ICHE 2014, Hamburg - Lehfeldt & Kopmann (eds) - © 2014 Bundesanstalt für Wasserbau ISBN 978-3-939230-32-8

Large-Scale Sediment Inflow and Bed-Variation from 12th Typhoon(2011) in the Asahi River Basin
Y. Tsukamoto

Graduate School of Science and Engineering, Chuo University, JapanT. Eguchi

The Kansai Electric Power Co., Japan

S. Fukuoka

Research and Development Initiative, Chuo University, Japan

ABSTRACT: In the Asahi River, heavy rainfall from 12th typhoon (2011) had caused the river channel
blockade by sediment inflow due to slope failures and debris flows. These phenomena make large amountof sediment discharge into the river shortly, and change the river channel characteristics, such as thechannel shapes, and grain size distributions. So, it is important to clarify the mechanism of sedimenttransports including large amount of sediment supply for the river management. In this study, first, weclarified points of slope failures and characteristic changes in river channel by large-scale sediment inflow using observed data and aerial photographs. In order to estimate the volume of sediment inflow, numerical analysis of slope failures and debris flows were made. Furthermore, we combined twodimensional flood flow model with river bed variation model and examined the sediment transport mechanism including the sediment supply in the Asahi River during the 2011 flood.
Keywords: Sediment inflow, Sediment transport, Debris flow, Slope failure, Numerical model, Stony-bed
river, Bed variation, Grain size distribution
1 INTRODUCTION
A heavy rainfall from 12th typhoon (2011) brought large-scale floods and slope failures in the Asahi Riverbasin. Sediment supply to the Asahi River due to slope failures changed channel shapes and grain size distributions of the river bed remarkably in the short period of time. Estimation of the amount of the sedimentsupply of debris flows by slope failures and understanding of sediment transports during the flood are important for the river management.
The Asahi River is a stony-bed river with a wide range of grain-size distributions. In the Asahi River, asediment bypass tunnel is installed for a measure against turbid water of a dam reservoir, and the reservoir
sedimentation. Fukuda et al. (2012) investigated the recovery mechanism of riffles and pools and flushingmechanism of reservoir sedimentation by using the two-dimensional flood flows and bed variation analysis
in stony-bed river (Osada et al. (2013)). However, sediment inflows from mountain streams were not considered in this model, because the flood discharge was relatively small compared with the 2011 flood.
The slope failures and debris flow simulation model to estimate sediment inflows from mountainstreams has been proposed by reserchers. For example, there are physically-based model of the SHETRAN(Bathust (2002)) and the SERMOW-Ⅱ (Hirasawa et at. (2012)) and so on. Ichikawa (1999) calculated thesediment runoff due to slope failures using by the kinematic wave-runoff model and slope failure model
considering an infinite length slope-stability analysis. And sediment inflows from mountain streams were
simulated by using debris flow equations which corresponded to each regime such as debris flow,
immature debris flow, and bed load (Takahashi (2001)). But, these researches have not considered enoughthe relation between flood flows and large-scale sediment inflow, and applicability to the design ofstructures for sedimentation. Therefore, it is important to clarify mutual relations of sediment transports inriver channel, mountain stream, dam reservoir and outlet structure.
The objectives of this study are to clarify the sediment transport mechanism based on observed data andto develop the two-dimensional flood flows and river bed variation analysis model clarifying the693relationship between flood flows and sedimet transports including the large amount of sediment supply ofdebris flows.
2 SEDIMENT YIELD AND RIVER CHANNEL CHANGE
2.1 Characteristics of the Asahi River basin
Figure 1 shows the plan form of the Asahi River basin and the flushing bypass tunnel. The Asahi River is astony-bed river with a wide range of grain-size distributions. The channel shapes are remarkably meandering, and the river bed gradients of upstream and downstream in Asahi Dam is about 1:80 and 1:100, respectively.
Asahi Dam is located at about 6.0km point of the Asahi River from the junction with the Kumano River.
The sediment flushing bypass tunnel indicated by the red line in Figure 1 was constructed to mitigate thelong-term turbid water and reservoir sedimentations in Asahi Dam Reservoir. The sediment flushing bypass tunnel is a 2,350m long, and 1:35 bed gradient.
Exit
Setoya River
Asahi River
2km
3km

1km

flow
Kumano River

Flushing Sediment
Bypass Tunnel

Asahi Dam

Sediment
Storage Dam

5km

Entrance

Upstream
Observation station

4km

Dam Reservoir
Downstream
Observation station
Miya Valley

Upstream Valley

Figure 1. Plan form of the Asahi River basin.

