Symposium on Measuring Techniques in Hydraulic Research I Delft I 22-24 Apri/1985

Hydrometry in sewer systems

1. Design and operation of a measuring network for urban hydrology on a small catchment in Brussels J.J.Petersl, A.Van Der Bekenl & A.Vanslambrouckl

2. Application of real time techniques in the Brussels sewer system J.M.Hiver2 & J.J.Peters2

3. Reliability of hydraulic field systems in sewers: Intercomparison of instruments A.BrulP, J.J.Petersl,2, J.M.Vandenbergh2 & A.Vanslambrouckl

Abstract : Several research projects in urban hydrology were recently started in Belgium. This paper contains three parts. The first part ·deals with a hydrologic telemetry network installed on the Campus of the Free University of Brussels (VUB). It compri ses a meteorological station, a set of hydraulic sensors installed in the rainwater drainage system, including a stormwater reservoir, and water samplers. Equipment is actually under completion. The second part of the paper deals with a telemetry system for quantity and quality measurements, placed in the sewersystem of the Northern part of the Brussels Region. Due to the complexity of this net­work, 19 hydraulic gauging stations were required in a first stage, containing water level or waterflow gauges. In addition, quality measurements are conducted by the Free University of Brussels (ULB). The whole system is linked with a cen­tral computer by means of four teletransmission substations. The third part of the paper treats with the results of an intercomparison between hydrometric instruments for sewers, jointly runned by the VUB, the LRHCh and the CRR-OCW. A market analy­sis allowed the selection of instruments best suited for sewers. Tables summar­i z ing the results of this survey are presented. Pn intercalibration test was con­ducted during at least six months on seven instruments of various trade marks and different working principles.

Abstract : Plusieurs recherches sont entreprises en Belgique dans le domaine de l'hydrologie urbaine - L'article comporte trois parti es. La premiere presente un reseau de mesures hydrologiques teletransmises si tue sur le Campus de la Vrije Uni ­versi tei t Brussel (VUB). Il comporte une station meteorologique, un ensemble de capteurs hydrauliques installes dans le res eau de collecte des eaux de pluie, y compris le bassin d ' orage, et des echantillonneurs d'eaux. La phase d'installation s' acheve actuellement. La deuxieme partie presente un res eau de' mesures hydraull­ques et de qualite teletransmises, installe dans le systeme de collecteurs (reseau unitaire) de la partie Nord de la Region Bruxelloise. La complexite de ce systeme necessitait !'installation, en premiere phase, de 19 stations de mesures hydrauli­ques comportant soi t des capteurs de ni veau, soi t des capteurs de vi tesse d' eau. Le reseau est complete par des mesures de la qualite des eaux, prises en charge par 1' Uni versi te Lib re de Bruxelles (ULB). L' ensemble est re lie a un ordinateur par 1' intermediaire de quatre sous-stations de telet.ransmission. La troisieme par tie presente les resultats d'une intercomparaison d'appareils hydrometriques pour egouts effectuee conjointement par la VUB, le LRHCh et le CRR-OCW. Une etude de marche a permis de selectionner les appareils les mieux adaptes aux conditions ren­contrees dans les egouts. Les tableaux resumes de cette analyse sont presentes . Une experience d'intercalibration a ete menee pendant plus de six mois sur sept ap­pareils de marque et de principe de fonctionnement different.

1Department of Hydrology, Free University of Brussels, Belgiu:n; '-Hydraulic Research Laboratory Chatelet, Belgium; 1Road Research Center, Brussels, Belgium

193

1. Design and operation of a measuring network for urban hydrology on a small catchment in Brussels

The Hydrol ogy Department of the Free Un iversi t y of Brussels, Belgium, started in 1981 with the installation of a telemetry network, on the sewer system of the Campus.

The main characterist ics of the sewer system are total drained area 22 ha impervious area total l ength conduits slope conduits diameters conduits rainfall design intens ity

for 10 min (T 10 year)

k17.2

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: CONDUIT

: WATERFLOW

k S.2 : NANHOL£ 5.2

: STORAGE BASIN : INVERTED SYPHON

: WEIR S,T : BUILDING S,T

l MS : INTE LLIGENT MEASURING STATION

Figure 1 Plan view of the sewer system and location of the measuring stations-Campus Free University Brussels (VUB).

The rain drainage system is connected with the Brussels sewer network through a bottleneck l imiting the discharge to 428 1/sec. In case of hi gh flow, t he exceeding discharge is diverted to a stormwaterreservoir by means of a weir. This storage basin is located just upstream th~ bottleneck at manhol e K 3.2. Its ~ximum capacity amounts 2900 m , with a storage-coefficient of 1660 m /m.

