River Flow 2020 – Uijttewaal et al (eds)© 2020 Taylor & Francis Group, London, ISBN 978-0-367-62773-7

Movement and discharge of sediments during the coordinatedsediment flushing operation of the Dashidaira dam and theUnazuki dam
Shoji Fukuoka
Research and Development Initiative Chuo University, Tokyo, Japan

Kohei Suzuki
Graduate School of Science and Engineering, Chuo University, Tokyo, Japan

ABSTRACT: The Unazuki dam and the Dashidaira dam, which are constructed in theKurobe River basin with a large amount of sediment yields, have conducted the coordinatedsediment flushing for the purpose of maintaining the function of the dam, preventing the degradation of the downstream riverbed and reducing coastal erosion. However, riverbed degradation and shortage of sediment with large diameters have been still in progress. In this paper,
we develop the riverbed variation model by floods that can reproduce the sediment movementfrom and in the Unazuki dam reservoir by using the cross-sectional data before and after thecoordinated sediment flushing during the flood. It was clarified that considerable amounts oflarge sediments such as 100mm and 50mm which contributes to the stability of the downstream river channel pass through a sediment flushing gate from the coordinated sedimentflushing analysis.

1 INTRODUCTIONThe Kurobe River is located in Toyama Prefecture, Japan (Figure 1) and is one of the mostrapid flow rivers in Japan. In addition, the Kurobe River basin is a rainy and snowy area, anda lot of sediment are yielded from the upstream mountain regions. Therefore, the comprehensive sediment control plan from the upstream to the coastal areas is being executed here. In
this situation, the coordinated sediment flushing has been conducted at the Unazuki dam andthe Dashidaira dam (7km upstream of the Unazuki dam) for the purpose of maintaining thefunction of the dams, preventing the degradation of the downstream riverbed and reducing
coastal erosion. Figure 2 shows the overview of the operation of the coordinated sedimentflushing in which the Unazuki dam (Ministry of Land, Infrastructure, Transport and Tourism) and the Dashidaira dam (Kansai Electric Power) conduct every event of floods. Deposited and inflowed sediments in the dams are discharged out the downstream river by thecoordinated sediment flushing. However, riverbed degradation has been seen in some downstream sections due to shortage of coarse sediment from the Unazuki dam. In order to solve
this problem, it is necessary to understand how the sediment yielded from the upstream is supplied to the downstream river.
Field observations and analytical studies have been carried out at the events of sediment flushing Unazuki dam and the Dashidaira dam. In the field observation, Sumi et al. (2005) and Kantoush et al. (2010) used 3D laser scanners to observe movements of sediments during thecoordinated sediment flushing of the Unazuki dam reservoir. It is shown that when the drawdown of reservoir water level is done, flood flows branch off in the reservoir, and in the free-flow
operation, sandbars appearing on the water surface moves slowly in the downstream direction.
Minami & Fujita (2008) conducted an analysis of the one-dimensional riverbed variation in theUnazuki dam, the Dashidaira dam, and downstream rivers. It was shown that the sediment1

Figure 2. The coordinated sediment ﬂushing.

Figure 1.

Location of the Kurobe River.

characteristics differ at the just downstream of the dam and further downstream. Taymaz et al.
(2017) propoced a method to improve sediment flushing efficiency of the Dashidaira dam byusing a 3D flood flow and bed variation model. However, the reproducibility of the analyticalmodel still has some issues, and it is not enough to clarify the sediment movement and sediment
discharge mechanism in the reservoir.
In this paper, we developed a numerical model that can reproduce sediment movement andbed variations between two dams by the coordinated sediment flushing by using the crosssectional data measured before and after the coordinated sediment flushing. And we clarify
the sediment movement in the Unazuki dam from the sediment discharge and riverbed variation at each time zone of dam operation.

