Study on Flushing Mechanism of Dam Reservoir Sedimentation
and Recovery of Riffle-Pool in Downstream Reachby a Flushing Bypass Tunnel
Tomoo Fukuda
Department of Science and Engineering, Chuo University, Tokyo, Japant-fukuda66@civil.chuo-u.ac.jp

Katsumi Yamashita
The Kansai Electric Power Co., Inc, Osaka, Japan

Kengo Osada & Shoji Fukuoka
Research and Development Initiative, Chuo University, Tokyo, Japan

ABSTRACT:In the Asahi River, riffles and pools had been scaled down in the downstream reach of Asahi Dam due to sediment deposition in Asahi
Dam reservoir. To improve this environmental problem, a flushing sediment tunnel was constructed so as to bypass Asahi Dam
reservoir. In this study, first, the authors demonstrated the recovery of riffles and pools and the grain size distribution in the downstream
reach by using the data of cross-sectional surveying, air photographs and field investigation. Next, authors’ two-dimensional model wasapplied to a series of flood events in the Asahi River. As a result, the flushing mechanism of Asahi Dam reservoir sedimentation, therecovery process of riffles and pools and the change in grain size distribution in the downstream reach were clarified by the model.
Keywords: Flushing Sediment Bypass Tunnel, Riffle and Pool, Numerical Model, Sediment Transport, Grain Size Distribution

1. INTRODUCTION
Sandbars, riffles and pools characterized in themeandering channel were seen in the Asahi River of the
Shingu River system, Nara Prefecture. However, inassociation with Asahi Dam constructed in 1978, someproblems had occurred such as occurrence of thelong-term turbid water, sediment deposition in the dam
reservoir and scale-down of riffles and pools in thedownstream reach of Asahi Dam. Moreover, sincegranitic white stones yielded from the upstream of Asahi
Dam had not been transported to the downstream, whitesandbars which characterizes the Asahi River had been
hardly seen in the downstream reach. To improve theseenvironmental problems, a flushing sediment tunnel wasconstructed so as to bypass Asahi Dam reservoir in 1998.
Sediments including white stones have been transported
effectively through the bypass tunnel to the downstreamof the dam, and so riffles and pools in the downstream
reach have gradually come to the condition before thedam construction. To develop a numerical model
explaining flood flow and sediment transport through thebypass tunnel is important for the design of structures forflushing sediment from other dam reservoir wheresediment deposition is serious.
The Asahi River is a stony-bed river with a wide range ofgrain size distribution from boulder to sand. Thus, therecovery of riffles-pools and grain size distribution in thedownstream of the dam must be reproduced by anumerical model which describes proper sedimenttransport and riverbed variation in the stony-bed rivers.
Osada and Fukuoka (2010, 2011) have developed a new
two-dimensional model for estimation of riverbed
variation by considering sediment transport mechanism
and bed surface unevenness in the stony-bed river.
Availability of this model was verified with the results ofthe field experiments carried out in the Jyoganji River. It
was clarified that the model is capable of explaining
sediment transport and grain size distribution in stony-bed
rivers.
In this study, first, the authors demonstrated the recoveryof riffles and pools by using the data of cross-sectional
surveying, air photographs and field investigation in the
Asahi River. Next, authors’ two-dimensional model isapplied to a series of flood events in the Asahi River.
Calculated results such as flood flows, sediment transport,
riverbed variation and grain size distribution as well as
recovery process of riffles and pools are verified byrespective observed data.

2. RECOVERY OF RIFFUL AND POOL IN THE
DOWNSTREAM REACH OF ASAHI DAM
2.1. Outline of the Asahi River and the Flushing
Bypass Tunnel
Figure 1 shows the Asahi River basin and the flushingbypass tunnel of Asahi Dam. Asahi dam has been

Kumano River(Totsu River)

Seto Dam
Setoya River

1km
Asahi River

Flushing sediment bypass tunnel(1998)

Exit

Setoya River

Asahi Dam(1978)
1km

Kumano River

2km
3km

5km
Asahi Dam

Entrance

Sediment
Storage Dam

4km

Asahi River

Upstream Observation
StationDownstream Observation
StationFigure 1. (Left) Asahi River Basin (Right) Plan View of the Flushing Bypass Tunnel
Exposed Rolling Stones and Rocks
across the River