A l ack of co-ordinat ion between several departments resulted in the construction of an inverted syphon at the crossing of a main pipe and a technical tunnel. The influence of this structure on the flow was not investigated. After a fl ooding in 1977 which inundated the basements of the

194

administration building, just before completion of the stormwaterreservoir, it was decided to start an hydrologic and hydraulic study of the Campus catchment.

The telemetry network consists of four independent "Intelligent Measuring Stations" (IMS), each of them operating several instruments and in telemetry connected to a mini-computer NCR- Tower 1632.

IrlTELLIGF.NT

MEASURING STATIONS

INSTRUMENTS

g ,. tel('phone wire

,----'------,/ PRiflTE R AND Pl.OM'ER

('--------+ TERMINAL

'-------__J~ CDC - COMPUTER

Figure 2 Structure of the telemetry network .

IMS 1 meteorological station, collects data about rainfall, windvelocity, radiation, air pressure, air temperature and air humidity .

IMS 2 situated at manhole K 3 . 2 , collects data about watervelocity in the bottleneck, water levels in manhole K 3. 2 and storage reservoir, *waterlevel inK 4 . 3 (downstream the inver ted syphon).

*IMS 3 and 4 :for waterlevel measur ements near the manholes K 9.2 and K 5.2 and upstream the inverted syphon.

This network will allow : 1) a hydraulic survey of the sewer system; 2) calibration of a rainfall - runoff and routing model, further study of the

Runoff Block and EXTRAN, both subroutines of SWMM; 3) survey about erosion and sedimentation phenomena in the conduits and the

inverted syphon; 4) a better knowledge of the behaviour of the sewersystem, which will lead to

a control strategy.

* not yet operation"l

195

The data are stored in a database and will be used as input for mathematical models. The programs ILSD and EXTRAN were already tested; it was attempted to include the inverted syphon in the calculations.

The flow in the bottleneck is the most difficult parameter to determine. Because of the high cost, an electromagnetic flowmeter could not be installed. An electromagnetic currentmeter was placed in the center of the pipe, only allowing determination of high flow.

The selection of the hydrometric instruments was based on a joint survey discussed in part 3 of this paper.

2. Application of real time techniques in the Brussels sewer system

In 1982, the Hydraulic Research Laboratory-Section Chatelet ( LRHCh), started a thorough hydraulic survey of the Northern sewersystem in Brussels. This was required for the development of a control strategy in view of the management of the network and the watertreatment plant to be constructed.

The structure of this network is very complex, combining sub- and supercritical flow conditions in a system where sewers, rivers and a canal are linked. In some areas, inundations occur repeatedly. (Figure 3)

Experience on flow gauging was already acquired during a previous study of the Southern part of the Brussels sewersystem with in situ measurements and local data acquisition. On the basis of this experience, it was decided to equip the Northern part with a telemetry system for quantity and quality measurements. The teletransmission system was constructed along a cable fitted to the ceiling of the 12 km long main pipe. 19 flow gauging stations were selected, equiped and connected to four telemetry substations and to the central processing unit.

On the base of previous experiences gained in the Brussels sewersystem, several types of waterlevel gauges were acquired and installed on one single site. In total, seven instruments were tested for at least six months. The continuous record of the signals made possible an intercalibration and an analysis of errors and defects, allowing the selection of the two types of instruments best suited : the pressure probe and the ultrasonic probe.

Other gauging stations will be equiped later. A joined study of available hydrometric instruments for sewers was conducted in cooperation with the Free University of Brussels ( VUB) and the Belgian Road Research Centre (OCW-CRR), it is presented in part 3 of this paper. In most of the stations, a stage-discharge relationship can be established. The highly unsteady and non-uniform flow occuring in some stations requires a continuous gauging with the velocity-area method. The continuous measurement of a flow velocity is almost impossible with the existing instruments. Therefore a new instrument was invented to detect the surface velocity. It consists of a paddl~-wheel mounted on a catamaran-type float. The rotation speed is converted into an analog outputsignal and the direction of the flow can be observed. The instrument is calibrated with "in si tu" measurements using other techniques, and the measured velocity must be multiplied by an experimental coefficient in order to obtain the cross sectional averaged velocity. Two prototypes were tested until now. The main working problems are related to the clogging of sewage on the paddles. Different designs of the paddle wheels were examined.(Figure 4)

196

The installed telemetry network for quantity and quality measurements wi ll permit the analysis of the hydraulic behaviour and of the pollution load during normal periods and during storms . This survey will result in proposals about the modifications of the networ k, inc luding the structures regulating the overflow to t he river or the canal.

The data collected at the gauging stations will serve as input for a rainfall-runoff, flow routing and other models.

3. Reliability of hydraulic field system in sewers intercomparison of instruments

The reliability of fieldsystems is strongly related to that of the instruments used. Most of the available instruments were not designed for use in sewer systems.

