Figure 1. Flume configuration (9-m long, 1.2-m wide) for (a) longitudinal distribution density experiments and (b) permeability experiments. (a) shows the change in jam quantity from a single jam to multiple jams. (b) shows the change in permeability for a single logjam from a more permeable jam to a less permeable jam with coarse particulate organic matter additions. Data were collected at sensors above and below the experimental reach. However, we use the “below jam(s)” sensor data for results and analyses because they best represent the combined effects of logjam distribution density and permeability changes.
A list of all flume runs is included as Table 2. We implemented tracer experiments with sodium chloride (NaCl) to characterize surface transient storage flow paths via fluid conductivity. We added 360-g of NaCl dissolved in 3.78 L of water to the flume at the injection point (Figure 1) via a pulse injection. The ability to resolve transient storage depends in part on the relative travel times through the mobile portion of the channel and transient storage zones, as well as the exchange rates between them. Given our expectation of short transport times (on the order of minutes) in this flume-scale system, we deemed a pulse injection sufficient to load even some the slower flow paths with salt. The first 3-m of the flume served as a mixing zone. The experimental section where jams and fluid conductivity sensors were placed spanned from 3.0 to 7.5 m (Figure 1). No jams or sensors were placed in the last 1.5-m of the flume (from 7.5 to 9.0 m) to minimize interference from backwater effects near the sediment screen. We measured specific conductivity at 10-s intervals during the tracer tests using Onset Computing HOBO U-24 fluid electrical conductivity loggers (data loggers). Data loggers recorded fluid conductivity measurements for 30 minutes following the pulse to allow ample time for the flume to return to background solute concentrations. Sensor placement was uniform across all flume trials. We deployed one data logger upstream of the NaCl injection point to measure background fluid conductivity, and two data loggers 0.3-m upstream and downstream of the logjam(s) in the monitoring reach, at 3 m and 7.5 m from the top of the flume (Figure 1). We only report data from the downstream sensor for clarity as it represents both the above and below logjam effects. The upstream sensor data are included in the Supplemental Information. The measured instream conductivity signals represent the combined effects of surface transient storage in the channel (e.g., backwaters, eddies) and subsurface transient storage via hyporheic exchange.
We set high and low discharge values that ranged over a factor of 10 (0.001 m3/s and 0.01 m3/s). Initial test runs suggested that tracer was flushed too quickly to measure with accuracy above 0.01 m3/s. Discharge readings were obtained from a flow meter with ±0.2% accuracy. Discharge for all flume runs was fully turbulent, allowing relaxation of Reynolds number scaling (Peakall et al., 1996). We ran replicates for all flume trials. A digital elevation model for the flume was constructed using structure from motion. Images were captured at regular downstream intervals with a camera mounted at a consistent elevation and we used ground-control points along the flume bed for additional adjustment. Images were processed using Agisoft Metashape Professional. The resulting digital elevation model had a resolution of less than 1 mm.