Kisiel A.

Mrowiec M.

I-Revues

SESSION 6.2 
NOVATECH 2007  1267 
Tubular storage tanks with internal throttle pipe 
for usage in densely urbanized areas  
Stockage souterrain des eaux pluviales dans des conduites 
circulaires avec un réducteur de débit, pour une utilisation en sites 
urbains denses  
Maciej Mrowiec, Adam Kisiel  
Institute of Environmental Engineering, Czestochowa University of 
Technology, Brzeznicka 60a, 42-200 Czestochowa, Poland 
email : mrowiecm@is.pcz.czest.pl, akisiel@is.pcz.czest.pl 
RESUME 
Cet article décrit la construction de bassins de rétention avec une tubulure interne 
d’étranglement conçue pour la gestion des eaux pluviales dans une zone urbaine 
dense. Ils peuvent être utilisés dans des systèmes d’égout séparatifs existants ou 
nouveaux (dans les systèmes unitaires avec certaines réserves), pour augmenter la 
capacité de stockage à un faible coût d’investissement. La recherche de vérification à 
l’échelle de laboratoire (pour un diamètre de conduite de stockage de 290mm) a 
confirmé la précision du modèle mathématique des bassins. Des simulations 
hydrodynamiques ont été effectuées en SWMM5 pour deux évènements de 
pluviométrie et un diamètre de conduite de stockage de 2m montre un rendement 
notablement plus élevé des constructions proposées par rapport à un bassin à 
chambre unique avec un orifice comme régulateur de flux.  
ABSTRACT 
The papers presents a construction of a detention tanks with internal throttle pipe 
designed for stormwater management in densely urbanized areas. They can be used 
both in existing and new separate sewer systems (in combined systems with some 
restrictions) to increase the storage capacity at low investment cost. Verification 
research at laboratory-scale (for storage pipe diameter 0,29m) have confirmed an 
accuracy of a mathematical model of the tanks. Hydrodynamic simulations have been 
made in SWMM5 for two rainfall events and storage pipe diameter equal to 2.0 
meters shows significantly higher efficiency of the proposed constructions in 
comparison to a single-chamber tank with orifice as flow regulator.  
KEYWORDS:  
Detention facilities, drainage systems, stormwater. 
SESSION 6.2 
1268 NOVATECH 2007  
1 INTRODUCTION 
Detention facilities are commonly used to manage the quantity of stormwater on 
existing urbanized as well as newly developed areas. They are intended to control 
peak flows, limit downstream flooding, and provide some channel protection (Degroot, 
1983). In areas where the drainage infrastructure already exists, retrofitting these 
systems to provide flow control can be prohibitively expensive (Saul and Ellis, 1990). 
In these cases, the in-line detention tanks can be alternatively applied, specially on 
densely built-up areas where the available space for on-site detention facilities is 
highly limited. The in-line tubular tanks are constructed in series with the sewer 
network and are controlled by a flow control device at their outlet . The detention 
reservoirs made of large diameter pipes (1,5÷3,0m) have been constructed tens 
years ago in many countries. Usually such constructions are realized as single-
chamber with the orifice as the flow regulator (Butler and Davies, 2000). The single-
chamber reservoirs have a number of disadvantages, the main are (Becker et. 
al,2001; Mrowiec and Kisiel, 2005b) : 
• maximum outflow rate is achieved only when the tank is full of stormwater,  
• main detention chamber is polluted frequently (in combined sewer systems -  
continouosly) causing a sediment disposal problems. 
The detention facilities with side weir are characterized by a better hydraulic 
performance because the detention chamber is filling when the outflow is equal near 
to maximum release flow rate.  
A new conception of tubular detention tanks is based on two main assumptions: 
• tank operates as in-line device (recommended for densely built-up areas where 
the available space on-site is limited), 
• the hydraulic performance is equivalent to the reservoirs with a side weir. 
Additional assumptions made for the constructions: low investment and maintenance 
costs, no external power required, minimize the number of mechanical parts and 
reliability issues have been taken into consideration. 
2 PROPOSED CONSTRUCTIONS OF THE TUBULAR STORAGE 
TANKS WITH INTERNAL THROTTLE PIPE 
2.1 The constructions overview 
Based on abovementioned assumptions the improved constructions of tubular 
storage tanks have been proposed: 
• reservoir with throttle pipe and float valve as the flow regulator (RPF) see fig.1a. 
• reservoir with throttle pipe and orifice as the flow regulator (RPO), 
• inversely sloped reservoir with throttle pipe and flap valve (RTI) see fig. 1b. 
All the construction have three chambers: an inlet, an outlet and storage but they are 
different in details. The construction of the RPF and RPO tanks is quite similar - an 
inlet and outlet chamber are connected by the throttle pipe, , lying on the bottom of 
the main chamber. The partition wall, separating the inlet and main detention 
chamber is also a weir of given crest elevation. The main difference between the 
RPO and RPF tank is the outflow regulation from the main storage chamber. RPF 
tank has the float valve regulator which holds stormwater during inflow phase while 
for RPO tank the flow regulation is performed by the properly designed orifice 
(optionally the Vortex regulator may be installed).  The difference causes less 
hydraulic efficiency for RPO tank, but it’s more reliable solution due to lack of 
mechanical parts. 
SESSION 6.2 
NOVATECH 2007  1269 
The RTI construction is specific because the main storage pipe is inversely sloped as 
well as the throttle pipe inside. It allows to limit the cover depth of the outlet channel 
but significantly changes the function of the inlet chamber - it can be arranged as grit 
chamber or as an infiltration one. The infiltration purpose is suitable only when the 
groundwater conditions are favourable (filtration coefficient >10-5 m/s)  and the 
stormwater contamination will not produce the accelerate colmatation. Otherwise the 
grit chamber is only option but then the RTI tank has a permanent pool of stormwater. 
This volume is usually less than 10m3 - insignificant in comparison to total available 
volume. 
Inlet channel
Weir
Throttle pipe
Storage pipe
Inlet ch
Outlet chamber
Outlet channel
Float valve
regulator Orifice
 