The Hydraulic Research Laboratory - Section of Chatelet (LRHCh) (Ministry of Public Works), the Department of Hydrology of the Free University of Brussels (VUB) and the Belgian Road Research Center (OCW-CRR) in Brussels organized a joined survey of the instruments actually available in Belgium and suited for installation in sewers :

1) water level measurements types pressure probe

ultrasonic probe bubble gauge dipping probe

2 ) watervelocity measurements types : e lectromagnetic

doppler 3) portable watersamplers.

For each type of instrument, a ques tionnaire was sent to the companies, inqu1r1ng the technical s pecificat i ons . The information was arranged i nto summarizing tables. (Tables 1 to 5).

For the pressure probes, special attention was pa id to the possible teletransmission of data. Therefore an analog output 4-20 mA is best suited because of the possibility to check the probe failures. Other important characteristics are the pressure compensation system, the enclosure·, the precision, the power supply, the maximum distance between transducer and transmitter and the price .

For the ultrasonic probes, the ultrasonic frequency, the beamwidth, the accuracy, the m1n1mum distance between transducer and watersurface and the integrationtime are considered to be the mos t important parameters. If a bubble gauge is used for level measurements, important cri teria are the maximum length of the t ube, the presence of an output signal and the autonomy of the installation by means of gas supply or compressor.

For the portable watersamplers, special attention was•paid to the maximum suction head, the autonomy and the possibility for time- or ·flow proportional sampling.

Tables 1 to 5 are based on the technical information collected between April 1984 and December 1984 (written and oral communications) . Prices are valid for Belgium and date back to the period mentionned (Information under reserve).

197

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198

Figure 4 Prototype of paddle-wheel current meter (LRHCh).

The telemetry systems runned by the institutions involved in this paper (VUB, LRHCh, CRR-OCW) are in their early state of development. Nevertheless, it is already possible to formulate some conclu~ions :

1. Most of the hydrometric instruments are not designed or suited for measurements in sewers.

2. The technical information provided by the companies about the instrumentation was sometimes found to be misleading, inaccurate or even erroneous ("flow gauge" indicated for a water level gauge).

3. The installation of the probes or sensors is often difficult, in some cases impossible due to the size or shape of the sewers (measurement of a dynamic instead of a static pressure if the probe is not correctly oriented; effect of sedimentation etc ... ).

4. The instruments and probes are not always waterproof and resistant to the agressive atmosphere encountered in some sewers.

5. There is already a standardisation of the analog output of many instruments : 4-20 mA, best suited for teletransmission.

6. There is an urgent need for development of instruments for telemetry which must be - robust, because of the harsh working conditions

- simple, the signal being processed in a central microprocessor or computer,

- cheap, (obviously) - easy to maintain.

Our experience learns that the cheapest instruments are often the most reliable. Some of them are installed already for sometimes more then two years, working correctly without requiring special maintenance. For the water level gauges, the pressure probe and bubble gauge are the most satisfactory. Mist in the atmosphere, foam at the water surface, electrical influences are often responsible for the malfunction of the ultrasonic probe. Other water level gauges such as dipping probes, float gauges etc... are difficult. to instal. Most of the current meters cannot be used in telemetry because of the solid or soluble wastes carried by the water. Attempts are made by the CRR-OCW to use electromagnetic probes mounted on a small sill on the bottom. Encouraging results were obtained, even if sediment is transported in the sewer. It is however difficult to dete~mine the coefficient needed to convert the measured velocity in a cross sectional velocity, which vary also with the flow rate.

199

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TABLE 3

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TABLE 4

LEVEL MEASUREMENTS DIPPING PROBES A . , 0-l 2'/, Oto50'

vdc12 - 248.000 vac220

B b , 0-l 2'/, -to•toso• vdc12 - 240.000

202

TABLE 5

WATER VELOCITY MEASUREMENTS E =ELECTROMAGNETIC 0 =DOPPLER

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4"

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vdc12 4-20

02 0,2-4 3 -10'•60° vac220-110 0 -20 4-20 245.000 DIGITAl OUTPUT 0-1

Bibliography

DERIEMAEKER, L. (1981) .

Intelligent meetstation deel II : Dagelijks protokol om de gegevens te bekomen . Analyse van de gecodeerde gegevens. VUB, dienst Hydrologie, november 1981, 14 p .

HIVER, J-M., VANDENBERGH, J - M. (1983). Etude de faisabilite relative a gestion automatisee du futur systeme d'assainis­sement du nord de la region bruxelloise . Rapport interne du Laboratoire de Recherches Hydrauliques, section de Chatelet, aout 1983, 13 p .

Intelligente meetpost. Deel I: Apparatuur en electronica. (1980) . VUB, Dienst Hydrologie, april 1980, 39 p.

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