2 CONDITIONS AND METHOD OF THE ANALYSIS
2.1 Analysis condition
Figure 3 shows water level hydrograph, and discharge hydrograph of two dams by the coordinated flushing gate operations in 2014. The peak inflow at the Unazuki dam is about 300m³/s.
The analysis is conducted from the Dashidaira dam (27.4km) to down the Unazuki dam(20.6km) as shown in Figure 4. The upstream end at the time of normal water level of the Unazuki dam reservoir is near 23.2km. The grain size distribution (Figure 4) used for the analysis is
the average value of bed material collected along the downstream channel of the Unazuki dam(14.0km-20.0km). The flood flow was calculated by the quasi-three-dimensional analysis
method of Takemura & Fukuoka (2019) which is suitable for steep rivers with boulders. The
riverbed evolution (bed load) was calculated using the two-dimensional bed variation analysis
method of Osada & Fukuoka (2012). The discharge hydrograph and the sediment graph at theDashidaira dam are needed to understand the sediment movement in the Unazuki dam reservoir. Figure 5 shows the flow chart for the calculation of the coordinated sediment flushing.

2Figure 3.

Operations of ﬂushing gate, water level hydrograph, and discharge hydrograph of two dam.

Figure 4. Analysis
distributions.

sectionsandgrain

size

Figure 5. Flow chart of the calculation methodof coordinated sediment ﬂushing.

2.2 How to obtain the sediment graph from the Dashidaira damFigure 6(a) and 6(b) show measured shapes of cross sections about 1.3km (No.7), 0.8km(No.5) in the Dashidaira dam reservoir before and after sediment flushing and assumed bedlevel.
Sediment deposited in the Dashidaira dam reservoir was eroded by up to maximum 8m dueto the drawdown and the free-flow operation. In order to reproduce such large erosion byanalysis, grain size distributions information is necessary. However, there were only reservoir
bed data before and after sediment flushing in the Dashidaira Dam reservoir. Therefore, inthis study, we estimated the sediment discharge graph from the flushing gate by making thefollowing five assumptions.
Before sediment flushing (Figure 7(a)), a large amount of deposited sediment in the Dashidaira dam reservoir is fine sediment. (2) When the draw-down ends, these fine sediments discharge out from the reservoir, and the coarser sediments were assumed to deposit on thebottom of the reservoir as shown by dotted line in Figure 6. The amount of suspended sedimentoutgoing from the flushing gate was calculated from the difference between the longitudinal reservoir bed elevations of before sediment flushing and at the start of the free flow. Then, suspended sediment graphs[silt (0.005mm to 0.075mm), fine sand (0.075mm to 0.25mm), medium3

Figure 6. Cross sectional shapes in the Dashidaira dam reservoir. 6(a)Cross sectional shape(No.7) 6(b). Cross sectional shape (No.5).

Figure 7. Observed and calculated water surface andriverbed proﬁles. 7(a). Before sediment ﬂushing. 7(b).
Free-ﬂow state.

sand (0.25mm to 0.85mm)] are determined to match the total amount of fine sediment, basedon the distribution of the suspended sediment concentration graph observed at the just downstream of the Dashidaira dam during the flood. (3) During the time of free-flow operation(Figure 7(b)), coarser sediments are scoured and transported, making the reservoir bed elevation formed after sediment flushing. Flood flow and bed variation analysis are performed so as
to take the initial reservoir bed topography at the start of free-flow operation. (4) Compare theanalysis with measured data and calculate by the correction of the initial reservoir bed elevation
until it almost matches the measured elevation. (5) The graphs of bed load and suspended sediment from the flushing gate obtained by the analysis are used as the upstream boundary conditions for the analysis of sediment movement of the Unazuki Dam.
Figure 8 (a) and 8(b) show the analysis results of the sediment graphs that passed throughthe Dashidaira dam flushing gate. The sediment discharged is 50mm, and the sediments withmaximum diameter are 300mm. When the water level is high, suspended sediment dischargedare not much. When the flow become in the a free-flow state and an open channel flow in theflushing gates, suspended sediments reach a peak value.
2.3 Sediment transport characteristics of the Unazuki Dam
Figure 9 shows CCTV (closed-circuit television) image at around 22.2km at 4:40 minutes on
July 15, 2014 flood. This CCTV image shows the flow in the free-flow states in the Unazuki

Figure 8. Calculated sediment graph that passed through the Dashidaira dam ﬂushing gate. 8(a).
Coarse sediment graphs 8(b). Suspended sediment graphs.

4Figure 9.

