The quality of field measurements in sewer systems relies on the spatial representativeness of the measurements. This study aims at field measurements of both velocity and total suspended solids (TSS) using a two dimensional sampler called Hydre. The device was implemented, tested and used in a sewer system for a range of hydraulic situations. This paper presents the principles of development and conception of the instrument. Analyses of the results demonstrate the ability of the device to provide robust TSS and velocity profiles in sewers for a range of flow conditions. L’évaluation des flux de polluants transitant dans les réseaux d’assainissement dépend notamment de la représentativité spatiale des mesures de vitesses et de concentrations en matières en suspension. Cette problématique a été étudiée expérimentalement par des mesures dans des réseaux urbains. Un échantillonneur bidimensionnel, appelé Hydre, a été développé et installé dans un collecteur. Il a permis d’étudier la répartition spatiale des vitesses et des concentrations pour une large gamme de contextes hydrauliques. Cet article présente la conception de cet échantillonneur. Les résultats sont présentés et discutés. Ils montrent la bonne homogénéité des concentrations dans la section mouillée

Cottineau, Louis-Marie

Larrarte, Frédérique

University of Queensland eSpace

International Meeting on Measurements and Hydraulics of Sewers, 2008, F. Larrarte and H. Chanson (Eds), Hydraulic 
Model Report No. CH70/08, Div. of Civil Engineering, The University of Queensland, Brisbane, Australia - ISBN 
9781864999280 
21 
 
A NEW VELOCITY AND TOTAL SUSPENDED SOLIDS MEASUREMENT 
DEVICE 
 
Frédérique Larrarte and Louis-Marie Cottineau 
Laboratoire Central des Ponts et Chaussées, Route de Bouaye, BP4129 44341 Bouguenais Cedex, 
France - Email : frederique.larrarte@lcpc.fr 
 
Abstract: The quality of field measurements in sewer systems relies on the spatial 
representativeness of the measurements. This study aims at field measurements of both velocity and 
total suspended solids (TSS) using a two dimensional sampler called Hydre. The device was 
implemented, tested and used in a sewer system for a range of hydraulic situations. This paper 
presents the principles of development and conception of the instrument. Analyses of the results 
demonstrate the ability of the device to provide robust TSS and velocity profiles in sewers for a 
range of flow conditions. 
 
Résumé: L’évaluation des flux de polluants transitant dans les réseaux d’assainissement dépend 
notamment de la représentativité spatiale des mesures de vitesses et de concentrations en matières 
en suspension. Cette problématique a été étudiée expérimentalement par des mesures dans des 
réseaux urbains. Un échantillonneur bidimensionnel, appelé Hydre, a été développé et installé dans 
un collecteur. Il a permis d’étudier la répartition spatiale des vitesses et des concentrations pour une 
large gamme de contextes hydrauliques. Cet article présente la conception de cet échantillonneur. 
Les résultats sont présentés et discutés. Ils montrent la bonne homogénéité des concentrations dans 
la section mouillée. 
 
Keywords: velocity profiles, total suspended solids, instrumentation, sewer. 
 
1. INTRODUCTION 
Two phase particle-laden flows have a wide range of applications related to water although the 
quantification of the solid mass remains a important question. For example, Pont et al. (2002) and 
Leecaster et al. (2002) discussed the spatial and temporal sampling representativity and strategy. In 
Europe, the May 1991 and October 2000 EC Directives and the national regulation define the total 
suspended solids (TSS) as a major source of pollution. A good management of sewer networks 
requires the minimisation of pollutant loads discharged to receiving waters, and this requires robust 
and accurate sensors to assess flow rates and TSS concentrations. Larrarte (2006,2008) described 
two devices developed and used in a man-entry sewer. The instruments gave a first series of data 
sets in a combined sewer without sedimentation. The practical experiences showed a number of 
problems with both samplers including the deployment difficulties, the corrosion, and the lack of 
22 
data on top of the bank. This paper presents the principles of development and application of a third 
sampler. Results are presented and discussed. 
 
2. MATERIAL DEVELOPMENT 
The samplers Cerbère and Orphée showed their usefulness for investigating dry and wet weather 
velocity and total suspended solid within sewers (Larrarte 2006,2008) but both systems had some 
disadvantages. For example, it was not possible to make measurements on top of the bank, in an 
area with a compound cross section. Further the system Cerbère experienced some corrosion 
troubles and it was not possible to adapt it to another experimental area. Also both systems worked 
in an area with mean velocities larger than 0.5 m.s-1. Based on these experiences, a new 2D sampler 
device, called "Hydre" was designed and built to investigate in a minimum of time both velocities 
and total suspended solid concentrations for a wide range of hydraulic conditions (Fig. 1). 
 
