J.J.Peters Hydraulic Research Laboratory, Chiitelet, Ministry of Public Works, Belgium, and Laboratory of Hydrology, Free University of Brussels VUB, Belgium
ABSTRACT : Flow gauging methods exist since long. New developments during the last decades concerned the methodologies, techniques and instrumentation. The enhanced use of electronics allowed new instrumentation based on, for example, electromagnetic, laser-optic and acoustic principles. After a short review of the actual situation, some needs for development are presented in relation with case studies, with special emphasis on problems encountered in developing countries. Beside the development of new instrumentation, prominence should be given to general and specific guidelines about their use in the various environments. Manufacturers must indicate clearly the limits of applicability of the instruments.
1 INTRODUCTION
Dr. W.R. WHITE has presented a very comprehensive review of methods for flow gauging in open channels (White 1987). Despite the large range of available standards, methods and instruments, engineers and hydroloe'ists have still to face new needs, especially in relation with the various environments of application.
This lecture is adressed to all users working in fields as different as sewers, very large or very small rivers, navigable canals, but also to the manu fac turers . Particular problems encountered in developing countries will be analy s ed : manu fac turers must be confronted with the special problems of use, installation and maintenance in difficult economic and climatologic conditions. The analysis of the needs for further developments will consider methods, techniques and instruments.
2 EXISTING METHODS
2.1 About the users
"Fi eld" users may be s ubdi vide d i n vah.ous groups , the objec t ives a nd work e nvi r onme nt s of whi c h a r e sometimes ex t reme l y different hydrologists interested in bulk flow determination at particular,
well choosen gauging sections; hydrographers interested in a detailed s urface flow field in areas of difficult navigation conditions; river engineers needing cross-sectional velocity di s tri butions at different time sc,.lt'> .~ i n re-l a t i on mental about about
with sediment transport; environengineers wanting information
the timeaveraged- as well a s the turbulent flow field for
analysis or prediction of mixing phenomena; urban hydrologists or urban hydraulicians, sanitary engineers concerned wi th mass flow and with hydrodynamic phenomena in non stationary conditions governing resuspension processes of sediment deposits; etc ..•
All these "field users" have at their disposal standards, guidelines, procedures and instruments, traditionally used in their organization. They generally do not question the choice of techniques or devices.
2.2 About the methods
Discharge measuring methods Table 1 summarizes the discharge measuring methods (see also Herschy 1978). Direct discharge determinations (mass per unit time) in the field are rare the electromagnetic discharge method and the tracer dilution method. The velocity-area method may also be
179
Table 1. Methods for flow gauging
Parameter commonly to be determined
Type of measurement
DIRECT METHOD
- Electromagnetic discharge gauging method
- Tracer dilution method
ALMOST DIRECT METHOD
h V A EMF
* (1]
c
* (1]
Calib. Single Continuous almost I non stationary
* * * * (a)
- Velocity - area method * * * * (a)
INDIRECT METHOD
- Slope - area method
- Stage - discharge method
Tracer (pulse) gauging method
- Flow measuring structures
special structures (specially designed) existing structures (not purposely designed)
- Mathematical models (flood routing)
h water level v = water velocity A cross-sectional area EMF electromagnetic field C tracer concentration
(1] (1 to n]
* (2]
*
* (1, 2 or more]
* * (n] (n]
considered as a (almost) direct method, needing the measurement of a velocity (field) and of the cross-sectional area. A third group comprises all methods where the discharge is determined indirectly, by measuring an effect, a result of the flow.
The choice of the method may be based on a wide variety of criteria - stationarity of the flow;
continuity of the gauging; - geometry and stability of the channel
(including alignment, vegetation, bed-
* * * (2]
* *
* * * (*) *
* * * (a)
* * * * * (n] (? l (? l
Calib. = calibration (1 to n] number of sampling points needed
* (*) (a)
necessary possible but not necessary continuous measurements not always possible under non stationary conditions
forms); - accessibility for installation and
maintenance; - availability; - economy (installation and maintenance
costs); - and others.
Except for the direct methods (electromagnetic and dilution) and for the tracer (pulse) injection method, the discharge is gauged through the measurement of water level(s) and/or water velocity(ies).
