Floodplain flow resistance in case of a sparse mixture of plants at low relative submergences

Vegetation affects flow hydrodynamics and transport processes across various spatial scales and is increasingly put forward as a powerful tool for nature-based solutions (NBS) in river management. Complex mixtures of different plant types at a wide range of densities are common on real floodplains, but such conditions are rarely properly reproduced in experimental investigations. This paper reports on the first results of an ongoing study, with a view on flow resistance associated with low relative submergences (H/hv = 1-2) and low vegetation densities (LAI = 0.12-0.58). Such experimental investigations are challenging due to the necessity of a high-accuracy sensor system suited for the bulk scale. A mixture of artificial elements was selected to mimic the behaviour of natural floodplain shrubs and herbaceous vegetation in a laboratory flume. Results suggested that also at very low LAI values the vegetative resistance increased linearly with LAI. An increase of H/hv from 1 to 2 caused a 25% to 50% reduction in the friction factor for very low to intermediate LAI, respectively.