Inlet channel
Weir
Throttle pipe
Storage pipe
Inlet chamber
Outlet chamber
Outlet channel
Flap valve
Infiltration bottom 
(optional)
 
Fig. 1. Izometric view of the detention tanks: a) RPF, b) RTI. The RPO tank has the same 
construction as RPF except the float valve.  
 
Both RPO and RPF tank can be applied to combined and separate sewer systems,  
while the RTI tank can be applied to separate ones only. 
SESSION 6.2 
1270 NOVATECH 2007  
2.2 Theoretical model of the proposed tanks 
Three main phases can be distinguished during typical wet weather conditions : 
• flow-through phase  (inflow QM is smaller than outflow Q0),   
• storage phase (inflow QM is greater than outflow Q0 and the difference (QR) is 
discharged to detention chamber),  
• emptying phase (inflow QM drops below Q0 and main chamber is released). 
 
KOh
L =L
α
0 T
hR
d TT , λ
WLF
i LT
H
P
RD
h
Hos
, μBP P
hKD
0ΔH H
 
Fig 2. The scheme of RTI tank for mathematical model. 
The flow-through phase is similar for all the tanks - during low flows, stormwater 
partially filling the inlet chamber and flows via the throttle pipe to outlet chamber. Due 
to its small volume, the inlet chamber is filled in relatively short time, while the main 
detention chamber remains empty.  
During intensive rainfall the inflow-rate increase cause the water overflow through the 
weir to the main storage chamber. During detention phase outflow through the orifice 
is restricted due to action of the float valve in the RPF tank. Water at outlet chamber 
lifts the float and simultaneously covers the orifice. In the RTI tank outflow from 
storage chamber is also limited by the flap valve located at inlet chamber.  
A mass conservation law is based on the model equations that describe the process 
of sewage storage in a reservoir (Gribbin, 2001; Durrans 2002). For the inlet chamber 
during storage phase (QM>Q0) the differential equation has the following form: 
0
0 M T R
(h )
dh Q dt Q dt Q dt
dt F
− −
=  
where:  QM  – inflow rate [m3/s],  
QT  – outflow rate through the throttle pipe [m3/s], 
QR  – outflow rate to the storage chamber [m3/s], 
h0  – depth at the inlet chamber [m], 
F(h0)  – area of free surface of water at the inlet chamber [m2]. 
simultaneously for the storage chamber: 
R
R R
(h )
dh Q
dt F
=  
where: hR – depth at the storage chamber [m], 
F(hR)  – area of free surface of water at the storage chamber [m2]. 
 
