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Arao, S.

Oda, K.

Oda, S.

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Une nouvelle chambre d’écoulement des eaux pluviales pour contrôler l'interception du débit

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NOVATECH 2016 
1 
New rainwater discharge chamber which can control 
intercepting flow rate 
Une nouvelle chambre d’écoulement des eaux pluviales 
pour contrôler l'interception du débit 
 
Shinji Arao*, Shuhei Oda**, Kohei Oda** 
 
*
  Dept. of Civil and Environmental Eng., National Institute of Technology, 
Matsue College, 14-4 Nishi-ikuma, Matsue, 690-8518, Japan 
(arao@matsue-ct.jp) 
**
  Waken Design Office Inc., 34-11 Shinjuku 1-chome, Shinjuku-ku Tokyo 
160-0022, Japan (s-oda@wsnet.co.jp), (kh-oda@wsnet.co.jp) 
 
RÉSUMÉ 
Récemment au Japon, des projets d'amélioration du système d'assainissement unitaire ont été mis en 
place pour atteindre les objectifs en matière de réduction de la pollution et des impuretés, ainsi qu’en 
matière de sécurité d'hygiène publique. Toutefois, il est difficile de contrôler l'interception du débit 
d'une installation de partage au moment d’une forte chute de pluie, même dans le contexte actuel des 
projets d'amélioration du réseau d'assainissement unitaire. Afin de résoudre ce problème, les auteurs 
ont développé une nouvelle chambre d’écoulement des eaux de pluie qui peut contrôler l'interception 
du débit à une valeur constante. L’utilité du mécanisme a été démontrée par des calculs numériques 
et des expériences sur un modèle (Shuhei ODA et al., 2014). Dans cette étude, l'utilité de ce nouveau 
mécanisme est démontrée par l'amélioration de la précision de la mesure du débit d'un modèle de 
partage qui est à une échelle 2,5 fois plus grande que celle utilisée dans les précédentes études 
(Shuhei ODA et al., 2014). Le résultat expérimental obtenu avec le nouveau modèle de partage 
indique qu'une erreur d’interception (un débit excessif d’égout intercepté/débit prévu d’égout 
intercepté) se situe dans la plage de 1 % à 2 %. 
 
ABSTRACT 
Recently in Japan, improvement projects of combined sewer system have been implemented to 
achieve the goals for reductions of pollution load and impurities and for security of public sanitation. 
However, it is difficult to control the intercepting flow of a diversion facility at the time of heavy rain 
even in the current combined sewer improvement projects. In order to solve this problem, the authors 
have developed a new rainwater discharge chamber which can control the intercepting flow rate at a 
constant value. Usefulness of the facility has been demonstrated by numerical calculations and model 
experiments (Shuhei ODA et al., 2014). In this study, utility of a new facility is demonstrated by 
improving flow measurement precision of a diversion model which is 2.5 times in scale as large as that 
used in previous study (Shuhei ODA et al., 2014). Experimental result with the new diversion model 
shows that an interception error (excessive flow rate of intercepted sewage/planned flow rate of 
intercepted sewage) is in the range of 1 % to 2 %. 
 
KEYWORDS 
Combined sewer, diversion chamber, interception accuracy, sewer management 
SESSION 
2 
1 INTRODUCTION 
Sewer sideweir with a throttling pipe is a simpe device to limit the discharge to treatment facilities 
(Giudice et al.,1999).In the conventional combined sewer system where intercepting and overflow of 
sewage are incorporated in a diversion chamber, the interception accuracy in an intercepting pipe is 
low during periods of heavy rainfall, and so it is difficult to achieve reliable sewage management 
around the world. In Japan, with the revisions of Enforcement Order of the Sewage Water Law in fiscal 
year 2003, the combined sewer emergency improvement projects were required as measures for 
reducing the pollution load on public water areas. The confluence improvement projects have been 
completed in fiscal year 2013 in small and medium cities, and the projects are carried on aiming for 
completion by fiscal year 2023 also in large cities. With the confluence improvement projects, in the 
diversion facility (JIWET, 2003, see Figure1) normally installed in the rainwater discharge chamber, 
the height of the overflow weir is made larger to increase the intercepting flow rate to the sewage 
treatment plant to be twice that of the conventional one to thereby reduce the pollution load on public 
water areas so as to achieve the goals for reducing by half the number of times of releasing the 
untreated released water from the overflow weir. Such an improved diversion facility still has a 
problem of increase in the ratio of the excessive interception ((intercepting flow rate - planned 
intercepting flow rate)/planned intercepting flow rate) with an increase in rainfall intensity, failing to 
control the flow rate of intercepted sewage to the sewage treatment plant, and is therefore still in a 
situation of having to release a part of the sewage untreated to the public water areas. To overcome 
the issue, it is necessary to introduce the diversion facility that can surely control the flow rate. 
The authors have developed, in previous studies (Shuhei ODA et al., 2014), a diversion device (in which 
three side overflow weirs and three orifices are 
combined, called a three regulating tanks model 
hereinafter) which can substantially fix the intercepting 
flow rate even if the inflow flow rate into the rainwater 
discharge chamber increases, and have verified its 
utility by numerical calculations and hydraulic model 
experiments. In this treatise, a model which is 2.5 times 
in scale as large as the experimental open channel 
used in the previous study (Shuhei ODA et al., 2014) is 
created and the measurement precision of the flow rate 
is improved, to reconfirm the utility of the three 
regulating tanks model. Further, intercepting 
characteristics of a two regulating tanks model (in 
which two side overflow weirs and two orifices are 
combined) are revealed to show that the two regulating 
tanks model is excellent as compared with the one 
regulating tank model in the conventional technology 
(see Figure1).                                                                 Figure1 Standard rainwater discharge chamber 
 
