Strange (stream)bed fellows: How does feature-scale hyporheic exchange depend on neighboring features?

Hyporheic exchange and biogeochemistry are increasingly prominent topics in fluvial hydrology due to the hyporheic zone's unique function as a hotspot for biogeochemical cycling. Improved understanding of the geomorphologic and hydrologic controls on hyporheic exchange may enhance active management of hyporheic zones for enhanced water quality (e.g., via stream restoration). Accordingly, many numerical and field studies have quantified the hyporheic exchange flows associated with natural and artificial hydraulic structures, such as pool-riffle-step sequences, bedforms, and cross-vanes. However, these studies treat each structure individually and assume that its impact on hyporheic exchange is independent from its context within a stream. In other words, these studies do not consider the impact of topology-the arrangement of individual structures in relation to adjacent structures-on their hyporheic exchange function. This assumption of independence may reduce the accuracy of hyporheic models, thereby diminishing the function of restoration projects designed to optimize hyporheic exchange and pollutant attenuation. Here we present results from a numerical modeling study of an idealized stream and hyporheic zone in COMSOL Multiphysics. We considered a repeated pool-riffle-step sequence, but systematically manipulated the height of individual features to evaluate their influence on other, neighboring structures. Our results demonstrate that the topology of the structures matters in most cases, especially when the spacing between structures is reduced. Each structure impacts hydraulic head gradients in both the upstream and downstream directions, with the length of influence depending on channel and sediment characteristics. If the structures have overlapping lengths of influence, they no longer function independently of each other. Surprisingly, changes to a neighboring structure A can have a larger influence on a given structure B than changes to the structure B itself. These results highlight the importance of considering multiple structures and their spacing explicitly rather than assuming that their effects are independent and additive. In particular, upscaling studies should consider these feedbacks explicitly to ensure that they accurately represent hyporheic flow and biogeochemical processes of interest.