E3S Web of Conferences 415, 06021 (2023)
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A simplified numerical model for evaluating sediment control byopen-type sabo dams in the Joganji River basin
Yusuke Yamazaki1*,Tomoyuki Noro2, Kenji Miwa3, Takahisa Mizuyama4, Masaharu Fujita5, Shusuke Miyata5, Akihiko
Ikeda2, Tomohiko Furuya6, Takahiko Nagayama7, and Takahiro Itoh8
1 Nippon Koei Co., Ltd., Land Infrastructure Development Operations, 5-4 Kojimachi, Chiyoda-ku, Tokyo 102-8539, Japan2 Sabo and Landslide Technical Center, 2-7-5 Hirakawacho, Chiyoda-ku, Tokyo 102-0074, Japan
3 Ministry of Land, Infrastructure, Transport and Tourism Hokuriku Regional Development Bureau, Tateyama Mountain Area Sabo

Office, 61 Bunazaka, Ashikuraji, Tateyama-machi, Nakaniikawa-gun, Toyama 930-1405, Japan
4 Professor Emeritus, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
5 Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
6 Nippon Koei Co., Ltd., Hokuriku Office, 5-1-43 Ekinishihonmachi, Kanazawa, Ishikawa 920-0025, Japan
7 Nippon Koei Co., Ltd., Osaka Branch Office, 1-2-5 Nishitenma, Kita Ward, Osaka 530-0047, Japan
8 Nippon Koei Co., Ltd., Research & Development Center, 2304 Inarihara, Tsukuba, Ibaraki 300-1259, JapanAbstract. The present study proposes a method to estimate sediment runoff by introducing a dam functionof the relationship between inflow sediment and sediment runoff through a slit dam. The model can process
rainfall runoff, sediment yield and runoff of a mountainous basin, and the model is applied to the upper
reaches of the Joganji River basin, which is known for its huge amount of sediment runoff and intense bed
variation because of the sediment yield caused by the earthquake in 1858. The performance of thecalculations of sediment control of the slit dam is evaluated by the model. The result indicates that sedimentdeposition is significantly changed by sediment runoff. The proposed method can be expected to evaluate
sediment transport with sabo dams on a basin scale.

1 Introduction
The Joganji River basin has experienced a lot ofsediment disasters because of the considerable sedimentyield of approximately 0.1 billion m3 that was caused by
mountain collapse as a result of the earthquake in 1858[1]. Some of the sediment movements caused by theearthquake built up natural landslide dams. Naturallandslide dams broke twice, 14 days and 59 days after
the earthquake, generating a large-scale outburst-floodand sediment deposition on the alluvial fan of theJoganji River [1].
Evaluating the effect of the open-type sabo dam (slitdam) on sediment runoff control and studying sedimenttransport within a basin with sabo dams and slit dams
are important [2,3]. The present authors have beenstudied modeling sediment deposition and sediment
runoff at slit dams and the dam installation method
through a model that processes rainfall runoff, sedimentyield, and runoff of a mountainous basin (Storm Induced
Multi-Hazards Information Simulator [SiMHiS]), which
was developed by Yamanoi and Fujita [4].
In the present study, we propose a method toestimate sediment runoff through a slit dam byintroducing a dam function of the relationship between
inflow sediment and sediment runoff through a slit dam.
The SiMHiS model with the proposed method is applied
to the upper reaches of the Joganji River basin toFig. 1. Schematic diagram of sediment runoff through theslit dam.

evaluate the performance of the calculations of
sediment control of the slit dam by the model.

2 Method
The SiMHiS is composed of rectangular unit slopes andlinear unit stream channels in its topography, in the samemanner as Egashira et al. [5]. Darcy’s law and theManning equation are applied to the lateral saturated
flow and overland flow on unit slopes, respectively.

* Corresponding author: a9651@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, 06021 (2023)
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01400

101200Rainfall (mm/h)

2030

1000

40800

50600

60
70

40080

Storm water runoff (m3/s)

(a)

200

900

10000

06

12180006

121800Time (h): July 17th to 18th, 2004)

Fig. 3. Temporal changes in gauged rainfall and simulated
storm water runoff.
Table 1. Parameters for dam function and patterns ofcalculations.
Parameter
T (hours)(b)

Rb1 (-)

ValueRun

400.5
210

Runs 0-3
Run 0Run 1
Run 2
Run 3

Rb1: Ratio of multiplayer to Qbin
Table 2. Particle-size distribution of supplied sediment (s)and bed material (b).
%%Diameter
Diameter
(m)
(m)
sbs
b0.0005751
20.099724
0.0021
2350.36
135
0.00751212
1.31
0100.027
612

Table 3. Hydrological and hydraulic parameters.

