Experimental Observations of Flow and Bed Morphology In A Meandering Compound Channel With Variable Density Floodplain Vegetation

Floodplain vegetation has been recognized for providing important ecosystem services to both humans and the aquatic and terrestrial organisms that inhabit riparian corridors. These ecological functions and ecosystem services often depend on floods, which produce hydrologic exchange flows – the transfer of water between a river channel and floodplain surface. Predicting the hydraulic nature of floods is difficult because of the complex flow patterns that develop over vegetated floodplains, at the channel-floodplain boundary, and within the active channel. In river restoration design and management applications, simulating oversimplified hydraulic conditions of floods may result in poor design and projects that ultimately fail to meet defined goals as channels respond to the stresses of a flood. To gain insight on how floodplain vegetation impacts flow and channel morphology during floods in meandering compound channels, we conducted a series of mobile-bed flume experiments in which discharge, flow depth, and floodplain vegetation density were varied. The flume centerline followed a sine-generated trace with a 30-degree crossing angle, and the channel was 1-meter wide inset in a 15.4-meter long, 4.9-meter wide basin. We tested bare and vegetated floodplain conditions with 2.67-centimeter diameter emergent, cylindrical elements at spacings of 3.0 units/m2 and 12.1 units/m2. Flow depths ranged from bankfull to a relative depth (Dr, the ratio of floodplain depth to bankfull depth) of 0.25. The bed was composed of mobile sand-and-gravel sediment with a D_50 of 3.3 mm. We measured bed morphology, the 3D flow field, and sediment transport for each experiment once conditions reached a state of quasi-equilibrium. At low relative depth, pool width increased with increased floodplain vegetation density, which may be attributed to an increase in the strength of secondary currents as floodplain vegetation steers flows toward the channel. At high relative depth (Dr = 0.25), bedforms became more pronounced with greater bar-pool relief as floodplain vegetation density increased. The flow field within the active channel was altered due to the hydraulic interaction of overbank flows with floodplain vegetation. Further flow field analysis and investigation of the mechanisms for morphodynamic response observed is an ongoing objective.