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SESSION 6.2 
NOVATECH 2007  1207 
Hydrodynamic behaviour of a stilling pond  
Comportement hydrodynamique d’un bassin de décantation  
Del Giudice, G.*, Della Morte, R.** 
* Department of Hydraulic and Environmental Engineering of University of 
Study of Naples Federico II, Via Claudio n.21 - 80125 Naples, Italy. Ph. +39 
081-7683435 / Fax +39081-5938936 / e-mail: giuseppe.delgiudice@unina.it 
** Department of Technology of University of Study of Naples Parthenope, Via 
Medina 40 - 80133 Naples, Italy Ph. +39081-5474874 / Fax +39081-5474882 
/ e-mail: renata.dellamorte@uniparthenope.it 
RESUME 
Le présent exposé propose une explication des comportements hydrodynamiques 
d’une structure particulière de bassin de décantation au moyen de la théorie du jet 
turbulent. Une série d’expériences employant des nombres de Froude différents et 
des largeurs de bassins de décantation différentes a été effectuée afin d’observer et 
de quantifier la croissance du jet de paroi et la perte de vitesse en fonction de la 
distance par rapport à l’entrée des effluents. De plus, les résultats expérimentaux 
montrent que les profils de vitesse axiale pour différentes sections du jet étaient 
pratiquement semblables avec, toutefois, quelques différences par rapport au profil 
du jet plan classique. Dans le cadre des paramètres testés, la largeur du bassin de 
décantation s’avère n’avoir aucun effet sur les caractéristiques du jet. 
ABSTRACT 
The proposed paper contributes to an understanding of hydrodynamic behaviours of 
a particular stilling pond structure by means of turbulent wall jet theory. A set of 
experiments with different Froude numbers and different stilling pond widths was 
used to observe and quantify the growth of the wall jet and the decay of the velocity 
scale with the distance from the inlet. Moreover the experimental results showed that 
the axial velocity profiles at different sections in the wall jet were almost similar, with 
some difference from the profile of the classical plane wall jet. In the range of 
experimental test parameters, the stilling pond width was found to be irrelevant on jet 
characteristics. 
KEYWORDS  
CSO Structure Design, Efficiency, Turbulent jet, Stilling pond. 
SESSION 6.2 
1208 NOVATECH 2007  
INTRODUCTION 
Combined sewer overflow (CSO) structures are used for diverting overflow discharge 
in water bodies during rainfall. Contemporary they provide for a small variation 
respect to the design value of discharge flowing to the treatment facilities. 
In the last few years it has realized that the common CSO structures are unable to 
control both water discharge and pollution simultaneously. Moreover many sewer 
systems experience a ‘first flush’ effect where significant volumes of pollutants arrive 
at an overflow (and are spilled) in the early part of the storm. The worsening of urban 
surrounding has led to consider either new CSO structures or to use the common 
ones along with a flow equalization basin or a rainwater storage tank. Such devices, 
eventually together with real time control (RTC) systems, allow to reduce the pollution 
loads into the receiving water bodies by favouring the sedimentation processes. 
The performance of basin or tank devices is different depending on their design 
volume. In fact significant volumes have an effect on both storm reduction peak 
(quantity) and sedimentation processes (quality). Conversely smaller volumes have 
mainly a quality effect.  
The stilling pond overflow structure represents a typical example of the latter solution. 
In fact by expanding the incoming flow into a rectangular chamber settleable solids 
fall to the invert, while floatables rise to the surface where they are directed by a 
transverse scumboard to a tranquil region on the surface near the inlet. Here they are 
stored until the end of the storm when the water level reduction allows to collect also 
the floatable solids toward the throttling pipe.  
An experimental investigation of stilling pond overflow structure has been conducted 
in the laboratory of the Department of Hydraulic and Environmental Engineering of 
University of Study of Naples Federico II. Although the laboratory model refers to 
design suggestions of several researchers (Sharpe e Kirkbride, 1959; Balmforth, 
1982; Balmforth ed al., 1994), it shows some innovative aspects. 
The experimental tests presented in this paper were performed to study the 
hydrodynamic behaviours of the stilling pond structure by investigating the developing 
wall jet due to the flow inlet expansion in a limited extent chamber. The results will be 
addressed to the optimal design of the stilling pond. 
The first set of experiments in clear water, with different Froude numbers and different 
stilling pond widths, was used to observe and quantify the growth of the wall jet and 
the decay of the velocity scale with the distance from the inlet. The results were 
studied and compared with that of a classical turbulent wall jet. The time-averaged 
axial velocity profiles at different sections in the wall jet were found to be quite similar, 
with some differences from the plane wall jet profile. 
1 EXPERIMENTS 
Experiments were performed in a 2.00 m long and a 0.36-0.70m wide Plexiglas 
stilling pond with a transversal embanked section and an approach pipe with an 
internal diameter D=150 mm. The dry weather flow channel of the stilling pond 
structure is semicircular (D = 150mm) with a lateral embankment slopes of 5%.  
For overflowing the exceeding discharge a frontal weir device has been used. The 
overflow weir is maximum 60 cm wide and 47 cm high from the throttling pipe bottom. 
A scumboard was located at different longitudinal distances Ls from the inlet, 
respectively at 9 and 11D, with a depth of 1D from the bed (Figure 1). 
The discharges were measured by flowmeters located in the supply lines and velocity 
distributions were measured along vertical sections at different longitudinal distances 
from the inlet (section S1 to S5 in Fig. 1), in the centerplane of the flume and at 8 
cross-sections. An acoustic profiler, Signal Processing DOP2000, was used to 
acquire the vertical distribution of local velocities.  
SESSION 6.2 
NOVATECH 2007  1209 
s1 s2 s3 s4 s5
B = 36 - 50 - 70
Scumboard   positions
L = 200
Ls = 9 D
Ls = 11 D
Pipe   inlet
D
 =
 1
5
28
58
88
118
148
7,
50
7,
50
40
Longitudinal section Transverse section
1 
D
 
