E3S Web of Conferences 415, 06020 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506020

Sediment control and logs capturing in sand pocket withcombination of sabo dam with large conduit and iron bars
Haruki Watabe1*, Satoshi Tagata2, Tatsuki Yuzawa2 and Takahiro Itoh1
1Nippon Koei Co., Ltd., Research & Development Center, 2304 Inariharakita Tsukuba-shi Ibaraki, Japan
2Nippon Koei Co., Ltd., Fukuoka Branch Office, 1-2-12 Higashihie Hakata-ku Fukuoka-shi Fukuoka, JapanAbstract. In the Otoishi River, a tributary on right side of the Akatani River in the Chikugo River basin, alarge amount of sediment and woody debris were deposited on riverbed due to the sediment disaster causedby the heavy rainfall in northern part of Kyushu Region on 5th to 6th July in 2017. As one ofcountermeasures to control the outflow of sediment and woody debris to the Akatani River, a sand pocket
is planned with the sabo dam with large conduit parts and the log broom works. In this study, hydraulicmodel test and flume test were carried out to obtain knowledges for design of the sand pocket. Iron bars are
installed vertically at the upstream front of the large conduit parts for capturing sediment and logs, and thoseintervals are desirable to be set in consideration of sediment diameter and sediment movement without thedam. Besides, it is necessary to consider capturing function and flow characteristics such as shockwave
when the log broom works is planned in setting of layout and dimensions.

1 Introduction

2 Experimental models
Two kinds of models in straight flume and hydraulicmodel were used. Model scale was 1/50, and some

(°)

(°)

OtoishiRiverJapanStudyarea
(°)
(°)

(°)

(°)

AkataniRiverKyusyu
RegionAkatani
River basin
(°)

(°)

Fig. 1. Study area.
Downstream end(Measurement : waterand sediment discharge)

Counter Main dam withdamlarge conduit

200m

In the Otoishi River, a large amount of sediment andwoody debris were deposited on the riverbed due to thesediment disaster caused by the heavy rainfall innorthern part of Kyushu Region on 5th to 6th July in2017. The Otoishi River is a tributary on right side of
the Akatani River in the Chikugo River basin, and it has
5.99km2 in basin area and 4.0km in length and 1/4 to
1/30 in bed slope. Accumulated rainfall depth for
24hours reached to 638mm at Tsurukouchi rain-gauge
station of Ministry of Land, Japan where is next to theOtoishi River basin. As one of countermeasures tocontrol the outflow of sediment and woody debris to theAkatani River, a sand pocket is planned in the Otoishi
River basin, and it includes the sabo dam with large
conduit and the log broom works. Some reseraches werecarried out on sediment control function using sabo damwith large conduits in the past [1].
In this study, it aimed to obtain knowledges forcontributing to detail design of the sand pocket. A flume
test was used to set the adequate vertical interval of theiron bar installed at the large conduits. In addition, ahydraulic model test was carried out to confirm theeffect of planar shape of river channel and the iron bars
on capturing of sediment and logs. Besides, the test wasconducted to evaluate the longitudinal layout of log
broom works in apron and transverse intervals of bars.

Otoishi River basin Basin area: 5.99km2

Upstream end(Supply : sedimentwater, and logs)

Channelworks

Measurement: water levelLow flowchannelLog
broomworks
ApronIron bars

Sandpocket400m

Fig. 2. Hydraulic model (plane view).

* Corresponding author: a6809@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, 06020 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506020

parameters were set to satisfy Froude similarity. Straightflume was used to set the vertical interval of the iron bars
installed at upstream front of the large conduit parts.
Length and width of the flume were 20.0 m and 0.6m,
respectively. Figure 2 shows the plane view of thehydraulic model with 8.0m long and 4.0m wide in model
values. Bed was fixed bed. Water, sediment, logs were
supplied from the upstream end of the model. Water andsediment discharge were gauged by bucket at thedownstream end, and water level was automaticallymeasured by servo-type gauge at the downstream end.

frequency distribution of the diameter. The ratio ofsupplying number of logs for L95 and L50 was set as 1:1.
Logs except for roots and crown were modeled byplastic stick. The specific weight of logs was referred tothe previous study [3], and it was set less than 1.0
because the tree species on site is mostly conifers.
Flume test aimed to set the effective intervals of theiron bars for confirming the capturing and runofffunction of the open parts of the dam for sediment andlogs capturing. Therefore, the peak flow discharge wasset to 100m3/s in clear water, that overflow does notoccur from the spillway, and the recession-typehydrograph was set in the flume test. In hydraulic model,
the hydrograph reproduced the flood in July 2017 wasapplied shown in Figure 4.