2.2 Objective flood
Figure 2 shows the rainfall heiehtgraph, the water level hydrograph, inflow and outflow discharge hydrographs in Asahi Dam Reservoir observed by the 12th typhoon (2011). The flood had three peaks and recorded the largest rainfall of totals about 1,500mm for three days.
Figure 3 shows the observed discharge hydrographs in upstream and downstream observation station.
The observed discharge hydrograph in upstream observation station was 708 m3/s, and in downstream observation station was 685 m3/s. The maximum annual discharge in upstream observation station was 219
m3/s and in downstream observation station was 244 m3/s. Hence, the flood was a large flood comparable
to three times of the annual maximum discharge.
30

32mm

41mm

39mm

800

1600

700

800

Hourly rainfall

10

cumulative rainfall

9/2 0:00

2000
1200

20

0
9/1 0:00
1000

9/3 0:00

9/4 0:00

9/5 0:00

9/6 0:00

400
0
9/7 0:00
470

Outflow discharge
Inflow discharge
Sediment bypass discharge
Water level

800
Discharge（m3/s）

Cumulative rainfall
1494mm

600

450
440

200

430

0

420
9/2 0:00

9/3 0:00

9/4 0:00

9/5 0:00

9/6 0:00

Upstream Observation station
Downstream Observation station

708m3/s
3685m /s

600
500
400

Maximum annual discharge
3
in upstream 244m /s

300
200

460

400

-200
9/1 0:00

Cumulative rainfall（mm）
Discharge（m3/s）

40

Maximum annual discharge
3
in downstream 219m /s

100
Water level（m）

Hourly rainfall（mm/hr）

50

410
9/7 0:00

Figure 2. Heiehtgraph and hydrographs in the Dam
Reservoir.

694

0
9/1 0:00

9/2 0:00

9/3 0:00

9/4 0:00

9/5 0:00

9/6 0:00

9/7 0

Figure 3. Discharge hydrographs at upstream anddownstream observation stations.

2.3 Sediment yield and river channel change
Figure 4 shows comparison between aerial photographs before and after the flood in the slope failure point
and the exit of the Miya Valley. The aerial photographs were taken after (September 2011 by Geographical
Information Authority of Japan) and before (May 2004 by Ministry of Agriculture, Forestry and Fisheries
of Japan) the flood. The investigation of slope failures points in the Asahi River basin showed the largescale slope failure (about 82,000 m2) at the Miya Valley (see figure 4 (a)).
Figure 4 (b) shows river channel change at the exit of mountain stream at the Miya Valley. In downstream of the exit, water course was narrowed by sediment deposition. In the upstream reach, the width ofwater surface was extended since river channel blockade raised water level. Fukuda et al. (2012) confirmed
the supply of the white-colored stones from upstream of Asahi Dam and recovery mechanism of riffles andpools by construction of the sediment bypass tunnel. However, a large amount of gray stones due to slope
failures were supplied and deposited on sandbars, and water course was rectilinear after the flood.
Before Flood (May 2004)

After Flood (Sep 2011)
・Deposition of the White Stones
・Riffles and Pools
3.0k
4.0k
Miya Valley
・Deposition of the Gray Stones
・Linearization of Water course
3.0k
4.0k
Miya Valley(a) State of slope failures(b) River channel change at exit of mountain streams

Figure 4. Comparison between aerial photographs before and after the flood.

Figure 5 shows the longitudinal distributions of the average and deepest elevations of the river bed observed before and after the flood. Large-scale slope failures were identified at the Miya Valley which met
the Asahi River at about 4.0km point. The average bed elevation was raised about 1.5m and deepest bed elevation about 3.0m around the exit of the Miya Valley.
The volume of sediment deposition in the upstream of the sediment storage dam is about 48,000m3, inthe Dam Reservoir is 299,800m3, and in the downstream of the Dam is 249,000m3. It is important to consider large-scale sediment inflow as an upstream boundary conditions of debris flows since the effect of thesediment inflow to the Asahi River from the Miya Valley is so large.
River Bed Variation（m）

9.0

Average bed elevation

6.0

Lowest bed elevation

Sediment Storage Dam
Dam Reservoir
Upstream of Dam
3
199,800m
48,000m3

Downstream of Dam
Sediment Deposition 249,000m3

Flushing Sediment Bypass Tunnel

3.0

（m）

0.0
0
-3.0

1000

2000

3000

4000

5000

Miya Valley

6000

7000

8000

9000

10000

11000

Upstream Valley

12000

Figure 5. Average and lowest bed elevations and sediment deposition volumes.