Sediment Storage Dam
Entrance

5.6km

5.7km

5.4km
5.2km

(a) May 1994
Figure 2. (Left) Entrance Viewed from Upstream(Right) Exit of Bypass Tunnel

Development of the
Sandbars by Depositionof White Stones

Exposed Rolling Stones

Exit
5.6km

5.7km

5.4km
5.2km

Levee free-board of
Sediment Storage Dam
EL.462m

Orifice(b) May 2004
Figure 4. River Situation Just Downstream of the Dam

Gate

EL.454m

4.4m
Bed Gradient18.5m

Figure 3. Structure of the Entrance

3.2km
3.4km

3.8km

3.0km

4.0km
4.2km

3.6km

operated from 1978 as the lower reservoir of thepumped-storage power generation dam located at 6.0km
section in the Asahi River. The flushing bypass tunnel
was constructed in 1998 to mitigate the long-term turbid
water, to reduce the sediment deposition in the reservoir
and to recover the riffles and pools in the downstream
reach. The flushing bypass tunnel is a 2,350m long, 3.8m
wide and 1:35 bed gradient. Figure 2 shows the entrance
and exit of the flushing bypass tunnel. At the entrance, asediment storage dam was constructed in order to prevent
sediment flowing into the reservoir. Figure 3 shows anentrance structure. An orifice has been constructed at theentrance. When the discharge from upstream is greaterthan 120m3/s, orifice flow occurs and prevent the clogging
by the sediment deposition in the flushing bypass tunnel.
In the upstream of the entrance, the bed gradient was
1:45 before the dam construction. However, since thesediment deposition into the dam reservoir had occurred
after the dam construction, bed gradient had come to be
low gradient, approximately 1:75. In the downstream
reach from just downstream of Asahi Dam to 5.0km
section, the bed gradient is steep slope (1:36), and the
bed gradient downstream of 5.0km section is 1:100 on
the average.

(a) May 1994

3.2km
3.4km

3.0km

(2) Riffle (1) Sandbar(3) Pool
3.8km

4.0km

4.2k

3.6km

(b) May 2004
Figure 5. River Situation from 3.0km to 4.5km

2.2. Change of Channel Condition Due to Sediment
Flushing in the Downstream Reach
Figures 4 and 5 show air photographs of the downstream
reach taken before and after the construction of theflushing bypass tunnel (May 1994, May 2004). Figure 4
shows the channel situation just downstream of Asahi
Dam. In 1994, the riverbed are visible to gray, and white

Bed Variation (m)(a) 1999－1998(a) White Stones deposited
on Sandbar ( (1) in Fig. 5 )(b) Riffle ( (2) in Fig. 5 )(b) 2001－1998(c) Pool ( (3) in Fig. 5 )Figure 6. Close-up View of Sandbar, Riffle and Pool(c) 2003－1998

Parcentage(%)

100
80
60
40

Sandbar
Pool
Riffle

20
0
10

100

Figure 8. Riverbed Variation in the Downstreamof the Dam (Based on 1998)
1000

Grain Size(mm)Figure 7. Grain Size Distributions of Sandbar, Riffle and Poolstones are not seen on the riverbed, since many rollingstones and rocks of the sedimentary rock which aresubstratum in this reach are exposed. In 2004, manyrolling stones and rocks are seen in the upstream reach of
5.3km section, on the other hand, white stones
discharged through the flushing bypass tunnel had
deposited, and alternate bars had developed downstreamof 5.3km section. Figure 5 shows the channel situationfrom 3.0km to 4.5km section. In 1994, flow channel
looks like a straight line all over the reach, and riffles andpools are rarely seen. In this reach, rolling stones are
smaller in size than that just downstream of the dam, andriverbeds are visible to gray in colour. It is consideredthat riverbed materials were composed of thesedimentary rocks. In 2004, deposition of white stones
are seen on sandbars. Riffles and pools are recovered by
the development of sandbars. Figure 6(a)~(c) areclose-up views of sandbar, riffle and pool shown in Fig.
5(b), May 2011. Figure 7 shows the grain size
distributions on the bed surface measured at sandbars,
riffles and pools in May 2011. In Fig. 6(a), black stones
seen in the right part are the sedimentary rock, and whitestones deposit on the black stones. Since the riffle has
usually high flow velocity, white stones (less than
150mm) are rarely seen and bed materials on the surface
are composed large black stones of the sedimentary rock.
Pools are located at the front of projected rock and outerbank of meandering channel as in Fig. 6(c). White stones
are seen on the bed surface of pools, and the bed material

