EXAMINATION OF WASHOUT CHARACTERISTIC OF Egeria densa DURING LARGE FLOOD

Abstract

 We had the largest flood flow in the observation history of Kawamoto Station in Gonokawa River in Hiroshima prefecture in July 2018. This study surveyed the cover degree of nonnative aquatic weed (Egeria densa) in the river segment (～38km) from Haji Dam to the three tributary junction section in Gonokawa River before and after the flood event, using unmanned aerial vehicles. Although the cover degree has significantly decreased after the flood in the whole area, the aquatic weed remained in certain locations. We conducted the reproduction simulation of the peak flow and examined relationships of the washout rate of the aquatic weed to dimensionless tractive force and flow velocity. Our results showed that the dimensionless tractive force was positively related to the washout rate, indicating that the cover degree could significantly decrease at sites with high dimensionless tractive force during flood.