Flow–plant–sediment interactions: Vegetative resistance modeling and cohesive sediment processes

Riparian vegetation growing on river banks and floodplains has pronounced impacts on the flow of water and the transport of substances, including fine sediment. These plant-mediated processes shape fluvial ecosystems and are essential in applications of environmental hydraulics, such as in compound channels with vegetated floodplains. The flow–plant–sediment interactions need to be estimated at different scales, but suitable parameterization of natural flexible vegetation for hydraulic analyses has been difficult. The objective of this thesis was to provide new insight on flow resistance and cohesive sediment processes in compound channels with riparian vegetation. The cross-cutting aim was to improve the parameterization of natural, flexible, foliated plants. The drag forces and the flexibility-induced reconfiguration were examined for five woody species in a laboratory flume. The flow resistance, net deposition and the suspended sediment transport were quantified under differing floodplain vegetation conditions (bare, grassy, woody) in a cohesive agricultural compound channel. The experimental investigations were accompanied with the application of existing and new models. The drag forces and reconfiguration of foliated woody plants were controlled by the leaf-area-to-stem-area-ratio, because both the foliage and the stem contributed notably to the plant-scale drag. The flow resistance of natural foliated vegetation was successfully modeled using a novel drag–density parameterization (Eqs. 18–23) that accommodates the reconfiguration and density separately for the foliage and the stem. The new parameterization improved the description of woody vegetation compared to the conventional approach of considering plants as rigid cylindrical elements. The flow resistance in the compound channel could be estimated by a two-layer model using the drag–density parameter and the vegetation height represented by the cross-sectional blockage factor. These same vegetation properties explained the net erosion and deposition of cohesive sediment on the floodplain, although deposition was supply-limited in long and dense plant stands. The timing and magnitude of cohesive sediment transport in the agricultural compound channel were governed by out-of-channel processes. As a practical implication, the sediment load transported in compound channels can be managed by appropriately maintained floodplain vegetation. In conclusion, straightforward approaches accompanied with a physically-based vegetation parameterization can be successfully used to describe the effects of natural flexible riparian plants on flow resistance and sediment deposition.