River Flow 2010 - Dittrich, Koll, Aberle & Geisenhainer (eds) - © 2010 Bundesanstalt für Wasserbau ISBN 978-3-939230-00-7

Impacts of sediment flushing on channel evolution and morphologicalprocesses: Case study of the Kurobe River, JapanS. A. Kantoush, T. Sumi & T. Suzuki

Water Resources Research Center, Disaster Prevention Research Institute, Kyoto University, Japan

M. Murasaki

Oyo Corporation, Japan

ABSTRACT: Flushing is vital for the preservation of long-term storage in reservoirs. However, downstream impacts can act as a constraint in the planning and operation of sediment flushing. In order to understand the processes and the impacts of flushing, two examples are given of the Unazuki and Dashidaira reservoirs, located at the Kurobe River eastern region of Toyama prefecture, in Japan. For bothreservoirs sediment inflow is extremely high compared to the storage capacity. During minimum poollevel the incoming floods erode a flushing channel in the deltaic deposits. The channel is gradually increased in width by bank-erosion processes during this period. The paper investigates the impacts offlushing on the dynamic dimensions of flushing channel, and the varying location. Furthermore, it clarify
the development of channel formation in both reservoirs. Quantitatively and qualitatively monitoringmeasurements during flushing have been conducted for sediment erosion process with 3D laser scanning,surface velocity (LSPIV), and suspended sediment concentrations. The flushing channel within Dashidaira reservoir has a stable profile and extends across the entire impoundment width of 170 m at distance of
640 m from the dam.
Keywords: Drawdown flushing, Flushing channel formation, Coordinated flushing, Kurobe River.

1 INTRODUCTION

Among these approaches, flushing is considered
the only economic approach to swiftly restore thestorage capacity of the reservoir with severe deposition. Basically, there are two types of flushing
operations with, and without drawdown, and optional techniques can be used with the complete
drawn flushing as shown in Figure 1.

1.1 Background
Dam construction disrupts the longitudinal continuity of the river system and interrupts the actionof the conveyor belt of sediment transport (Kondolf, 1997). Upstream of the dam, all bedload sediment and all or part of the suspended load is deposited in the stagnant water of the reservoir(reducing reservoir capacity) and upstream of thereservoir in reaches influenced by backwater.
Downstream, water released from the dam possesses the energy to erode the channel bed andbanks, but has little or no sediment load. The rapid reservoir sedimentation not only decreases thestorage capacity, but also increases the probabilityof flood inundation in the upstream reaches due toheightening of the bed elevations at the upstream
end of the reservoir and the confluences of the tributaries (Liu et al., 2004). In order to remove andreduce reservoir sedimentation, many approaches
such as flushing, sluicing, dredging and water andsoil conservation are developed (ICOLD, 1989).

Types of Sediment Flushing in Reservoirs
With Drawdown

Without Drawdown

More effects along and across the basin (flushing channel)

Very local effects (flushing cone)

CompleteFloodseasonemptying

PartialNonflood Under pressure
Seasonminimum
emptying operation level

Dam outlets are
opened for very
short time
Free flowoverspillway

Additional techniqueswith complete drawdown

Auxiliary
channelMechanically

Lateral,Longitudinal, andin both directions

Bulldozers topush deposits
into flushingchannelFigure 1. Classification of sediment flushing techniques

Drawdown is the lowering the water levels in areservoir. Hydraulic flushing involves reservoir
drawdown by opening the bottom outlet to generate and accelerate unsteady flow towards the outlet (Morris and Fan, 1998). This accelerated flow
possesses an increased stream power and conse1165

quently eroding a channel through the deposits
and flushing the fine and coarse sediments
through the outlet. During this process a progressive and a retrogressive erosion patterns can occurs in the tail and delta reaches of the reservoir,
respectively (Batuca and Jordaan, 2000). Among
reviewed literature, sediment removal from reservoirs (White, 2001) generally only affords the
problem of sediment flushing inside the reservoir.
However, there is scarce information about experiences on prototypes and especially in respect toflushing channel formation. One of the phenomena in reservoirs that is not well investigated andtheoretically explained is the formation of flushing channels in the delta of the reservoir (Sloff etal., 2004). In many reservoirs these channels canbe found, e.g. see Figure 2, as they are a common
feature of deltaic deposition in wide reservoirs.

