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.