CCTV image taken at 22.2km (4:40 on July 15th).

dam reservoir, where the river is wide, and large sandbars are seen. During the period fromthe free-flow state to the refill state, riverbed structures do not change much because of sandbars hardly change and flow rate are relatively low. For this reason, the position of the thalweg during free flow operation is almost similar to the cross-section surveyed often the coordinated sediment flushing. In this study, the initial riverbed shape was set using the planar
position of the thalweg seen from CCTV images and the cross-sectional shape measured aftercoordinated sediment flushing. The other cross sections used those measured at intervals of
200m before the coordinated sediment flushing. In the downstream of the Dashidaira dam,
the maximum suspended sediment diameter was observed between 0.25mm and 0.85mm.
Therefore, the sediments with diameter of 0.85mm or less is assumed to be suspended sediment and were calculated by the depth integrated two-dimensional advection diffusion equation. However, further study about diameters dividing suspended load and bed load must beinvestigated.
The sediment discharge is greatly influenced, depending on the reservoir water level and thegate operation time of the two dams. Therefore, the gate operations were divided into threetimes zones (Figure 3): ①The Dashidaira dam and the Unazuki dam are in the drawdown
state ②The Dashidaira dam is in the free-flow state, and the Unazuki dam is in the drawdown
state ③The Dashidaira dam and the Unazuki dam are in the free-flow state.
3 CALCULATATION RESULTS AND CONSIDERATION
3.1 Dashidaira dam: Drawdown state, Unazuki dam: drawdown state
Figure 10 shows calculated results of longitudinal distributions of bed load in the Unazuki
dam. During the period of ①, There is almost no sediment discharge from the dam. But thereis large discharge from the dam, the riverbed at the downstream of the Dashidaira dam isscoured and transported. The maximum sediments with size of 300 mm are deposited around22.6 km in the Unazuki dam reservoir. From this, it is considered that in the time zone ①, thebed load that was deposited in downstream of the Dashidaira dam before present coordinatedsediment flushing is transported further downstream.
3.2 Dashidaira dam: Free-flow state, Unazuki dam: drawdown state
Figure 11 shows calculated profiles of the water surface and riverbed in the Unazuki dam.
Figure 12 shows the calculated longitudinal distributions of the coarse sediment (bed load).
Similar amount of sediment transport discharged from the dam are seen in the upstream river
channel, too. It is considered that the sediment from the Dashidaira dam is provided to theUnazuki dam reservoir by the coordinated sediment flushing.
Figure 13 shows calculated longitudinal distributions of the suspended sediments of different sizes from the Dashidaira dam. Suspended sediment of 0.01mm passes through the Unazuki dam while depositing in the river channel between Dashidaira dam and Unazuki dam.
However, the larger suspended sediment (0.1mm,0.5mm) deposit in the river channel area and5

Figure 10. Calculated longitudinal distributions of the bed load.

Figure 11. Calculated water surface proﬁles and riverbed proﬁles in the Unazuki dam.

Figure 12. Calculated longitudinal distributions of Figure 13. Calculated longitudinal distributions
the bed load.
of the suspended sediment.

does not reach the reservoir. This is because fine sediments are not included the initial riverbed
material of the Unazuki dam. The result of bedload analysis shows that the 2 mm and 10 mmparticle sizes are moving as shown in Figure 12. Therefore, it is considered that the 0.1 mm
and 0.5 mm grain sizes are transported not only by suspended load motion but also by bed
load motion. The analysis accuracy is controlled by the initial grain size distribution and themoving mode of sediments.

63.3 Dashidaira dam: Free-flow state, Unazuki dam: free-flow state-refill state
Figure 14 shows calculated water surface profiles and riverbed profiles in the Unazuki dam. It
can be seen that the Unazuki dam reservoir behaves like a river in the time of free flow gate
operation. Figure 15 shows the calculated bed topography and depth-average velocity vector
in the Unazuki dam reservoir at 23:00 on July 14th. It is seen that the main flow velocity
increased due to reduced cross-sectional areas by sandbars appearing on the water surface.
Figure 16 shows the calculated longitudinal distributions of the bed load during free-flowstate of the Unazuki dam. In the wide cross-section area(22.6km-22.2km), the finer sedimentgroup in the bed load was hardly deposited, and the sediment discharge 100mm and overdecreases in this section. This is due to the fact that the depth-average velocity is about 2.5m/s, which is slower than that of the upstream river channel. Figure 17 shows the calculated sediment graphs for particles of each diameter that passed through a sediment flushing gate of theUnazuki dam for 10 minutes. About 12 particles with a maximum diameter of 300mm passthrough the Unazuki dam for 10 minutes in free-flow state. It can be seen that 50mm passes
10,000 and 100mm passes about 1,000. There are considerable sediment discharges of 100mm
and 50mm. In addition, there is a time difference in the sediment discharge of 300mm, 100mm
and 50mm. From a result of the grain size distribution collected at just downstream of the theUnazuki dam, particles of 100 mm on the average and the maximum 200 mm were observed.
These particles almost correspond to particles the calculated maximum sediment diameter.

