MULTI-SCALE HYPORHEIC INTERACTIONS IN POOL-STEP-RIFFLE SEQUENCES: IMPLICATIONS FOR FIELD STUDIES AND STREAM RESTORATION (Invited Presentation)

There is growing interest in hyporheic exchange and biogeochemistry due to the hyporheic zone’s unique function as a hotspot for attenuation of anthropogenic contaminants. However, we still cannot reliably predict hyporheic exchange fluxes and residence times in unstudied reaches. Many numerical and field studies have characterized hyporheic exchange associated with individual natural and artificial hydraulic structures (i.e., local scale), and some also consider uniform gaining and losing conditions (i.e., regional scale). 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 intermediate scale 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 contaminant attenuation. Here we present results from a 2D numerical modeling study of stream-hyporheic interactions in COMSOL Multiphysics. We considered one idealized pool-riffle-step series, and one surveyed reach from WS01 in the HJ Andrews Experimental Forest. In both simulations we systematically manipulated the height of individual features to evaluate their influence on neighboring structures. Our results demonstrate that the topology of the structures matters: each structure impacts hydraulic head gradients in both the upstream and downstream directions, with the length of influence depending on channel characteristics. 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. We place these results in the context of a hyporheic restoration in Seattle, WA with documented multi-scale interactions, and their relevance for improving experimental design and hyporheic function.