E3S Web of Conferences 415, 02008 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341502008

Hydraulic model test on channel shifting and yielding woody
debris on the fan after sediment disaster in the past
Kenji Hashimoto1*, Haruki Watabe1, Hiroshi Kisa1, Tomoyuki Hoya1, Takahiro Itoh1, Yasuharu Watanabe2, andRyosuke Akahori3
1Nippon Koei Co., Ltd., Research & Development Center, 2304 Inarihara Tsukuba-shi Ibaraki, Japan2Kitami Institute of Technology, Civil Infrastructure Course, School of Regional Innovation and Social Design Engineering, 165

Koen-cho Kitami-shi Hokkaido, Japan
3Aichi Institute of Technology, Department of Civil Engineering, Faculty of engineering, 1247 Yachigusa, Yakusa-cho, Toyota-shi

Aichi, JapanAbstract. The re-movement of sediment and woody debris in torrents after huge sediment transport in thepast could cause new flood and sediment disasters due to heavy rainfall events. However, it is not clear how
re-movement of logs effects on bed variations such as bars and river channel divergence. In present study,
hydraulic model tests were carried out referring to the magnitude of floods, that was over the plan size, inTottabetsu River basin in August in 2016. These tests include different magnitudes of flood, suppling logs
from bed/side bank erosion, and existence of sabo facilities. The key results were as follows: The presenceof logs by bed/side bank erosions influences on the patterns of flows and sediment transport, because
deposition of logs affects formation of bars. Difference of the magnitudes of the floods affects the activenessof the interaction between logs and sediment transport. In additions, the locations and slits of the sabo dam
need to consider hydraulic conditions and characteristics of woody debris for appropriate control of sedimentand woody debris.

1 Introduction
The re-movement of sediment and woody debris in
torrents and rivers after huge sediment transport anddeposition in the past could cause newly flood andsediment disasters due to future heavy rainfall events,
because there are a lot of volume of sediment and woody
debris deposition everywhere along the river andtorrents.
There were heavy rainfalls in the Tottabetsu Riverbasin due to the Typhoon No.10 (Lionrock) on 28th to30th August in 2016. Figure 1 shows the location of theTottabetsu River. The river is a tributary of the TokachiRiver in Hokkaido in Japan, and has 44.0 km in length,
161 km2 in basin area and 1/37 in bed slope. The basin
area is 134 km2 in upstream of the study area.
Accumulated rainfall depth was 532 mm for three days
at around 40.0 mm/h at the Tottabashi rain-gauge stationof Ministry of Land, Japan and the value exceeded theprevious record that was 314 mm in September in 2001
[1]. Huge amount of sediment and logs were transported
and resulted in a lot of those deposition. The sabo masterplan there needed to renew due to the sediment disasterin 2016.
Re-moving of woody debris yielded by bed and sidebank erosion due to future floods could induce theflooding by blockage of piers and constriction of theriver channel. In present study, hydraulic model tests
were carried out focused on evaluating supposed
phenomena, which can be taken place after sedimentdisasters in the past. Supposed factors in tests are theremoving of woody debris, bed variation and side bankerosion, river channel divergence and so on, andFig. 1. Study area.

hydraulic model tests were carried out at the several
magnitudes of flood referring to floods in Tottabetsu
River basin in 2016.

2 Experimental model and conditions
2.1 Hydraulic model
Figure 2 shows hydraulic models of the Tottabetsu River
with the 1/70 in the model scale. In the prototype scale,the length of the channel of study area is about 2.80 km
and the width of the channel varies approximately from50.0 m to 350 m including several narrow and wide flow

* Corresponding author: a9456@n-koei.co.jp© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 02008 (2023)
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https://doi.org/10.1051/e3sconf/202341502008

sections with meanders. The depth of the movable bed
was set to about 8.00 m. The mean longitudinal gradientof the bed is about 0.800 degrees (= 1/70). Two opentype sabo dams were set in Run1. Detailed informationis shown in Fig. 2. The No. 1 sabo dam is re-constructingfrom closed to open-type sabo dam. Specification of thismodel test was based on the Froude similarity.
2.2 Measurement and conditions
Water, sediment discharge rate and total volume of
sediment runoff were measured at the downstream endof the channel. The plane view and overhead view of theflow patterns and sediment deposition were taken bycamera using UAV and scaffold, respectively. Water
levels were measured at No. -3, No. -5, and thedownstream end of the channel by servo-type level
meters as shown in Fig. 2. Tracing dye (white paint) wasused for visualization of flow patterns. Photographs
captured by camera after each test were used to generate
the three-dimensional topographic data. White papers
with 80.0 × 80.0 mm were used for tracer of transverse
surface velocity profiles of water. Logs were modelledby plastic materials for conifers floating on the waterand submerged broadleaf trees, and the specific weightof those log models is 0.952 and 1.20, respectively. Logs
were measured at downstream end and on the bed
surface after experimental runs. Deposited logs weremeasured transversely and vertically along several
measurement lines.

