Flume Experiments with Refractive‐Index Matched Sediment Revealing the Impact of Two In‐Line Channel‐Spanning Logs on Hyporheic Exchange

In‐channel wood, a critical component of forested rivers, has the capacity to enhance hyporheic flow. This process facilitates the continuous exchange of gases, solutes, and nutrients across the sediment‐water interface, regulating pollutant transport and biogeochemical cycles in rivers. When two wood structures are in close proximity, the hyporheic flows induced by each log can interact, yet such effects remain largely uncharacterized. In this study, we investigated the impact of two in‐line channel‐spanning logs with a vertical gap above the sediment‐water interface on hyporheic flow through laboratory experiments conducted under various conditions. Specifically, we measured water surface profiles, surface flow fields, and hyporheic flow fields around logs with different center‐to‐center distances (). Our results demonstrated that when the center‐to‐center distance between two logs was less than 10 times the log diameter, the wakes of the two logs interfered with each other, resulting in a decrease in both hyporheic flow rates and the difference in water surface elevation. Furthermore, we demonstrated the relationship between the pattern of log‐induced hyporheic flow and the surface flow regime. Our results suggest that the hyporheic flow pattern induced by logs can be inferred from measurements of the surface flow patterns. Our findings will contribute to an improved estimation of hyporheic flow induced by logs distributed along river channels.