The impact of natural floodplain vegetation on flow resistance and fine sediment transport

Suitable implementation of sustainable river management strategies and Nature-Based Solutions requires detailed understanding of the impacts of riverine vegetation on flow characteristics, sediment transport, water quality, river health and ecology. Modelling of the flow resistance of floodplain vegetation has received substantial attention, as many research and engineering purposes require reliable estimates. Existing drag force and flow resistance models are based on highly simplified representation of vegetation. In particular, there remains a lack of adequate description of the effects of reconfiguration and relative submergence. Subsequently, many models are used outside their developed domain, resulting in poor predictions. This thesis extends current knowledge on flow resistance of flexible flood-plain vegetation mixtures as well as on sediment transport processes under vegetated settings. The specific objectives were to 1) quantify vegetative drag and measure flow resistance of vegetation mixtures of woody plants and a herbaceous understory to evaluate existing resistance equations; 2) develop leaf area index (LAI)-based flow resistance models suitable for both emergent and submerged conditions, and 3) advance knowledge on transport and net deposition of fine sediments under the effect of seasonal variability in plant properties. The LAI-based models developed in this thesis provide for accurate predictions of vegetative flow resistance of mixtures of flexible floodplain vegetation also at low relative submergences. Most accurate predictions are obtained when a reliable estimate of the deflected vegetation height hd is available and the newly introduced von Kármán scaling factor α is used. A workflow for implementation of the proposed LAI-based approaches was developed for modelers and practitioners. The newly introduced LAI-based drag ratio approach can be used for estimating the influence of seasonal variation in foliage on sediment transport. The presence of foliage increased the net deposition in the vegetated areas and enhanced deposition near the main channel-floodplain interface by¬ two to five times. The new insights allow for improvements in future predictions of suspended sediment transport and deposition in partly vegetated, main channel-floodplain or bank/riparian vegetation settings. The potential applications range from flood inundation studies to river restoration, floodplain reforestation, channel design and management of riverine environments.