Hydraulic and geomorphic processes in an overbank flood along a gravel-bed, meandering river: implications for chute formation

Friday, 19 December 2014
Lee Harrison1, Thomas Dunne2 and Burch Fisher2, (1)NOAA, Santa Cruz, CA, United States, (2)Univ California Santa Barbara, Santa Barbara, CA, United States
Hydraulic interactions between rivers and floodplains produce off-channel chutes, whose presence can increase the ecological diversity of the valley floor. Detailed studies of the hydrologic exchanges between channels and floodplains are usually conducted in laboratory facilities, and studies documenting chute development are generally limited to qualitative observations. In this study, we use a reconstructed, gravel-bedded, meandering river as a laboratory for studying these mechanisms at field scale. Using an integrated field and modeling approach, we quantified the flow exchanges between the river channel and its floodplain during an overbank flood, and identified locations where flow had the capacity to erode floodplain chutes. Hydraulic measurements and modeling indicated high rates of flow exchange between the channel and floodplain, with flow rapidly decelerating as water was decanted from the channel onto the floodplain due to the frictional drag provided by substrate and riparian vegetation. Peak shear stresses were greatest downstream of the maxima in bend curvature, along the concave bank, where terrestrial LiDAR scans indicate initial floodplain chute formation. A second chute has developed across the convex bank of a meander bend, in a location where sediment accretion, point bar development and plant colonization have created divergent flow paths between the main channel and floodplain. In both cases, the off-channel chutes are evolving slowly during infrequent floods due to the coarse nature of the floodplain, though rapid chute formation would be more likely in finer-grained floodplains. The controls on chute formation at these locations include the river curvature, cross-stream position of the high velocity core, erodibility of the floodplain sediment, and the density of riparian vegetation.