Meandering channels in laboratory flumes

1. Meandering channels in laboratory flumes Christian Braudrick with Bill Dietrich, Leonard Sklar, & Glen Leverich

2. Chutes Problem • Bars are essentially passive in current meandering models and they therefore the models cannot be used to predict channel response to changes in discharge or sediment supply • Creating meandering channels in the laboratory has proven difficult without strength channels braid, channels with cohesive materials stop migrating • Bank erosion occurs faster than bars can create new floodplain • Open chutes on the inside of bars are the locus for braiding photo courtesy Gary Parker

3. Outline • Experimental design and methods • Experiments on the necessary conditions for meandering • Experiments on bar morphology and sediment supply • Ongoing experiments at RFS • Experiments at OSL • What next

4. Hypotheses If slope, discharge, and channel dimensions are appropriate (following Parker, 1977), gravel-bed meanders also require: • Bank strength to slow bank erosion to allow time for the bars to grow • Fine sediment (sand) to fill chutes and the downstream end of bars • Overbank flows to allow the bar to grow to the floodplain elevation

5. Hypotheses If slope, discharge, and channel dimensions are appropriate (following Parker, 1977), gravel-bed meanders also require: • Bank strength to slow bank erosion to allow time for the bars to grow - Provided by alfalfa sprouts • Fine sediment (sand) to fill chutes and the downstream end of bars Lightweight plastic sediment • Overbank flows to allow the bar to grow to the floodplain elevation - Test both a variable hydrograph and a steady hydrograph

6. Experimental conditions • Flume is 16.5 m long and 6.1 m wide • Basin slope = 0.0046 • Basin filled with sand with a D50 of 0.85 mm • Initial channel geometry = 40 cm X 1.9 cm • Froude number = 0.55, Reynolds number = 4500 • Feed sand (scaled gravel) and lightweight plastic (scaled sand) 16.5 m 6.1 m flow

7. Measurements • Coarse and fine feed fed independently • Discharge measured with a v-notch weir • Topography measured with laser-camera system • Overhead cameras record position every five minutes • Water surface profiles measured with point gauges (now using TiO2 and laser-camera system) • Velocity measured with both floats, dyes • Load cell to measure sediment flux at the downstream end (did not work in first experiment).

8. The life cycle of alfalfa at RFS • Seed flume with low flow on • Water by hand and with low flow until seeds germinate • After germination turn on grow lights-12-14 hours a day • From seed to sprout takes about one week • Alfalfa starts to die after about 20 hours of run time (over the course of about 3 days) • After the alfalfa dies, start over again • Building too cold in winter for alfalfa

9. The experiment • Consists of a 71 hour, variable peak hydrograph (1.8 + 2.7 l/s + early tests up to 4.5 l/s) and a 64 hour steady peak hydrograph (1.8 l/s) • The variable experiments used plastic feed with a median diameter of 350 microns and specific gravity=1.5 • The steady flow experiments used plastic feed with a median diameter of 350 microns and specific gravity=1.3 • Sprouts seeded at ~1.2 sprouts/cm2 • Coarse feed turned off periodically to prevent aggradation at upstream end of the flume

10. Time-lapse video blue =fed sediment white=fine sediment (plastic) brown=floodplain derived sediment

11. Bars migrated both downstream and laterally • Curvature developed from upstream to downstream • Curvature redeveloped following cutoffs • The bend wavelength was about 14 channel widths

12. Width stabilized after 40 hours • High flow tests caused channel width to increase • Sinuosity increased in first 50 hours, and reached a quasi-steady value • Meandering maintained during steady peak flow, but flow was overbank

13. Bars were built by erosion of the upstream banks. Little fed coarse sediment propagated past the first bend • Chutes were plugged at their upstream and sometimes downstream ends and the water within them was nearly still • Fine sediment deposited at the downstream end of bars, as levee-like overbank deposits, and at the upper elevation of bars • Fine sediment plugged abandoned channels Sediment dynamics

14. Close up of a bar From recent experiments

15. Comparing migration rates to the field-Time scaling Time scaling in flumes depends on your variable of interest Here we adopt Froude-scaling where we assume time scales as the square root of the scaling factor (1/50-1/100) (see Yalin, 1971 Parker et al. 2003) where i is the frequency of flood flows, and  is the scaling factor Our 138 hour experiment therefore represents 40-57 days of flood flows in the field. Assuming an average of 8 days of bankfull flow per year in meandering streams (Andrews and Nankervis, 1995; Dunne and Leopold, 1978) these experiments represent 5-7 years of high flow.