3 Experimental conditions

Percentage passing by mass (%)

Experiments focused on 2 contents shown in Table 1.
Table 2 shows the experimental conditions in prototype
value. Figure 3 shows the grain size distribution curvein composition of riverbed and slope failure area.
This study focuses on bed load, and grain size
diameter less than 2.0mm in prototype value wasexcepted from the curve. The representative values ofgrain size diameter are as follows: d95 = 366.5mm, d60 =30.1mm. Herein, the symbols of d95 and d60 show thevalue of 95% and 60% of sediment diameter for thepercentage passing by mass. The dimensions of logs
were set based on field survey and the previous study [2].
The value of 95% and 50% of the distribution curve forlog length are shown as L95 and L50, and those values
were set to 8.3m and 5.9m, respectively. Those values
show 1/3 times of length of fresh woods because woods
are broken in process of outflow [3]. The representativevalue of log diameter was set to 0.261m as 50% of

Run2

Specifications of logs

Log length: L95 = 8.3m, L50 = 5.9m
Log diameter: 26.1cm
Log species: Conifer
Specific weight: 0.952

Flux sedimentconcentration

0.010

Supplying rate of logs

Setting in accordance with water
discharge

Discharge (m3/s)

4,000

100
80

3,000602,000

40

1,0000123456Time (hr)78910Fig. 4. Hydrograph, sediment discharge rate and temporalchanges of supplying number of logs in hydraulic model test.

4 Setting of vertical intervals of ironbars

0m3/s at 1.6 h
Duration time: 1.6 h
140m3/s (Peak) at 4 h
13.5m3/s at 10 h
Duration time: 10 h

d95 = 366.5mm, d60 = 30.1mm

5,000

1200Contents

Sediment diameter

1000

6,000

0100m3/s (Peak) from 0 h to 0.95 h

1/30 ( = 1.91°)

Hydrographsediment discharge rate
Supplying number of logs(Total)

20

Table 2. Experimental conditions.

Bed slope at upstream

100140

Flume test for setting the vertical intervals of theiron bars
Hydraulic model test for confirming thecapturing and runoff function of the sand pocket
for sediment and logs

Hydrograph(Hydraulic model test)

0.1
1
10
Sediment diameter (mm)160

Contents

Hydrograph(Flume test)

0.01

Supplying number of logs（logs/h）

Run1

100
90
80
7060
50
40
30
20
1000.001

Fig. 3. Grain size distribution curve in composition ofriverbed and slope failure area.

Table 1. Experimental runs.
Run No.

Grain size distribution curve (prototype)
Grain size distribution curve without fines (prototype)
Grain size distribution curve without fines (model)

Area ratio of open part against large conduit part isdefined as ∑ 𝐴𝑛 / At, in which ∑ 𝐴𝑛 : Total area of open
part, At: Area of large conduit part (Figure 5). Four
patterns of the area ratios were set based on grain size
diameter.
Clearance

Bottom ofArea of large conduit part：Atclearance
Total area of open part：A1+A2+A3
Area ratio of open part against large conduit part :Ironbars
A3
A2
A1/ AtFig. 5. Schematics and definition for area ratio of large
conduit parts and intervals of iron bars.

