Hyporheic exchange in streams : the influence of hydraulic and morphologic characteristics

Abstract

The hyporheic zone (HZ) is a region of saturated sediment where surface water, groundwater flow, and sediment actively interact with one another. The flow into and out of the HZ is called hyporheic exchange (HE), which is crucial to the physicochemical and biological processes in streams. Various geomorphic features such as pool-riffle sequences, meanders and alternate bars strongly influence the surface flow and induce significant HE in a stream. This study aims to systematically investigate the influence of various hydraulic and morphologic characteristics on the HE in streams with various geomorphic features, including a straight pool-riffle stream, a meandering pool-riffle stream and a straight stream with alternate bars. A literature review was first performed, and using previously published data, empirical equations for estimating various HE characteristics (i.e., mean depth, median residence time and flux) in a single pool-riffle sequence were derived. Then, laboratory models were built to investigate the HE using tracer experiments. And numerical models of both surface and groundwater flow were developed and validated with the laboratory data. Consequently, the validated numerical models were used to evaluate the influence of controlling factors on HE characteristics (i.e., flowrate, volume and flux-weighted mean residence time) in different streams. Key factors examined in pool-riffle streams included stream discharge, streambed amplitude and wavelength, sinuosity, bed slope and groundwater flow. For the straight stream with alternate bars, factors of stream discharge, bar width and spacing, bed slope and groundwater flow were evaluated. Results of the straight pool-riffle stream show that the HE flowrate and volume on stream banks (i.e., in floodplain) and streambed are significant and comparable for large stream discharge and the impacts of key factors are similar. The stream discharge, bedform amplitude and bed slope are found to directly affect the flow state in the pool, and the flow state accordingly determines the influential patterns of the three factors. The HE flowrate and volume increase linearly with discharge when the flow state in the pool remains subcritical. If the flow state in the pool transfers from supercritical to subcritical condition as bedform amplitude increases for large discharge or bed slope, the HE flowrate increases with amplitude in a certain range but above a specific magnitude it becomes insensitive to amplitude. Results of the meandering pool-riffle stream show that the HE flowrate on stream bank becomes larger than that on streambed for large stream discharge and sinuosity, and the HE on stream bank is more sensitive to these two factors. Effectiveness of sinuosity in inducing HE (i.e., HE flowrate divided by thalweg wavelength) has the lowest values at the sinuosity of 1.11. Results of the straight stream with alternate bars show that the alternate bars induce significant HE on both stream banks and streambed, whose flow pattern is similar to that in the straight pool-riffle stream. This study overall elucidates the basic processes that control the HE in streams with various geomorphic features and is beneficial to our understanding of the riparian ecosystem and the development of stream restoration techniques.