 
Figure 1 - Two-dimensional sampler Hydre in the combined sewer, view from upstream 
23 
 
Figure 2 - Measurement head of the two-dimensional sampler Hydre 
 
The measuring head was equipped with two flow meters Nivus PVM-PD and a sampling pipe (Fig. 
2). Validation tests showed that the flowmeters Nivus PVM-PD operated within a rather small 
sampling volume (0.03 m long and 0.015 m diameter) positioned at 0.1 m in front of the transducer 
(see Appendix). In practice, a basic problem arose from drifting materials coming in contact with 
the transducers and clogging the sensors. Since the sensors were not visible, clogging proved 
difficult to detect and two pipes were devised to clean the velocity sensors with compressed air 
prior to each reading. The sampling pipe was also cleaned with compressed air before each 
sampling. At a sampling location, the data set included the mean value of two replicates, with each 
replicate being the mean value of the instantaneous velocity gauged over a 10-second period. This 
procedure complied with the NF EN ISO 748 Standard. Once velocities were simultaneously 
measured at both transducers, the vacuum sampling was triggered. 
The device Hydre was installed in a combined sewer located in a public park of Nantes (France). 
The site was selected to investigate the influence of the velocity field on TSS concentrations 
because an intermittent deposit was previously observed. The sewer had an egg-shaped section with 
a bank (Fig. 3). The mean velocity Vm ranged from 0.3 to 0.5 m.s-1 for water levels of respectively 
0.53 to 1.01 m. Dry weather conditions corresponded to water levels below 0.85 m with velocities 
below 0.60 m.s-1. 
Hydre was implemented before each measurement campaign (Fig. 4). The transverse movement is 
based on displacement along a horizontal rail. The position is controlled with an ultrasonic sensor 
(Fig. 5). The sensor movements were remotely controlled from above the ground level with a 
computer system (Fig. 6). In a sewer, the wetted area could be sampled from the bottom to a 
maximum elevation of 1.5 m which encompassed roughly 90% of the situations encountered during 
a given year. 
 
24 
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
2,4
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5
y (m)
z 
(m
)
bank
0.0   0.1  0.2  0.3   0.4  0.5  0.6  0.7  0.8   0.9   1.0  1.1  1.2   1.3  1.4  1.5
2.4
2.
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Z 
(m
)
z 
(m
)
Z 
(m
)
 
Figure 3 - Sewer cross section looking upstream 
 
 
Figure 4 - Installation of the two-dimensional sampler through a manhole 
 
25 
 
Figure 5 - Two-dimensional sampler Hydre on its rail and detail of the ultrasonic sensor used to 
control the transverse location 
 
Sampling pipe
Cover of the 
sampling bottle
Neck of the sampling bottle
(inside the careening)
Computer
Flowmeter head
 
Figure 6 - Parts of the sampling device located above the ground. 
 
3. RESULTS 
The flow is considered as fully turbulent and subcritical when the Reynolds number 
)4/(DVR hme ν=  is greater than 105, where Vm is the flow velocity, Dh the hydraulic diameter and 
26 
ν is the water kinematic viscosity with ν = 10-6 m2.s-1. 
 
3.1 Velocity fields 
The channel was narrow and shallow. The aspect ratio maxh/bAr =  was between 1.4 and 2.1, 
where b is the free surface width and hmax the maximum water level in the main channel during the 
measurements. In such narrow channels, the maximum velocity was clearly located below the free 
surface during dry weather conditions (Fig. 7). The results were consistent with the earlier velocity 
measurements of Larrarte (2006) in another egg-shaped sewer. On top of the bank, a local dip 
phenomenon could also be seen, that means that the maximum velocity is located below the free 
surface. Bonakdari et al. (2008) showed numerically that this velocity dip phenomenon was related 
to strong secondary currents of the second kind of Prandtl, and the flow was fully three 
dimensional. Figure 8 shows an example of velocity profiles measured during a storm event when a 
combined sewer overflow was active about 1 km upstream of the measurement area. 
 