1.3 Aim of the Study
The lack of knowledge regarding flushing channel
evolution and its morphological processes, initiated a study to channel formation, applied toflushing processes in Dashidaira and Unazuki reservoirs in Japan. The study aims at investigating
the causes of these observed behaviors, and findmeasures to control and predict the channel characteristics. Therefore this study is focusing mainly on track the patterns of reservoir erosion
through field and experimental modeling. Finally,
quantitatively and qualitatively monitoring techniques during flushing are discussed.
2 CASE STUDY OF THE KUROBE RIVER
Japan has geologically young mountains, steep
and short rivers with flashy flow regimes, anddensely populated floodplains. Japan suffers fromfrequent heavy storms. The combination of steep
catchments and heavy storms results in widespread hill slope failures and landslides and extensive flood discharges.
2.1 Study Area
The Kurobe River originates in mountainous areas
with peaks elevation above 3,000 m. The mainriver flows for about 85 km into Toyama Bay as
shown in Figure 3. The average river bed slope
ranges from 1/5 to 1/80 is about 1/30. The meanannual precipitation ranges from 2400 mm to4100 mm. Figure 3 shows an outline of the Kurobe River System.
The Kurobe River is the only river in Japan
where full scale dam-linked sediment flushing isdone by performing sediment flushing well linked
to sediment flushing at an electric power dam(Dashidaira Dam) installed upstream from the
Unazuki Dam. Both dams have an extremely large
amount of sediment inflow; therefore, they were
the first in Japan which was built with sedimentflushing facilities (sediment flushing gates).

Figure 2. Flushing channel in reservoir upstream of Gebidem dam during drawdown operation

1.2 Relevant Parameters for Flushing Channel
The characteristics of the flushing channel when
the reservoir is fully drawdown are location,
width, side and longitudinal slopes, and shape.
Some of these characteristics are investigated experimentally in shallow reservoir geometries withdifferent shapes (Kantoush and Schleiss, 2009).
The width of the flushing channel Wf is estimatedas proposed by the World Bank RESCON model,(2003) (REServoir CONservation), and Atkinson(1996). To predict Wf, Atkinson, (1996) suggests
the empirical relationship (Wf = K Qf0.5) based on
prototype measurements in four reservoirs of China, United States, and India. In this equation, Wf(in meters) is the flushing-channel width, K is acoefficient depending on bed material (in this case
K is 12.8) and Qf is the discharge (in m3·s-1) thatwill form the channel.
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Japan Sea

Asahi Town
Niigata
Pref.

N

Kitamata Dam
Unazuki Dam
Dashidaira Dam
Koyadaira Dam(a)Sennindani Dam

Toyama Prefecture

Kurobe Dam
Kurobe Reservoir

KurobeRiver

Dashidaira reservoir
gross capacity
9.01 MCM

NaganoPrefecture

Dashidaira dam(b)

Gifu Prefecture

Figure 3. Kurobe River site map and location of Dashidairaand Unazuki dams

Figure 4. (a) Photo of Dashidaira dam during draw down
flushing; (b) Plan view of Dashidaira reservoir topographic
map

2.2 Dashidaira dam and its flushing facility
Figure 4(a) shows the Dashidaira reservoir, damsite, and downstream reach during flood. The concrete gravity dam with a height of 76.7 m is providing 124 MW. The dam is located 26 km upstream the river mouth. Dam was constructed by
Kansai Electric Power Company in 1985. Figure4(b) shows the topography of the Dashidaira reservoir with initial gross and effective capacitiesof 9.01 MCM and 1.66 M.m3, respectively. The
small capacity reservoir has an elongated shapewith quasi-uniform width.
The Unazuki Dam is a multipurpose dam
planned mainly to prevent floods on the Kurobe
River. The dam height is 97.0 m, completed in2001 by the Ministry of Land, Infrastructure andTransport. The upstream side of the Unazuki dam
and inlet of two sediment scouring gate are shownin Figure 5(a), during draw down. The sedimentscouring channels are both equipped with flushinggates with dimension of 5.0 m x 6.0 m. The threegates in upstream, intermediate, downstream sides
are high pressure slide gates. The purposes offlushing facilities are to sustain the original functions of the dam and maintaining sediment routing
system within the reservoir. Unazuki reservoir(Figure 5(b)) has larger gross and effective capacities comparing with Dashidaira of 24.7 MCM
and 12.7 MCM, respectively.