(c)Parameter
ValueParameter
Value
D1, D2 (m)0.1, 0.5
α (-)
5
κ1, κ2 (mm/s)
3, 0.2n (m-1/3 s)
0.05
λ (-)
0.467
Em (m)
0.6
N (m-1/3 s)
0.3
Ed (m)
0.6
D: depth of soil layer, κ: saturated hydraulicconductivity, λ: porosity, N: equivalent roughness
coefficient, α: regime coefficient, n: Manning’sroughness coefficient, Em, Ed: depth of mixing layer anddeposition layer, respectively.

of the slit is difficult because the spatial resolution of theunit stream channel is not sufficient to express the slitdam topographically. Therefore, the method to estimate
the sediment runoff requires an approach that is
independent of the topographic properties.
Relationships between inflow sediment andsediment runoff through the slit dam are formulated
focusing on the duration time at which the backwater isformed and sediment runoff during the duration time.
Figure 1 shows a schematic diagram of sediment
runoff through the slit dam, where Q is the runoff, T isthe duration during which sediment runoff is stopped by
the backwater, t1 and t2 are the times at which thebackwater is formed and cleared, respectively, Qb is therunoff of the bed load, Qbin is the bed load runoff at theFig. 2. (a) Location of the Joganji River basin, (b) locationsof a rain gauge, sabo dams, and the Myoju Sabo dam within
the Joganji River basin with contour lines every 500 m, (c)unit channels and unit slopes of the Joganji River basin.

The sediment transport formula of Ashida-Michiue [6]is applied for the bed load and suspended load
calculations of unit stream channels.
Setting up the effect of a slit dam usingtopographical properties such as dam height and width2

E3S Web of Conferences 415, 06021 (2023)
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Figure 2(c) shows unit channels and unit slopes. The
area of the target basin is approximately 200 km 2, andthe elevation ranges from 300 m to 3,000 m. There are
twelve main sabo dams and slit dams within the basin.
Sediment runoff, deposition, and water level areobserved around the Myoju Sabo dam (slit dam with ashutter [7]).
The topography model for the SiMHiS is built using
elevation data provided by the Geospatial Information
Authority of Japan (GSI).
3.1.2 Target storm event

Fig. 4. Temporal changes in Qb from Runs 0 to 3. Solid lines
indicate temporal changes in Qb and dashed lines indicate
accumulated Qb.

A storm event was selected focusing on peak runoff andduration time for the backwater was formed at theMyoju Sabo dam. The design peak runoff for the Myoju
Sabo dam is approximately 2,000 m3/s, and thebackwater is expected to be formed by 100 m 3/s.
A storm event that occurred from 17th July to 18th
July in 2004 was selected. Hourly rainfall data provided
by the Japan Meteorological Agency (JMA) was usedfor rainfall runoff. Figure 3 shows temporal changes inhourly rainfall as determined by the rain gauge (Fig. 2).
The maximum 24-hour rainfall was 174 mm, and themaximum hourly rainfall was 45 mm.
Simulated runoff is shown in Figure 3. Peak runoff
is approximately 800 m3/s, which is half of the design
runoff of the Myoju Sabo dam. Since this implies that
overflow of the slit dam does not occur, Equations (1)and (2) are within the application range. From theviewpoint of the field survey, it is considered that while
runoff exceeds 100 m3/s (red line in Fig. 3), thebackwater is formed. Therefore, from 23:10 of the 17th
to 7:20 of the 18th, the backwater is assumed to beformed and the time accounts for a quarter of the total
runoff event.