size are smaller than that of sandbars and riffles. From
the result of the field investigation, bed materials at
riffles have been composed of coarse sediments of thesedimentary rock, and white stones are not easily
deposited there. In contrast, white stones are easily
remained in pools.
Figure 8 shows riverbed variation from bed elevation
measured in 1998. Figure 8(c) corresponds to thesituation of Fig. 4(b) and Fig. 5(b) taken May 2004.
Flood peak discharges beyond 250m3/s in 2001 and 2003
seem to cause larger riverbed variation and riverbed
degrades from 3.8km to 5.0km and aggrades downstreamof 3.8km.

3. NUMERICAL MODEL AND ANALYSISCONDITION
The authors investigate effects of the bypass tunnel andhow riverbed and grain size distribution recovered byusing the numerical model. The numerical model
combined unsteady two-dimensional flood flow analysis
with two-dimensional riverbed variation analysis forstony-bed rivers has been developed by Osada andFukuoka (2010, 2011). This model is constructed byfocusing on sediment transport mechanism and bed
surface unevenness in stony bed rivers. The detail of themodel was explained in the Osada and Fukuoka’s paper(2010).
The analysis was performed from the junction with the
Kumano River to 2km upstream of the entrance as shownin Fig. 1(Right). Cross-sectional forms observed in 1998
were used to the initial form for the computation. The
numerical model was applied to floods occurred from1999 to 2003. Water level and discharge have beenmeasured at observation stations upstream anddownstream of the dam. Discharges from the flushingbypass tunnel and gates of the dam have also been
observed. The upstream boundary condition was given
by observed discharge hydrographs, and the downstream
boundary condition given the uniform flow depth
because water level had not been observed near thedownstream end. Figure 9 shows computational grids at
the area of the entrance and exit. To take into account theshape of the entrance and exit, the computational grid
was developed more in detail than that of other area. The
resistance of the orifice constructed at the entrance wasgiven by the drag force so as to minimize difference
between observed and calculated discharge hydrographs
through the bypass tunnel. Figure 10 shows the grain size
distribution observed upstream and downstream of thedam. Initial grain size distribution used for calculation
was determined based on observed data as shown in Fig.
10. Equilibrium sediment transport was given as the
upstream boundary condition.

variation in the upstream of the entrance during thesecond flood in 2001. Figure 13 shows longitudinal waterlevel and riverbed elevation profiles upstream of theentrance each time ((A)~(D)) shown in Fig. 11. Time in
Fig. 13(B) displays as orifice flow. The water level
upstream of the entrance was nearly constant and stones
and gravel were not transported near the entrance but
deposited in the sediment storage reservoir. At the time(C), water surface slope varied to the steep slope, andsediment deposited upstream entrance was transported tothe entrance. Riverbed elevation profile at that time isshown in Fig. 12(b). In the time between (C) and (D), thewater surface profile just upstream of the entrance wassteep, then, large amount of stones and gravel deposited

Elevation(m)Elevation(m)Exit

Entrance

50m
50m

50m
50m

Sediment Storage
Dam

Figure 9. Computational Grid (Left)Exit (Right)Entrance

4. RESULT OF ANALYSIS ANDCONSIDERATION

100

Percent finer (%)

90

Figure 11 shows calculated and observed hydrographs of
upstream flood discharge and flow and sediment
discharge through the flushing bypass tunnel from 1999
to 2003. Figure 11 also shows total discharge hydrographfrom the bypass tunnel and the gates of Asahi Dam. The
result of the analysis shows that large amount of
sediments was discharged through the bypass tunnel notin the orifice flow period but recession period when flood
discharge is less than 70m3/s. This phenomenon was also
demonstrated by Harada et al. (1998) by the laboratory
bypass tunnel experiment. Figure 12 shows riverbed