(a)

Unazuki reservoir grosscapacity 24.7 MCM

Unazuki dam(b)
Figure 5. (a) Photo of Unazuki dam during drawdown flushing; (b) Plan view of Unazuki reservoir topographic map

2.3 Monitoring techniques for sediment flushing
Innovative methods for sediment monitoring techniques should be developed in order to evaluate
environmental influences and effectiveness of se1167

diment flushing. Several parameters are measured
during and after flushing, namely: 3D topography,
2D surface velocities, cross section bathymetry,turbidity, and sediment transport rate. More details about these techniques are shown and explainby (Sumi et al., 2004, 2005). Figure 6 shows fieldmeasurement of channel evolution and morphological processes during a coordinated sedimentflushing operation at the Unazuki Dam reservoir

Sand barin 2006. These measurements included surface velocity using LSPIV (Large Scale Particle Image
Velocimetry) performed by using CCTV camera,in combination with geographic feature measurement with a 3D laser scanner. A high concentrated
SSC is also observed downstream of the dam.
.

Side bank

Section 5

Riegle LMSZ210, 2-300 m,
2.5 cm accuracy

Line 7
Ononuma in Unazuki reservoir

3D laser scanner

Figure 6. Photo of monitoring measurements within Unazuki reservoir by using 3D laser scanner (Sumi et al., 2004)

processes and channel formation. Four sets of experimental tests are administered in Dashidaira reservoir after two years of dam completed. The
purposes of Dashidaira physical models are to
predict the sediment and flow patterns in equilibrium state during flushing especially with washload effect, and moreover, to examine different
test procedures and find the optimal sediment material can be used for Unazuki dam. The test concept and the bed material initial conditions areshown in Figure 8. The initial sedimentation condition is built in the model with silica sand andstyropor granulate as shown in Figure 8. The diameter of the silica as bed load is the same as the
prototype. The styropeor material was selected as
wash load due to the similarity in density and sizewith the field.

Discharge (m3 /s)

Averagerainfall (mm)

0
5
10
15
20
700

Average hourly rainfall in Dashidaira reservoir (mm)Dashidaira

51 hrsInflow discharge
Outflow discharge

500

100

12 hrsUnazuki

700

Discharge (m3/s)

320

300

-100

300

60 hrs235

200

Inflow discharge
Outflow discharge
Water level-300

12 hrs0215

Suspended sediment at Dashidaira
Suspended sediment at Unazuki

SSC (mg/l)

30,00025,00020,00015,00010,0005,000
03 LABORATORY EXPERIMENTS

Water level (El. m)

Based on laboratory tests, numerical simulations,and previous engineering practices at Dashidaira
dam; the coordinated sediment flushing operation
rules for Dashidaira and Unazuki reservoirs are
established. Since 2001, when the Unazuki Dam
was completed, sediment flushing and sluicing
have been conducted coordinately for the twodams. Dashidaira dam previously completed has
been executed sediment flushing in single since1991. The coordinated sediment flushing would
be carried out when the inflow at Dashidaira reservoir become firstly larger than the designateddischarge of 250 m3/s in the rainy season around
June to July. In the next flood events after flushing, whenever a flood excess of 480m3/s, sedimentsluicing is performed.
Figure 7 shows actual sediment flushing operation performed in July 2006. Erosion phase withfree flow state is lasting for 12 hours to flush out
sediment volume of 0.24 MCM. The flushing timefrom the beginning of drawdown to the end at Dashidaira dam is 51 hrs and 60 hrs at Unazuki dam.
It is a key to the coordinated sediment flushingand sluicing to do these operations at the sametime in Dashidaira and Unazuki dams that are located longitudinally.

