2 STREAM GAUGING USING VELOCITY - AREA METHOD
 

2.1 CURRENT METER VELOCITY GAUGING

The principle of this method consists of determining a velocity field in a cross section of a stream and computing the discharge using known geometric relationships.

In reality, flow speed is never uniform in a given cross section of a stream. So it is a good idea to assess the velocity field in a number of different verticals spaced along its width. In addition, we get a profile of the cross section by taking a width measurement and depth measurements at several points of the cross section.

There are some recommendations regarding the measurement itself, the equipment used and the calculation of the discharge from the raw measurements taken. Keep in mind that stream gauging is made up of two phases: the first has to do with measurement the physical parameters (width, depth, speed); the second is the calculation of the discharge as a function of the recorded measurements.

The following recommendations are suggested to improve the accuracy of current meter gauging.
 

2.1.1 Choosing the measuring section

The geometric dimensions of the measuring section should be cleanly defined to cover all the streamflow.

This measuring section should be as rectilinear as possible. Its location should be far from natural or artificial obstacles or bends in the streambed. The measuring section should be perpendicular to the flow of the stream. When this is not possible, the widths of the biased section can be adjusted for correction.

The flow must be as regular as possible. Avoid taking measurements in converging or diverging flow areas that are oblique to the direction of the flow or that are in backwash or dead flow areas. If these conditions exist, try to estimate the amount of error caused. Some adjustment will then be made to the raw measurements.

The depth of the water should be sufficient that the equipment can be properly submerged. The measuring section itself should not present any disproportionate vertical or horizontal variation. In order to limit uncertainties, we are looking for the best compromise between sufficient depth and measurable water speed including low flow conditions.

The location of the section should not be obstructed by any obstacles immediately upstream or downstream (immersed tree trunks or branches, rocks, plant growths, etc.) which would affect the measurement. Small modifications may be made at the site of the measuring section as long as they do not affect the section where the limnigraph is situated e.g., building small dikes to channel water flow, cleaning up stones, roots or vegetation on the bottom and on the banks. In this case the measurement can be taken only after waiting enough time the stream flow has stabilised after modifications. All the same, if it is necessary to clean out moss or dead leaves with a sill, the measurement should be taken before the cleaning. Then, it would be a good idea to join up the discharge to the top water level after the cleaning (don't forget to wait for stabilization)

Remember: take enough time in choosing your measuring section. A badly chosen section will never yield a high quality gauging. The measuring section should stay the same in each type of discharge measure.
 

2.1.2 Equipment

Measuring equipment (cup or propeller style current meters) should be adapted for the speeds to be measured. Choose carefully the propeller in accordance with the ranges of speed expected to measure (pay attention to the manufacturers recommendations). In particular, the water speed should be sufficient to turn the propeller in good conditions. To limit uncertainty of the measurement, stream water speed should be greater than .05 m/sec for the most sensitive propellers. The propeller's pitch should be the lowest compatible with impulsion counter used. Likewise, the weight of the sounding weight should be adapted to the measuring conditions and water speed.

The weight and bulk of the measuring equipment are such that you must take care to set up its access and operation in the most ergonomic way possible.

Don't forget that the current meter/propeller/support assemblies should be tried out and calibrated in the lab. You must follow the calibrated configuration and avoid mixing and matching current meter parts that have not been calibrated together.

The equipment must be in good working condition, especially current meter axles, axle bearings, propellers as well as the propellers themselves. Also, don't hesitate to change often (ideally it should depend on the number of hours of use and the turbidity of the fluid) the oil ensuring the water tightness of the current meter. The propellers should not have worn or chipped blades. They should spin easily on the water meter axle without having to be tapped along. You should not be able to feel resistance to the rotation motion; stopping the rotation of a propeller should be as regular as possible and always happen in an easy natural way (this is easy to test). Moreover, on C31s, you can do a test called "back rotating the propeller" (a small reverse rotation after stopping).
 

CURRENT METER MAINTENANCE

Equipment maintenance begins with its storage. When transported it should be disassembled as much as possible or at least well blocked and protected to reduce vibrations. Also make sure that the propeller does not spin too much in the air.

Before each use, verify the propeller's condition.

After a campaign of measuring operations or one long measuring operation, change the oil.

C31 type current meter

Check and clean the axle and the bearings with white spirits (essence H) once a week. Using an ultrasound machine makes cleaning much easier.