3 ESTIMATION OF SEDIMENT INFLOW VOLUME
3.1 Estimation of the occurrence time by slope failures
Large-scale sediment supply due to slope failures and changes in river channel geometry appear in thetemporal changes in water surface profiles. We estimated the occurrence time of slope failures from twodimensional flood flow analysis using cross-sectional bed shapes observed before and after floods andcomparison of observed and calculated water levels.
695

The analysis was performed from the junction with the Kumano River to 2.0km upstream of the entrance of the sediment bypass tunnel. The initial bed forms were given based on cross-sectional bed shapes
measured before (2011) and after (2012) the flood. The numerical model was applied to the flood occurredin September 2011. Water level and discharge have been measured at observation points at upstream anddownstream of Asahi Dam. In downstream observation point, water levels were not measured because thewater gauge was damaged by sedimentation after the large amount of sediment inflow. The upstream
boundary condition was given by observed discharge hydrographs at the upstream observation point. The
downstream boundary condition was given by the uniform flow depth. The manning roughness coefficientswas n=0.035 so as to explain the observed water-level hydrographs in the observation points.
Figure 6 shows the comparison between observed and calculated water-level hydrographs and occurrencetime of slope failures. In early stage, the calculated water levels using channel cross-section before theflood was similar to observed one in upstream observation point. And about 25 hours later, the calculated
water levels using channel cross-section after the flood was in rather good agreements with observed one.
On the other hand, in the downstream observation point, the calculated water levels using channel crosssection before the flood were similar to flood peak. And about 66 hours later, the calculated water levels
using channel cross-section after the flood explained observed one. Since changes in geometrical conditionof river channel appears in the observed water levels, it is assumed that the large sediment supply from theMiya Valley has been occurred at about 25 hours in upstream and about 66 hours in downstream observation point, respectively.
365

10

364

20

363

Occurrence time

Water level(m)477Hourly rainfall
obs
cal (after flood)cal (before flood)

476
475
474

30
40
50

360
359

472

70

471

80

470

90

356

469

100

355

20

30

40

50
60
70
Time（hour）

80

90

100

Hourly rainfall
obs
cal (after flood)cal (before flood)

361

60

10

Occurrence time

362

473

0

0

357

(a) Upstream observation point

40
50
60
70
80
90
100

0

110

20
30

data
missing358

10Hourly rainfall（mm）

0478

Hourly rainfall（mm）
Water level(m)

479

10

20

30

40

50
60
70
Time（hour）

80

90

100

110

(b) Downstream observation point

Figure 6. Occurrence time of slope failures predicted from observed and calculated water level hydrographs.

3.2 Estimation of the sediment inflow volume
3.2.1 Sediment runoff analysis model due to slope failures
The volume of sediment runoff was calculated by the kinematic wave runoff model which simulated surface and subsurface runoff and the stability analysis assuming the slope of infinite length (Ichikawa(1999)). The equation of the kinematic wave runoff model is written as:
𝜕𝑞 𝜕ℎ
+
=𝑟
𝜕𝑥 𝜕𝑡
𝑎ℎ
𝑞=�
𝛼(ℎ − 𝑑)𝑚 + 𝑎ℎ

(1)(ℎ ≤ 𝑑)(ℎ > 𝑑)(2)

Where, q: unit width discharge, h: water depth, d: subsurface layer thickness, r: rainfall intensity, 𝑎 =𝑘𝑠𝑖𝑛𝜃/𝛾, k: coefficient of permeability, 𝛾: effective porosity, 𝜃: slope gradient, 𝛼 = √𝑠𝑖𝑛𝜃/𝑛, m=5/3, 𝑛:manning roughness coefficients.
The safety-factor is defined the ratio of the shear stress 𝜏 (Eq.(3)) bringing slope failures to the resisting
stress 𝜏𝑟 (Eq.(4)) by assuming the slope of infinite length. In this study, we assume the slope failures are
occurred when safety-factor become less than 1.0 after the occurrence time of slope failures predicted fromobserved and calculated water level hydrographs (see figure 6).
𝜏 = {𝛾𝑠𝑎𝑡 ∙ 𝐻 + 𝛾𝑡 ∙ (𝐷 − 𝐻)} ∙ 𝑠𝑖𝑛𝜃 ∙ 𝑐𝑜𝑠𝜃𝜏𝑟 = 𝑐 + (𝜎 − 𝑢) ∙ 𝑡𝑎𝑛𝜑= 𝑐 + {(𝛾𝑠𝑎𝑡 − 𝛾𝑤 ) ∙ 𝐻 + 𝛾𝑡 ∙ (𝐷 − 𝐻)}𝑐𝑜𝑠 2 𝜃 ∙ 𝑡𝑎𝑛𝜑
696