0,00
0,10
0,20
0,30
0,40
0,50
0,60
0,70
0,80
0,90
1,00
1,10
0,00 0,10 0,20 0,30 0,40 0,50 0,60
Velocity(m/s)
di
st
an
ce
 fr
om
 th
e 
bo
tto
m
 (m
)
y = 0.30 m
y = 0.50 m
y = 0.69 m
y = 0.70 m
y = 0.89 m
0,0
0,3
0,6
0,9
1,2
1,5
1,8
2,1
2,4
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6
(m)
(m
)
banquette
B
h
0. . 0.2 0.3 0.4 0.5 0.6
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
di
st
an
ce
 fr
om
 th
e 
bo
tto
m
 (m
)
(m
)
 
Figure 7 - Field measurements of vertical distributions of velocity for dry weather conditions 
 
27 
0,00
0,10
0,20
0,30
0,40
0,50
0,60
0,70
0,80
0,90
1,00
1,10
0,00 0,10 0,20 0,30 0,40 0,50 0,60
velocity (m/s)
di
st
an
ce
 fr
om
 th
e 
bo
tto
m
 (m
)
y = 0.30 m
y = 0.50 m
y = 0.70 m
y = 0.89 m
y = 1.10 m
y = 1.30 m
0,0
0,3
0,6
0,9
1,2
1,5
1,8
2,1
2,4
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6
bank
B
h
0.0 . 0. 0.3 0.4 0.5 0.6 
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.
di
st
an
ce
 fr
om
 th
e 
bo
tto
m
 (m
)
 
Figure 8 - Field measurements of vertical distributions of velocity during storm event conditions 
 
3.2 TSS concentrations 
The water samples were sieved at 2 mm and analyzed to get the TSS and volatile matter 
concentrations (in g.m-3) with respect to the French and European standard NF EN 872. In most 
cases, the samples were analyzed twice to evaluate the uncertainty associated to the analysis 
process. The precision of TSS and volatile matters measurements was about 10 g.m-3. The amount 
of particles greater than 2 mm was less than 5% in mass. The flowmeters measured also the 
temperature, which varied from 11 to 22°C. About 12 dry weather and 4 rainfall campaigns were 
conducted between March 2006 and November 2007. They gave respectively 500 dry weather and 
200 rainfall event concentrations. 
Figure 9 shows that the TSS concentrations were between 100 and 350 g.m-3. For dry weather days, 
the result was close to the earlier observations of Larrarte (2008), 5 km downstream. During rainfall 
events, including storm events, the concentrations remained also lower than 350 g.m-3. No influence 
of storms was noticed, and this differed from the observations made downstream by Larrarte 
(2008). 
Figure 10 shows the ratio of volatile to total suspended solids concentrations. A high ratio of 
volatile matters to TSS indicates a large component of organic matter. More than 70% of particles 
of that urban area showed a large organic component and the meteorological context had no 
influence on the results. 
 
28 
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 50 100 150 200 250 300 350 400
cumulative suspended solids concentrations (mg/l)
dry weather
rain event
 
Figure 9 - Cumulative total suspended solid concentrations. 
 
0%
10%
20%
30%
40%
50%
60%
30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 85% 90% 95% 100%
% of organic matters in the TSS concentrations (in mass)
 dry weather
 rain events
 
Figure 10 - Percentage of organic matter in the TSS concentrations (in mass). 
 
Figure 11 shows the ratio of particles smaller than 125 microns to total suspended solids. The data 
ranged between 55% and 85% for most samples. The ratio tended to be larger during rainfall 
conditions than dry weather days. But the rainfall conditions corresponded to only 3 rain events and 
the result needs to be confirmed. 
 
29 
0%
10%
20%
30%
40%
30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 85% 90% 95% 100%
% of fine particles in the TSS concentrations (in mass)
 dry weather
 rain events
 
 
Figure 11 - Percentage of particles smaller than 125 microns in the TSS concentrations (in mass). 
 
The spatial distribution of pollutants was investigated. Figure 12 shows the TSS concentrations 
measured on various verticals within the cross section. The data corresponded to a dry weather day 
with a water level of 0.57 m and a 0.02 m thick sediment deposit below the sampler. The mean 
velocity in the cross section was equal to 0.37 m.s-1 and this was below the commonly accepted 
self-cleansing velocity. There were vertical concentration fluctuations but no concentration gradient 
was noticed. The results presented were consistent with those of Ahyerre at al. (2001) for the 
suspended solid concentrations. The concentrations obtained by Ahyerre et al. (2001) in the 
sediment were higher than those obtained by Worhle and Brombach (1991) close to the bottom. 
 
