BED MORPHOLOGY IN AN EXPERIMENTAL MEANDERING COMPOUND CHANNEL WITH VARIED FLOODPLAIN VEGETATION DENSITY

River restoration designs often overlook the complex hydraulic conditions that occur during floods. Emergent floodplain vegetation and other roughness elements interact with flow and can enhance or diminish the channel forming hydrodynamic forces that mobilize sediment and produce varied bed morphologies. However, limited research has explored the role of floodplain vegetation on channel-floodplain hydrodynamic interactions and subsequent morphological responses. Thus, our work is motivated by these knowledge gaps to investigate the effect of varied floodplain vegetation densities on in-channel hydraulics and bed morphology through laboratory experiments. Here we present preliminary results from a flume experiment which took place at the Colorado State University Engineering Research Center. We performed seven runs in a 30-degree crossing angle meandering compound channel with mobile sediment and various floodplain densities and flows. The active channel was 1-meter wide and inset in a 15.4-meter long, 4.9-meter wide basin. We tested bare and vegetated floodplain conditions with 2.67 cm 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 (the ratio of depth over the floodplain to bankfull depth) of 0.25. The bed was composed of mobile sand and gravel sediment with a of 3.3 mm. We surveyed the bed morphology using structure-from-motion photogrammetry and observed that, independent of relative depth, the bar-pool relief and sediment transport rate increased with vegetation density. Sediment transport for all flows above bankfull were higher than those at bankfull conditions, which is a distinct result when compared with other similar meandering compound channel studies with mobile beds. Further analysis of the flow field will elucidate the mechanisms responsible for this morphologic response.