Fig. 1. Sketch of the investigated stilling pond structure (lengths in cm). 
 
A set of experiments in clear water, with different approach Froude numbers, 
Fo=Uom/(gD)0.5, and different stilling pond width, B, were conducted. Table 1 shows 
the primary details of the experiments. In Table 1, Uom is the mean velocity at the pipe 
inlet and Re the Reynolds number of the jet. 
 
Experiment B [cm] Re Fo Uom [m/s] 
E1 36 49,000-85,000 0.21-0.37 0.33-0.56 
E2 50 49,000-85,000 0.21-0.37 0.33-0.56 
E3 70 49,000-130,000 0.21-0.57 0.33-0.87 
Table 1. Primary Details of Experiments 
2 RESULTS 
In order to define the hydrodynamic behaviour of the jet, one must first predict the 
characteristics of this flow phenomenon in terms of wall jet growth and velocity scale 
decay with the distance from the inlet. Fig. 2 shows the velocity profiles in the forward 
as well as in the reverse flows at several sections x for experiment E3 with D/B=0.2 
and Ls/L=0.85; y is the distance above the bed, D is the diameter of the circular jet 
and U is the local mean velocity. The maximum reverse velocity at any station was 
found to occur near the water surface, except for section 1 (s1 in Fig.1 – x = 11 cm) 
(Ead and Rajaratnam, 2002, 2004). 
Velocity measurements in the forward flow show clearly the structure of the wall jet. 
To test for the similarity of velocity profiles, the maximum velocity Um at any station 
was chosen as the velocity scale, and the length scale b is equal to y where U=0.5Um. 
Figure 3 shows a consolidated plot of the data for the E2 experiments with D/B= 0.3 
and Ls/L=0.85 and 1. The velocity profiles in the forward flow are quite similar but 
somewhat different from the classical plane wall jet profile (Schwarz and Cosart 1961; 
Rajaratnam 1976). In fact in the present data, the distance y corresponding to Um/U=1 
is in the range 032÷0.40 times b, whereas for the classical plane wall jet the value is 
0.16b. Moreover the upper end profiles are lower than the plane turbulent wall profile 
due to the interaction between the forward and backward flows (Ead and  
Rajaratnam, 2002).  
 
 
SESSION 6.2 
1210 NOVATECH 2007  
0
10
20
30
40
50
-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
11
41
71
101
131
u (m/s)
x (cm)
y (cm) D/B=0.2
1
 
Fig. 2. Velocity profiles for D/B=0.2 and Ls/L=0.85 
 
0
0.5
1
1.5
2
2.5
3
3.5
0 0.2 0.4 0.6 0.8 1 1.2
41
71
101
131
y/b
U/Um
x (cm)Ls/L = 0.85 ÷ 1 B=50 cm
_____ PWJ
 
Fig. 3 Similarity profiles for the forward flow. PWJ plane wall jet profile (Rajaratnam, 1976) 
 
Figure 4 shows typical velocity profiles in the developed flow region, for all the 
measurements in section 5 (s5 in Fig. 1) and for different stilling pond widths. In 
Figure 4, for each stilling pond width, the velocity profile corresponds to the average 
profile valued in the whole range of Fo. Figure shows the negligible effect of the 
experimental stilling pond widths on the velocity distributions.  
 
