MECHANISM OF VEGETATION DESTRUCTION DUE TO A LARGEFLOOD IN THE COMPOUND MEANDERING RIVER
Takahisa Gotoh and Shoji Fukuoka
1

Graduate Student of Science and Engineering, Chuo University
1-13-27, Kasuga, Bunkyo-ku, Tokyo, Japan (goto_510@civil.chuo-u.ac.jp)
2
Professor, Research and Development Initiative, Chuo University
1-13-27, Kasuga, Bunkyo-ku, Tokyo, Japan (sfuku@tamacc.chuo-u.ac.jp)

ABSTRACT
Two types of flow can be seen in compound meandering channels, which are simple
meandering channel flow and compound meandering channel flow. The compound
meandering channel flow has a characteristic that maximum velocity filament passes the innerbank side around the peak discharge of large floods. Therefore, large flood flows may destructvegetations on sandbars formed near the inner bank. In the Ota River, flood discharge bytyphoon 0514 was largest in the past 50 years. Vegetations on sandbars formed near the innerbank were destructed by this flood. On the other hand, vegetations on the outer bank side of
the flood channel were remained. The authors observed temporal changes in water surface
profiles during the flood, vegetations and bed scouring before and after flood. The objectiveof this study is to make clear the mechanism of vegetation destruction in compound
meandering channels by 2-D flow analysis using the observed temporal changes in water
surface profiles and field data.
Keywords: flood, vegetation destruction, temporal changes in water surface profiles,
compound meandering channel, bed scouring
1.

INTRODUCTION

Vegetations in rivers have important roles to make the good river environment. They
supply habitats and corridors in aquatic life, and are essential elements composing riverlandscapes. On the other hand, vegetations give rise to serious problems with respect to theflood control such as water level increase due to their large resistance. So, it is required toestablish a vegetation management in rivers. Fukuoka and Fujita (1992) had developed
quasi-2D flow analysis method, in which effects of vegetations on flows are considered by noflux within vegetation clusters and shear stress exerted on the interface between the flow andvegetation. Maeno et al (2005) proposed unsteady 2D flow analysis method, in which effects
of vegetations on flows are considered by drag force. Shimizu et al (2002) showed process ofvegetation destructions based on the degradation of movable river-bed. Yagisawa and Tanaka
et al (2008) indicated the relation between river beds conditions and vegetation destructions
based on threshold drag moment of vegetations and calculated the threshold shear stress forvegetation destructed. They suggested that the investigation of local scouring around thevegetations is needed for understanding mechanism of vegetation destruction. But theimportant issues still remain, which include evaluation of effects of vegetation destruction on
flood flow behavior and the depth of bed scouring causing vegetation destruction, although
effects of bed scouring on vegetation destruction have been suggested. In this study, we
attempt to solve above the issues by observation data of temporal changes in water surface

1

profiles during the flood, vegetation conditions and river beds profiles before and after flood.
Compound meandering rivers which are generally adopted in the design of rivers in Japan
have two types of flood flow: simple meandering channel flow and compound meanderingchannel flow (Okada and Fukuoka, 2004). Large floods assume to be compound meanderingchannel flow at the peak discharge and may destruct river vegetations on sandbars formednear the inner bank. The flood of the Ota River due to typhoon 0514 was the largest in thepast 50 years. The flood flow destructed most of vegetations on sandbars formed near the
inner bank side. The objective of this study is to make clear the mechanism and the cause ofvegetation destructions on compound meandering channels by detailed investigation before
and after the flood and unsteady 2-D flow analyses using the observed temporal changes inwater surface profiles.

2.

OBSERVATION OF VEGETATIONS AND THE FLOOD IN OTA RIVER

Temporal changes in water surface profiles were observed at 200m intervals of
longitudinal distance by the water level gauges installed on either bank (see Figure 1).
Our gauges were not possible to measure water level beyond the design water level.
Discharge hydrograph was measured using floats at Yaguchi-daiichi discharge observation
station. In 2003, we investigated the situation of vegetations growing on flood channels andsandbars. The measurement items were positions, height, species and density of vegetations
and measured data are stored in databank (see Figure 2).
: Water level observation station：Discharge measurement station(m)

Yaguchi-daiichi
C
12.0km

11.0km
9.0km

FLOW
10.0kmVegetation growth area(ｍ）

Figure 1. Channel conditions of section studied

Position of vegetation

9.8km

9.6km

9.4km
9.2km
9.0km

Figure 2. Situation of vegetation growth

2

8.0km

Therefore, we can discuss positions and causes of vegetation destruction by surveying after
the flood. Surveying demonstrated that Salix chaenomeloides grew in front of sandbar andMorus alba on sandbar of higher elevation to compared Salix chaenomeloides growth area.