3

2006/7/1

6

912

15 18 21 0
3Time (hrs) 2006/7/2

6

9

12

15

18

21

0

2006/7/3

Figure 7. Coordinated Sediment flushing operation in 2006

3.1 Experimental facility installation and setup
Laboratory experiments for Dashidaira and Unazuki reservoirs are set up to examine the flushing
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Water level (El. m)

2.4 Analysis of coordinated sediment flushing
operations of Dashidaira and Unazuki dams

3.2 Erosion and flow patterns during flushing

Figure 8. Initial bed condition and sediment material
Baffle plate
pipe
Drain

Inlet of water supply

Dashidaira
dam
Settlingbasin

Six layers ofconcrete blocks

Six layers concrete blocks CB
Wall: six layers CB

Cement plastering

The final bed morphology obtained from experiment is used as the initial topography for clear
water test. Here, clear water without sediment isintroduced into the basin to investigate the further
bed evolution. Figure 11 shows the flow with vectors and the bed topography after 11.9 hrs offlushing. Reduction of water surface generates
high flow velocity near the outlet; the flow starts
to erode a wide channel. At that stage, a significant amount of sediment deposits was flushed
through the reservoir, and the initial channel deepened as a result of the strong jet flow and erosion.

(a) Plane View

Stream gauging tank(b) Cross Section View

Backwater section

Stone
Concrete blockpipe(C) Longitudinal view

Figure 9. Dimensions and laboratory setup of Dashidairadam (a) Plan view; (b) Cross section; (c) Longitudinal view

Bed load is deposited
Width of flushing channel is 36.3 m

The experiments were carried out at PWRI,
Public works Research Institute, the Ministry ofConstruction, Japan. Schematic view of the experimental setup and cross section of Dashidaira reservoir are shown in Figure 9. The model scale ofDashidaira and Unazuki dams is 1:62.5. Two different test procedures with several clear water inflows and flushing durations are used in bothdams. A discharge of 200 m3/s imposed without
sediment. By the flushing inflow, a channel iseroded in the sediment layer. The physical modelof Dashidaira reservoir was constructed only for
flushing test experiments. Therefore, the original
bed profile is apparently different from the field
profile (Figure 10). But, the initial bed deposition
profile before flushing is the same as the actualone. The result of the flushing test is shown in
Figure 10.

Width of flushing channel is 61.3 m

Figure 11. Morphological patterns after flushing within Unazuki reservoir after t =11.9 hrs

The flushing channel location was clearly visible by the re-suspended sediment which was rapidly released during 11.9 hrs (Figure 11). The
flow trajectory path is eroded until the scouring
channels bottom outlet. Fine suspended sediments
were carried and deposited on the right and on theleft sides along the reservoir. After the channel isformed its location is rather stable, and difficult toshift as its depth enforces the flow pattern to follow the channel path.
3.3 Comparison of laboratory and field data

Elevation [m]After flushingwhite Styropor 0.4mm(d=0.385mm, σ=1.059)

Within Dashidaira reservoir, the actual and laboratory test of flushing channel is schematicallyshown in Figure 12.
By the clear-water inflow a channel is erodedin the placed sediment layer. The actual and laboratory has almost the same channel formation
way, except at the wider section before dam. The
meandering behavior at the wider location within
reservoir has opposite direction of the channel.
The location of the channel is difficult to be predicting in the wider region within the reservoir.
Moreover, the bank failure of the flushing channel
creates relatively steep side slopes as shown in
Figure 13. The side slope is almost constant within Dashidaira reservoir. Bank failure caused by

Before flushing
No.5 Silica sand(d=0.496mm,σ=2.65)
Angle of repose inwater : 30°

Original bed
Water level

Distance from Dashidaira Dam [m]Figure 10. Longitudinal bed profiles before and after flushing laboratory test along Dashidaira reservoir

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hydraulic overpressures associated with rapid
drawdown. The eroded channel profile has a trapezoidal cross section (Figure 13).