SESSION 6.2 
NOVATECH 2007  1211 
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.0 0.2 0.4 0.6 0.8 1.0 1.2
U/Um
y/b
36
50
70
B (cm)
 
Fig. 4. Velocity distribution in the developed flow  
 
Having found that the velocity profiles in the forward flow are quite similar, it is 
necessary to study the variation of the velocity scale Um and the length scale b with 
the distance x. Figure 5 shows the decay of the maximum velocity with the 
longitudinal distance from the inlet. The maximum velocity in the central plane Um 
remains almost equal to the exit velocity Uo at the pipe inlet up to the end of the 
potential core beyond which it decrease with x. For a water depth ratio h/D=3 (h is the 
water depth in the chamber, constant for the whole experiments), the potential core 
length, for a round wall jet with Fo >1, is in the range 3.5÷4 times A0.5 (Rajaratman 
and Humphries, 1983); that is between 0.4 and 0.5 m from the pipe inlet. Fig. 5 shows 
the decay of the maximum velocity with the longitudinal distance from the pipe inlet 
for a with ratio D/B=0.21 (Experiment E3).  
The maximum velocity at the pipe inlet was varied from 0.4 to 1.05 m/s and Um was 
measured in a number of six sections or less, depending on scumboard longitudinal 
position Ls. The experimental potential core length is greater than 24 cm and less 
than 54 cm that is in the range of Rajaratman and Humphries tests (1983). 
Fig. 6 shows the decay of the maximum velocity Um, at any section, in terms of the 
maximum velocity of the jet at the inlet pipe, Uo, with the normalized distance from the 
inlet x/A0.5 for a with ratio D/B=0.30 (Experiment E2). A0.5 is the characteristic length 
scale for the jet (Fischer et al. 1979) where A is the cross-sectional area of the pipe 
inlet. The decay of the maximum velocity is almost linear with the distance and the 
slopes increase with the reduction of the scumboard length ratio Ls/L. The same trend 
was found for a with ratio (D/B) of 0.21 and 0.42 (tests E3 and E1) confirming the 
irrelevance, in the range of experimental test parameters, of stilling pond width on jet 
characteristics. A new length scale, x/A0.5.Ls/L, was used to take in account the effect 
of the length ratio Ls/L on maximum velocity decay (Fig. 7 for D/B=0.42). All 
experimental data are well interpreted by a linear equation (Fig. 7) with a correlation 
coefficient of 0.91. 
 
SESSION 6.2 
1212 NOVATECH 2007  
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Fo=0.23 Ls/L=1 Fo=0.23 Ls/L=0.68 Fo=0.23 Ls/L=0.85
Fo=0.33 Ls/L=1 Fo=0.33 Ls/L=0.68 Fo=0.33 Ls/L=0.85
Fo=0.42 Ls/L=1 Fo=0.42 Ls/L=0.68 Fo=0.42 Ls/L=0.85
Fo=0.70 Ls/L=1 Fo=0.70 Ls/L=0.68 Fo=0.70 Ls/L=0.85
U m (m/s)
x (m)
Ls/L=0.6 Ls/L=0.85D/B=0.21
 
Fig.5 - Maximum velocity decay (D/B=0.21) 
 
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10 12
Fo=0.23 Ls/L=1 Fo=0.23 Ls/L=0.68 Fo=0.23 Ls/L=0.85
Fo=0.33 Ls/L=1 Fo=0.33 Ls/L=0.68 Fo=0.33 Ls/L=0.85
Fo=0.42 Ls/L=1 Fo=0.42 Ls/L=0.68 Fo=0.42 Ls/L=0.85
U m /U o
x/A 0.5
D/B=0.30
 
Fig.6 – Dimensionless maximum velocity decay (D/B=0.30) 
 
SESSION 6.2 
NOVATECH 2007  1213 
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10 12 14
Fo=0.23 Ls/L=1 Fo=0.23 Ls/L=0.68 Fo=0.23 Ls/L=0.85
Fo=0.33 Ls/L=1 Fo=0.33 Ls/L=0.68 Fo=0.33 Ls/L=0.85
Fo=0.42 Ls/L=1 Fo=0.42 Ls/L=0.68 Fo=0.42 Ls/L=0.85
Serie3
D/B=0.42
x/A 0.5 L/L s
U m /U o
U m /U o = -3.1 · 10 -2 (x/A 0.5 L/L s )+1.1
ρ 2 =0.91
 