3.

CHANGES IN TEMPORAL WATER SURFACE PROFILES DUE TO
VEGETATION DESTRUCTION

3.1

Situation of vegetation destruction

Okada and Fukuoka (2004)’s studies concerning compound meandering channel
proved two types of flows: simple meandering channel flow and compound meanderingchannel flow. They proposed the diagram to judge flow regime of the compound meanderingchannel by the relation between sinuosity S (=meander length/meander wavelength) andrelative depth Dr (=flood channel depth/main channel depth). The discharge of the Ota Riverflood by typhoon 0514 was largest in the past 50 years and water level of the flood reachednear the design flood water level (see Figure 3). Figure 4 shows the diagram plotted by thesinuosity and the relative depth at the peak discharge of the Ota River. The flood flow aroundthe peak discharge was considered as compound meandering channel flow.
-50

Lateral distance（ｍ）
50 100 150 200 250 300 350

016

Relative depth Dr

Water-level (m)Ota River

H.W.L. 13.22ｍ

14
12
10
8
6
4

Compound meanderingchannel flow
Simple meanderingchannel flow

2
0

9/6
12:00

9/6
18:00

9/7
9/7
0:00
6:00
Time (hour)

9/7
12:00

9/7
18:00

Sinuosity S
Figure 4. Flow classification diagramof compound meandering channels

Figure 3. Observed water-levels and crosssection at discharge observation station

10.0kmFlow

9.0km

Yaguchi-daiichi
8.0km

12.0km

11.0km

13.0kmArea of vegetation destructed
Area of vegetation remained
Figure 5. Location of vegetation destruction

3

Figure 6. Vegetation destruction on sandbar
The location of vegetation destructions in the Ota River is shown in Figure 5. The
vegetation destructions by typhoon 0514 were largest in scale. The vegetation on sandbars
formed near inner bank was destructed due to maximum velocity filament passing the innerbank side. On the other hand, the vegetation on flood channel near the outer bank remained.
Okada and Fukuoka (2004) showed that the compound meandering channel flow gave rise toscouring of sandbars formed near the inner bank. Figure 6 shows the vegetation destructions
due to bed scouring. Roots of vegetations are revealed due to bed scouring aroundvegetations.

3.2

Observed Temporal changes in water surface profiles

River characteristics such as channel shape, bed configurations, and vegetations
appear in the observed temporal changes in water surface profiles of flood flows (Fukuoka,2005). Figure 7 and Figure 8 show observed temporal changes in water surface profiles in therising period and the receding period of the flood, respectively. The comparison of thesefigures shows when vegetations were destructed by the flood flow. In the flood rising period,
effects of the vegetation clearly appear in water surface profiles, which have mild slope withvegetations and steep slope with no vegetation. On the other hand, in the flood receding
period, effects of the vegetations do not appear in the water surface profiles in the floodreceding period.
From the comparison between field inspection after the flood and the observed temporalchanges in water surface profiles, vegetations on sandbars of the inner bank side seem to havebeen destructed between a.m. 2:00 and a.m. 3:00 around peak discharge, in which the water
surface profile becomes irregular. In the following sections, detailed mechanism of vegetation
destruction is discussed by the two-dimensional unsteady flow analysis (Fukuoka and
Watanabe, 2004) using observed temporal changes in water surface profiles.