360
350
normal waterlevel 343m

340
Delta front that formed at CS 8 initially,is crashed and deposited at CS 6-2

Elevation （EL.m）

No.8

No.6-2*Width of flushing channel is 30-40 m*Width of flushing channel is 50-80 m

Experiment
Actual*results in experiment

330
320

spillway
325m

July 2008, after flushing
July 2009, after flushing310

May 2009, before flushing

August 2004, after flushing300

May 2004, before flushing290

December 1994

Dashidaira dam
280
(a)
-500
-0

Original river bed in 1985
500

1,000

1,500

2,000

2,500

3,000

Distance from dam（m）
280

Figure 12. Comparison between experimental and actual
flushing channel formation within Dashidaira reservoir

270

Equilibrium sedimentation profile

Elevation（EL.ｍ）

260
December 2003

250

normal water level 245m

240spillway 231m
230

Sept. 2009, after flushing
May 2009, before flushing

224

220

August 2004, after flushing
May 2004, before flushing

210
200
(b)
-1,000

Original river bed, December 1999

Unazuki dam
0-

1,000

2,000

3,000

4,000

5,000

Distance from dam (ｍ）

Figure 14. Actual field longitudinal bed profiles variations
in time along (a) Dashidaira reservoir; (b) Unazuki reservoir

High sediment deposition is formed in May
2004 within Dashidaira reservoir as shown in Figure 14(a). Afterward in August 2004, by continuous sediment flushing another bed profile isformed. Longitudinal profiles before and after
flushing in 2009 are shown in Figure 14(a). On
the other hand in Figure 14(b), the sedimentationin Unazuki dam progresses mainly by the coarse
sediment inflow from both sediment flushing ofDashidaira dam and a tributary river. The flushingefficiency calculated from the water consumption
including the discharge during drawdown and thesediment volume flushed out is about 2%. For thisreason, it can be estimated that sediment flushingby using a natural flood discharge in the rainyseason is executed regardless of the previous
year's amount of sedimentation to prevent the sediment from changing in quality in the reservoir
every year, and in that case while doing an enoughdilution.
Figure 15 shows the amount of flushed sediment from the Dashidaira Dam and the amount ofaccumulated sediment at the Unazuki Dam foreach year. Due to regular sediment flushing, theDashidaira Dam has been maintaining present sedimentation volume equivalent to about 45% of
the gross storage capacity, which is equilibrium
state. The accumulation of sediment at the Unazu-

Figure 13. Photograph, looking downstream, of eroded
channel at the end of experiments of Dashidaira reservoir

4 FIELD OBSERVATION AND DATA
4.1 Evaluation of sediment flushing effects
The effects of sediment flushing in the Kurobe
River should be evaluated. The suitable condition
to perform the flushing operation is the taking
place of a natural flood more than a constantscale. The previous and present longitudinal sediment profiles of Dashidaira and Unazuki dams
are shown in Figures 18(a) and (b), respectively.
Dashidaira dam was constructed fourteen years
before Unazuki dam. Therefore, the Dashidaira
Dam is currently at an equilibrium state in termsof its sediment, and the quantity of passing
through is approximately one million cubic meters
yearly. However, sediment is still being accumulated at the Unazuki Dam. While the majority of
sediments of grain size larger than 2 mm aretrapped at the reservoir, about 70% of the sediment that has grain size smaller than 2 mm, issluiced.
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7,0006,000

Wide channel widthBankfailureFlushed sediment from Da shidaira da m
Accumulated sediment volume at Unazuki dam

Side
bank
erosionDegradation
July 2004
Cross section

Figure 16. Flushing channel formed in Dashidaira reservoir

5,000

4.3 Evolution of flushing channel formation
inside Unazuki reservoir

4,000
3,000

Several field measurements are conducted in orderto evaluate channel formation during drawdown
flushing within Unazuki reservoir. By using 3D
laser scanning technology (Sumi et al., (2004),
Sumi et al., (2005)), erosion-deposition process of
sediment in reservoirs and rivers are measured asshown in Figure 6. Moreover, morphologicalchanges in river channels during and after flushing are investigated.

2,000
1,000
0

1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007

Cumulative sediment volume (x103 m3)

ki dam is reduced in 2006, and as the size of sediment materials is stabilized, coarse sediments
are reaching up to the dam. Therefore, thesecoarse sediments are also beginning to passthrough the dam in addition to the fine sediments
produced upstream.