Fig. 2. Hydraulic models.

Sediment discharge rate (m3/s)

Water discharge rate (m3/s)

Run1876543210800
700
600500400300200
1000051015

2025Time (h)303540

Supplying number of logs (logs/h)

Fig. 3. Water and sediment discharge rate in Run1-2 andRun1-3.

The representative values of grain size diameter are
as follows: d95 = 285 mm, d60 = 70.4 mm. Herein, thesymbols of d95 and d60 show the value of 95 % and 60 %of the percentage passing by mass. The values of 95 %and 50 % of the frequency distribution for log length are
shown as L95 and L50, and are set to 16.1 m and 8.00 m,respectively. The representative value of log diameter
was set to 0.300 m as 50 % of frequency distribution ofthe diameter.
Logs supplied in the tests were consistent withconifers and broadleaf trees, which were the ratio 1:9,
while the length of each type of trees was consistent with
16.0 m and 8.00 m in the ratio of 1:1. Sediment
discharge was set in accordance with equilibrium
concentration of 0.003. Total supplying number of logs
were set to about 20,000 logs referring on therelationship between logs volume and total runoff
volume of logs in Run1. Number of logs in Run2 was
set to 0.25 times of that in Run1 as the total supplying
number of logs. Logs are supplied in case of Run1-1,Run1-2 and Run2-2, and logs discharge rate aredetermined depending on water discharge. Water andsediment discharge rate and supplying number of logs
are shown in Fig. 3, Fig. 4 and Table 1.
Hydrograph with process of decreasing to increasingstage was set in Run1-2 and Run1-3 to model logs runoff
and removing in each stage after huge flood. Water
discharge was supplied steadily in Run1-1, Run1-2
Run2-2 and Run2-3. Those values were plan size (770m3/s) and the excess probability for return period ofabout 20 years, respectively. In Run1-1 and Run2-1,
steady flow rate was supplied for forming the riverbed
before each run.

Run1Run1-2

Supplying number of logs
Run1-3

3000
2500
2000
1500
1000
50000510152025Time (h)303540Fig. 4. Temporal changes of supplying number of logs in
Run1-2 and Run1-3.
Table 1. Experimental conditions.
RunNo.

Water
discharge
rate (m3/s)
and duration(h)Sediment
discharge
rate (m3/s)
and sedimenttransport
concentration

Run
1-1

770 m3/s,
6.00 hours

2.32 m3/s,
0.003

Run
2-1

Hydrograph,and 17.0
hours shownin Fig. 3
Hydrograph,and 22.2
hours shownin Fig. 3
650 m3/s,
33.3 hours

Temporalchangesshown in Fig.
3, and 0.003
Temporalchangesshown in Fig.
3, and 0.003
1.96 m3/s,
0.003

Run
2-2

650 m3/s,
17.2 hours

1.96 m3/s,0.003

Run
2-3

500 m3/s,
24.0 hours

1.50 m3/s,0.003

Run
1-2
Run
1-32

Supplyingnumber of logs(logs/h)
2,487 logs/hfrom upstream end(During one hour)Temporal changes
shown in Fig. 4,
from upstream end
Without
supplied logs
Without
supplied logs
314 logs/h (total)
192 logs/h fromthe upstream end,
122 logs/h fromthe side banks
Without
supplied logs

E3S Web of Conferences 415, 02008 (2023)
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sediment discharge rate, and the temporal change had
few fluctuations of runoff.
The reason for the large deposition of logs at the No.
2 sabo dam is that the ratio of the slit width to the lengthof logs (= L95) at the No. 2 sabo dam is about 1:3.10,
Whereas, the ratio of the No. 1 sabo dam is about 1:1.60,
and means easy passing through the slit. The change of
sediment runoff take place at about 5, 27 hours inraising/decreasing stage in Run1-2 and Run1-3,respectively. Flow discharge is 400 m 3/s at that time, andresults in the stopping of sediment runoff at the No. 2
slit dam and in the starting of sediment runoff at the No.
1 sabo dam without intercept of water level.
It is desirable to set appropriate multiple layout ofdams and setting of slit width based on plane topography
characteristic of sediment and logs runoff.

