Log jams alter gradients in hydraulic head, increase the area available for hyporheic exchange by creating backwater areas, and lead to the formation of multiple channel branches and bars that drive additional exchange. Here, we numerically simulated stream‐groundwater interactions for two constructed flume systems—one without jams and one with a series of three jams—to quantify the effects of interacting jam structures and channel branches on hyporheic exchange at three stream flow rates. In simulations without jams, average hyporheic exchange rates ranged from 2.1 × 10−4to 2.9 × 10−4 m/s for various stream discharge scenarios, but with jams, exchange rates increased to a range of 1.3 × 10−3–3.5 × 10−3 m/s. Largely due to these increased hyporheic exchange rates, jams increased stream‐groundwater connectivity or decreased the turnover length that stream water travels before interacting with the hyporheic zone, by an order of magnitude, and drove long flow paths that connected multiple jams and channel threads. Decreased turnover lengths corresponded with greater reaction significance per km, a measure of the potential for the hyporheic zone to influence stream water chemistry. For low‐flow conditions, log jams increased reaction significance per km five‐fold, from 0.07 to 0.35. Jams with larger volumes led to longer hyporheic residence times and path lengths that exhibited multiple scales of exchange. Additionally, the longest flow paths connecting multiple jams occurred in the reach with multiple channel branches. These findings suggest that large gains in hydrologic connectivity can be achieved by promoting in‐stream wood accumulation and the natural formation of both jams and branching channels.
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 () below 0.06 and increased by a factor of five with increasing at . 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 . Further, we observed that at , 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.
more » « less- NSF-PAR ID:
- 10476027
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 59
- Issue:
- 11
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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