If necessary, the bearings or axle can be replaced. Use white spirits to dissolve and eliminate the protective grease. These substitutions do not change the current meter's parameter .

C2 type meter

To clean the bearings, shake the axle/bearing assembly vertically in a bath of pure benzene.

After cleaning, during periods of nonuse, the parts should be stored without oil.

A careful check of the propeller/meter/impulsion counter assembly will allow you to verify its mechanical condition. Two check-ups per year would seem sufficient. It is just a matter of spinning the propeller and seeing how many rotations it makes before stopping. Then you examine the results on an abacus (see annex1).

It is fundamental to ensure a regular program of maintenance on the equipment and, replace, if necessary the propellers. If this is done, regular propeller recalibration becomes unnecessary.

Concerning impulsion counters, they should allow you to work point by point or in an integrated system. It would be wise to keep an extra set of batteries at hand so you don't have an electricity outage during "the gauging operation of the century". Likewise, particular care should be take with the electrical connections (cables, current meter, counter, etc.) Here also, don't forget to include in each vehicle, the spare parts necessary for each piece of equipment you carry.

For all of the support materials (graduated wading and sounding rods, winches, sounding weights, boats etc.), they should be properly maintained and stored in such a way that they will be operational even after relatively long periods (several months) of inactivity. The winch cable, the abscissa cable in case of a boat gauging operation, should be regularly checked for corrosion, rupture or twisting. This is an important aspect that should never be overlooked for it is a guarantee of safety and reliable operation for not only all of the data collection equipment, but also the personnel involved. Beams and ferry cables require specific maintenance. The normal standards regarding lifting machinery are not applicable to these systems.

This quality approach implies that there be a record which shows that equipment checks have been properly done.

We recommend, for example, that:

Lastly, the vehicle used to transport the men and equipment should be adapted for operations that need to be done and the distances that need to be covered. It should be sufficiently comfortable and practical to permit and even facilitate measuring operations. With this in mind, the signalization designed to protect the vehicle and its personnel should also be carefully maintained. Protective clothing, adapted for the elements as well as hip waders, should be replaced regularly. Finally, all vehicles should be equipped with a first aid medical kit.
 
 

Equipment problems should never get in the way of measuring operations. Equipment in good working order is indispensable. Extra equipment is necessary, especially in terms of consumables such as spare parts and parts which wear out. "Annex" equipment, which facilitates the measuring process, making it easier and more reliable is also one of the key elements for a successful measuring operation.

When faced with unexpected circumstances however, you may have to rely on more primitive less accurate methods of measurement. In a flood situation, it is better to obtain information of medium quality than no information at all.
 

2.1.3 Techniques

Without having to rewrite all the literature concerning the techniques of stream gauging (see bibliography), it seems important to recall some fundamental elements. These have to do with the choice of verticals, the number of points per vertical and the measuring time per vertical.

Gauging is a compromise between time of measure and precision of the measurement. The transportation time being generally longer than the actual measurement time, you must have an idea of what will be a sufficient amount of time to devote to the measuring operation. We will discuss gauging in flood conditions later.

The choice of verticals

Some people recommend that verticals be regularly spaced, others that the number of verticals be constant, no matter what width the stream, and others still, that verticals be spaced at variable intervals.

Vertical spacing should take into account the following principles:


An insufficient number of verticals usually leads to an under estimation of the discharge.

The spacing between each vertical should vary in a way that is inversely proportional to the variations in depth and water speed. The higher the vertical and/or horizontal gradient, the closer together the verticals should be.

This is explained by the fact that, when the data is examined, each vertical unit's profile is applied to an area on either side of it. The farther apart the vertical units are, the larger this "application zone" is. By reducing the space between verticals, we reduce the "application zone" and more accurately represent the actual variations of the stream bed and water speeds. We can thus better square significant lateral variations with unit flow data. The more regular the stream bed and discharge, the more regularly we can space the vertical units.
 
 


Point by point measuring

The number of points per vertical unit must take into account the depth, the vertical variations in water speed and the equipment used to take the measurements. It should, in any case, permit the most accurate representation possible of reality.

Here also, it is practically impossible to establish one universal rule. The points can be spread out evenly on a vertical axis, taking more measurements from the bottom half. The points should be spaced closer together if there is a strong vertical speed gradient, so as to best be reconciled with these variations.