Figure 14. Calculated water surface proﬁles and riverbed proﬁles in the Unazuki dam.

Figure 16. Calculated longitudinal distributions
Figure 15. Calculated bed topography and water of the bed load sediment discharge during free-ﬂow
depth-average velocity vector in the Unazuki dam state.
reservoir at 23:00 on July 14th.

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Figure 17. Calculated sediment graph of bed load of the number of particles that passed through theUnazuki dam ﬂushing gate in 10 minutes. 17(a). 100mm,50mm. 17(b). 300mm.

Figure 18. Calculated suspended sediment graph that passed through the Unazuki dam ﬂushing gate.

Figure 18 shows the calculated result of the time graph of suspended sediment that passed
through the Unazuki dam sediment flushing gate. The peak of suspended sediment discharge
passing through the Unazuki dam appears at the time near the peak sediment discharge (7/1413:00) of the Dashidaira dam. In the time of the draw-down operation, suspended sedimentsfrom the Dashidaira dam are considered to pass through the Unazuki dam.

4 CONCLUSIONS
Characteristics of sediment movement in each time zone were investigated based on numericalanalysis and field observation results. The main conclusions and future works are given as
follows.
(1) we developed a numerical model that can reproduce sediment movement and bed variations by the coordinated sediment ﬂushing.
(2) Different of characteristic sediment transport have been seen under each gate operation of
the Dashidaira dam. When the drawdown of stored water was performed, the sedimentdeposited downstream of the Dashidaira dam is transported. Most of the 0.01mm passes
through the Unazuki dam while depositing in the river channel during free-ﬂow state.
And, coarse sediment from the Dashidaira dam is provided to the Unazuki dam reservoir
by the coordinated sediment ﬂushing.

8(3) The sediment movement in the Unazuki dam reservoir were examined by the numericalmodel using the ﬂood hydrograph and sediment graph from the Dashidaira dam as boundary conditions. It was clariﬁed from the coordinated sediment ﬂushing analysis, that themaximum particle size with 300mm diameter passed through the sediment ﬂushing gate at
the event of relatively small ﬂood ﬂow. In addition, bed load, such as 100mm and 50mm,
which contributes to the stability of the downstream river channel, passed a lot downstreamof the Unazuki dam.

REFERRENCES
Kantoush, S. A., Sumi, T. & Suzuki, T. (2010).: Impacts of sediment ﬂushing on channel evolution
andmorphological processes: Case study of the Kurobe River, Japan, River Flow, 1165–1173.
Minami, S. & Fujita, M. (2008).: The sediment responses in the downstream by sediment ﬂushing of thereservoirs, Annuals of Disas. Prev. ResInst. Kyoto Univ., No.51 B.
Osada, K. & Fukuoka, S. (2012).: Two-dimensional riverbed variation analysis method focused on themechanism of sediment transport and the bed surface unevenness in stony-bed rivers, journal of Japan
Society of Civil Engineers Ser.B1(Hydraulic Engineering), Vol.68, No.1, pp. 1–20.
Sumi, T., Murasaki, M., Nagura, H., Tamaki, H .& Imai,T. (2005).: Study on measurement oferosion-deposition process of reservoir sediment during ﬂushing operation (part 2), journal of Japan
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Takemura, Y. & Fukuoka, S. (2019).: Analysis of the ﬂow in undular and hydraulic jump stilling basinsusing non-hydrostatic quasi-three dimensional model considering ﬂow equation on boundary surfaces,
journal of Japan Society of Civil Engineers Ser.B1(Hydraulic Engineering), Vol.75, No.1, pp. 61–80.
Taymaz. E, Sumi, T., Kantoush, S. A., Kubota, Y., Haun, S. (2017).: Three-Dimensional Numerical
Study of Free-Flow Sediment Flushing to Increase the Flushing Efﬁciency, Proceedings of the Institution of Civil Engineers-Water Management, doi: 10.3390/w9110900.

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