Effect of floodplain vegetation on flow and transport of cohesive particles in an environmental two-stage channel

Kaisa Västilä, Juha Järvelä, Johanna Jalonen

Friday 3 july 2015

8:30 - 8:45h at North America (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 14F. Environment - Ecohydraulic

Cohesive sediments and sediment-bound substances, such as phosphorus, affect the water quality, habitats, and technical functioning of channels. This study focused on investigating the interaction between floodplain vegetation, flow, and transport of cohesive particles in an 11 m wide, 190 m long compound test reach. Our two-year monitoring included detailed vegetation analyses by manual sampling and terrestrial laser scanning, cross-sectional surveys, sediment analyses, and continuous monitoring of flow and suspended sediment concentration (SSC) at both ends of the test reach. The flow resistance of the test reach was successfully estimated by a process-based model accompanied with a physically-based characterization of the natural vegetation. Total phosphorus exhibited a strong correlation with SSC, which allowed computing the transported and deposited phosphorus loads from the respective sediment loads. The deposition of cohesive sediment, and thus particle-bound phosphorus, was greater at higher water levels. Simulation results showed that the discharge within the vegetation and on the floodplain increased with water level, leading to greater availability of sediment. Annually, 5.5% of the incoming suspended clay and silt was deposited on the 190 m long floodplain, and the height was the best vegetation property for explaining the net deposition. The annual amount of deposited sediment-bound phosphorus was estimated to be an order of magnitude greater than the amount of phosphorus in the biomass of the 0.4 m high floodplain grasses. Overall, this research showed the potential of controlling the transport of cohesive sediment and sediment-bound substances using two-stage channel designs.