In all possible measuring situations, it is better to avoid verticals with only one point of measure unless it is impossible to do otherwise, such as when measuring a marginal part of the total flow (at the edge of a zone where the water speed is extremely slow) or for safety reasons (surface speed measurements in flood conditions).

To correctly take into account the flow heterogeneity in time, the duration of a point measurement should be more than 30 s if the propeller speed is greater than 2 t/s ; the duration should be more than 40 s if the propeller speed is lower than 2 t/s.

As for the vertical units, it is advisable to place them as close as possible to the surface and bottom in order to minimize coefficients and surface effects.

Measurement by integration

Integration techniques depend on measuring the average speed at a vertical unit by moving a constant speed meter from surface to bottom (or vice versa). This allows the measurement to be done more quickly.

The ISO standard only deals with measurements of more than 2m in height. The integration speed must not be greater than 5 % of the water speed. The measurement must last more than 60 s.

Measurement integrating a wading rod and a small sized current meter is a French innovation. The results are comparable to point by point measurements. The speed of the raising should be adapted to that of the water and measurement time should be more than 30 s, even up to 45 s. This technique becomes viable when water depth is 15 cm or greater and is well adapted to vertical irregularities linked to the increasing proliferation of plants.

Comparison of the two methods

Point by point gauging enables a posteriori an evaluation of the measured velocity field. It is just as precise but richer in data than the integration method, which is nonetheless, faster to do.

Measurement by integration should be used:

In stream gauging, it is difficult to apply a particular set of fixed rules. The number of vertical units, the number of points for each vertical or the speed of vertical integration all should be adapted to the characteristics of the section. The main concern is to represent reality as closely as possible. So in the case of a high gradient, there must be a greater number of measuring points.
 

Special advice

In the case of irregular and/or widely divergent variations, you should note the height of each vertical. In this case, the discharge calculation will have to be made using the independent vertical method. No matter what, the beginning level and the ending level of the gauging must be recorded.

When using a boat for gauging, the safety rules should be followed, not only when setting up the operation but also when taking the actual measurements. Safety vests are imperative during this activity. Measurements from a boat allow us to resolve problems and uncertainties associated with taking measurements from a bridge (the effects of pilings, upstream surveillance, etc.). This method of measurement requires more time and effort to prepare but the time needed for the measurement itself can be reduced considerably. The system for attaching the boat to the abscissa cable should include a quick release clip allowing the boat to be freed rapidly, if necessary.

Measurements in high water conditions are dangerous. During flooding, when we cannot do gauging, other data can be usefully collected: photos, the water line, surface water speed, observations about the flooding conditions (see checklist in annex). Flow measurements during flooding should be taken as quickly as possible in order to minimize the variation in water level (in case of rapid rise, for example) and to minimize the exposure of equipment and personnel to the risks of floating debris and other possible dangers.

When doing gauging in high water conditions from a bridge, the safety rules are equally important, especially concerning road signs and conditions of waterway congestion. Whenever possible, it is a good idea to use a single span bridge for this work.

This is not always possible, notably on large rivers which are usually crossed by multi-span bridges. It is advisable to make the water speed measurement as close as possible to the arches. To the extent possible the gauging should be done on the downstream side of the bridge with the measurement taken at a point vertically(directly) in front of a piling. When a measurement must be taken on the upstream side of the bridge you must take one measurement for each arch and, of course, add them to get the total flow.

In the case of strong flooding conditions, it is imperative that the measurements include all of the flow. So be sure that overflow runways or secondary branch , even far removed from the stream in question, are not functioning.

Likewise, within reasonable safety limits, measurements must be taken of lateral overflows. Most of the time it is more of an estimation than an actual measurement, depending on how far laterally the flooding goes. One way of estimating is to count steps laterally away from the streambed, then to calibrate the steps on dry to make the calculation. It is useful to take note of fixed reference points (poles, trees, etc.) compared to where the verticals are placed.
 

2.1.4 Calculations

At the measurement site, the field checklist (see the French version ) should be filled out. On this checklist, in addition to information concerning location, date, time, people involved and conditions of the operation, all other information about equipment used as well as particular observations which could be useful in the calculations will be noted. Obviously, this includes numerical data for the calculations: abscissas, the depths of banks and verticals, correcting coefficients, propeller speed in number of spins and time of measurement or integration time, etc. This worksheet could be reused in 10, 20 or 30 years. Make the writing readable, the numbers clear and legible for future use.