2

E3S Web of Conferences 415, 06020 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506020

Log broom works trap the logs gradually and waterlevel in the apron increases. Logs stay in the apron, andflow angle of logs are easily change to right-angle
against the flow direction. Then, logs are easy to becaptured by the works, and the works is finally
blockaded. Water level in apron increases due to theblockade, and the effect of the high velocity andshockwave on the works becomes small because
submerged hydraulic jump is formed.
Figure 8 shows logs trapping in differences ofinterval of the iron bars. Capturing ratio of logs isdefined as trapping number of logs by the works against
supplying number of logs at upstream end. The intervals
are set to 2.5m and 5.0m, and the value of 5.0m is
approximately 0.5 times of log length L95. The test givesthat the interval of 2.5m is more effective for thecapturing of the logs than that of 5.0m.
Sediment passes through the intervals because theinterval is set larger than sediment diameter of d95, andsediment easily become depositing in the apron if theworks is blockaded by logs.

Il : Interval ofbottom clearance
100Sediment
runoff
95
rate

9085

Logs runoff rate (%)

Il=1,500mm

1098765432
10

Il=1,000mm (0.604)
Il=1,000mm(0.663)

Il=500mm

Sediment runoff rate (%)

Figure 6 shows the relationship between the ratio∑ 𝐴𝑛 / At and runoff rate of sediment and logs. The result
gives that the effective area ratio for capturing function
differs in sediment and logs. The values of effective area
ratio for sediment and logs are 0.6 and 0.663,
respectively. In this study, area ratio of 0.663 (intervalof bottom clearance:1,000 mm, other intervals:500m) isapplied, and the ratio is effective setting for both ofsediment and logs capturing.

80
0.5
0.6
0.7
0.8
0.9
1.0
Area ratio of open part agaisnt conduit part( ΣAn / At )Logs
runoffrateFig. 6. Relationship between area ratio of open part and runoff
rate of sediment and logs.

Figure 7 shows the flow patterns around the sabo
dam in decreasing stage of hydrograph. Logs are trappedby the iron bars installed at upstream front of the large
conduit parts. However, sediment runs out from thebottom of the clearance in decreasing stage of thehydrograph because the clearance is not blockaded
completely. Even if the iron bars are installed at the large
conduit parts, the function of sediment runoff indecreasing stage of flood is not prevented if the verticalintervals of the iron bars are set adequately.
Flow pattern from left side
in upstream area of the dam

Interval of iron bars：2.5m
Capturing ratio of logs：64.2%

Interval of iron bars：5.0m
Capturing ratio of logs：26.3%

5.2 Flow patterns
Figure 9 and Figure 10 shows the flow patterns of sandpocket at few hours intervals. Figure 11 shows the flow
pattern of the apron.
In 3 hours, the main flow is formed along left sidebank of the sand pocket because the flow angle fromupstream area tends to be left in the plane view. Theflow angle depends on the direction of channel works
and low flow channel in the plane view. The flow
collides to the upstream front of the main dam, and it
flows in clockwise in the sand pocket. Besides, overflow
from the spillway is not occurred, and logs are trappedby the iron bars installed at the large conduit parts.

Log

FlowDamFig. 7. Flow patterns around the sabo dam in decreasing stageof hydrograph.

5 The capturing function ofpocket for sediment and logs

Main dam

Fig. 8. Capturing function of logs in difference of transverse
interval of iron bars.

Flow pattern in downstream
area of the dam

FlowIronbars
Sediment runs out from bottom of clearance

Log broomworks

sandLog broomworks

Main damwith large conduit
Log

5.1 Longitudinal layout of log broom works andinterval of iron bars
Jet is formed in the apron because water passes throughthe large conduit parts. It induces high velocity andshockwave in apron, and those flow characteristics
could cause the damage of the works. Therefore, the log
broom works is desirable to set the location where theinfluence of flow characteristics around the apron on theworks is decreasing. There are some logs that are moveparallel to the interval of iron bars and a few logs tend
to pass through the works in initial stage of the test.

Flow inclockwise

3hoursFig. 9. Flow pattern in sand pocket in 3hours.

Peak discharge is shown in 4 hours. Most ofsediment deposits in left side of the sand pocket because
the main flow is formed along the left side bank. Though
the logs overflow from the spillway of the dam due to3

E3S Web of Conferences 415, 06020 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506020

blockage of the large conduit parts by the logs, the logs
are trapped by the log broom works installed in theapron (Figure 11).
Main flow gradually shifts to right side bank because
sediment deposits between left side bank to center of thesand pocket. Sediment deposit along the main flow andmain flow gradually shifts in response to sedimentdeposition.

changes of sediment discharge. Sediment discharge isreduced by about 90% at peak stage of discharge.

