Mechanisms of maintaining cobble-bar vegetation - focused on the intensity of disturbances and the refugia of the vegetation during large floods -

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A vegetation monitoring study and a field survey using an image velocimetry of river flows as well as a fluid-dynamic numerical simulation have been conducted to find the effects of disturbances caused by floods of different intensities and return periods on a cobble-bar vegetation that is sustained by these disturbances. It has been found that there is an area near the downstream edge of a cobble bar where the bed shear force stays below the critical level required to move bed material and vegetation during very large flood events. This area coincided with the location where some vegetation actually survived large floods and served as a 'refugia'. Furthermore, it has been suggested that this special condition needed to maintain the cobble-bar vegetation is created by the morphologic characteristics of a sharp bend, a shortcut secondary channel and a bar with a steep-sloped downstream edge.