4

10
9

B
J B
J
J
B
J
B B B
B
B B B
B B B B BJ
B B B B
B 23:00
J
B
J B B
B
B B
J B J J
J 23:30
B
B
J J
B
J
J
B
J
B J
B B B BJ
B 0:00J
J J
J 1:00
B 2:00Vegetation

水位（ｍ）

Water level (m)

8
7
6
5
B
J
B B B
J
B J J
3J

4
2
1

8

8.5

9

9.5

10

10.5
11
縦断距離（ｋｍ）

11.5

12

12.5

13

Longitudinal distance from the river mouth (km)Figure 7. Observed water surface profiles in the flood rising period
10

Water
level (m)
水位（ｍ）

9
8

J 3:00
J 4:00
J 5:00
J 6:00

7
6
5

J J

4

J
J
3J J
J
2J J
J

1

Vegetation

8

J
J
J
J

J
J
J
J

8.5

J J
J
J J
J

J
J J J
J
J
J J J
J J J J JJ J J J
J
J

9

9.5

J
J
J
J

10

J
J
J
J

J
J
J
J

10.5

J
J
J
J

J
J
J
J

11

J
J
J
J

J
J
J
J

J J
J
J
J
J
J J J J
J J J
J J

11.5

12

J
J
J
J

12.5

JJ
JJ
JJ
JJ

13

縦断距離（ｋｍ）
Longitudinal distancefrom the river mouth (km)Figure 8. Observed water surface profiles in the flood receding period
3.3

Unsteady 2D flow analysis using observed water surface profiles

The calculation is conducted based on the method of Fukuoka and Watanabe et al(2004). Equations of motion (1), (2) and equation of continuity (3) are as follows. The
effects of vegetations on flood flows are represented by vegetation permeability coefficient Kof equation (4).
h~~~∂U ~ ∂U ~ ∂U ~ ~ ~∂ζ∂ζ~ ∂θ ξ ~ ∂θ ξ
+ Uh ~ + Vh ~ + J (V − U cos θ ηξ )(Uh ~ + Vh ~ ) = − gh( ~ + cos θ ηξ ~ )
∂t
∂η
∂η
∂η
∂ξ
∂ξ
∂ξ

