H4) Transient storage increases at higher discharge for all scenarios.
Numerical model simulations and flume runs suggest that discharge has complex effects on transient storage, likely depending on the timescale of storage. Both flume and models show an increase in backwater when discharge increases (Figure 3 and 6). We observed that increasing the discharge to the flume both can store more surface water and elevate the surface water level in front of a logjam (Figure 6). Similarly, the submerged area of the model increased by about 20% at higher flows for all configurations (Supplemental Table 5).
Mean arrival times of surface and subsurface flow paths decrease with increasing discharge in both the flume and model. In the flume, this indicates quicker surface water flow paths with greater discharge. In the model, this indicates faster subsurface exchange rates when discharge increases (Figure 2, 5, and 7). Across flume trials, skew significantly increases at low discharge compared to high discharge (p<0.0001 averaged over the number of jams or averaged over permeability). Numerical model simulations suggest that lower discharge also facilitates a greater magnitude of skew as logjam characteristics increase in complexity (i.e., when more jams are present and when a single jam is less permeable) (Figure 2, 5, and 7). We infer that that there is more Gaussian flow at high discharge as solutes moves quickly through the system and the breakthrough plots show a lack of heavy tails (see Supplemental Plots). Consequently, at low discharge, we observe more heavy tailing behavior. This may be a matter of how easy it is to resolve that smallest fraction of flows that stick around the longest and overall sensitivity in the flume/model. However, the combined results also suggest a more complex behavior that may depend largely on surface water flow in the backwater zone.
4 Discussion
Our primary objective was to evaluate the influences of changes in (i) logjam longitudinal distribution density, (ii) logjam permeability, and (iii) discharge on transient storage. Using a combination of physical and numerical modeling allowed for better constraint of both surface and subsurface flow paths. Results suggest that increasing the longitudinal distribution density and decreasing the permeability both increase transient storage, regardless of the discharge. This fits with what we expected to see based on past work. Ader et al. (2020) found that the effects of large wood on surface and subsurface transient storage may be very local, implying the need for multiple wood pieces and logjams to influence segment-scale transient storage. Our observations confirm this inference and suggest that multiple logjams lead to more distributed flow paths and more pervasive reach-scale exchange. Studies evaluating hyporheic interactions around beaver dams (e.g., Lautz et al.,2006; Briggs et al., 2013) indicate that stream reaches with beaver dams exhibit a greater degree of hyporheic interaction. Our results showing transient storage around low permeability logjams with permeability similar to that of a beaver dam support this understanding that a more tightly packed logjam can enhance transient storage.