16. Bank migration rates were very fast Our migration rates scaled to 0.5-0.7 channel widths per year much higher than field rates, which are generally range from 0.01-0.1 widths per year We hypothesize that higher alfalfa densities would increase strength and decrease the migration rate

17. 120 hrs 132 hrs Cutoffs The channel cut off five times. All cutoffs all involved chutes to some degree, either migration into open chutes, or connecting breakout channels with downstream reaches. We propose that limiting chute development would limit cutoffs Breakout connects with chute 127 hrs 126 hrs cutoff Breakout channel, promoted by upstream aggradation

18. Summary of necessary conditions experiments • We were able to create a self-formed, single-thread meandering channel that maintained a stable width • Meandering was maintained under the steady peak flow conditions, but the flow was slightly overbank • Channel geometry was similar to channels in the field • The channel cut off five times and rebuilt curvature after cutoffs • Cutoffs limited sinuosity development, sinuosity was a maximum of just under 1.2 • Fine sediment was crucial-filled the downstream end of bars, plugged chutes, and plugged cut off channels • Bank migration rates were very fast relative to channels in the field

19. What next?How does sediment supply affect bar morphology and migration rate • We want to test the hypothesis that increased topographic steering-induced stress alters bar morphology. • Use alternate bars, bars in sinuous fixed bank streams, and the meandering flume. • We hypothesize that increased supply will cause bars to grow and increase the area of topographic steering, while decreased stress will decrease the area of topographic steering • With the Barflies, modify an existing hydrodynamic model (MD_SWMS) to include supply effects, and provide data to test the model

20. Ongoing experiments RFS • Can we increase the sinuosity by changing the bank strength to slow down erosion? • Increase bank strength by increasing the alfalfa density by 2.5-3X (2.8-3.5 sprouts/cm2) • Increase discharge to increase sediment flux • Increase discharge to increase bank stress (to still get erosion) (Qw=2.6 l/s) • Start with a sinuous channel • Two runs with similar conditions- • Run 1-sediment feed error caused aggradation • Run 2-excavated existing channel

21. 0 hours 13 hours very strong banks limit erosion to just downstream of apex

22. 13 hours 13 hours-after planting Vegetation strongly influences channel morphology

23. 13 hours 21 hours

24. 21 hours Aggradation Very high curvature bends developed 33 hours- end of run Aggradation

25. Second experiment 0 hours 20 hours aggradation Narrowing initial channel doubled the bar frequency-still got aggradation upstream

26. Ongoing experiments RFS • Higher bank strength (and slower erosion rates) limited chute development • The sinuosity increased (up to 1.5), with erosion focused just downstream of the bar apex • We are struggling with continued aggradation in the middle of the flume. • Will get in one more run prior to freeze up.

27. OSL experimentswith Susannah Erwin, Peter Wilcock, Anne Lightbody, Kristen Sweeney, and Dan Mielke • This Fall we conducted a test of supply and bar morphology linkages at the OSL. • Flume experiments at field scale. • We measured topographic response going from no supply, to a moderate supply, to a ~5X increase in supply and back to moderate supply again • Other measurements include velocity, water surface elevation, sediment flux, and grain size data. • Here, I will just show some preliminary data

28. Approximate cross section locations XS1 XS3 XS5

30. Preliminary Results • The bar shrank considerably when supply was turned off (not shown) • Turning on the supply created a bar that extended around the middle three cross section, with a trough on the inside edge of the bar • Increasing the supply by ~5 times caused the bar to extend upstream and downstream. The bar swelled, the pool shallowed 10-15 cm, and the trough to disappear

31. What next • Create a self-formed channel in meandering flume and insert fixed walls to test bar response to changes in supply-may need to decrease alfalfa density somewhat • Test the effect of increased supply on alternate bars at RFS straight flume-is 5X too large a supply increase? • Working with the bar flies, provide data to modify MD_SWMS (a quasi 3-d hydraulic model) to include supply effects