Time (years)

Figure 15. Comparison of flushed sediment volume at Dashidaira dam and accumulated sediment at Unazuki dam

4.2 Morphological process of Dashidaira
The flushing efficiency changes widely by various
factors such as configuration of reservoir, elevation of sediment flushing gates, volume and grain
size of deposited sediment, discharge rate during
sediment flushing, duration time from the start ofdraw down flushing and so on. During flushingprocesses two types of erosion pattern can occurs;
a progressive erosion in the tail reach of the reservoir, and a retrogressive erosion in the delta reachof the reservoirs. In order to predict the flushingchannel width and location, it is important to understand the erosion process of sediment within
reservoir.
Figure 16 shows erosion process of deposited
sediment in Dashidaira reservoir in 2004. It can befound that longitudinal and lateral erosions
created by river bed degradation and side bankerosion. A meandering flushing channel in the accumulated sediment is also formed. Not all the sediment is flushed out during the free flow duration. However, the sediment on both banks isgradually fall down by the stream bank erosion.
During very high discharges in 2004 the channel
was able to break through the sand bar with bend
cut-off and forming a wide channel section. Nevertheless, these cut-off events can only occur
when water levels at the dam are not completely
draw down, because then the flow becomes concentrated in the existing deep channel. The flushing operation has a significant contribution to themaintenance of the reservoir capacity to extend
the dam life time.

Unazuki dam
Scouring gate

Scouring channel

3D laserscanner data
during flushingon 3rd July 2005

Figure 17. 3D laser scanner measurements of deposition andwater surfaces within Unazuki reservoir

The morphological topography with the water
surface is clearly visible in Figure 17. Field measurement of time and spatial variations of water
surface during drawdown flushing in night time isalso conducted. Figure 18 reports a snapshot interms of water elevation profiles within Unazuki
reservoir along the scouring channel. The water
surface elevation along the measuring line isshown in Figure 18. This figure shows how theshock waves generated by the scouring gate propagated over the sediment bed. The observed flushing wave propagates along the measuring line atdifferent time steps. Figure 18 indicates the 3D laser scanner capability to reproduce the global evolution of the sediment profile as a function of theflushing durations, with high accuracy.

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2h00
2:00
2h04
2:04

223
2:10
2h10

222

2h13
2:13
2h18
2:18

221

2:23
2h23

2h27
2:27

220
219

2:34
2h34

218
0

20

40

60

80

100

120

140

160

180

Distance along the measuring line (m)Figure 18. Time series of measured water surface elevations

Elevation (m)Time and spatial variations of reservoir surface
velocities during drawdown and flushing period
were also measured by LSPIV. These data can beconverted to actual velocities by the 3D laser
scanner data. Both reservoir water level and flow
velocity changing during the drawdown. Sidebank erosion process of the sand bar formed in the
Unazuki reservoir and water surface profiles nearby banks are shown in Figure 19. The location of
the measured cross section (section 5) is shown in
Figure 6. Based on these results, it can estimate
the height of side bank of 1.0-1.2 m is eroded withthe speed of 7.5 m/hr. Slope of water surface is also estimated as 1/75 and water waves generated
by the antidune are also observed.

45

40

Bed features of sand bars -22.4-22.6
Side bank -22.8
-23
-23.2
Side bank slope
Water -23.4
surface -23.6
-23.8
-24
-24.2
-24.4
35
30
25
20 (m )

15h02
15h15
15h30
15h44
16h01
16h16
16h31
16h45
17h00

Cross section distance (m)Observed channel width, Wf (m)Laboratory and field measurements for a large V
shape gorge reservoir show that channel formationin reservoirs is particularly controlled by the flow
and deposition patterns. It is therefore highly dependent on the boundary conditions, the initial
flow conditions and the geometry. On the other
hand, the sensitivity of flow patterns creates opportunities to redirect the channel direction bymeans of small interventions in the flow field(e.g., operation of water-levels at gates, or constructions that redirect the flow within the reservoir).
Channels formed by flushing in reservoir sediment deposits correlate well with flushing discharge. Figure 20 shows the relationship and thecomparison of observed flushing channel widths
in Dashidaira and other four reservoirs. The fitted
line is described by Wf = K Q0.5 =12.8 Q0.5 (section 1.2), where the width is determined by theflow and is independent of sediment size. The
flushing channel width in Dashidaira reservoir is
170 m at distance of 640 m from the dam andflushing discharge of 250 m3/s. Therefore, Sumi
and Iguchi (2005), suggest that K value for Japanese reservoirs is 6.4, to be reduced by twice.