Fig. 5. Temporal changes in Zb from Runs 0 to 3.

inlet of the unit stream channel, Qbout is the bed load
runoff at outlet of the unit stream channel, and Qb1 is thebed load runoff when the backwater cleared. The valueof Qbout before t1 and after t2 can be calculated by theequilibrium bed load equation. However, it isinappropriate to apply the equation during the period
from t1 to t2 because of the complicated local variationof the topography and hydraulic condition around theslit dam.
Therefore, in the present study, Qbout is defined asfollows:

3.1.3 Settings
In order to test the relationship between sediment runoff
from the slit dam (the Myoju Sabo dam) and sedimentdeposition in the slit dam, variables T and Rb1 areintroduced. Here, T is the duration time when sediment
runoff is stopped by the backwater, and Rb1 is the ratioof Qbin to obtain Qb1 (Qb1 = Rb1 Qbin). Table 1 shows thevalues of T and Rb1, which is the multiplier to Qbin used
to obtain Qb1 and patterns of calculations.
The Myoju Sabo dam is located around the center ofthe unit channel. Therefore, bed load runoff from theoutlet of the unit channel is regarded as bed load runoff
from the Myoju Sabo dam, and sediment deposition of
the unit channel is regarded as sediment deposition ofthe dam. Since the height of the dam is not consideredin this calculation, changes in the river bed may exceed
the dam height under particular conditions. Bed load
runoff from the Myoju Sabo dam is calculated using
Equations (1) and (2).
Assuming the suspended load does not contribute toriver bed change in the steep river of the present study,
the suspended load is not considered. Therefore only
bed load is supplied at the outlet of two upper unit
stream channels in the equilibrium condition. Table 2

𝑄𝑏𝑜𝑢𝑡 (𝑡1 ≤ 𝑡 < 𝑡1 + 𝑇) = 0(1)𝑡 − (𝑡 + 𝑇)
(2).
𝑄𝑏𝑜𝑢𝑡 (𝑡1 + 𝑇 ≤ 𝑡 < 𝑡2 ) = 𝑄𝑏1
𝑡2 − (𝑡1 + 𝑇)

The parameters are to be defined by flume tests. Since
the relationships between inflow sediment and sediment
runoff through the slit dam differ depending on whether
the overflow of the slit dam occurs, the equations are outof the application range when the overflow of the slit
dam occurs in the present study.

3 Condition and settings
3.1.1 Joganji River basin
A mountainous basin located in the upper reaches of theJoganji River basin was selected for application of theproposed method. Figure 2(a) shows the location of theJoganji River basin. Figure 2(b) shows locations of arain gauge, sabo dams, and the Myoju Sabo dam within
the Joganji River basin with contour lines every 500 m.

3

E3S Web of Conferences 415, 06021 (2023)
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Referencesshows the particle-size distribution of supplied sedimentand bed material. That of supplied sediment isdetermined by the sieving tests for sampling materials at
the bare slope within the target area, and the distributionof bed material is determined by the iterative
calculations during several ten years. The hydrologicaland hydraulic parameters are determined by general
settings for mountainous areas in Japan (Table 3).
The initial condition for the stream bed is set bysimulating 30 mm of hourly rainfall for 24 hours
followed by no rainfall for 24 hours.

1.
2.
3.
4.

5.

4 Results
Figures 4 and 5 show the temporal changes in bed load
runoff through the Myoju Sabo dam (Qb) and elevationof the sediment deposition of the Myoju Sabo dam (Zb),respectively. The amount of degradation of elevation(Zb) becomes larger according to an increase in Rb1, asshown in Runs 1 to 3. The result indicates that sedimentdeposition is significantly changed by sediment runoff.
The effect of the sediment runoff on changes inelevation was significant in Run 3. The total amounts ofsediment volume without porosity in Runs 0 to 3 were
3.83×105 m3, 0.92×105 m3, 1.08×105 m3, and 2.19×105
m3, respectively.
Introducing sediment runoff calculation into existingmodels becomes easier using the simulation in thepresent study with surveyed topography and bed load
data.

6.

7.

5 Conclusion
The present study proposed a simplified method toestimate sediment runoff through a slit dam byintroducing a function of the relationship between
inflow sediment and sediment runoff through a slit dam.
The model is applied to the Joganji River basin, which
is known for its huge amount of sediment runoff andintense bed variation because of the sediment yieldcaused by the earthquake in 1858.
The result indicates that sediment deposition issignificantly changed by sediment runoff. At the sametime, the result means that the effect of sabo facilities
such as the sabo dam can be considered by introducing
the proposed dam function in the calculation of basin
scale by the unit-slope and unit-channel model (e.g., theSiMHiS). Although additional verification will berequired, the method is expected to evaluate sedimenttransport with sabo dams in a basin scale. The authors
are going to improve the function based on flume tests
and sediment-hydraulic approaches.

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