This phase of recording on paper is obligatory, even if the examination of evidence is to be done right there in the field in real time. In any case, you should plan a printing at the same time as you record the data. You should get into the habit of this practice but that also means to reexamine and rethink about the materials currently used.

Different practices in making calculations could, for a given gauging operation, lead to significantly different results.

Concerning the data recording itself, it would seem very useful to use a visual graphics program for measuring verticals profiles (for point by point measurements), for discharge and for wetted sections. These representations should remind the measure taker of what he "saw and measured in the field" and therefore, to notice possible anomalies. This software program should be easy to install and use and be compatible with standard output devices such as colour laser printers.

Archiving flow measurement data is equally necessary. This should allow the accumulation of data and also be easy to use in order to access "historical" data in the event an error is discovered later on.

Archiving and storage is as valuable for field worksheets as it is for calculation worksheets. Saving hardcopy records is imperative and backing up computer file records is highly recommended. It would be wise to have computer storage of all raw data concerning a gauging operation, not just the rating curve data. Computer files on gauging operations should offer the possibility of multi-variable sorting which would allow data to be used in ways that the original measuring operation had not intended (for ex. drainage basin approach). The quality of data recording facilitates the eventual use of the data.
 

2.2 THE ELECTROMAGNETIC (EM) CURRENT METER

2.2.1 The principle

Inside the submerged sensor, an induction coil creates a magnetic field between two fixed electrodes; the movement of the water, the conducting fluid, within this magnetic field, produces voltage proportional to its speed. (Faraday's principle). This induced voltage is electronically processed by the measuring unit and converted into information that can be used by the operator.

The speed displayed on the counter represents an average, measured over a fixed time that is set by the operator- 1-120 seconds for the Flo Mate, 2-60 seconds for the Sensa Ott- but it can also be measured instantly (in hydrobiology applications, for example).

The current meter is theoretically 100% accurate since the current speed measured is perpendicular to its axis; however the angle of inclination to the water surface has been observed to have an effect on the measurement.

2.2.2 Setting up

The sensor is attached to a pole whose diameter depends on the type of meter used- 10 mm for the Flo Mate, 20 mm for the Sensa Ott- with or without data recorder. Assembly on a sounding weight is not currently possible.

For the Ott model, the manual says that it doesn't matter whether the sensor is placed above or below the electrodes: it will not change the measurement.; we have found that people prefer nevertheless to keep the electrodes above the sensor.

The measuring technique used is the point-by-point current meter method, choosing the number of verticals in the velocity field and the number of points per vertical.

The electromagnetic current meter is good for measuring water velocity in low speed currents or grassy conditions where micro-current meter type current meters are not very efficient.

The ranges of measure vary according to the model:

- for the Sensa, from .005 to 1.5 or 2.5 m/s depending on the setting. According to the manufacturer it is accurate to 1.5% of the measured speed.

- For the Flo Mate, from .15 to 6 m/s with a 2% margin of error. The stability at zero, is ± 1.5 cm/s

For negative speeds, the Sensa gives a signal and the Flo-Mate will actually measure them within certain limits. In the case of the former, values can be determined by simply turning the meter around to make the measurement which is accurate to a few cm/s.
 

2.2.3 Results of experience

The EM current meters are preferred to micro-current meter models used in point-by point measurements. Flow measurement results are comparable for a given measuring section .In a single point, the dispersion of observed results is slightly wider, though the reasons for this are not clear at this time.

Advantages

Measurements in slow water conditions are possible, under 5 cm/s, for example, but you shouldn't have illusions about accuracy in such conditions since the putting the measuring device in the water creates a thermal convection current of its own, on the order of a few mm/s.

Measuring in the presence of grassy growths poses no particular problems, a considerable advantage over the micro-current meter models but that does not eliminate the problem of representitivness of the points chosen. All the same, in heavy waters, the risk of deterioration is minimal.

The same sensor measures a larger, though limited, range of speeds. The absence of movable parts seems to be an advantage and he speed measurement read out is direct. But even if the EM current meter is a bit more resistant to shock than the micro-current meter type, it is not invulnerable to deterioration.

It provides the velocity value directly.
 

