Hyporheic Exchange in a Straight Stream with Alternate Bars

Hyporheic exchange (HE) is essential to riverine ecosystem, water quality, and energy cycling in a stream due to its governing role in physicochemical and biological processes in hyporheic zone (HZ). Alternate bars, characterized by a repetitive sequence of diagonal bars with a pool-riffle bedform, are common geomorphic features that generally form in straight streams. Following similar laboratory and numerical methodology as Huang and Chui (2021, ) who investigated the HE induced by pool-riffle sequences, this study focused on the HE induced by alternate bars (i.e., geomorphic features on stream banks instead of streambed) where the influences of key factors were systematically examined. The results show that alternate bars strongly influence surface flow and induce significant hyporheic flows in both stream banks and streambed, whose patterns, magnitudes, and influential patterns of key factors could be similar to the HE induced by pool-riffle sequences. The HE flowrate increases linearly with discharge, bed slope, and bar spacing. However, influential patterns of bar width on HE vary, and the bar width changes the influential patterns of discharge and bed slope on HZ scale, highlighting the influence of bar width in the complex interactions among discharge, bed slope, and bar width. All indicators of HE characteristics generally increase with bar spacing and follow exponential relationships with groundwater flow. Findings in this study lay the foundation in research on riparian ecosystems and water quality in the streams with alternate bars and help guide the design of bars during river restorations.