RESEARCH ON THE DESTRUCTION OF HERBACEOUS VEGETATION BY FLOOD FLOW ON SAND BAR IN RECOVERING PROCES

Abstract

This research focuses on examining destruction of the herbaceous vegetation on the sand bar in the Kitagawa River. The sand bar in Kawasaka area was excavated at the beginning of 2000, and the vegetation on the sand bar started recovering in spring in 2001. A flood occurred on Oct. 16, 2001 and destroyed the vegetation on the sand bar. The methods of examination are (i) drawing the distribution map of the vegetation on the sand bar, (ii) surveying the condition of the sand bar, (iii) testing the tension strength to cut off the stalk in the middle of it or to extract the root of the vegetation, (iv) simulating the two dimensional steady flow in Kawasaka area at the peak of the flood and (v) estimating the maximum size of transported bed material on the sand bar and the drag force on the canopy. From these investigations, the followings are clarified. That is, the bed material with smaller diameter than 2cm can be transported in the flood. The transportation may destroy some parts of the vegetation on the sand bar. However, the other parts of it can be destroyed by the tension strength due to the drag force on the canopy.