 ~
∂θ ξ 
∂ ~
∂θ ξ
1∂ ~
~
( J dξ ⋅ hτ~ξη ) − J h(−τ~ξξ cos θ ηξ + τ~ξη ) ~ +(−τ~ξη cos θ ηξ + τηη ) ~ 
 ( J dη ⋅ hτ ξξ ) +∂η ∂ηJ  ∂ξ
∂ξ
~~~∂V ~ ∂V ~ ∂V ~ ~ ~∂ζ ∂ζ~ ∂θ η ~ ∂θ η
+ Uh ~ + Vh ~ + J (U − V cos θ ηξ )(Uh ~ + Vh ~ ) = − gh(cos θ ηξ ~ + ~ )
h
∂t
∂η
∂η
∂ξ
∂ξ
∂ξ ∂η
− τ 0ξ +− τ 0η +

 ~
∂ ~
∂θ η
∂θ η 1∂ ~~( J dξ ⋅ hτ~ηη ) − J h(−τ~ξξ + τ~ξη cos θ ηξ ) ~ + (−τ~ξη +τ~ηη cos θ ηξ ) ~ 
 ( J dη ⋅ hτηξ ) +∂η∂η J  ∂ξ
∂ξ



(

)

(

)

∂h ∂ ~∂ ~~~+J dη ⋅ Uh +J dξ ⋅ Vh = 0
∂t ∂ξ
∂η
∂
∂
∂
∂
,
=
~=
~
d
d
∂
∂
∂η
ξ
ξ
η
η
∂ξ

J

(τ 0ξ , τ 0η ) = (

gn2
1

+

(1)

(2)
(3)

(4)

gha
~ ~
) u 2 + v 2 (U , V )K2h3ha = min(h, htree )~ ~~~~u 2 + v 2 = J 2 (U 2 − 2UV cos θ ηξ + V 2 )5Table 1. Vegetation permeability coefficients and Manning roughness coefficients

Calculated water level
23:00
1:00
0:00
2:00
14.5

Observed water level
1:00
23:00
2:00
0:00

13.5

水位（ｍ）

Water level (m)12.5

Vegetation

11.5
10.5
9.5
8.5
7.5

B
J
B J J
J
J
B J B
J
J
B

J
B
B
J
B J
J
J
J
B
J
J
BJ
B
B
J
B B B
BB

B
J
JJ
J
J
J
B
J
B
B
J J J B
B J J
J
J B
B B
J
B
J
B
B
B
J J J B B
B B
B B

6.5
5.5
7.4

8.4

9.4

10.4
縦断距離（ｋｍ）

11.4

12.4

Longitudinal distance from the river mouth (km)Figure 9. Observed and calculated water surface profiles in the flood rising and peak
period14.5

Calculated water level
5:00
3:00
6:00
4:00

11.5
10.5
9.5
8.5
7.5

BB
B J
J
B B
B
J
B B
J J B BB
B
J
J
B
B
B J J
B J J
J
J
J
J
B
B B
B
B B
J J J J B B J J JJBJ
B B J J
B
B
J J J
J J
J
B B B JBJ
B J J J J
BB JJVegetation

12.5

水位（ｍ）

Water level (m)13.5

Observed water level
3:00
5:00
6:00
4:00

B B B
B
J J
J J
B

6.5
5.5
7.4

8.4

9.4

10.4
縦断距離（ｋｍ）

11.4

12.4

Longitudinal distance from the river mouth (km)Figure 10. Observed and calculated water surface profiles in the flood receding period

6Discharge (m3/s)

Calculation
Observation

Time (hour)
Figure 11. Observed and calculated discharge
Manning’s roughness coefficient n is given by ordinary values of channel resistance tomain channel and flood channel. And K is determined so as to agree with the observed water
surface profiles. Characteristics of the water surface profiles in the flood receding period are
different from these of the flood rising period. The difference arises mainly from the change
in vegetation resistance associated with the large scale vegetation destructions. Therefore thevegetation permeability coefficients would change between a.m. 2:00 and a.m. 3:00 aroundthe peak discharge. Table 1 shows the vegetation permeability coefficients and Manning’s
roughness coefficients used in the analysis.
The vegetation permeability coefficients in the flood receding period are large comparedwith these of the flood rising period. The analytical grid is divided longitudinally into 224
cells and divided laterally into 23 cells. The boundary conditions are observed water level
hydrographs at 8.6km and 11.6km (Yaguchi-daiichi discharge observation station). Figure 9
and Figure 10 show observed and calculated water surface profiles in the rising period and thereceding period of the flood, respectively. The calculation results show a good agreementwith observed temporal changes in water surface profiles. Figure 11 compares with calculated
discharge hydrograph observed and discharge hydrograph measured at the Yaguchi-daiichi
discharge observation station. The above proves that the vegetation on the sandbar formednear inner bank were destructed between a.m. 2:00 and a.m. 3:00 around the peak discharge.
We believe that the vegetation on sandbars formed near inner bank would be destructed in theflood of the design scale of the Ota River since the flood of the design scale slightly is lager
than its by typhoon 0514.

4.

MECHANISM OF VEGETATION DESTRUCTION

4.1 Vegetation destruction by bed scouring on sandbar formed inner bank side
The velocity distribution and maximum velocity filament by the calculation result at a.m.
2:00 is shown in Figure 12. Figure 13 shows cross-section at 9.4km point, and compareschange in the cross-section before and after the flood. The spatial distribution of the bed
variation before and after the flood is shown in Figure 14. Maximum velocity filament occurs
on inner bank sandbar formed around 9.4km point where vegetation growth was denser andvegetation destruction was severe. Velocity has near 4.0(m/s) there. The maximum bed

7CC9.0km4(m/s)

10.0kmYaguchi-daiichi

9.4km

11.0kmVegetation growth area

Maximum velocity filament

Figure 12. Velocity distribution and Maximum velocity filament

Elevation (m)Before flood

