spatial pattern of head dissipation around the screen.

We address these research gaps for a uniform, isotropic streambed material. Constant-head injection tests were performed in laboratory apparatus reproducing the case of radial well flow. We analyzed the performance of a series of drilled-holes screens by studying the effects of four screen parameters (length; number, diameter, and spacing of holes) on F. We also investigated the spatial pattern of head dissipation in the medium, experimentally determining the effective radius of dissipation, which in turn allowed for direct estimation of BR's shape factor.

We were able to correctly estimate the fairly large K value of our medium with drilled-holes mini-piezometers, even though the flow immediately past the screen holes was clearly transitional or turbulent. Using either Hvorslev's shape factor, as modified by Chapuis for bottom-plugged wells, or else our experimentally determined F yielded remarkably similar K values, as long as the proportion of openings was above 6% of total screen area. However, the shape factor proposed by BR should not be used, as it consistently underestimates K. We suggest that injection tests in mini-piezometers with drilled holes can be used to estimate K, as long as the correct shape factor is used.