Laboratory Characterization of Turbulent Hyporheic and Free Surface Flows Associated with Bars in Gravel Streams and Related Efforts to Improve Measurement Techniques

Abstract

Bars and pool-riffle sequences are common occurrences in gravel streams, and a major driving force affecting the exchange of groundwater and free surface water in upper river reaches. Such topographic features induce downwelling and upwelling hyporheic flows, essential to the preservation of the river ecosystem. The primary goal of this work is to address the lack of data on hyporheic flows induced by bars and their effect on the streamflow. An extensive laboratory study detailing both the hyporheic and free surface flows is presented. The work is limited to the study of a two-dimensional axisymmetric bar, 1.6 m long and 0.06 m high, over a substrate of finite thickness and subjected to a 0.18 m streamflow depth. It is found that over the upstream side of the bar, the hyporheic flow abstracts a significant amount (10.1%) of water from the free surface flow. Through the combined effect of suction and injection, a pronounced shear layer forms 0.01 - 0.015 m away from the bed surface, leading to intense turbulence activity on the downstream side of the bar. In particular, the region around the location where the hyporheic flow divide line meets the bed surface appears as a preferential region for the generation of eddies, that ultimately grow considerably as they are conveyed downstream, before dissipating. This study is also used as an opportunity to investigate means of acquiring more accurate measurements of streamflow instantaneous velocities using the Nortek Vectrino Profiler. For this purpose, two additional experimental studies are presented. These deal with the minimization or elimination of the saw-tooth pattern in vertical profiles of pertinent quantities (velocities and turbulence stresses), and ways to minimize the instrument's interference region near the bed.