In recent years, forestation is progressing in the downstream Asahi river area. The trees are densely grown in
the river channel, and the effects of impeded water flow are being studied. For this study, in order to elucidate
the wash-out mechanism of vegetation during flooding, the field surveys and flood flow analysis wereconducted for both areas. The numerical results of this study particularly provide useful information to improve
the reproducibility of the vegetation wash-out phenomenon by setting the wash-out limit condition throughfocusing on vegetation height and tractive force and incorporating the resistance reduction due to vegetation
wash-out in the flood flow analysis.

STUDY SITE AND METHODOLOGY
Study site and data collection
The target reach was measured using the present ALB system with laser sensors (Leica Chiroptera II; Leica Corp.)
during a time with a normal water level in a normal water season (November , 2017, and January , 2018). Figure
1 presents ALB data acquisition range in Asahi river and Hyakken river, shows analysis area.
Classification by ALB data and Ortho-image before and after flooding
Clustering was performed on the ALB point cloud data contained in the 2 m mesh, and the river was divided into
4 labels. Figure 2 depicts a comparison on the classification of ground covers and vegetation species based on
ALB data, and ortho-image taken by drone before flooding, and it was observed that vegetation classifications
were almost consistent. Figure 3 shows the ortho-image, taken by drone, of the wash-out situation of the Asahi
River after flooding (July 2018). Comparing with Ortho-image before and after flooding, we can make sure thesituation of wash-out vegetation.
2017/11 ALB
2018/1 ALB

Tamagashi area： bamboo cutting（2014)

Haraojima Water Level
Observatory (Hyakken River
analysis downstream end)

flow

flow

16k

11k4

14k

Analysis
upstream end

10k8k2

Aioi Bridge Water
Level Observatory(Asahi River analysis
downstream end)

Gion area

12

3

45

Herbs Residual
草残存 草流出Herbs Washout

3.5
3
2.5
2
1.5
1
0.5
0

Case1

Case2
0

123
τ*50/τ*c50

45

Figure 4. Variation of the non-dimensional tractive
force by particle size (Nakahara, figure 2 .).
17k4

Case1

24

6

8

10Vegetation height ℓ(m)Figure 5. Model diagram (only wood) of the limit condition
for the residual and wash-out considering vegetation height .

Case2

Case2

Case2

Bamboo forests

1.0km

Trees Residual

Nakahara area： Sandbar devaluation(2018)

17k4
trees
herbs
Young bamboobamboo
Bare ground and 16k
low waterway

Trees and herbs washout
Bare ground andlow waterway

flow
16k

17k4
flow

17k4

flow

flow
16k

16k

15k

17k4

17k4

flow

17k4

Herbs Residual

Figure 1. ALB data acquisition range in the target study area.

Gion area

Trees Residual
木残存 木流出
Trees Washout

0flow

12k

17k4765432107 06543210

τ*85/τ*c85

INTRODUCTION

Examination of residual and wash-out conditions of vegetation
Firstly, unsteady flood flow analysis without vegetation wash-out was performed within the analysis range
shown in Figure 1. As a result, it was found that it is desirable to use the share force to judge the vegetation
residual. Figure 4. shows variation of the non-dimensional tractive force by particle size. From the figure 4., it
can be seen that the 85% particle size is easier to consider for the residual judgment. In addition, when thevalue of τ*85/τ*c85 exceeds 2, it was confirmed as the vegetation wash-out.
Flood flow analysis considering resistance change caused by vegetation wash-out
The model diagram of the limit condition for the residual and wash-out of vegetation, considering 85%
particle size as a representative value, is shown in Figure 5. Case 1 is a condition in which the limit of thewash-out is constant of 2 (=τ*85/ τ*c85) regardless of vegetation height, and Case 2 changes the limit value
gradually depending on the vegetation height. It was taken into consideration that the herbs were notdependent on vegetation height, and a certain value of 2 was imposed as a limit condition. The condition ofvegetation species and ground covers based on the flood flow analysis as depicted in Figure 6. The simulated
results demonstrated that the proposed model can improve the reproducibility of the wash-out situation
after the flooding (figure 3.), compared with models that did not incorporate both the woody vegetation
height distribution and transition process of woody vegetation wash-out during flooding. In addition, inFigure 6, using this model could show the possibility of vegetation wash-out before flood peak time.
τ*85/τ*c85

K. YOSHIDA, S. MAENO ,K.NAGATA, MD. TOUHIDUL ISLAM, Okayama University, Okayama, Japan
S. NIGO, Chugoku Regional Development Bureau, MLIT, Hiroshima, JapanRESULTS AND DISCUSSION

τ*85/τ*c85

Flood simulation model coupled with woody vegetation wash-out
in the lower Asahi River, Japan

16k
15k

flowflow

Flood Peak time

Tamagashi area

15k

15k

5 hours before
Peak time

15k

3 hours before
Peak time

Flood Peak time

Figure 6. The condition of vegetation species and ground covers based on the flood flow analysis.

CONCLUSIONS
16k

16k
Nakahara area

15k

Figure 2. Classification by ALB data (left), and ortho-image(right) before flooding (May 2018) in Asahi River (14.5 KP17.4 KP).

15k

Figure 3. The wash-out situation of the Asahi Riverafter flooding

In this study, taking the 2018 Asahi River flood as an example, the local topography, ground cover, andvegetation conditions were reproduced with ALB data, and the residual and wash-out conditions ofvegetation during the flooding were confirmed. In addition, although the relationship between the wash-out
situation and the drag of vegetation at the peak of flood flow and the tractive force acting on the ground wasconsidered, it was difficult to organize the limit conditions of vegetation wash-out. Therefore, a simple model
was developed considering vegetation height in addition to the tractive force, and the flood flow analysis
was performed taking into account the resistance reduction due to vegetation wash-out.