6 ConclusionsThe results support the following conclusions.
(1) The dimensions of intervals of the iron bars installed
at upstream front of the large conduit parts wereexamined by the flume tests. The effective area ratio of
the large conduit parts differed between the function of
capturing sediment and that of logs (Sediment: Effective
area ratio is 0.604, log: Effective area ratio is 0.663).
The dimensions were set effective for both of sedimentand logs capturing shown as follows: Effective area ratio
is 0.663, bottom interval of clearance is 1,000mm, otherintervals are 500mm.
(2) The functions of the iron bars are as follows: Cut off
sediment discharge, preventing from rapid sediment
runoff from large conduit parts in decreasing stage,
trapping sediment and logs. Interval of bottom partshould be set to thickness of sediment moving layer sothat sediment runs out from bottom of clearance indecreasing stage of flood. Other intervals are desirable
to set less than log diameter or maximum grain diameterfor trapping sediment and logs.
(3) The iron bars trap logs if the overflow from thespillway is not occurred. The log broom works installedin the apron trap the logs that overflow from the spillway
or are running out from the large conduit parts.
(4) Flow in clockwise is formed along the left side bankin the sand pocket until 5 hours, and sediment also
deposits along the main flow. Main flow gradually shifts
from left side to right side bank due to the deposition ofsediment, and flow in counter-clockwise is formed in
final stage of hydrograph. Those depend on the planar
layout of the channel works and transverse shape of thesand pocket. It induces the flow in clockwise or counterclockwise by collision of flow to the main dam, andsediment deposits with channel shifting.
Present experimental data helps facility design ofsand pocket for sediment control such as sedimentcapturing and sediment runoff.

Log broom works

MainDamFlow inclockwise

4hours(Peak discharge)

Log

Fig. 10. Flow pattern in sand pocket in 4hours (Peakdischarge in clear water: 140m3/s).
10hours
Main dam withlarge conduit parts
Log broom works(Interval of ironbars : 2.5m)

ApronCounter dam

Fig. 11. Flow pattern in apron.

5.3 Capturing ratio of sediment and logs
Table 3 shows the capturing ratio and runoff ratio ofsediment and logs. The capturing ratio of logs at apron
is 65.2% because water overflow the spillway and log
broom works trapped a large amount of the logs. If thewater level is lower than height of the spillway, logs aretrapped by the iron bars installed at upstream front of thelarge conduit parts or deposited in sand pocket. While,
capturing ratio of sediment increases to around 90% in
the sand pocket and apron. Figure 12 shows the temporal
Table 3. Capturing and runoff ratio of sediment and logs.
Sand pocket(%)

Apron(%)

Runoff(%)

Sediment

87.0

2.80

10.2Logs

20.6

65.2

14.2

We gratefully acknowledge Northern Kyushu Heavy Rainfall
Reconstruction Branch Office, Chikugo-gawa office of River,
Ministry of Land, Infrastructure, Transport and Tourism forsupporting this study.

References1.

Sediment discharge (m3/s)

6.0

Sediment
discharge rate

5.0
4.0
3.0

2.

Sediment
discharge atdownstream
end2.01.0
0.001234 5 6Time (h)7893.

10Fig. 12. Temporal changes of sediment discharge atdownstream end.

4T. Mizuyama, M. Irasawa, A. Fukumoto, M.
Kobayashi, Sediment Control Effect of a Sabo
Dam with Large Drainage Conduits, Journal of theJapan Society of Erosion Control Engineering, 42,
2, 29-34 (1990) (in Japanese).
K. Yazawa, Specific volume of Japanese woods,
Journal of Japanese Forestry Society, 28, 8, 457459 (1941) (in Japanese).
T. Mizuyama, Y. Ishikawa, M. Fukuzawa, A study
on mechanisms of movement and accumulation offloating logs and their countermeasures, Report ofPWRI, 183-3, 71-156 (1991) (in Japanese).