Fig.7 – Influence of length ratio Ls/L on dimensionless maximum velocity decay (D/B=0.30) 
 
The growth of the length scale b of the jet (or half-width) with distance is shown in Fig. 
8a e 8b respectively for Ls/L=0.85 and 0.68 along with the circular wall jets in stagnant 
flow law (Rajaratnam and Pani, 1974; Rajaratnam, 1976). The growth rate law is valid 
for all experimental data except for the sections near the scumboard. 
 
Figure 8 Variation of jet half-width with distance. 1) Circular wall jets in stagnant flow (Rajaratnam 
and Pani, 1974) 
 
3 CONCLUSION 
The experimental tests presented in this paper allowed to predict the hydrodynamic 
behaviour of the stilling pond as a particular turbulent wall jet phenomenon, due to the 
approach pipe in the chamber, when the ambient fluid has a limited extent. The 
experimental results showed that the axial velocity profiles at different sections in the 
wall jet were almost similar with some difference from the classical plane wall jet 
profile. In the experimental range parameters, the stilling pond width was irrelevant on 
jet characteristics. The decay of the maximum velocity Um, at any section, in terms of 
0.0
1.0
2.0
3.0
4.0
0 2 4 6 8 10 12
36
50
70
Ls/L=0.6b/D
x/D
B (cm)
1
b)
 
0.0
1.0
2.0
3.0
4.0
0 2 4 6 8 10 12
36
50
70
Ls/L=0.85b/D
x/D
B (cm)
1
a)
SESSION 6.2 
1214 NOVATECH 2007  
the velocity Uo at the inlet, with the longitudinal distance x, was described by a 
function of x/A0.5 and the scumboard length ratio Ls/L. 
The growth rate of the length scale b of circular wall jets in stagnant flow was found to 
be valid except for the sections near the scumboard. 
Further tests to analyze the effect of the confined ambient fluid on the wall jet volume 
and momentum flux are actually in progress. 
LIST OF REFERENCES  
Balmforth, D.J.: Urban drainage system, Proceedings of the first international seminar on urban 
drainage systems – Southampton, September, 1982. 
Balmforth, D.J.: Saul, A.J. e Clifforde, I.T.: Guide to the design of combined sewer overflow 
structures, Report No FR0488, FWR Foundation for Water Research – November, 1994. 
Della Morte, R., Del Giudice, G., Gargano, R. and Rasulo, G.: Sull’efficienza qualitativa degli 
scaricatori di piena dotati di bacino di calma. Atti del XXIX Convegno di Idraulica e 
Costruzioni Idrauliche, Trento, settembre 2004 (In Italian). 
Ead, S.A. and Rajaratnam, N.: Plane turbulent wall jets in shallow tailwater. Journal of Eng. 
Mechanics ASCE, Vol. 128, n.2, 2002. 
Ead, S.A. and Rajaratnam, N.: Plane turbulent wall jets on rough boundaries with limited 
tailwater. Journal of Eng. Mechanics ASCE, Vol. 130, n.10, 2004. 
Fischer, H., List, E.J., Koh, R., Imberger, J, Brooks, N.: Mixing in inland and costal waters. 
Academic Press, New York, 1979. 
Frederick, M.E. e Markland, E.: The performance of stilling ponds in handling solids, Symposium 
on storm sewage overflows, Institution of Civil Engineers, London 1967. 
Rajaratnam, N.: Turbulent jets. Developments in water science n.5, Elsevier N.Y., 1976. 
Rajaratnam, N. and Humphries, J.: Journal of Hydraulic Eng. ASCE, Vol. 109, n. 11, 1983. 
Rajaratnam, N. and Pani, B.S.: Three dimensional turbulent wall jets. Journal of Hydraulic 
Division ASCE, Vol. 100, n.1, 1974. 
Sharpe, D.E. and Kirkbride, T.W.: Storm water overflows: The operation and design of a stilling 
pond, Proc ICE, Vol. 13, August, 1959. 
Wu, S. and Rajaratnam, N.: Effect of baffles on submerged flows. Journal of Hydraulic Eng. 
ASCE, Vol. 121, n.9, 1995. 


Comportement hydrodynamique d'un bassin de décantation

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