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Cohesive sediment processes in vegetated flows: Field study in an environmentally preferable agricultural channel
Kaisa Västilä, Juha Järvelä

Many of the environmental impacts caused by agricultural drainage are connected to the anthropogenic disturbances to natural cohesive sediment processes. Sediment processes can be steered towards the natural equilibrium by allowing the drainage channels to have vegetation and variable channel geometry. To advance environmentally preferable hydraulic engineering, a good understanding of the cohesive sediment processes in such channels is essential. In this presentation we analyze the factors and processes affecting cohesive sediment erosion and transport in a vegetated agricultural channel, and describe the seasonal differences in the processes. The results were obtained during the first year of a 3-year field study at the formerly dredged Ritobäcken Brook in Sipoo, where a floodplain was excavated to improve the drainage of the surrounding agricultural fields. Study reaches were planted with different floodplain vegetation, and the development of the cross-sectional geometry and flow resistance are measured in the reaches. The brook?s discharge and suspended sediment concentration (SSC) have been continuously monitored during one year. We found that the unsteadiness of the flow exerted a major control on the sediment transport during the rising stage. Although notable SSC peaks occurred after most major rains, the greatest sediment loads were transported during the relatively short autumn high flow and spring snowmelt periods. The process understanding and reliable field data provided by our study can be used in the development and validation of modelling tools for sediment transport and river morphology. Such models are needed for planning sustainable river management strategies and designing environmentally preferable channels.

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