Katsushi Ishida, Toshiyuki Sakurai and Tetsuya Sumi

Abstract
The Koshibu Dam was constructed in 1969 and managed by the Ministry of Land,Infrastructure, Transport and Tourism, Japan. At the Koshibu Dam, the sediment bypass
tunnel (SBT) project was launched in 2000 with the aim of preventing sedimentation in
the reservoir and ensuring the continuity of sediment transport at the river. The SBT
was completed in September 2016.
After completion of the SBT, we carried out a total of eight bypass discharges, twice in2016, three times in 2017 and three times in 2018. In this paper, we mainly report onmonitoring results of discharges in 2017.
In 2017, we discharged a maximum discharge rate of 183 m3/s. This discharge rate isabout 50% of the planned maximum discharge rate of the SBT 370 m3/s and equivalent
to the average annual maximum discharge rate scale. As a result of monitoring, it wasconfirmed that there was no problem in gate operation, no significant abrasion damages
in the facility and no significant environmental change in the downstream river. It wasestimated that about 34,000 m3 of sediment was bypassed to the dam downstream bythree times operations in 2017. We could take the bypassed sediment grain size
distribution and the sediment transport characteristics during bypass operations.
Based on the lessons in 2017, we will try to improve the SBT operation by more precise
analysis on the relationship among inflow discharge rate, gate operation and sedimentinflow to the tunnel. Monitoring results on bypassed sediment budget and grain size,
abrasion damage on the tunnel invert as well as changes in downstream riverbed
morphology and aquatic communities will be also studied to feedback to the operation.
Keywords: Sediment bypass tunnel, gate operation, monitoring, sediment budget,abrasion damage

1Outline of the Koshibu dam and its SBT facilities

The Koshibu Dam is an arch type concrete dam of 105 meters high for flood control,
irrigation and power generation constructed in 1969. Figure 1 shows the outline of theKoshibu dam and its SBT facilities. Regarding the background and history of the project,specifications and structure of the facilities of the Koshibu Dam and its SBT, pleaserefer to the past reports (Kashiwai, J., Kimura, S. (2015), Takeuchi et al. (2017), Sakurai
et al. (2018)).

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Figure 1: The outline of the Koshibu dam and its SBT facilities

2The trial SBT operation method and monitoring method in 2017

In order to understand the change of sediment transport dynamics and river environment
and to decide the formal operation method of SBT, trial operation of the SBT andmonitoring has been carried out. Specific operational rules during the trial period andmonitoring method for sediment budget are discribed below.

2.1 The trial SBT operation method
The policy of the trial SBT operation method as of 2017 was to discharge as much as
possible from the SBT during flood to the extent that it does not affect the water use
capacity of the reservoir. Specifically, when the inflow dsicharge rate during flood
exceeds 60 m3/s, the gate of SBT is opened and discharge is started. After that,discharge from the SBT as much as possible within the range of discharge amount in theflood control rule of Koshibu Dam. When the inflow discharge rate decreases below 60
m3/s, the gate of SBT is closed and the discharge from SBT is finished. Regarding thistrial operation method, the formal operation method will be examined reflecting themonitoring results of future trial SBT operations.

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2.2 Sediment budget monitoring method
In order to understand the sediment budget in the SBT operation during flood, it isnecessary to confirm the sediment quantity by each grain size with regard to "Daminflow", "Bypass outflow" and "Dam outflow".
The budget of the gravel is estimated from the riverbed survey upstream anddownstream of the bypass tunnel, before and after floods, and from the riverbed
material investigation. Survey will be also conducted after the flood season. Since it isnecessary to get data in more detail at the diversion pool and the energy dissipator,
precise surveys will be conducted before and after the bypass operation. In addition, asthe first case of the SBT in Japan, sediment observation devices (plate type sensor etc.,
Sakurai et al. (2017), Koshiba, T., Sumi, T. (2017), Koshiba et al. (2017)) wereinstalled on the bottom of the tunnel outlet part.
With regard to the sand, it is difficult to observe all amount of the sand transport during
flood. Therefore, using the observed data as a verification data, the amount of the sandtransport is estimated from the plane two-dimensional river bed variation model
analysis. For water sampling at the tunnel outlet, water intake pipes were installed on
the training wall surface, in order to collect the discharged water at different water
depths.
The amount of the silt transport is estimated by combining the discharge rate data withthe suspended sediment concentration and the grain size distribution obtained by water
sampling at the tunnel upstream and downstream points.

3

Trial operations and monitoring results in 2017

After completion of the Koshibu dam SBT, trial discharge of the SBT during flood wascarried out 8 times by September 2018. It was conducted twice during the floods causedby the typhoon and the front line in 2016, three times during the floods caused by thetyphoon in 2017, three times during the floods caused by the typhoon in 2018. The trial
discharge conducted in September, 2018 was the maximum discharge rate and thelongest time discharge so far. The maximum discharge rate of the SBT was 195 m3/s
(about 50% of the design maximum discharge rate of 370 m3/s), and the discharge time
was about 69 hours. In this report, we describe about the trial discharges in 2017 whenthe monitoring data arrangement has been completed.

3.1 Trial operations in 2017
Hydorographs of three trial operations in 2017 are shown in Figure 2 to 4. And Figure 5
shows photographs of flow conditions in the inlet and outlet part of the SBT during thethird trial operation.

