A geomorphic and hydraulic investigation in the context of floodplain revegetation; based on a soil bioengineering application on the Mattole River, Petrolia, California, USA

As fluvial, riparian and floodplain ecosystem functions are recognised for their role supporting fisheries and ecological values, recovery of streamside vegetation is increasingly important in river 'restoration'. Fluvial geomorphology and hydraulic engineering do not yet account well for the role of vegetation in fluvial processes. This research addresses the need for greater understanding of woody riparian vegetation influences on the hydraulics of overbank flow and floodplains sedimentation. Original hypotheses, research design, and data collection were generated by the student to address this gap in knowledge. A soil bioengineering design was constructed on the Mattole River, California, to revegetate the floodplain for better fish rearing habitat. Field data collection was carried out on this unregulated river for two flood events. The sediment samples resulting from a 1.25-year flow permitted the field testing of an hydraulic flume model of vegetation trapping efficiency. From velocity profiles measured during a I5-year storm event, the bed shear stress reduction caused by the vegetation was computed to be approximately 70-90%. A survey conducted in the UK and internationally evaluated from literature, hydraulic researchers and practitioners of river revegetation, the extent of and gaps in knowledge with regard to river bank stabilisation using live vegetation. A flume flow visualisation study simulated the hydraulic behaviour observed on the Mattole floodplain, which enabled characterisation of flow behaviour through a porous filter medium. Results of this research indicate that flexible woody stems have a profound 'calming' effect on overbank flow. These effects are propagated in the downstream direction at least five and as much as ten times the width of the baffle, much further than previously indicated. This research suggests that flexible vegetation is extremely effective in trapping fine (clay) sediments, contrary to general understanding and of importance for fish habitat. For hydraulic reasons, constructed zones of shrubs, such as the siltation baffle, could be spaced further apart than current design practise indicates