Experimental Observations of Floodplain Vegetation, Bedforms, and Sediment Transport Interactions in a Meandering Channel

Floodplains provide important ecological, hydrological, and geomorphic functions within river corridors. During overbank flows, complex hydrodynamic conditions occur as water exits and re-enters the channel and interacts with hydraulically rough floodplain vegetation. However, the extent to which floodplain vegetation influences channel-altering hydrodynamic forces and thus bedform topography and sediment transport is poorly understood. We address this knowledge gap and present the results of flume experiments where we measured bedform topography under varied floodplain vegetation conditions at two overbank flow relative depths. The experiments were conducted in a 1-m wide meandering compound channel inset in a 15.4 long, 4.9-m wide basin. The channel bed was a mobile sand-and-gravel mixture with a median sediment size of 3.3 mm, and sediment transport occurred only within the channel. We tested bare and vegetated floodplain conditions with 2.7-cm diameter rigid emergent vegetation elements at spacings of 3.0 and 12.1 units m(-2). We performed a moving-window analysis of topographic surface metrics including skewness, coefficient of variation, and standard deviation, as well as topographic patch analysis of area and contagion to measure changes in bedform heterogeneity as flow depth and vegetation density were varied. Our results show that both greater density vegetation and larger flows can increase bedform topographic heterogeneity. These findings suggest that floodplain vegetation and natural hydrologic regimes that include overbank flows can enhance stream habitat complexity. Designing for the effects of established vegetation conditions and prioritizing floodplain vegetation planting may be useful for river managers striving to achieve successful biomic river restoration.