(3)

(4)

Where, c: adhesive force , 𝜎: vertical stress of soil , u: pore-water pressure of soil , 𝜑: internal frictional
angle of soil, 𝛾𝑠𝑎𝑡 : saturated unit weight of soil, 𝛾𝑤 : unit weight of water, 𝛾𝑡 : wet unit weight of soil, D:
subsurface layer thickness, H: subsurface layer water depth.

3.2.2 Sediment inflow analysis model due to debris flow
The sediment inflows from mountain stream to the river channel was simulated by using debris flow
equations which corresponded to each regime such as stony debris flow, immature debris flow, and bed
load (Takahashi (2001)). The debris flow was calculated by one dimensional bed variation Equation (5)(7).
𝜕ℎ 𝜕𝑞
+
=𝑖
𝜕𝑡 𝜕𝑥
𝜕ℎ
𝜕(𝑈𝑞)
𝜕(𝑧𝑏 + ℎ)𝜏𝑏+𝛽= 𝑔ℎ𝑠𝑖𝑛𝜃 − 𝑔ℎ𝑐𝑜𝑠𝜃 −𝜕𝑡
𝜕𝑥
𝜕𝑥
𝜌𝑡
𝜕(𝐶ℎ) 𝜕(𝐶𝑞)
+
= 𝑖𝐶∗
𝜕𝑥
𝜕𝑡

(5)

(6)

(7)

Where, i: erosion and deposition velocity of the bed, β: momentum correction coefficient, U: crosssectional average-flow velocity, g: acceleration of gravity, 𝑧𝑏 : bed level, 𝜏𝑏 : bed shear stress, 𝜌𝑡 =𝜌(1 − 𝐶) + 𝜎𝐶, 𝜌: density of water, σ: density of soil, C: volume concentration of debris flow, 𝐶∗ : volumeconcentration of sediment deposit.
The resistance laws in the river bed are given as follows:
For stony debris flow (when C ≥ 0.4𝐶∗ )𝜎𝐶∗ 1⁄3𝑑 2 2
𝜏𝑏 = � � �� � − 1� � � 𝑈
𝐶8ℎFor immature debris flow (when 0.01 < C < 0.4𝐶∗ )𝜌𝑡𝑑𝐿 2
𝜏𝑏 = �� � � 𝑈2
0.49 ℎFor bed load (when C ≤ 0.01 or h/d ≥ 30)
𝜌𝑔𝑛2
𝜏𝑏 = 1⁄3 𝑈 2
ℎ

(8)
(9)
(10)

Where, d: representative grain size of soil particle, n: manning roughness coefficient.
The erosion and deposition velocity of the bed and equilibrium concentration are given as follows:

For erosion of the bed (when C < 𝐶∗ )𝐶∞ − 𝐶 𝑞
𝑖 = 𝛿𝑒(11)𝐶∗ − 𝐶∞ 𝑑
For deposition of the bed (when C ≥ 𝐶∗ )𝐶∞ − 𝐶 𝑞(12)
𝑖 = 𝛿𝑑𝐶∗ ℎ𝜌𝑡𝑎𝑛𝜃𝑤𝐶∞ =(13)(𝜎 − 𝜌)(𝑡𝑎𝑛𝜑 − 𝑡𝑎𝑛𝜃𝑤 )Where, 𝛿𝑒 : erosion velocity coefficient, 𝛿𝑑 : deposition velocity coefficient, 𝜃𝑤 : internal frictional angle.