180
Flow velocity measurements There are many cases where the flow velocity only is needed, without a discharge determination : local (point) velocities in eulerian coordinates or flow paths in lagrangian coordinates. The techniques are generally the same as these used for the discharge measurement.
About the techniques and instruments To illustrate the variety of available techniques and instruments, we will limit this presentation to water level and water velocity determination (table 2 and 3).
The different techniques may be subdivided in : - mechanical measurements; - electrical measurements; - wave travel time measurements; - pressure measurements; - electromagnetic force measurements; - tracer travel time measurements; - heat dissipation measurements.
3. NEEDS FOR FURTHER DEVELOPMENTS
During the last decades, electronics improved the more common methods electronic pulse counters for propeller current meters, radiopositioning etc ••• Furthermore, new methods were invented, for example the moving boat, an interesting alternative of the velocity-area method.
New methods were invented or became operational because of progress in electronics the electromagnetic discharge gauging, the acoustic flow meter, the laser flow meter, the electromagnetic discharge gauging , e t c ....
rod floats (velocity integrated over a certain depth)
-Tracer
Renewable energies, such as wind or solar energy, teletransmission techniques (by cable, radio and satellite), microelectronics a . o . opened new perspectives for flow gauging in remote areas •
To describe the needs for further developments is a kind of a wager. It depends so closely on every particular case, on every environment that the following description is not meant to be complete or precise, but rather to develop some ideas in relation with own or known experiences.
To begin with, some examples of cases where needs were identified : - gauging in wild, alluvial rivers
(remote areas); - gauging in large canals or canalised
rivers; - gauging in sewers.
3.1 Wild alluvial rivers
Wild alluvial rivers, with their ever changing morphology, their eroding banks, their sometimes high floods pose peculiar problems.
First of all, it is often not easy to find good discharge gauging sections and water level gauging stations.
Furthermore, there is . often a lack of topographic data and no bridges or cableways available.
Discharges may be determined with a velocity area method : - exploration of the velocity from an
anchored vessel, sometimes from a bridge or cableway;
- moving boat; - float tracking.
Stage may be determined with different types of instruments, but installation problems due to changing shoaling or bank caving are difficult to overcome.
Possibilities of new developments are :
a. positioning
Topographic positioning can be supplanted by radiopositioning, which became more effective, more accurate and less costly during the last years; there is a need for lighter, portable and low energy devices.
Float gauging is in some cases the only applicable method in non navigable areas or when the flow field is complex. Automatic float tracking must be improved with radio positioning, topographic positioning (eventually by night with lighted floats), radar or aerial photographies. Float tracking is also a complementary technique to bathymetric surveying, providing the flow field. Aerial photographies, generally taken from an aircraft or from an helicopter are expensive. In sane cases remote operated cameras hanged at a captive balloon provide a relatively inexpensive solution to the positioning problem.
b. flow gauging technique
In large alluvial rivers, flow gauging with current meters happens to be difficult or inaccurate because of the varying direction of the currents in the gauging section, because of turbulence, floating debris, or the presence of bedforms, etc.
A first need concerns additional guidelines, on the one hand some more general, on the other hand some more specific, taking into account different local working conditions.
A second need concerns analysis of available information to optimize operation procedures. A well known problem is the choice of the number of verticals, of the number of samplings per vertical and of the sampling time in each station. Such an analysis could improve the existing guidelines. A standard procedure is not always the best !
A third need concerns the development of currentmeters with indication of the direction of the flow and eventually of the depth; these exist, but are generally expensive.
A fourth need concerns an improvement of the moving boat method. In rivers with floating debris (weed, brush-wood) the currentmeter may be continuously caught and its functioning disturbed. The design of a two component electromagnetic flo"~.meter, measuring the flow in magnitude and direction at the surface of a well profiled immerged device could be investigated .
A fifth need could concern the design of instrumentation to measure accurately the velocity profile near the bed in relation with bedfonns and sediment transport measurements. Similar devices were designed by oceanographers, but their use in rivers poses peculiar problems.
c. stage
There is a need for new instruments, which could be installed in a safe place (land, island or stable shoal, away from eroding banks) with the sensor placed at a remote location, under water, at a known fixed level. Pressure transducers and pneumatic (bubble) gauges may fit these requirements. Only the former may operate over long periods with a renewable energy source (wind or solar energy).This available instrumentation was generally developed for the industry and its usability in open field must be checked.
