Physical and numerical modeling of river channel morphology often consider the influence of a single discharge or a series of individual discharges assumed to be in normal, steady conditions. However, the rate of change between these discharges may also affect channel morphology. Rapid flooding has been linked to poorly sorted, less armored beds compared to more gradual floods, but the influence on morphology is rarely considered. In addition, installation of vegetation is common in restoration projects though it is not always clear how this will impact morphological features such as bars. Here we present results from a set of flume experiments and 2D modeling designed to investigate the influence of hydrograph shape and vegetation on the morphology of a forced bar in sand-bed channel. Flume experiments were conducted in the Outdoor Stream Lab at Saint Anthony Falls Laboratory, Minneapolis, MN. We ran three falling limb only hydrographs with different recession rates (10, 30, and 70%). Minimum discharge, total volumetric water discharge, and estimated sediment transport capacity were held within 10% between runs. The ratio of sediment supply to estimated transport capacity was also held constant at all times. The 10% run peaked at 150 L/s, while the 30% and 70% runs peaked at 284 L/s. The 30 and 70% runs were repeated with vegetation (Juncus and Carex) that mimicked vegetation established at approximately bankful height. Similar initial conditions for all runs were established by running the flume to equilibrium at constant flow and feed rates. Detailed bar topography/bathymetry data were collected before, during, and following each run. Bar morphology at the conclusion of recession hydrographs indicated that bar development declines as recession rate increases. Both with and without vegetation, the faster recessions resulted in bar morphology that was less distinct. This observation is supported by bar-top widths and areas that both declined as recession rate increased. Comparison of pre and post-run bars show deposition over the majority of the bar during the gradual recession (10%). In contrast, faster recessions (30 and 70%) produced deposition in a smaller area on the bar top and scour along the edge, also indicating that faster recession rates may suppress bar development. The addition of vegetation to the bar-top reduced bar development compared to runs without vegetation. For both the 10 and 30% recession rates, bar-top area and width were less with vegetation. This likely resulted because, while deposition was promoted within the rougher patch, bar area was lost to scour promoted adjacent to the patch. One result of erosion adjacent to the patch was formation of a small cut-off channel that went across the bar-top on the inside of the meander during both vegetation runs. In addition, during the 30% recession run a significant portion of the downstream end of the bar was lost to scour, producing a smaller bar with irregular morphology.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- 1825 HYDROLOGY Geomorphology: fluvial;
- 1862 HYDROLOGY Sediment transport;
- 1813 HYDROLOGY Eco-hydrology