3.2.3 Numerical analyse of slope failures and debris flows
In slope failure model, the slope unit data was created by 50m mesh based on 5.0m mesh data (Basic map
information by Geographical Information Authority of Japan). Moreover, the slope direction and gradientwere divided from the drainage paths in each mesh. The parameters of slope failure model are shown in table-1. Since the characteristics of soil such as adhesive force and an internal frictional angle were unknown, we set values used in other researches by slope failure model. The thickness of slope unit was assumed 0.6m in the Miya Valley and 0.2m in the Upstream Valley by the columnar section of soil in Mt.
Shakagatake.
697

In debris flow model, longitudinal and cross-sectional shapes were made by the 5.0m mesh data. The analysis was performed 1.5km reach in the Miya Valley and 1.2km reach in the Upstream Valley. The longitudinal distance for the calculation was set about 50m-70m. The parameters of the debris flow model are
shown in table 2. Since the characteristics of sediment inflow from mountain stream in the Asahi River basin were unknown, we set values of other researches used by debris flow model. Sediment supply due toslope failures was modeled assuming the process of temporal sedimentation and blockade in mountainstream.
Table 1. Parameter of slope failure model.
______________________________________________
Thickness of slope unit (Miya Valley) 0.6m〃
(Upstream Valley)
0.2m
Equivalent roughness coefficient
0.035
Porosity
0.4
Coefficient of permeability
0.005m/s
Adhesive force
2.5kgN/m2
Internal
frictional angle
20°_____________________________________________

Table 2. Parameter of stony debris flow model.
______________________________________________
Thickness of sediment deposit
3.0m
Density of coarse particles
2,650kg/m3
Density of water
1,000kg/m3
Volume concentration of the bed
0.65
Erosion velocity coefficient
0.0050
Deposition velocity coefficient
0.0003
Manningroughnesscoefficient
0.050_____________________________________________

40

250

41mm

32mm

20
0
8/31 21
300
250
200
150
100
50
0
0

9/1 21

9/2 21

9/3 21：Miya valley：Upstream valley

9/4 21

9/5 2140(m3/s)

Total sediment inflow(Miya valley)
Total sediment inflow(Upstream valley)
sediment inflow(Miya valley)
sediment inflow(Upstream valley)200

215,000m3

8

150

6

100

4

50

2
0

0

60

80

Time(hr)

100

120

10

9/6 21

25,000m3

20(103m3)

39mm

0

140

Figure 7. Observed hourly rainfall and calculated
sediment runoff volume.

Sediment inflow

60
Total sediment inflow

Sediment runoff
volume (103m3)

Hourly rainfall(mm/hr)3.2.4 Result of analysis
Figure 7 shows the hourly rainfall and calculated sediment runoff volume by the slope failures. In the Miya
Valley, slope failures were caused at the third peak of the rainfalls and the sediment runoff volume wasestimated about 215,000m3. In the Upstream Valley, the sediment runoff volume was about 25,000m3.
Figure 8 shows calculated sediment inflow from the Miya Valley to the river channel, and its total volume.
The total volume of the sedement inflow was estimated about 195,000m3 in the Miya Valley and about
3,7000m3 in the Upstream Valley. In the Miya Valley, the total volume of sediment inflow was equivalent
to about 78% of volume of the sediment deposition in the river channel at the down stream of the Dam( 249,000m3, see Figure 5 ).

20

40

60

80

Time(hr)

100

120

140

Figure 8. Calculated sediment inflow frommountain streams.

4 RIVER BED VARIATION ANALYSIS CONSIDERING LARGE-SCALE SEDIMENT INFLOW
4.1 Concept of the river bed variation analysis considering large-scale sediment inflow
We conducted two-dimensional river bed variation analysis in stony-bed river considering large-scale sediment inflow as boundary conditions. Figure 9 shows the concept of the process of aggradation by sediment�����
inflow from mountain stream. Average height of each particle size Z
𝑝,𝑘 was estimated by using pick-up
rate 𝑉𝑝,𝑘 and deposit rate 𝑉𝐷,𝑘 and each particle size function on the bed surface 𝑃𝑘 . In order to consider
the process of river bed aggradation and temporal river channel blockade, the volume of sediment inflow
𝑉𝑖𝑛,𝑘 was assumed to deposit with deposit rate 𝑉𝐷,𝑘 (Eq.(14)). The sediment deposition section was set in
the remarkable section of sedimentation based on observed data indicated by the red frame in figure 11.
�����𝜕𝑍
𝛼2 �𝑉𝑃,𝑘 − �𝑉𝐷,𝑘 + 𝑉𝑖𝑛,𝑘 ��
𝑃,𝑘
=−
𝜕𝑡
𝛼3
𝑃𝑘

Where, 𝛼1 , 𝛼2 : 2-D and 3-D shape factors of the particles (=𝜋/4, 𝜋/6), 𝑘: each particle size.
698

(14)