2 EXPERIMENTAL DEVICE 
Figure2 and Photograph1 show the outline of the experimental device (the model in a scale of 1/13 of 
the assumed original size). As a conventional type (one regulating tank model) in which a open 
channel  (open channel gradient is horizontal) made of an acrylic resin having a length of 2.625 m and 
a width of 0.500 m is combined with the side overflow weir for diversion and the orifice for interception, 
a side overflow weir having a weir length of 1.000 m and a weir height of 0.150 m was installed and 
one orifice (three kinds of diameter Φ = 0.0269 m, 0.0197 m and 0.0150 m) was installed slightly 
downstream of the overflow weir. Note that the vertical distance from the weir top portion to the orifice 
center was set to 0.0505 m. Further, as shown in Figure2, in a three regulating tanks model of a new 
diversion device, the total length 1.000 m of the overflow weir was trisected so that each weir length 
was 0.333 m, and the orifices having almost the same diameter as that of the orifice used in the one 
regulating tank model were installed at three points slightly downstream of the trisected overflow weirs, 
respectively. Note that the vertical distance from the open channel bottom surface to each orifice 
center was set to 0.0995 m. Further, in a two regulating tanks model, the total length 1.000 m of the 
NOVATECH 2016 
3 
overflow weir was divided so that the upstream weir length was 0.667 m and the downstream weir 
length was 0.333 m. Table1 lists the dimensions relating to the side overflow weir and the orifice used 
in the one regulating tank model, the two regulating tanks model, and the three regulating tanks 
model. 
 
 
 
 
 
 
 
 Figure2 Outline of experimental device        Photograph1 Experimental device with three regulating   
                          (three regulating tanks model)                       tanks (viewed from downstream) 
 
Table1 Dimensional data regarding side overflow weir and orifice 
 
 
 
 
3 EXPERIMENTAL RESULT 
Figure3 shows the experimental results in the one regulating tank model (conventional technology), 
the two regulating tanks model (new technology), and the three regulating tanks model (new 
technology). The vertical axis in Figure3 represents the magnification of the inflow flow rate (Qi) into 
the rainwater discharge chamber with respect to the planned intercepting flow rate (Q0) and is given in 
Equation (1). The horizontal axis represents the excessive ratio (%) of the intercepting flow rate (Q2) 
with respect to the planned intercepting flow rate (Q0) and is given in Equation (2). 
Sewage flow rate ratio (inflow magnification) (1) 
Interception error (%)                          (2) 
As illustrated in Figure3, in the one regulating tank model (conventional technology) in which the 
orifice diameter is varied into three kinds, the interception error increases with an increase in sewage 
flow rate ratio in any of the cases. At a sewage flow rate ratio of 30, the interception error drastically 
decreases from 29% to 1% in Type D (an orifice diameter of 0.0269 m) of the three regulating tanks 
model as compared with Type A of the one regulating tank model. Further, at a sewage flow rate ratio 
of 80, the interception error decreases from 25% to 2% in Type F (an orifice diameter of 0.0150 m) of 
the three regulating tanks model as compared with Type C of the one regulating tank model. From 
these facts, it has been reconfirmed that the three regulating tanks model has extremely excellent 
functions capable of controlling the intercepting flow rate as substantially achieving the target. On the 
other hand, regarding Type G of the two regulating tanks model in which the orifice diameter is set to 
Elevational view 
Plane view 
Orifice 
Third tank Second tank 
First tank 
Type of model Weir length Orifice diameter φ Weir height
Type A 0.0269m 0.15m
Type B 0.0197m 0.15m
Type C 0.0150m 0.15m
Type D From the downstream 0.0269m, 0.0268m, 0.0268m From the downstream 0.15m, 0.196m, 0.252m
Type E                                    0.0197m, 0.0198m, 0.0198m                                   0.15m, 0.202m, 0.258m
Type F                                    0.0150m, 0.0150m, 0.0150m                                   0.15m, 0.207m, 0.265m
Type G From the downstream 0.0269m, 0.0268m From the downstream 0.15m, 0.207m
Type H                                           0.0197m, 0.0198m                                         0.15m, 0.207m
Type I                                           0.0150m, 0.0150m                                         0.15m, 0.207m
One regulating
tank model
1m
Three regulating
tanks model
0.333m×3
Two regulating
tanks model
Upstream 0.667m,
Downstream 0.333m
SESSION 
4 
0.0269 m, the interception error is about 6% at a sewage flow rate ratio of 30, which is larger than that 
of the three regulating tanks model. However, that is quite smaller than the interception error of 29% of 
the one regulating tank model.   
 
