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- Title
Impacts of Channel‐Spanning Log Jams on Hyporheic Flow.
- Authors
Huang, S. H.; Yang, J. Q.
- Abstract
In‐stream wood structures, such as single logs, river steps, and debris dams, are known to drive hyporheic flow, defined as the flow that goes into the subsurface region and then back to the free‐flowing surface water. The hyporheic flow plays an important role in regulating water quality and biogeochemical cycles in rivers. Here, we investigated the impact of a channel‐spanning porous log jam, representing piles of wood logs, on hyporheic flow through a combination of direct visualization and theories. Specifically, we developed a method using refractive index‐matched sediment to directly visualize the hyporheic flow around and below a porous log jam, formed by piles of cylindrical rods, in a laboratory flume. We tracked the velocity of a fluorescent dye moving through the transparent sediment underneath the log jam. In addition, we measured the water surface profile and the spatially varying flow velocity near the log jam. Our results show that the normalized log jam‐induced hyporheic flux remained smaller than 10% at Froude numbers (Fr $\mathit{Fr}$) below 0.06 and increased by a factor of five with increasing Fr $\mathit{Fr}$ at Fr>0.06 $\mathit{Fr} > 0.06$. We combined the mass and momentum conservation equations of surface flow with Darcy's equation to explain the dependency of the log jam‐induced hyporheic flux on Fr $\mathit{Fr}$. Further, we observed that at Fr>0.06 $\mathit{Fr} > 0.06$, the water surface dropped noticeably and the turbulent kinetic energy increased immediately on the downstream side of the log jam. These findings will facilitate future quantification of hyporheic flow caused by channel‐spanning porous log jams. Plain Language Summary: Log jams are trees that frequently fall and accumulate in rivers. Field surveys and numerical simulations suggest that log jams slow down the surface flow and drive hyporheic flows, which are bidirectional flows that go into the riverbed and back to the surface water. Hyporheic flows carry pollutants and nutrients and thus play a critical role in water quality and river biogeochemical cycles. Despite the importance of hyporheic flows, the quantitative characterization of log jam‐induced hyporheic flows remains incomplete. In this study, we conducted experiments in a water‐recirculating flume with a log jam model that resembles piles of wood logs commonly found in rivers. We injected a fluorescent dye into a transparent sediment bed made from hydrogel beads and visualized the flow within the sediment bed. Our experimental results show that log jams can increase the hyporheic flow rate by one order of magnitude. Further, we developed a theoretical model to explain the hyporheic flow induced by a log jam. Our experimental results and theoretical model will facilitate the evaluation of the impact of log jams on the fate and transport of nutrients and contaminants in rivers for future restoration projects. Key Points: The hyporheic flow velocity in a channel with a log jam was two orders of magnitude higher than that of a bare channelAt Froude numbers (Fr $\mathit{Fr}$) above 0.06, the nondimensional log jam‐induced hyporheic flux increased by a factor of five with increasing Fr $\mathit{Fr}$Similar to the hyporheic flux, the water surface profile and turbulence intensity changed noticeably at Fr $\mathit{Fr}$ above 0.06
- Subjects
BIOGEOCHEMICAL cycles; FLOW visualization; FLOW velocity; FROUDE number; STREAM restoration; CONSERVATION of mass; DAMS
- Publication
Water Resources Research, 2023, Vol 59, Issue 11, p1
- ISSN
0043-1397
- Publication type
Article
- DOI
10.1029/2023WR035217