Disadvantages

In case of problems on the cable, reparations are difficult. The weight of the Ott meter, several kg, is a problem; the Flo-Mate weighs only 1.65 kg.

The electrodes must be frequently cleaned; preparation and reinitialization after a prolonged period of non-use are also constraining factors. Ott recommends recalibration every two years whereas the Flo-Mate has a procedure that sets it back to zero.

Drag is greater than with a micro-current meter model which causes some problems for correctly judging water depth in certain conditions such as shallow water or slow current, etc.
 

2.3 THE ACOUSTIC DOPPLER CURRENT PROFILER (ADCP)
 

2.3.1 The Principle

The Acoustic Doppler Current Profiler (ADCP) makes vertical profiles of water speed using acoustic energy. This borrows from oceanographical techniques in the field of hydrometry. The ADCP has four transducers that emit ultrasound signals independently of each other; these signals are transmitted in groups of pings.

The basic measuring theory is that the varying quantities of suspended sediment in water move at the same speed as the water itself. The ultrasound signals are transmitted, then reflected by the suspended sediment back to the ADCP with a frequency delay proportional to the speed of the water current.

The signal is used in two ways; the elapsed time of the rebound allows you to calculate the depth of the section measured and the change in frequency allows the calculation of average water speed in the section measured.

The ADCP measures:

Crossing from one bank to the other, the ADCP integrates the various data collected and thus calculates the stream's discharge.
 

2.3.2 Set up

The ADCP is made up of:

The ADCP is most frequently set up and operated from a boat which crosses the stream to be measured; the recommended speed for crossing is less than 4 km/h, keeping in mind, the slower the speed, the greater the possibility for accuracy with an optimum speed of 2 km/h.

The ADCP measures its own speed relative to the stream bed with the aid of a gyroscope and a pendulum to correct for verticality and the water current discharge measurement is independent of the device's movement.

Given the rapidity of the examination of the velocity area, it is imperative that several crossings be made to establish a significant measurement sampling: the final accuracy of the gauging comes from the abundance of measurements. Moreover, the repeatability of the measured discharge constitutes a good indication of its quality.

The operators have on board with them a program which allows them to configure the operational modes of the ADCP, to acquire and examine the collected raw data (depth, average speed, etc.) and to test the correct operation of the measuring process.

The operator notes the salinity and temperature of the water; variations in these characteristics are automatically compensated by the ADCP sensor.
 

2.3.3 Limits

The driver of the boat should be concerned exclusively with the task of carefully handling his craft. Movements by the other members of crew should be reduced as much as possible to keep a horizontal position.

Do not allow bubbles near the transducers. Keep the boat speed low. If another craft passes, allow a waiting period before taking the next measurement.

It is advised not to take a measurement in the downstream vicinity of a waterfall. Finally, in water with concentrations of suspended sediment greater than 1.5 g/l, the ADCP will not be able to detect the bottom and will give erroneous readings.

The transducers work both as transmitters and receivers; the ADCP switches back and forth from one function to the other. A certain distance is necessary to allow for the dampening of the transducer vibrations created by the transmission. The transmitted acoustic signal is made up of several distinct impulses with slight time lapses between them. Then the signal must be filtered, to reduce risk of confusion between the end of one impulse and the beginning of the next, which could yield incoherent data. All these processing steps eliminate any link between the measurement itself and the results of the calculations.

The main limitation in using the ADCP is that it doesn’t measure all of the different water speeds from the surface to the bottom and from bank to bank. Indeed, the transducers must be kept continuously submerged at about 25 cm beneath the surface, which leaves the first layer of water unexamined. The bottom layer measurement is likewise limited by interference between emitted soundwaves and those rebounding off the bottom of the stream.
 



Streamflow speed

These unexamined layers are reconstituted by a software program; this mode of examination of "the extremities" (the top, along the banks and near the bottom) requires a comparative calibration gauging that establishes the parameters of the measured section.



This apparatus will only work properly in depths of greater than 1m. To obtain high quality measurements, it is preferable that the shape of the measured section be as close as possible to a rectangle with relatively steep banks at least 20 meters apart.

The range of water speed in which these measurements can be accurately made goes from several m/s (whatever does not jeopardise the safety of the crew) to as low as 10 cm/s. Less than 10 cm/s, the values will be displayed but their accuracy will have to validated.
 

