High Gradient Experiment

The tilting bed flume was set at 3% for the high gradient experiments. This slope was

representative of a longer reach in Wood’s Creek (combining the two surveyed sections for more data). A sediment mix based on the scaled representative high gradient stream (step pool) grain size distribution (with an adjustment to account for surface armoring) was created by combining sediment mixes from local quarries (Figure 3.11). Based on the field surveys, structures (ribs, riffles, steps, cascades and boulders) were placed along the flume. The scaled mean and standard deviation of the structure spacing was maintained as was the distribution of each type of

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structure, by developing a randomization script in MATLAB to place structure types along the flume. Structures greater than five bankfull channel widths away from the culvert were glued in place to facilitate experiment resetting. Structures within five channel widths from the culvert were reset for each run and allowed to adjust.

A target bankfull discharge appropriate for the high gradient system was selected using Manning’s equation. The discharge was set by trial and error to the bankfull elevation. A hydrograph was developed by selecting a target maximum flood stage and stepping from below bankfull to bankfull to two overbank flows. Each step on the hydrograph was 30 minutes, except for the highest flow, which was only maintained for 15 minutes because it was difficult to keep up with the manual recirculation (Figure 3.12). For each run, sediment recirculation was

continuous. Because material moving through the stream was larger than what the recirculation system could handle, large (>1 cm) sediment was collected off of a screen at the downstream end of the flume and recirculated by hand at the top of the flume. The effect of culvert filling was tested for both steady state bankfull flows and for a simulated hydrograph (see Figure 3.13). In addition, the effect of structures on bed stability within the culvert was tested by installed structures for both a bankfull and hydrograph run. During each experiment, continuous bed and water surface elevation data were collected down the middle of the flume using the data

acquisition cart outfitted with a sonar probe and ultrasonic transducer. Bed data collected during the run, are spotty, however, as the turbulent flow created surface waves and the sonar could not be set too close to the bed to avoid damage. If the sonar was out of the water, no data was

collected. After each experiment, the final bed was scanned using a high-resolution laser scanner. Pre- and post- bed photos were taken (Figure 3.14). Velocity data were collected after the

experiment over a filled culvert bed with a side looking ADV.

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Figure 3.13. Comparison of equilibrium (no culvert), initial, and final bed elevation along the channel midline for the high gradient experiment.

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Figure 3.14. Final topography for equilibrium (no culvert), filled, non-filled and filled with structures initial conditions for the high gradient experiment with bankfull flow. Z is measured in mm from the probe to the bed.

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Figure 3.15. Surface bed material collected a) before flow (bulk mix), and b-d) at the end of each bankfull run for the high gradient experiment.

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Figure 3.16. Final topography for equilibrium (no culvert), filled, non-filled and filled with structures initial conditions for the high gradient experiment with a simulated hydrograph. Z is measured in mm from the probe to the bed.

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Figure 3.17. Surface bed material collected a) before flow (bulk mix), and b-d) at the end of each hydrograph run for the high gradient experiment.

To monitor the stability of structures in the hydrograph experiment with structures installed in the culvert, each set of two structures were marked with a different color of paint. Figure 3.18 illustrates the initial structure set up (based on the spacing and elevation of field data) and final bed image after the hydrograph run. Structures near the end of culvert (blue and red) were least affected by the flood, while structures near the culvert entrance (orange and white) were greatly degraded.

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Figure 3.18. Pre- and post- photographs of structures placed in the culvert. Flow is from bottom to top. Each set of two structures was color coded to track the movement of

structures.

Depth-averaged velocity measurements for each flow rate were collected in the middle of the culvert and upstream of the influence of the culvert with a side-looking ADV (Table 3.3). All velocity measurements in the culvert were collected under the filled culvert with structures scenario. The first flow rate was below bankfull, the second was at bankfull and the 3rd and 4th were above bankfull flow.

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Table 3.3. Depth-averaged velocity and depth measurements collected in the middle of the culvert and upstream of the culvert for each flow rate in the hydrograph for the high gradient with structures experiments. All culvert measurements were collected for the filled with structures case.

CULVERT UPSTREAM Flow Rate (lps) V (m/s) H (cm) V (m/s) H (cm) 19.5 0.56 8.8 1.02 8.1 36.2 0.84 9.6 1.16 14.3 52.8 1.28 12.8 1.26 15.8 69.2 1.18 14.8 1.3 16.8

Simultaneous bed and water surface measurements were collected during each run. However, because of the large bedload, the sonar probe had to be set high enough to be safe. This resulting in spotty sonar data and therefore, these data are not included below. An analysis of water surface data may help us to track the location of hydraulic jumps, waves, etc. through time to get some information on the bed movement.