Flood wash-out conditions of an exotic and invasive plant, Eragrostis curvula, in Arakawa River, Japan

Abstract

The effect of an exotic and invasive plant, Eragrostis curvula, on the threshold of gravel movement and wash-out conditions of the plant due to flood was investigated in the midstream Arakawa River, Japan. Under various hydraulic conditions (grain diameter, bed slope, and water depth) and plant growth characteristics (clump diameter and plant density), the Shields parameter of the gravel in the plant-vegetated area was estimated using the drag characteristics of the plant measured directly in field and wind tunnel experiments, and the plant's effect on friction velocity was evaluated. The removal threshold of E. curvula could be defined when the friction velocity around the plants was equal to the critical friction velocity of d 84 grain diameter at which 84% volume passed through the sieve. The threshold condition was found to be well expressed by the relationship between the bed slope direction component of the water weight and the drag characteristics due to the plant MD c 1.5, where M is the plant density and D c the clump diameter of E. curvula.