After flood

1

Destructed vegetation
Sparse zone

8

A BC

9.4km

9.2km

9.6km

Scouring

4

SandbarFlood-channel

A
B
C

0
-4
-50

Sandbar

5

150
250
350
Lateral distance (m)Figure 13. Cross section at 9.4km point

Flood channel

Vegetation destructed area
Figure 14. Change in bed profiles
before and after the flood

scouring which reaches 2-meter in depth occurred around the vegetation A on sandbar at thissection (see Figure 13). The longitudinal bed scouring occurs near the vegetations A on
sandbar and vegetations C beside flood channel (see Figure 14).
There is a sparsevegetation zone B in the middle of the sandbar around the 9.4km point (see Figure 13 andFigure 14). The bed scouring on sandbar was formed near the inner bank side due to the flood
flowing there about peak discharge. Hence, it is concluded that the main mechanism ofvegetation destruction is the bed scouring around the vegetations on the sandbar.
4.2. Difference of bed scouring by floods scale
Figure 15, Figure 16 and Figure 17 show aerial photographs in the neighborhood of 9.4km
after 1972, 1993 and 2005 floods, respectively. In 1972, the vegetations on sandbar formed
inner bank side have not grown yet (see Figure 15). The vegetations have grown densely on
sandbar since 1990 (see Figure 16). The maximum flood discharge in 1972 was similar to7200(m3/s) of the 2005 flood and 6800(m3/s). Scouring occurred on sandbar formed innerbank side (see Figure 15 and Figure 17). On the other hand, the flood scale in 1993 which
maximum discharge was 3900(m3/s) was smaller than 1972 and 2005 floods. Therefore
scouring would not occur on sandbar formed the inner bank side. Three cases of floods showthat vegetation destruction occurs only in large scale flood. Hence, we believe that the8vegetation destruction on is seen due to the sandbar bed scouring around the vegetations,when the scale of design flood happen in the Ota river.

9.4km

9.6km

9.2km

Figure 15.

Sandbar bed scouring by 1972 flood
9.4km

No Scouring

9.2km

9.6km

9.0km
SandbarFlood-channel

Figure 16.

Sandbar bed scouring by 1993 flood

Scouring

9.4km

9.2km
9.0km

9.6km

SandbarFlood channel

Figure 17. Sandbar bed scouring by 2005 flood

95.

CONCLUSION

The following conclusions were drawn on vegetation destruction by flood.
(1) The effects of vegetation destruction appear in temporal changes in water surface
profiles of flood flow. In flood rising period, water surface profiles were irregular inshape due to the presence of vegetations. But water surface profiles in flood receding
period were relatively smooth for the vegetation destruction.
(2) Unsteady 2D flow analysis using measured temporal changes in water surface profiles
gives an information that how much discharge destructs vegetations and whenvegetations were destructed.
(3) Survey of bed scouring and the vegetations before and after the flood showed that the
bed scouring around vegetation on the inner bank sandbar gave rise to vegetation
destruction.

REFERENCES
Okada,S., and Fukuoka,S. (2004). Characteristics of Flood Flow, Sediment Discharge and
Bed Topography in Compound Rivers, Journal of Hydraulic, Coastal and Environmental
Engineering, JSCE, No.754/Ⅱ-66, pp.19-31.
Fukuoka,S. (2005), Flood Hydraulics and Channel Design, Morikita Publishing Company.
Fukuoka,S. and Fujita,K.(1990). Hydraulic Effects of Luxuriant Vegetations on Flood Flow,
Report of Public Works Research Institute Ministry of Construction, Vol.180-3.
Fukuoka,S., Watanabe,A., Hara,T. and Akiyama,M. (2004). Highly Accurate Estimation of
Hydrograph of Flood Discharge and Water Storage in River by Using an UnsteadyTwo-Dimensional Flow Analysis Based on Temporal Change in Observed Water Surface
Profiles, Journal of Hydraulic, Coastal and Environmental Engineering, JSCE, No.761/Ⅱ-67,pp.45-56.
Maeno,S., Watanabe,S. and Fujituka,Y. (2005). Improvement of Modeling of Flow Analysis
Using easily obtained Vegetation Characterritics, Journal of Hydroulic,Coastal andEnvironmental Engineering, JSCE, No,803/Ⅱ-73,pp.91-104.
Shimizu,Y., Kobatake,S. and Yoshikawa,T. (2002). Study on the Destruction Criterion of
Riparian Trees in a Gravel-bed River Caoused by Flood Distarbance, Annual Journal of
Hydraulic Engineering, JSCE, Vol.46, pp.953-958.
Yagisawa,J., Tanaka,N. and Fukuoka,S. (2008). Analysis of the Threshold Drag Moment andShear Stress at Flood Events for Uprooting Trees Vegetated on Gravel Bed Bars, Advancein River Engineering, Vol.14, pp.139-144.

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