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Figure 2: Hydrographs of the first trial operation in 2017

Figure 3: Hydrographs of the second trial operation in 2017

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Figure 4: Hydrographs of the third trial operation in 2017

Figure 5: Flow conditions of the SBT inlet (left) and outlet (right) at third trial operation in 2017(2017/10/29 17:00, SBT discharge rate: about 50 m3/s)

In the first operation, the maximum discharge rate from the SBT was about 100 m3/s. At
the time when the inflow dishcarge rate became 60 m3/s (SBT operation finish judgment
reference dishcarge rate), the SBT discharge was terminated and it was a short-term
discharge of about 3 hours.
In the second operation, the largest past SBT discharge rate at the time 183 m3/s wasdishcarged. The discharge time was about 10 hours, and after the peak of the inflow, the報－160

inflow discharge rate was 60 m3/s or more, but the SBT discharge was terminated inorder to recover the water use capacity.
In the third operation, since the water use capacity was kept at the beginning of theflood, when the inflow amount was less than 60 m3/s, the SBT discharge was startedearlier. Although the maximum SBT discharge rate was not so much as 86 m3/s, about
49 hours of discharge, which is the longest at the time, was carried out.

3.2 Results of monitoring in 2017
As a result of trial operation of SBT in 2017, regarding structual aspects, wear of thesurface mortar part was observed over the entire bypass tunnel and aggregate exposure
was confirmed at a part of invert concrete, but there was no serious damage (Figure 6).
And there was no major damage to near the inlet part protected by the steel lining andrubber steel. There was no accumulation of sediment or driftwood in the tunnel.
Moreover, regarding SBT gate operation, it was confirmed that the gate could bereliably closed without sediment or driftwood being clogged.
As for the riverbed environment, the sediment supplied to the downstream by the SBT
discharge reached the Tenryu River (main river) junction, and the increase of the sandand gravel was confirmed in the river bed downstream of the Koshibu dam.
Regarding the biological environments, major changes in attached algae, benthicorganisms, fish and rare land plants have not been confirmed at the time of monitoringin 2017 due to SBT discharge.

Figure 6: Abrasion damage situation of the tunnel invert after the trial operation in 2017報－161

4Results of sediment budget monitoring in 2017

As a result of examination by combining survey results before and after trial operations,riverbed material investigation, water sampling analysis during trial operations and river
bed variation model analysis, the sediment volume passed through the SBT in three trial
operations in 2017 was estimated about 34,000 m3.
The sediment budget for each sediment grain size group is shown in Figure 7 together
with annual average sediment budget plan of the Koshibu dam SBT. During the trial
operations, about 67,000 m3 of sediment flowed from the upstream, about 15,000 m3 ofsediment accumulated in the diversion pool and about 34,000 m3 of sediment wasbypassed. The propotion of sand in the bypassed sediment was as large as 70%. In
addition, in the operations, the amount of reservoir sedimentation was suppressed toabout 5,000 m3, and about 31,000 m3 of sediment was supplied to the downstream river.
Compared with the annual average sediment bugdet plan (the left in Figure 7), since theflood frequency was smaller than usual and the scale of flood eventsr were small in
2017, the annual estimated inflow sediment volume about 150,000 m3 wasquantitatively small. Although the ratio of bypassed sediment and inflow sediment of
trial operations in 2017 was about 50%, which was close to the planned ratio.

Figure 7: The sediment budget for each sediment grain size group (left: annual average sediment budget
plan of the Koshibu dam SBT, right: sediment budget during the trial operations in 2017)報－162

5Conclusions

The Koshibu dam SBT has been completed in 2016, and several trial discharges havebeen carried out so far. Although it was a relatively short time, the SBT discharged 183
m3/s, which is about 50% as compared with the planned maximum discharge rate of 370
m3/s in 2017. It is close to the average annual maximum outflow discharge scale.
As a result of monitoring, it was confirmed that there was no problem in gate operation,
the facilities had no noticeable abrasion and damage, and no significant environmental
change was confirmed. Moreover, it was able to understand approximaterly thesediment budget and the characteristics of sediment transport through the SBT.
In the future, we will continue monitoring, accumulate monitoring results of SBT
discharge in various flood events, make an effort to understand the damage situation ofthe facilities, the change of river environment and sediment budget. We will try toimprove the SBT operation by reflecting the monitoring results.

References
Takeuchi et al. (2017). Monitoring scheme for sediment bypass tunnel at the Koshibu Dam. 2nd
International Workshop on Sediment Bypass Tunnels, Kyoto Japan.
Kashiwai, J., Kimura, S. (2015).
sediment bypass. Proc. First International Workshop on Sediment Bypass Tunnels, VAWMitteilungen 232, Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich
Switzerland, 45-54.
Sakurai et al. (2017). Bedload monitoring with impact plates at Koshibu sediment bypass tunnel. 2ndInternational Workshop on Sediment Bypass Tunnels, Kyoto Japan.
Koshiba, T., Sumi, T. (2017). Experimental study on measurement method of gravel discharge rate inhighspeed flow using plate-type sensor. 2nd International Workshop on Sediment Bypass
Tunnels, Kyoto Japan.
Koshiba et al. (2017). Field experiment of bedload transport rate measurement at sediment bypasstunnel. 85th Annual Meeting of ICOLD, Prague, Czech Republicio.

Authors (Text style: Authors Address)
Katsushi Ishida
Tenryu River Integrated Dam Management Office, Chubu Regional Development
Bureau, Ministry of Land, Infrastructure, Transport and Tourism, Japan
Toshiyuki Sakurai (corresponding Author)
Japan Dam Engineering Center, Japan
Email: sakurai@jdec.or.jp
Tetsuya Sumi
Disaster Prevention Research Institute, Kyoto University, Japan報－163