3 Logs capturing due to slit dams
Figure 5 shows the plane view of topographicalvariation in Run1-3, Run2-2 and Run2-3.
In case of Run1-3, the logs were deposited on theinner edge of channel and front edge of the sand bar. Theslit of No. 2 sabo dam trapped about 80 % of the totallogs, that were almost broadleaf trees. The increasing ofwater level caused by slit blockage took place in about
200 m upstream of the No. 2 sabo dam. The main
channel downstream of the No. 2 sabo dam wasconvergent on the right bankside without channel
shifting. A little volume of conifer deposited in the slitof No. 1 sabo dam without slit blockage.
Figure 6 shows the temporal change of the sediment
discharge rate measured at the downstream end. In Run1,the sediment runoff was less than half of the suppliedFig. 5. The plane view and topographical variations in Run1-3, Run2-2 and Run2-3.

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E3S Web of Conferences 415, 02008 (2023)
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https://doi.org/10.1051/e3sconf/202341502008

point bars and is affected by deposited logs at theboundary between flow water and edge of sand bars.
There are almost broadleaf tree, which occupy around
90 % in logs number, in upstream reach of Tottabetsu
River basin. The broadleaf tree is usually submerged inflow water on the moving sediment particles. Sedimentdeposition takes place significantly in the wide flow
areas and sediment deposition is control by the wideflow area.
(2) Braided channel is quite active in wide and narrow
flow area at 500 m3/s in which hydraulic condition is in
the boundary between double bar and alternate bar
formation. The braided channel takes place everywhere
and whenever in wide flow area, and the interaction is
active between logs and sediment movement. The
woody debris and sediment particles could betransported well at the flow discharge (500 m3/s) thoughcapacity of sediment transport is less than that of 650
m3/s.
(3) Two sabo dams were arranged in the area inhydraulic model test to evaluate effects of woody debris
deposition on sediment and logs capture. Huge amountsof logs are captured and deposited in the area of
upstream sabo dam due to broadleaf tree that is a lot of
the rate for total logs, though the sabo dam is a slit dam.
The results could emphasize that the location and theopening of the slit need to consider hydraulic conditionand characteristic of woody debris for appropriate
control of sediment and woody debris. As shown inintroduction, we focused effects of woody debris on
sediment transport on the fan area after huge magnitude
of woody debris events. Though mechanism needs to beclarified for woody debris flow in steep slope torrents,
we will try to evaluate the interaction between steep
torrents and fan area, taking into account effects of sabo
facilities, debris flow mode and so on.

4 Influences of channel shifting on logs
deposition
Let us discuss interaction between logs deposition andflow patterns in Fig. 5 and 6. Logs tend to accumulate
along the water edge of flow channels or at front edge ofsand bars in Run2. Channel shifting is dominant inRun2-2, and effects of logs deposition along the wateredge on channel shifting and sand bars appear, because
logs deposition could decide water flow patterns. Speedof channel shifting is also affected by logs deposition
near the water edge of channel, and results in almost
fixed channel edge and increasing sediment runoff bysediment discharge rate in Run1 and Run2. Braidedsmall channels are formed everywhere in Run2-3.
Trigger of braided channel depends on the rate of flow
width to depth and additionally on logs deposition.
Capacity of sediment transport decreases due tosediment and logs deposition by braided channel, andresults in aggradation of the bed and decreasing of
sediment runoff to downstream reach.
Present experiments focused on sediment runoff andeffects of logs on sediment transport after huge sedimenttransport over the plan size. The values of sediment
runoff vary from 0.300 to 0.750 for the inlet sediment
discharge rate in Run2-2 and from 0.333 to 0.666 in
Run2-3. Sediment runoff tends to increase due to mainly
channel shifting and due to secondly braided channels
corresponding flow rate, and the effect of logs on
sediment runoff is significant. There are the variationsof sediment runoff in time-developing. That means
usual monitoring needs to carry out in the site for future
floods.
The study is going to consider in detail for therelationship between bed variation and runoff rate of
sediment and woody debris, and for those deposition.

Authors should be thankful for students in graduate andundergraduate school of Kitami Institute of Technology fordata analyses and great helps in hydraulic model test.

5 Conclusions
Present experimental study was carried out supposing
woody debris and sediment transportation after huge
sediment transport and deposition in the past. The
results obtained in present flume tests are as follows:(1) Channel shifting of main channel takes place in wide
flow area, and the flow width is determined by wide andnarrow flow area at 650 m3/s in which double bar isformed. The flow channel becomes one channel andReferences
1.

T.Kudo, N. Nagano, A. Matsuoka, T. Hayakawa,T. Kamijo, Y. Matsuyama, N.Osanai, M. Kasai,
Journal of the Japan Society of Erosion Control
Engineering, 73 (6), 3-11 (2021), (in Japanesewith English Abstract)

Fig.6. The temporal change of the sediment discharge rate, measured at the downstream end in Run1 and Run2.

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