4. CONCLUSION 
A two-dimensional sampler device was developed to improve the reliability of measurements in 
sewer systems. The sampler allowed the measurements of both velocity and total suspended solids 
within the cross section of a sewer for a wide range of hydraulic conditions. 
The analysis of the results recorded by the sampler demonstrated the ability of the device to 
characterise both the velocity and TSS fields. The measurements showed that a dip phenomenon 
occurred in the narrow channel with a second maximum over the bank, there was no concentration 
gradient even though a deposit existed at the time of the sampling, and the particles were mainly 
organic and fine whatever the meteorological conditions. 
Field experiments must be extended to include other rainy days and storm event measurements and 
to investigate the relation between sediment deposit and hydraulic conditions. 
 
30 
0,00
0,10
0,20
0,30
0,40
0,50
0,60
0,70
0,80
0,90
1,00
1,10
0 50 100 150 200 250 300 350
concentration (mg/l)
di
st
an
ce
 fr
om
 th
e 
bo
tto
m
(m
)
TSS < 2 mm y = 0.40 m
TSS < 2 mm y = 0.60 m
TSS < 2 mm y = 0.79 m
0,0
0,3
0,6
0,9
1,2
1,5
1,8
2,1
2,4
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6
bank
B
h
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
di
st
an
ce
 fr
om
 th
e 
bo
tto
m
(m
)
 
Figure 12 - Vertical distributions of the TSS concentrations for a dry weather day 
 
5. ACKNOWLEDGEMENTS 
This work was supported by the Direction of Research and Scientific Affairs of the Minister of 
Equipment. The author would like to thank the technical staff of both the Laboratoire Central des 
Ponts et Chaussées - Division Eau & Environnement and Division Métrologie et Instrumentation, 
and the Direction de l’Assainissement de la Communauté Urbaine de Nantes for their valuable 
contributions to these experiments. 
 
6. NOMENCLATURE 
mxh
bAr =  aspect ratio 
b width of the free surface (m) 
C concentration (g.m-3) 
Dh hydraulic diameter (m) 
hmax maximum water level (m) 
Re Reynolds number 
Sm wetted area of the vertical cross-section (m2) 
Vm mean velocity (m.s-1) 
y transverse distance (Fig. 3) (m) 
ymax maximum width of sewer channel (Fig. 3) (m) 
ν water kinematic viscosity (m2.s-1) 
 
7. REFERENCES 
AHYERRE, M., CHEBBO, G., and SAAD, M. (2001). "Nature and Dynamics of Water Sediment 
31 
Interface in Combined Sewers." Journal of Environmental Engineering, ASCE, Vol. 127 No. 3, 
pp. 233-239. 
BONAKDARI, H., LARRARTE, F., JOANNIS C., and LEVACHER, D., (2008). "Champ de 
Vitesses et Contraintes de Cisaillement dans un Collecteur d'Assainissement." La Houille 
Blanche, No. 3, pp. 20-25. 
LARRARTE, F. (2006). "Velocity Fields in Sewers: an Experimental Study." Flow Measurement 
and Instrumentation, Vol. 17, 282-290. 
LARRARTE, F. (2008). "Suspended Solids within Sewers: an Experimental Study." Environmental 
Fluid Mechanics, Vol. 8, No. 3, pp. 249-261. 
LEECASTER, M.K., SCHIFF, K., and TIENFENTHALER, L.L. (2002). "Assessment of Efficient 
Sampling Designs for Urban Stormwater Monitoring." Water Research, Vol. 36, pp. 1556-1564. 
PONT, D., SIMONNET, J.P., and WATER, A. V. (2002). "Medium-Term Changes in Suspended 
Sediment Delivery to the Ocean: Consequences of Catchment Heterogeneity and River 
Management (Rhône River, France)." Estuarine, Coastal and Shelf Science, Vol. 54, pp. 1-18. 
WOHRLE, C., and BROMBACH, H. (1991). "Probenahme im Abwasserkanal." Wasserwirtschaft, 
Vol. 81, pp. 60-65. 
 
8. APPENDIX 
It can be noticed on Figure 2 that the two-dimensional sampler Hydre is intrusive. The velocity 
field around the measurements head was computed with a three-dimensional Navier-Stokes solver. 
Figure A-1 shows the influence of the measurement head on the velocity field at 0.05 m upstream. 
This influence becomes negligible at 0.1 m upstream of the head that was the location of the 
ultrasonic flowmeter sampling volume. 
 
 
32 
(A) 0.05 m upstream of the head 
 
(B) 0.10 m upstream of the head 
Figure A-1 - Flow field around Hydre sampler head 
 


A New Velocity and Total Suspended Solid Measurement Device

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A New Velocity and Total Suspended Solid Measurement Device

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