C108

Investigation of geomorphology and habitat influenced by the sediment bypass tunnel operation in
the downstream of Koshibu Dam〇Yixuan LIU, Sohei KOBAYASHI, Tetsuya SUMI, Yasuhiro, TAKEMON, Sameh A. KANTOUSH

Introduction
Dams obstruct the continuity of sediment
movement in the river system, resulting in reservoirsiltation, which can impact reservoir operations anddiminish water storage (Kondolf et al., 2014).
Reservoir siltation can lead directly to a reduction inreservoir storage capacity. The siltation of sediments inreservoirs not only affects the service life of thereservoirs and negatively impacts their functions (e.g.
flood control, hydropower generation, irrigation, etc.)
but also leads to a lack of sediments in downstream
reaches to maintain a typical river ecosystem.
A sediment bypass tunnel (SBT) is a technique for
dealing with reservoir sediment siltation that involves
diverting sediment that normally flows into thereservoir from upstream to downstream through an
artificial outflow tunnel. Sediment released
downstream by the SBT causes changes in riverbed
morphology as well as a reestablished proclivity todeposit previously deteriorated portions. SBT can beconstructed on existing dams and effectively deal withwater storage shortages caused by reservoir water level
declines.
This research investigates the interrelationships
between channel structure, habitat structure, andbiodiversity in the downstream of the Koshibu Dam
under the effect of SBT operations by analysing theyearly changes in physical and biologicalcharacteristics including years before operation,
during operation, and after suspension of operation.
Method(1) Study Area

Figure 1 Map of Koshibu Dam SBT and downstream

Koshibu Dam was built in 1969 in Nagano
Prefecture, Japan. The total capacity of the reservoir

is 58 million m3. The reservoir has a total storage
capacity of 58 million m3 and is utilised forhydropower generating, irrigation, and flood control(Kashiwai and Kimura, 2015). The Koshibu River islocated in a region of significant erosional activity
due to both excessive precipitation and seismic
events (Auel et al., 2017). The SBT was built in 2009
to improve sedimentation and became operational in2016. The application of SBT was suspended in 2020
due to the need for repairs and was commissioned
again in the summer of 2023. This research is mainly
focused on the downstream of the Koshibu Dam. The
total channel length is about 4 km from the outlet of
the SBT to the downstream confluence with the main
Tenryu River.
(2) Imagery analysis for RSCC
This research analysed images from 2014 to2022 on a year-to-year scale. All images were taken
in November of the year. The periods ofphotographing are a relatively low-water, morestable period. Channel number (CN) and sinuosity(S) were the main parameters used to categorise theriver.
(3) HEC-RAS hydrological simulation
One-dimensional river flow simulation wasconducted to understand erosion/depositional potentialof the channel during flood flows (50m3/s, 100m3/s,200m3/s) and habitat conditions during normal flows(1m3/s).
The US Army Corps of Engineers created HECRAS, which is frequently used for analysing thehydraulic characteristics of rivers (Brunner, 2016). Thismodel requires the use of river cross-section data andflow and can output hydrological information such as
flow velocity, slope, shear stress, water depth, etc., foreach cross-section. The water surface profile wascalculated using the energy equation below.

Where Y = water depth; Z = channel elevation; V =average velocity; α = velocity weighting coefficient; he= energy head loss; g = gravitational acceleration; The
subscripted numbers represent the individual crosssections (here 1, 2 as an example).

Result(1) RSCC change from 2014 to 2022

increased in 2018.
(2) Shear stress under different flow conditions
At 1.8 km and 3.4 km, the shear stresses were lower
all year under high flow conditions. In contrast, highbed shear stresses were prevalent at 1.0 km. In 2016,
riverbed shear stress was higher in almost all sections
than in other years. In contrast, riverbed shear stressdecreased in all river sectors in the years after 2016.
However, this kind of change is not shown to besignificant under low flow conditions. Riverbed shear
stress was reduced overall under low flow conditions,but in 2018, the riverbed shear stress at 3km was muchhigher than the average. A similar situation occurred at
4.6 km upstream in 2016. In addition, under the low
flow condition, high and low shear stresses mostly
alternate in 200m increments. However, there was littlechange in bed shear stress in the 1.6km to 2.6km
portion of the river in 2020.
Conclusion

Figure 3 Historical change of channel number and sinuosity in Koshibu River

Figure 2 Shear Stress change in different sites and years

The downstream of Koshibu Dam was dominatedby the Single type channel (CN=1). As shown in Figure
2, before SBT was applied, the channel type wasrelatively uniform, and the straight channel occurred
more often than the sinuous channel within each unit.
The diversity of RSCC has increased to a certain extent
after the application of SBT started in 2016. Althoughstill predominantly single channel, many sections,particularly around the bed protection weirs,
significantly increased CN and sinuosity. In particular,
both the number of channels and the meandering

Through annual analysis of the channel
morphology downstream of Koshibu Dam, it isevident that sediment replenishment via SBT
contributes to ecological restoration to a certainextent. The once continuous single-channel structureof the Koshibu River has been enriched withsediment, resulting in a diverse array of channel
types. Notably, changes in channel morphology,
particularly in the vicinity of the bed protection
weirs, exhibited a more pronounced response tosediment inputs, with a higher degree of alteration inchannel number and sinuosity compared to anotherriver reaches. Examining specific channel change
patterns concerning yearly flow and sediment supplyis crucial.
Reference
Auel, C., Kobayashi, S., Takemon, Y., Sumi, T., 2017.
Effects of sediment bypass tunnels on grain size
distribution and benthic habitats in regulated rivers.
Int. J. River Basin Manag. 15, 433–444.
https://doi.org/10.1080/15715124.2017.1360320
Brunner, G.W., 2016. HEC-RAS River Analysis
System: Hydraulic Reference Manual, Version 5.0.
US Army Corps Eng. Eng. Cent. 547.
Kashiwai, J., Kimura, S., 2015. Hydraulicexamination of Koshibu dam’s intake facilities for
sediment bypass., in: First International Workshop
on Sediment Bypass Tunnels. ETH Zurich, pp. 45–53.
Kondolf, G.M., Gao, Y., Annandale, G.W., Morris,G.L., Jiang, E., Zhang, J., Cao, Y., Carling, P., Fu, K.,
Guo, Q., 2014. Sustainable sediment management inreservoirs and regulated rivers: Experiences fromfive continents. Earths Future 2, 256–280.