 
 
 
 
 
 
 
Figure3 Relationship between sewage flow rate ratio and interception error 
Figure4 illustrates the relationship between the flow rate of inflow water and the overflow water depth 
at the upstream end of each weir in the three 
regulating tanks model.  When the flow rate of 
inflow water was 0.00934 m
3
/s, the overflow water 
depth was 0.060 m at the weir of the third tank, 
0.0045 m at the second tank, and 0.0012 m at the 
first tank. As described above, the water depth only 
slightly increases at the first tank, so that the water 
level in the regulating tank at the most downstream 
can be fixed to almost a constant value. Further, as 
illustrated in Figure4, the increase in water level in 
the second tank is considerably suppressed.                Figure4 Relationship between flow rate of inflow 
                                                                                         water and overflow water depth 
4 CONCLUSIONS 
In the three regulating tanks model (new technology) in which three sets of the side overflow weirs and 
the orifices are installed, the interception error (the excessive ratio of the intercepting flow rate with 
respect to the planned intercepting flow rate) falls within a range of 0.5% to 2%. In the two regulating 
tanks model in which two sets of the side overflow weirs and the orifices are installed, the interception 
error falls within a range of 3% to 8%, though the interception error is larger than the interception error 
of the three regulating tanks model.  
LIST OF REFERENCES 
Giudice, G. D. and Hager, W. H. : Sewer Sideweir with Throttling Pipe, Journal of Irrigation and Drainage 
Engineering, Vol.125, No.5, pp.298-306, September, 1999. 
Japan Institute of Wastewater Engineering and Technology: Combined Sewer Improvement Measure Filtering 
Screen Technical Manual, pp.7-13, 2003 (in Japanese).  
Shuhei Oda, Kohei Oda and Shinji Arao: Technical Theory and Verification of Flowing Water Diversion in New 
Rainwater Discharge Chamber, Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 
Vol.70, No. 2, pp. 49-59, 2014 (in Japanese). 
Shuhei Oda, Kohei Oda and Shinji Arao: Study on Optimization of Combined Sewer System with New Diversion 
Chamber, 13
th
 ICUD, Kuching, Sarawak, Malaysia, 2014. 
 
1.00
10.00
100.00
0.10 1.00 10.00 100.00
Q
i/
Q
0
(Q2-Q0)/Q0×100(%)
29％
1％
30.0
12％
20％
4.3％ 6％
80.0
25％5％2％ Two regulating
tanks model
One regulating
tank model
Orifice diameter = 0.0150m
Orifice diameter = 0.0197m
Orifice diameter = 0.0269m
Three regulating
tanks model
Measured value Type of model
Type A 
Type B 
Type C 
Type D 
Type E
Type F 
Type G 
Type H 
Type I 
One regulating
tank model
Three regulating
tanks model
Two regulating
tanks model
0
1
2
3
4
5
6
7
0 1 2 3 4 5 6 7 8 9 10
O
v
er
fl
o
w
  
w
a
te
r 
d
ep
th
 i
n
 t
h
e 
u
p
st
re
a
m
 e
n
d
 o
f 
ea
ch
 w
ei
r(
m
)
Inflow flow rate(m3/s)
Type DThird tank
Second tank
First tank
×10-2
×10-3


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Une nouvelle chambre d’écoulement des eaux pluviales pour contrôler l'interception du débit

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