SESSION 6.2 
NOVATECH 2007  1271 
To calculate the overflow discharge QR, the standard relationship between the flow-
rate and depth for the rectangular weirs can be used with assumption that weir is: 
sharp crested, non submerged and no contracted. Transitional flow-rate QT can be 
described on the basis of Bernoulli equation, comparing the total energy at the inlet 
and the outlet chambers.  
For the RPO tank the storage phase have a different course -  the outflow rate Q0 is a 
sum of flow through throttle pipe QT and orifice QZ, so the balance equation for 
storage chamber have a form (Mrowiec and Kisiel, 2005a): 
R
R R Z
(h )
dh Q Q
dt F
−
=  
where: QZ  – flow-rate through the orifice. 
Therefore the total outflow rate from the tank is a sum of QZ and QT and relationship 
between them also determine the required storage capacity of the reservoir.  
When inflow-rate is less than  outflow-rate (QM<Q0), the inlet chamber is evacuating 
quickly (RPF and RPO tanks) due to its small volume. In the RPF tank the float valve 
gradually opens the orifice allowing to release retained stormwater while in the RPO 
tank the orifice have no mechanical regulator. The flow-balance equation for storage 
chamber: 
R
R Z
(h )
dh Q
dt F
−
=  
To calculate discharge through the orifice a standard equation for such devices can 
be applied. In the RTI tank the stored stormwater is released through the flap gate at 
inlet chamber controlled by the hydrostatic forces (as difference between depths at 
inlet and storage chamber).  Then, via the throttle pipe, flows to the outlet channel. 
 
  
Fig. 3. Model of the RPO tank during verification research: a) inlet chamber b) outlet chamber - 
the main detention chamber with throttle pipe is visible. 
 
The formulated mathematical models have been verified in laboratory conditions by 
means of physical models. Main dimensions of the models: length L=2,10 m, storage 
diameter DR=0,29 m (total volume of 0.14 m3), throttle pipe diameter dT=0,03m The 
research was conducted for the inflow-rate ranged from 0,2 to 2,0 dm3/s taking into 
consideration different values of flow reduction factor β (defined as the ratio of 
maximum outflow to maximum inflow-rate). The flows relative error ranged from 1% to 
12% but the highest values occur during release phase, when the depths were 
smaller than throttle pipe diameter (dT). Generally the models allow to estimate main 
parameters with fairly good precision, enough to engineering purposes. 
SESSION 6.2 
1272 NOVATECH 2007  
3 HYDRODYNAMIC MODEL OF THE PROPOSED TANKS 
In order to evaluate the hydraulic efficiency of the proposed constructions in the 
particular drainage systems, the single event simulation have been done using 
SWMM5 application. Densely built-up watershed of the Czestochowa city, routing 
stormwater to Warta river, was chosen to compare the hydraulic properties of the in-
line reservoirs made of large diameter pipes. The simulation have been done for two 
rainfall events (both hydrographs have been simplified to linear functions):  
• 1-hour rainfall, treated as critical rainfall, 
• 4-hour rainfall with two peak flows. 
 
 
 
Fig. 4. Scheme of proposed detention tanks used in SWMM5: a) single chamber  b) RPO (RPF 
has additional control rules for the orifice), c) RTI.   
 
Figure 4 presents schemes of the tubular tanks to run a hydrodynamic simulation  in 
SWMM (dynamic wave routing: 1sec). Primary model assumptions were:  
• 1-hour rainfall is a critical for the single chamber tank (the tank is completely 
filled), 
• maximum outflow rate from the tanks: 0,145 m3/s 
• storage chamber diameter: 2,0m; length: 150m; slope: 0,5% (except the RTI tank 
which is inversely sloped and the outlet channel elevation was 0,55m higher), 
• the throttle and orifice diameters were different for each reservoir to meet the 
outflow requirements, 
• for the RTO tank the flow-ratio QT/Q0=0,67. 
 
