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

Figures & data

Figure 1 Photograph of a clump of E.curvula and the definition of key parameters in this study. D _c: clump diameter (m), h _v: clump height (m), M: clump density (number of clumps / m²)

Table 1 Experimental conditions for drag force measurement on E. Curvula

	Reference diameter of clump D c (m)	Velocity U (m/s)	Relative height^a h v−def/H	Reynolds number, R e ^b
Wind tunnel (Exp. 1)	0.07 – 0.12	5.0 – 15.0	0.8 – 0.9	2.2 × 10^4 −1.2 × 10^5
Wind tunnel (Exp. 2)	0.07 – 0.12	5.0 – 15.0	0.2 – 0.5	2.2 × 10^4 −1.2 × 10^5
Field exp. (Exp. 3)	0.04 – 0.20	0.2 – 0.8	0.8 – 1.2	1.5 × 10^4 −1.6 × 10^5
^a h v_def is the deflected plant height (m).
^b R e = UD c/ν where, ν is the kinematic viscosity (m^2/s).

Figure 2 Measurement of drag force acting on E. curvula and a clump model. (a) Wind tunnel experiment, (b) schematic of the measurement method in a river (L ₁, L ₂ is 65 cm, 45 cm, respectively, in this study)

Figure 3 Relationship between relative plant height to water depth and the drag coefficient C _d of E. curvula. h _v-def is the deflected plant height, H is the water depth

Figure 4 Field investigation site and differences in vegetation density and clump size of E. curvula. (a) Arakawa-ohashi (AR): SF is fringe region of the vegetation, SC is the centre region of the vegetation (in SF and SC region, clump size and the density is low, D _c = 10–20 cm), DF is fringe region of the vegetation, DC is the centre region of the vegetation (in DF and DC region, clump size and the density is high, D _c = 30–50 cm), (b) Kumagaya-ohashi, KU: clump size and the density is low, D _c =10–20 cm

Figure 5 Characteristics of E. curvula, relationship between: (a) D _c and shoot height (h _v (m)), (b) D _c and plant density (clumps/m²), and (c) D _c and bending stiffness EI _M (E: Young's modulus (N/m²), I _M: second moment of area (m⁴))

Table 2 Conditions for calculating friction velocities around E. curvula

	Notation	Value	Unit
Water depth	H	1.0 – 6.0	m
Bed gradient	I	1/1000 –1/375	–
Particle diameter	d 50 ^a	0.9 – 2.9	cm
	d 84 ^a	2.3 – 6.4	cm
Reference diameter of a clump	D c	0.1 – 0.5	m
Density of clumps	M	^b	Number of clumps /m^2
^a d 50 and d 84 are grain sizes at which 50% and 84%, respectively, of the material weight is finer.
^b M is changed depending on D c.

Figure 6 Local scour around E. curvula. (a) Relationship between the flood water depth (H (m)) and the local scour depth (h _s (m)), and (b) photograph of the scouring situation around E. curvula after a flood. D _c is the clump diameter (m), L _R is the root penetrating depth (m)

Figure 7 The comparison of the duration time in which τ _e exceeded τ _c84 at the AR and KU. τ _e: the shear stress considering the clump diameter and plant density of E. curvula, τ _c84: is the critical shear stress of d ₈₄. The site characteristics are shown in Table 3. SC, DC and SF are defined in Figure 4. The blank symbols mean E. curvula did not wash out due to flood. The filled symbol mean washed out due to flood

Table 3 Field and river characteristics used in this study

						Maximum water depth at flood event H (m)	Particle diameter (cm)
Abbreviation	Location	Flood event	Latitude	Longitude	Riverbed gradient		d 50	d 84
AR	Arakawa River	October 2006	36°08′N	139°21′E	1/375	1.0 – 1.3	0.9 – 2.8	3.2 – 6.4
		September 2007				2.8 – 3.1
KU	Arakawa River	September 2007	36°08′N	139°20′E	1/375	2.9	3.6	6.8

Figure 8 Relationship between D _c and threshold size of movement_, d _s-crit, as a function of the number of clumps, M (clumps/m²). The dashed line shows the mean diameter at the investigated site (H = 2 m, I = 1/400, d ₅₀ = 1 cm)

Figure 9 Relationship between water weight on the bed (slope direction), ρgHI, and the representative parameter for growth and the drag, MD _c ^1.5. (a) Schematic of the relationship between (MD _c ^1.5)_crit and (ρgHI)_crit, (b) change of (ρgHI)_crit,as a function of D _c (I = 1/375, d ₅₀ = 0.9 cm)

Figure 10 Validation of the wash-out condition of E. curvula. (a) Fringe region in Arakawa River, (b) centre region in Arakawa River. The site characteristics are shown in Table 3