Increasing Streambed Permeability Around Vertical Steps to Enhance Hyporheic Exchange

In-stream structures can be used to promote hyporheic exchange, that is, the exchange of water and solutes between the stream and the underlying streambed. The modification of streambed permeability around these structures has been proposed to enhance their effectiveness in promoting hyporheic exchange. However, the topic has not been thoroughly studied yet. In this work, the effect of the increase of permeability around in-stream structures has been assessed via numerical models. Different combinations of variations in permeability and geometry of sediments around in-stream structures (steps) were tested to evaluate their potential for river management for improving hyporheic exchange. Structure performance is evaluated on the basis of hyporheic flow, vertical exchange velocity, and residence time distributions. Results showed that even when step-induced exchange velocity values in homogeneous sediments are small, the increase of permeability around the step can effectively enhance hyporheic flow. The numerical simulations showed that a longer high-permeability region resulted in smaller values of exchange velocity. The overall hyporheic flow was slightly larger for lengths of 0.3–0.7 m compared to 0.1 m, and the median residence time was slightly longer for lengths of 0.3–0.7 m compared to 0.1 m. The results also indicated that including low-permeability blocks upstream and downstream of high-permeability regions have no effect on hyporheic exchange. Additionally, the results indicate a moderate influence of the length of high-permeability region on hyporheic exchange and point out the importance of considering the trade-off between increased hyporheic flow and higher costs for replacing a larger volume of sediments.