80

Initial Condition of Grain Size
Upstream Reach
Downstream Reach

1.1k
0.35k
Upstream fromNear
the Entrance
the Entrance

70
60
50

5.35k
5.3k
5.2k
5.0k
4.9k
4.75k

40
30
20
10

Downstream of
the Dam

0
0.1

1

10

100

1000

Grain Size(mm)Figure 10. Grain Size Distribution Used for Calculation
Sediment Volume (m3)Q(m3/s)

2000

1999

300

2001

(A) (B)

3,000

(D)

(C)

200

2,000

100

1,000

0

0
0

10

20

0

10

Time (hour)

20 0

10

20

Sediment Volume (m3)Q(m3/s)

2003

2002

300

3,000

30

40

50

60

Discharge
Upstream of Dam(Obs.)
ダム上流流量(実測)Dischargeof Bypass Tunnel(Obs.)
トンネル流量(実測)

200

2,000

Dischargeof Bypass Tunnel(Cal.)
トンネル流量(解析)

100

1,000

Total
Outflow Discharge(Obs.)
ダム合計放流量(実測)0Sediment
Discharge(Cal.)
流出土砂量(解析)

0
0

5

10

15 0

10

20

30

40

50

Time (hour)

Figure 11. The Relationship between Flood Discharge Hydrograph and Sediment Discharge Volume

Z(m)Entrance

Sediment
Storage Dam
50m

50m

50m

(a) Bigining of Second Flood in 2001 (b) After 19 Hours, Second Flood in 2001 (c) After 32 Hours, Second Flood in 2001
(Figure 11. (D))(Figure 11. (C))(Figure 11. (A))
Figure 12. Time Variation of the Bed Elevation Upstream of the Entrance (Second Flood in 2001)
464

Elevation (m)Orifice
460
458
456
454

357.5
Water
Level
Bed
Elevation

Water Level (m)

(A)
(B)
(C)
(D)
(A)
(B)
(C)
(D)

462

357.0
356.5
356.0
355.5
0

452

20

40

60

Time (hour)
20

40

60

80

100

Longitudinal distance (m)Figure 13. Variation of Water Level, Bed Elevation, Second Flood in 2001((A)-(D) correspond to the times indicated in Fig. 11.)
40
30
20
10
0
150

60

20

4

Grain Size (mm)Figure14. Percentage of Each Grain Sizein Discharged Sediments

was discharged from the bypass tunnel. Then, bed
elevation profiles varied from Fig. 12(b) to Fig. 12(c).
The riverbed elevation profile upstream of the entrance
was almost the same before and after the floods. It
demonstrates that the flushing bypass tunnel displays agood performance. Figure 14 shows the percentage ofeach grain size in discharged sediments. The stone-classmaterial of 150mm of 8%, and the other grain sizes of
30% were contained in them. These percentages arealmost the same as the grain size distribution of initialcondition upstream of the dam, thus each grain sizegroup upstream of the dam were properly transported in
the downstream river.
Figure 15 shows the comparison with observed andcalculated water level hydrographs at 4.3km section.
Although the calculated water level hydrographs were
estimated slightly low as compared to the observed datain the recession period of the floods, calculated results
were able to reproduce the observed data. Figure 16
shows sediment discharge of each grain size group in

Figure 15. Water Level Hydrographs at the Downstream
Observation Stations (4.3km) in 2001
Sediment Discharge (m3/s)

0Percentage of Discharged
Sediment (%)