1:50
1h50

224

Elevation (m)5 DISCUSSIONS

Unazuki
宇奈月ダム
dam

Flushing
跳水
waves

225

Sanmenxia
Guanting
GuemseyBairaDashidaira

1000

Wf = 12.8Qf0.5 (Atkinson , 1996)

100

10
1

10

100

1000

Flushing discharge, Qf (m3/s)

Figure 20. Comparison of Dashidaira dam flushing channel
width with Atkinson relationship of four dams

Figure 19. Time series of flushing channel side bank erosion
processes within Unazuki reservoir

To successfully apply the channel formation
processes for removing deposits, the location of
the channel and its width can be changed by modifications of the flow pattern. Before a channel isformed these interventions can be much smaller
than that after its formation, because the channel
attracts the main flow, and stabilizes the flow pattern. The present study has new information regarding the dimensions of channels of Dashidairadam, in the laboratory and the field. The width ofthe channel is in accordance with the riverine
width, flow pattern, water depth, discharge, andthe bottom outlet arrangement. A larger volume in
the reservoir generally generates a higher flushingflow depth and velocity in the flushing channel.

From the viewpoint of the comprehensive sediment management in a sediment routing system,
monitoring of quantity and quality of sedimenttransport during these flushing events in rivers
and reservoirs is also very important. Based onfield observation of turbidimeter, movement of thesuspended sediment load discharged from Unazuki dam during flood, flushing and sluicing periods
are clarified.

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Reservoir sedimentation management in Japan isentering a new era. Although there are still technical problems to be solved, it is believed that theimportance of sediment management will increasingly grow. Assessing issues, depending on eachcase, of dam security, sustainable management ofwater resources and sediment management in asediment routing system, an effective sedimentmanagement plan should be studied.
REFERENCES
Atkinson, E., 1996. The feasibility of flushing sedimentfrom reservoir. HR Wallingford Report OD137.
Batuca, D. G. and Jordaan, J. M., 2000. Silting and desiltingof reservoirs. A. A. Balkema, Rotterdam.
Kantoush, S. A. and Schleiss, A. J., 2009. Channel formation in large shallow reservoirs with different geometries
during flushing. Journal of Environmental Technology,
Volume 30 Issue 8, 855-863.
Kondolf, G.M., 1997. Hungry water: effects of dams andgravel mining on river channels. Environmental Management, 21 (4), 533–551.
Liu, J.; Minami, S., Otsuki, H., Liu, B and Ashida, K., 2004.
Prediction of Concerted Sediment Flushing. J. Hydr.
Engrg. Volume 130, Issue 11, pp. 1089-1096.
Morris, G. L. and Fan, J., 1998. Reservoir sedimentation
handbook: Design and management of dams, reservoirs,
and watersheds for sustainable use. McGraw-Hill.
Sloff, C.J., Jagers, H.R.A., Kitamura, Y., 2004. Study on thechannel development in a wide reservoir. Proc. River
Flow, Napoli, Italy, 811-819.
Sumi, T., Murasaki, M., Fujinaga, K., Nagura, Y. and Tamaki, H., 2004. Study on Measurement of ErosionDeposition Process of Reservoir Sediment during Flushing Operation. Annual Journal of Hydraulic Eng.,
Vol.48, 1147-1152 (Japanese).
Sumi, T., Murasaki, M., Nagura, Y., Tamaki, H. and Imaki,T., 2005. Study on Measurement of Erosion-Deposition
Process of Reservoir Sediment during Flushing Operation (Part 2). Annual Journal of Hydraulic Eng., Vol.49,
1033-1038(Japanese).
Sumi, T. and Iguchi M., 2005. Technical Feasibility Studyof Sediment Flushing in Reservoirs using the RESCON
Model. Journal of Japan Society of Dam Engineers15(2), 92-105 (Japanese).
The World Bank, 2003. The RESCON Approach. Reservoir
Conservation Volume I.
ICOLD. 1989. Sedimentation control of reservoirs. International Committee of Large Dams.
White R, 2001. Evacuation of sediments from reservoirs.
Thomas Telford Publishing, London.

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