3.2. Gauging in large canalised rivers
Many rivers were canalised for navigation. Due to the increase of the crosssectional area (widening and deepening) and the operation of weirs and locks, it is often impossible to record continuously the discharges with a simple method such as the stage-discharge or the slope-area method. The acoustic flow meter became a very interesting solution to this non stationary problem, but some difficulties still remained :
182
influence of an irregular velocity distribution on the accuracy;
- influence of water density stratification (due for example to thermal pollution) on the propagation of the acoustic waves;
- influence of water quality (solid content, gases, plants and debris); influence of bed changes in very shallow or very wide cross sections. Large efforts are actually devoted to
ameliorate this promising technique. Calibration at very low flow rates (a
few centimeters per second) is another difficult task, beyond the possibilities of most rotating element current meters. Electromagnetic current meters can cope with this problem, but they are very costly. Vertical axis meters with Savonius rotor have a very low start-up velocity but are also too costly. New and cheaper instruments of this type were recently developed; efforts should be maintained.
3.3. Gauging in sewers
Sewers became, during the last decade, a very interesting field for testing and development of flow gauging methods, techniques and instruments. The choice of these is limited by the quality of the water sensors, propellers and other devices are rapidly spoiled or clogged by grease, oil, paper, plastics and other pollutants.
The size and shape of the sewers determines also this choice. Many catchments are actually instrumented for research, design or management (Maksimovic 1986). Continuous . gauging i s frequently required. In unitary systems, the flow is clearly non stationary during storm events and sediment deposition di sturbs the normal use of methods and instruments.
now in methods,
are most
The experience gained until sewers have shown whi ch techniques and instruments adapted.
In almost stationary conditions, the slope-area, stage discharge and electromagnetic discharge gauging methods are best suited. Also special structures or exi s ting structures may be used for flow gauging, although special structures such as weirs or flumes er ea te an obstruction and additional resistance to the flow, not always allowed.
The velocity- area method is generally not appropriate for continuous gauging. In non stationary conditions, electro-
magnetic gauging is best suited, but limited to pipes with small diameters. For larger sewersizes, special structures with a control section, such as an abrupt drop, may be used as long as there is no backwater effect.
Needs for further development of methods are :
Electraaagnetic discharge meter
If the method is well suited for full pipe flow, its application in open channel (sewer) flow is limited by following factors :
a. the difficult installation in existing sewersystems;
b. the quality of the effluent spoiling the electrodes and modifying the inductance;
c. the difficulty of in situ calibration.
When new sys tems are built, station may be inserted in siphon.
Experiences with in si tu could serve to establish including requirements for lations .
Velocity-area methods
the gauging an inverted
measurements guidelines,
new instal-
For larger sewers, velocity-area method different ways
the continuous can be used in
a. sane manufacturer s have developed electromagnetic current meters instal,led on the bottom, combined with water level gauges. The use of these devices is rendered difficult by sedimentation, but also because there is no clear relationship between the average velocity and the measured one. Experiences were conducted with the currentmeter installed on a profiled sill.
b. experiences are going on about the use of floating surface currentmeters , drumtype or paddle - wheel type. This kind of instruments was prefered because of the more stable relationship between average and measured velocities. The paddlewheel type is little affected by pollution or floating debris, but its starting velocity remains high because of the heavy construction, due to the rough working conditions .
183
c. slope area and stage discharge relationships may be used easily. Main problems concern the stability of the relationships because of a.o. silting or erosion (calibration problem) 1 but concern also the installation, power supply and maintenance difficulties of level gauges. Intercomparative tests showed that pneumatic (bubble) gauges, pressure transducers and acoustic level gauges are most suited 1 but cannot be employed in all cases. Further developments of instruments are related to their robustness, low energy consumption and reliability.
d. flow measuring structures • specific structures These structures are generally designed for open channel flow such as rivers and canals. Their application in sewers requires special precautions due to the already mentioned difficulties. There is a need to develop structures which are less affected by the pollution elements, presenting the lowest additional resistance to the flow and easy to install, preferably disassemblable.