The grain size distributions of the Asahi River and sediment inflow is shown in figure 10. The grain size
distributions of sediment inflow were given by field survey in 2013. In the river channel, the grain size distributions surveyed by 2000 were used. Equilibrium sediment transport rate was assumed for the upstream
boundary conditions.
flow

Remarkablesedimentation
seciton100

Sediment inflow
from mountain stream

5.35k
5.30k
5.20k
5.00k Obs.(2000)
4.90k
4.75k
Cal. River channel
Cal. Sediment inflow

90
80
Percent finer（％）

Total sedimentinflow

time

70
60
50
40
30
20
10

Set the volume of sediment inflow inremarkable sedimentation section

0
0.1

10
Grain size（mm）

100

1000

Figure 10. Grain size distributions used foranalysis.

Figure 9. Concept of the river bed variation analysis
considering large-scale sediment inflow.

4.2

1

Results and considerations

Figure 11 shows the comparison between the calculated and observed river bed variation contours before
and after the flood. The height of the calculated sediment depositions was about 1.0m lower than the observed one through the area in the exit of the Miya Valley. In the upstream of the Dam, the height of calculated sediment deposition was similar to observed one at the exit of the Upstream Valley, but the height ofcalculated sediment deposition was lower than observed one near the entrance of sediment bypass tunnel.
As shown below, this model cannot explain the extension of deposition area and process of sediment deposition since the large inertial force in sediment inflow has not considered, and thus cannot reproduce
properly the height of the sediment deposition. However, calculated bed variations reproduce the sedimentdepositions of the observed data, especially the sediment depositions and temporal river channel blockadearound the exit of the Miya Valley. Figure 12 shows the distribution of grain size D80. After the flood,larger size materials were transported in sediment deposition area due to sediment inflows.
CalculatedObserved

Entrance

Bed variation(m)Bed variation(m)Upstream Valley

Upstream Valley

Observed

Calculated

Bed variation(m)Bed variation(m)

3.0k

3.0k

4.0k

4.0k
Remarkable
Sedimentation
sectionMiya Valley

Entrance

Miya Valley(b) Upstream of the Dam(a) Downstream of the Dam

Figure 11. Comparison between calculated and observed river bed variation contour (After the flood).
Entrance

After flood
Grain size D80 (m)Grain size D80 (m)

3.0k
4.0k
Miya Valley

Upstream Valley

After flood(a) Downstream of the Dam(b) Upstream of the Dam

Figure 12. Distribution of grain size D80.

Figure 13 shows the calculated longitudinal peak water level. Flood marks were observed by field survey atthe left-bank in downstream of the Miya Valley, and the calculated longitudinal peak water level repro699

duced observed one. We used the observed peak water level as a reference water level at downstream observation point, because the water gauge was not available by sedimentation.
The analysis gives overall explanation for the flood flows and the bed variations during 2011 flood considering large-scale sediment inflow from mountain stream.
370

480
Miya Valley

Wate level（m）

360

Upstream Valley

470※Reference value

465

355
Calculated peak water level
Flood markObserved peak water level (dowmstream)
Average bed elevation(after flood)
Average bed elevation(bofore flood)

350
345
340
3000

Sediment
Storage Dam

Wate level（m）

475

365

3500

4000
Longitudinal distance（m）

4500

5000

(a) Downstream of the Dam

460

455
450
9100

Calculated peak water level
Observed peak water level (upstream)
Average bed elevation(after flood)
Average bed elevation(bofore flood)

9600
10100
Longitudinal distance（m）

10600(b) Upstream of the Dam

Figure 13. Calculated longitudinal peak water level.

5 CONCLUSION
In this paper, we clarified the sediment transports mechanism including the large sediment supply of thedebris flows due to the slope failures in the Asahi River basin during the 2011 flood.
Main conclusions are drawn as follows.
1. Observed data showed that river channel characteristics (channel shapes, grain size distribution) remarkably changed by large-scale sediment inflow due to slope failures and debris flows.
2. The volume of sediment inflow was estimated by slope failure model and debris flow model. They werecombined with two-dimensional flood flows and river bed variation analysis model to clarify the relationship between flood flows and river sediment transports accompanying the large amount of sedimentsupply.
3. It was shown that the analysis gave overall explanations for flood flows and bed variations during 2011
flood considering large-scale sediment inflow from mountain stream and that to consider the large-scale
sediment inflow in river bed variation as boundary conditions was important to solve properly the problem treated in the paper.
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CHANGING WORLD-Need for Knowledge Transfer across the Generations & the World, Kyoto, Japan.
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