SESSION 6.2 
NOVATECH 2007  1273 
00:45 01:35 02:25 03:15 04:05 04:55 05:45
0
20
40
60
80
100
120
140
160
Fl
ow
 [d
m
3 /s
]
Time, [h:min] 
 Singlechamber
 RTI
 RTO
 RPF
 
Fig. 5. Outflow hydrographs from the different types of the tanks (hydrograph of the 1-hour rainfall 
above the curves). 
 
00:50 01:45 02:40 03:35 04:30 05:25
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
D
ep
th
, [
m
]
Time elapsed [hh:mm]
 SingleChamber
 RTO
 RPF
 RTI
 
Fig. 6. Depth at storage chamber for particular type of tank (hydrograph of the 4-hours rainfall 
above the curves). 
SESSION 6.2 
1274 NOVATECH 2007  
Simulation results for critical (design) storm confirmed the results obtained for 
hydrographs tested at laboratory-scale. The single-chamber tank geometry was 
shaped to be completely filled. The efficiency of RPF and RPO tanks was significantly 
higher than for single chamber ones – the volume required can be reduced by as 
much as 34% (RTI tank) and 37% (RPF tank). For the RPO tank the possible volume 
reduction is about 18% because it is some kind of compilation between the single-
chamber and the RPF tank.  
The second rainfall event characterized by significantly lower peak flows but 4-times 
longer duration cause the following maximum filling-rate for each tank: single-
chamber: 63%, RPO: 31%, RPF and RTI: 10% (excepting the permanent pool in the 
RTI tank). Such results shows the potential efficiency of proposed constructions 
during the long-term simulations should be greater than for single critical storm. For 
many low-intensity rainfall events the multi-chamber constructions allow to avoid the 
stormwater discharge to storage chamber. Therefore a problem of sewage solids 
deposition can be reduced to minimum as well as the maintenance costs. 
4 CONCLUSIONS 
Proposed constructions of the in-line detention tank with throttle pipe located inside 
the storage chamber make possible to better utilization of storage capacity in 
comparison to single-chamber tanks for equivalent performance. Theoretical flow 
route model has been developed for each type of the tank and successfully verified at 
laboratory-scale. Hydrodynamic simulations in SWMM5 have been made for the 
innovative tanks on sample catchment show the potential volume reduction from 18 to 
even 37% in comparison to single-chamber tubular reservoir. All the presented 
constructions may be applied in existing and new sewer systems at relatively low 
capital costs and assure lower maintenance costs due to lower cleanse needs. 
LIST OF REFERENCES  
Becker M., Prinz R., Liebig T., Effectiveness of a stormwater tank with overflow with a 
downstream stormwater holding, KA-Wasserwirtschaft, Abwasser, Abfall 1/2001, pp.16-24, 
Brombach H. (2002) Sewerage System and Stormwater Tanks as Reflected in Statistics, KA - 
Wasserwirtschaft, Abwasser, Abfall 4/2002, 
Butler D. and Davies J.W. (2000). Urban drainage. E & FN Spoon, London. 
Degroot W. (1983) Stormwater Detention Facilities: Planning, Design, Operation and 
Maintenance; American Society of Civil Engineers,  
Durrans T. (2002) Stormwater Conveyance Modeling and Design, Haestad Press, 
Gribbin J. (2001) Introduction to Hydraulics & Hydrology: With Applications for Stormwater 
Management, Delmar Learning, 
Mrowiec M., Kisiel A.: Simulation model of an in-line storage tank with throttle pipe. The 10th 
Scientific Conference “State of Art Trends of Development and Challenges in Civil 
Engineering”, Koszyce, 11-13 September, 2005. 
Mrowiec M., Kisiel A.: An innovative construction of an on-line storage tank. 10th International 
Conference on Urban Drainage, Copenhagen, 21-26 August, 2005. 
Saul A.J. and Ellis D.R. (1990) Storage tank design in sewerage systems. Proceedings of the 5th 
International Conference on Urban Storm Drainage, Osaka, Japan, July, 713-718. 


Stockage souterrain des eaux pluviales dans des conduites circulaires avec un réducteur de débit, pour une utilisation en sites urbains denses

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Stockage souterrain des eaux pluviales dans des conduites circulaires avec un réducteur de débit, pour une utilisation en sites urbains denses

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