A flume investigation of the influence of flood recession rate and vegetation patches on channel bar morphology
Investigations of the influence of flow stage on channel morphology often focus on the impacts of peak or minimum discharges. Similarly, restoration efforts on heavily regulated rivers and predictions about the response of unregulated channels to climate change often focus on the effects of peak flows (e.g. scour depth, grain sizes mobilized) and minimum flows (e.g. habitat availability). Rarely considered are the impacts of the rate of change in flow stage on the rising and falling limbs of flood events. We investigated the influence of discharge recession rate during hydrograph falling limbs and the coupled influences of recession rate and vegetation patches on the morphology of a forced channel bar in the sand-bed Saint Anthony Falls Outdoor Stream Lab. We ran three hydrograph falling limbs with different recession rates (10%, 30%, 70%), but held the minimum discharge, total water volume, and estimated sediment transport capacity within 10% between runs. The 10% recession run started from a peak flow of 150 L/s, while the 30% and 70% recession runs started from 284 L/s. We held the ratio between sediment supply and estimated transport capacity constant through runs by setting the sediment feed rate equal to the estimated transport capacity. Similar starting conditions for all experiments were established by running the channel at constant discharge and sediment feed rate to equilibrium, which was satisfied when cross-sections stabilized and down-stream bar growth ceased. The 10% and 30% runs were repeated with vegetation (Juncus and Carex) installed in a dense patch on the equilibrium bar. Measured changes in bar topography showed that higher peak flows increased the maximum elevation of deposition at the upstream end of the bar. Repeat bed scans during runs suggested that peak flows deposited sediment at the bar head and receding flows redistributed this sediment across the bar, with less redistribution for faster recession rates. Greater redistribution may have produced a lower bar head elevation for the 30% recession compared to the 70% recession. On the final bar topography we objectively identified the boundary between bar top and bar side by locating each cross-sectional point where the width:depth ratio increased rapidly over a small increase in elevation. With the bar top and side differentiated we then quantified mean bar top widths and side slope angles for each run. Results suggest that bar width decreases with faster recession rates, with average bar-top widths of approximately 980, 660, and 540 mm for the 10, 30, and 70% recession rates, respectively. The average bar side slope also decreased as recession rate increased, with slopes of approximately 0.28, 0.24, and 0.22 (m/m) for the 10, 30, and 70% recession rate runs, respectively. The addition of vegetation on the bar promoted deposition within the vegetation patch, increasing bar height, but also produced scour along the patch edge compared to equivalent runs without vegetation. These study findings provide needed insight for designing flow releases on managed rivers and suggest that larger peaks will increase bar height, gradual recession rates redistribute sediment more effectively than faster recession rates, and vegetation will stabilize sediment within patches but scour the bar adjacent to vegetation patches.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- 1825 HYDROLOGY / Geomorphology: fluvial;
- 1856 HYDROLOGY / River channels