• existing structures It is often possible to design a sewersystem in such a way that flow gauging becomes easy, using structures such as abrupt drops, passages from sub- to supercritical flow, through a simple level measurement in a control section.
In a general way, specific guidelines for sewer gauging are needed.
e. mathematical modeling In some cases of existing, complex systems, the installation of a taletransmitted flow data acquisition network, connected to a central processing unit allows the use of mathematical models to follow, on-line, the hydraulic behaviour of the system. Analysis of ongoing experiences in this field could be useful to new installations. Guidelines are necessary to resume these experiences as well about the methods as about techniques or instruments.
4. SPECIFIC NEEDS OF DEVELOPING COUNTRIES
Problems of flow gauging in developing countries are related to their economic, technical and even social environments.
Some of them have skilled and trained technicians specialised in gauging,
eventually in the use of sophisticated equipments, but not necessary in their maintenance or repair.
Other countries are in the phase of starting up measuring networks and do not have experience with the methods, techniques and instrumentation available.
Furthermore, the practical problems encountered in some undeveloped areas may be very specific : flow gauging in arid zones, in tropical rivers, in large unstable deltas, etc.
There are differents ways to help developing countries to overcome these barriers.
4. 1. Guidelines
International organisations such as .WMO, ISO, IASH, IAHR should promote the further development of guidelines, manuels and other documentation about flow gauging in general or in specific cases.
In the past, WMO has already conducted very use full intercomparati ve studies about flow gauging instruments (Starosolszky 1977).
We should not forget the importance of guidelines for gauging procedures in remote areas with very simple techniques and instruments compass 0 self-made floats, sounding by hand, etc.
There is also an urgent need for clear reviews of available techniques and instruments, so that a sound choice can be made.
4. 2. Methods
Flow gauging methods must be developed, improved or adapted to specific local conditions. Discharge may, for example, be measured by the moving boat method with simple instrumentation, eventually in combination with a cableway.
already techniques
have to be
The possibilities of the mentioned aerial photographic (satelites, captive balloons) further investigated.
4.3. Instrumentation
The manufacturers of instrumentation must be faced with the specific needs such as robustness, simpleness, easy maintenance. The production costs of electromechanical equipment are actually higher than those of full microelectronical devices. Nevertheless, considered over
184
the life time of the instruments, it could be more interesting to buy instruments which could be maintained by local people, easily and with low cost.
4.4. Data processing
Manuals for data processing are needed with special attention for error detection. It is possible to organise the measurements in such a way that validation becomes · easy . Statistical techniques allow correlation analysis, for example between data of two or more water level gauging stations .
5, CONCLUSIONS
There is clearly a need for further development in the field of flow gauging in open channels. If some needs are similar to those found in laboratory conditions, the ones encountered in the field may be quite different. Sophistication is not necessarily synonym of improvement. Efforts must be devoted to learn people which methods, techniques ~nd instruments are adapted to their particular requirements, depending on the objectives.
Many problems may be eas ily solved if the hydraulics are well · understood, Therefore guidelines should remember the basic principles .involved,
As users, we are sometimes upset by the
presentation, by manufac turers , of a level gauge as a "flow discharge measuring device". Limits in use must be stated clearly.
Developments and improvements have to be tested in the field, eventually with the help of users or international organisations.
An important domain for further development is flow gauging in relationship with sediment transport measurements. But this subject is so large that it could be possible to devote a whole seminar or workshop to it.
REFERENCES
Herschy, R.W. 1978. Hydrometry. Principles and Practices. Chichester, John Wiley & Sons.
Maksimovic, C. & M. Radojkovic 1986. Urban drainage catchments. Selected worldwide rainfall-runoff data from experimental catchments. Oxford, Pergamon Press .
Starosolszky, 0. & L. Muszkalay 1977, Report on intercomparison of principal hydrometric instruments (first phase, 1972- 1975). Geneva, WMO.
White, W.R. 19787 . Discharge measuring methods in open channels. Short course on discharge and velocity measurement. IAHR Section on hydraulics instrumentation, Zurich . Balkema, in preparation .
