Morphological characteristics of flood refugia of cobble-bed vegetation

Periodic flooding alters riverine vegetation community composition; moderate flood disturbance maintains communities at an early semi stage, whereas severe floods may remove entire communities. After disturbance, plants may recolonize riverbeds from refugia. Here, we investigated the morphological characteristics of refugia where riverine plants survived large floods. At three sites, plant communities growing on cobble-bars were surveyed, and the hydraulic characteristics of each site were analyzed. The populations of an indicator plant. Anaphalis margaritacea sub-species yedoensis (Franch. et Savat.) were assessed after major floods in 2004 and 2009. At each survey, the particle size and bed composition were measured, and the critical shear-stress required to move particles at each site was calculated. A one-dimensional flow and the average and local shear-stress at ten different discharges, including that of the largest flood were calculated. The results demonstrate that the A. margaritacea community is maintained at locations where the local shear-stress at peak discharge in large-floods does not exceed a critical value. These conditions were achieved in locations with particular morphological characteristics, where the river is not constrained by levees and is restricted by a downstream narrow-pass; in this reach river width increases with discharge thus limiting the increase in water depth with increasing discharge and allowing the establishment of refugia where plant species may survive large floods. Flooding and reestablishment from refugia maintain a characteristic riverbed community in equilibrium. (C) 2012 International Association for Hydro-environment Engineering and Research, Asia Pacific Division. Published by Elsevier EN. All rights reserved.