Observed
Calculated

0.010

Discharge200m3/s
Discharge100m3/s
Discharge50m3/s

0.008
0.006
0.004
0.002
0.000
350

240

150

60

20

4

Grain Size(mm)Figure 16. Sediment Discharge of Each Grain Size
Associated with change in Flow Conditions

different flow conditions. These data are the calculated
results averaged from 3.0km to 4.0km sections. When the
discharge is 200m3/s, stone-class materials (d > 75mm)
are actively transported in comparison with gravel-classmaterials, and large stones such as rolling stones(d=240mm, 340mm) can also be transported. When thepeak discharge was larger than 200m3/s, outflow
discharge is limited by 100m3/s to regulate the waterlevel in the reservoir as shown in Fig. 11. Sedimenttransport rate in 100m3/s was less than that in 200m3/s.
However, the grain size group from 240mm to 60mm
were actively transported as shown in Fig. 16. White
stone materials (d < 150mm) can be transported enoughby 100m3/s. In case of 50m3/s, 60mm gravel-class group
was mainly transported.
Figure 17 shows calculated riverbed variation
corresponding to observed one shown in Fig. 8. The
calculated results reproduce the processes of shifting ofdeposition area to the downstream and development of
bed scouring from 4km to 5km, but calculated

5.0k
Bed Variation (m)5.0k
Deposit Amount (m)(a) 1999－1998

3.0k

3.0k
Riffle

4.0k
5.0k

4.0k

Figure 18. Distribution height of Sediment Deposition,
1999~2003

5.0k(b) 2001－1998

3.0k

RiffleGrain Size (mm)

4.0k
3.0k

5.0k

Riffle4.0k
Riffle(c) 2003－1998

3.0k

Figure 19. Distribution of Calculated Mean Grain Diameter,
2003

4.0k

seen in the actual riffles in Fig. 6(b), calculated results
seem to reproduce bed conditions.
Figure 17. Calculated Riverbed Variation in the Downstreamof the Dam

sedimentation distribution in the downstream from 3.6km
differs from observed result. Figure 8 shows that
sediment from upstream deposited in this reach, but
calculated sediments deposited intensively in the innerbank side of meander reaches. There are a lot of rockprojections and narrow sections in this reach, while thenumerical model is not considered enough these
influences on sediment transport.
Figure 18 shows the distribution height of the sedimentdeposition discharged through the flushing bypass tunnel
from 1999 to 2003. Figure 19 shows distribution of
calculated mean grain diameter after 2003 floods. Since
the initial condition of mean grain diameter was 160mm,
the blue colours indicate grain size reduction and red
colours armouring. Flushing sediments deposited mainly
around 5.0km and in the downstream reach of 3.6km.
Figure 5(b) shows that a large amount of depositing
white stones is found in the downstream reach of 3.6km.
The discharged sediment through the bypass tunnel until
2003 had been mainly deposited in this reach. It isconsidered that the sediments tend to deposit around5.0km because river bed slope changes around this reach.
Although sandbars by the deposition of white stones
around 5.2km had been formed, the calculation could notdescribe it. The calculated result of mean grain diameter
distribution downstream of 3.6km except for few points
demonstrates grain size reduction due to deposition ofwhite stones. On the other hand, white stones are hardly
seen in riffles (e.g. around 4.2km and 3.7km) as shownby red circles in Fig. 18. Since white stone was not also

As above, it was clarified that the numerical model iscapable of explaining quantity and quality of sedimentflushed by the bypass tunnel, development of sandbar
and recovery of riffles and pools in the downstream reachin the Asahi River.

5. CONCLUSION
To elucidate mechanism of sediment flushing by bypass
tunnel of Asahi Dam, development of riffles and pools
and change in grain size distribution downstream of thedam were studied by using the field investigation data
and the numerical model. The numerical model gave agood explanation for the mechanism of sediment flushingby the bypass tunnel and the processes of recovery of
riffles and pools and change in grain size distribution bydeposition of white stones in the downstream reach.
REFERENCES
Osada, K. and Fukuoka, S. (2010). Analysis of
Two-Dimensional Riverbed Variation in Channels with
Stony Beds, 11th International Symposium on River
Sedimentation, South Africa, CD-ROM
Osada,K., Kobayashi,M. and Fukuoka,S. (2011). Study ofFlood Flow and Sediment Transport in the Satsunai River
with Spur Dikes, Advances in River Engineering, JSCE,
Vol.17,pp.149-154
Harada, M., Kokubo, T. and Deno, T. (1998): Hydraulic
Properties of Sediment Transport in The Tunnel of
Sediment-Bypass System of a Reservoir, Proceedings of theJapan Society of Civil Engineers, No.600/II-44, pp. 69-84.