Effects of stream discharge, alluvial depth and bar amplitude on hyporheic flow in pool-riffle channels

Hyporheic flow results from the interaction between streamflow and channel morphology and is an important component of stream ecosystems because it enhances water and solute exchange between the river and its bed. Hyporheic flow in pool-riffle channels is particularly complex because of three-dimensional topography that spans a range of partially to fully submerged conditions, inducing both static and dynamic head variations. Hence, these channels exhibit transitional conditions of streambed pressure and hyporheic flow compared to previous studies of fully submerged, two-dimensional bed forms. Here, we conduct a series of three-dimensional simulations to investigate the effects of bed topography, depth of alluvium, and stream discharge on hyporheic flow in pool-riffle reaches with variable bed form submergence, and we propose three empirical formulae to predict the mean depth of hyporheic exchange and characteristic values of the residence time distribution (mean and standard deviation). Hyporheic exchange is predicted with a three-dimensional pumping model, and hyporheic flow is modeled as a Darcy flow. We find that the hyporheic residence time is well approximated by a lognormal distribution for both partially and entirely submerged pool-riffle topography, with the parameters of the distribution defined by the mean and variance of the log-transformed residence time. Depth of alluvium has a substantial effect on hyporheic flow when alluvial depth is less than a third of the bed form wavelength for the conditions examined.