Conclusion

The ADCP is not in competition with other equipment but expands the range of hydrometric techniques already available. It is a tool well adapted to large, slow moving rivers, relatively deep and wide. For these kinds of waterways, the current meter gauging could be too time consuming, too complicated in terms of river bank variation or navigation and sometimes virtually impossible to gauge because of their very slow water speeds.

Therefore, typical ADCP applications would be in downstream sections of large rivers such as the Soanne, seine, Garonne, Rhone, etc.

To our knowledge, there are currently five hydrometric organizations which have acquired ADCPs: the DIREN Rhone-Alpes and Ile-de-France, the CNR, EDF/DTG and ORSTOM (IRD). Each of them has done several gauging operations which have yielded the following observations:

Because the equipment is American, most of the documentation and manufacturer contacts are in English. The 1997 price of an ADCP was about 650 000 F (99 000 Euros), 150 000 F of which is for spare parts. A simplified version of this device was to have come on the market for about 160 000 F (24 300 Euros).
 

2.4 FLOAT GAUGING

This method is used only measuring surface speeds or, more exactly, speeds in the top stratum (about the first 20 cm) of the measured stream.

2.4.1 The floats

In the simplest of cases, the use of natural floats: tress, large branches or other floating masses heavy enough to be really representative of the flow speed. Extra care is required when spotting and identification of floats is done by upstream and downstream teams.

In the case where man made floats are needed, empty plastic 1.5 litter water bottles seem well adapted for the job. Don't forget to add a bit of ballast, sand or dirt, to the bottom of the bottle so it floats in an upright position with conical top out of the water (minimum surface exposed to wind). I t is also recommended to use Florescent spray paint on the bottles to make them easier to see.
 

2.4.2 Operating mode

When in flooding conditions, it is a good idea to identify and plan a float measuring run before a crisis situation arises. The length of this run should be long enough that the floats will need 30-50 seconds to travel from the beginning to the end. It should be located in a reach that is a straight run of water, far enough away from bends, bridges, etc., that the flow of water is not too disturbed.

The two chronometer (or clocks) method seems to be the most efficient for this measurement since it allows us to identify float positions and trajectories in relation to a bank. It entails the timing of floats by two teams (up and down stream). The floats are timed over the total course distance by both teams. In addition, each team times the floats between two intermediate markers placed diagonally (upstream and downstream) in mid-course (see diagram). It is these intermediate times that allow us to calculate the position of the floats.



The speed will be equal in relation to the length of the measuring run by the average of the two total times recorded by the two teams.

We suppose that the markers AB and CD are perpendicular to the flow of the stream and that AC is parallel. If this is not the case- if AB and CD form an angle x to the perpendicular of the flow direction, then we have to correct the calculated positions of the floats by cos x.

The calculation of float position is the simple application of Thales theorem on the two triangles ADC and ABC.

In the triangle ADC:

tA = time to cover total distance la. Ta= time to cover distance L

In the triangle ABC;

tc = time to cover total distance l-lc. Tc= time to cover distance L

By repeating this operation at different positions along AB, we can determine "virtual" verticals and calculate the floats on each one. In fact, we have the examined velocity area of the surface.

The average flow speed thus obtained will be multiplied by the average value of the wetted section, obtained by the minimum transversal profiles of AB and CD.

Conclusion

The float measurement method should only be used when it is impossible to use classic current meter methods. This method, used wisely, and in a conscientious manner will result in a very discharge estimation. This means that the teams must be well trained and thoroughly prepared with respect to the field conditions and the equipment.
 
 

2.5 DILUTION GAUGING

(often called chemical gauging)

As a reminder, let us point out the fact that there are two methods of dilution gauging: the instantaneous injection method and the constant injection method.

The principle of these two methods is the injection of a tracer solution of a given concentration at a specific point along a stream of water. Then the concentration of the solution is monitored downstream along a distance far enough to ensure that it is sufficiently mixed with the water. The discharge is then deduced by comparing the concentrations of the injected solution with the water sample taken downstream.

There is no conflict between this method and use of classic current meter methods. To the contrary, they are complementary. Dilution gauging is interesting when the measuring section is varies greatly over short distances, when current is quite turbulent or when the use of classic techniques poses a safety risk to personnel.

Therefore, when velocity area methods are not appropriate, dilution gauging methods may be employed.
 

It is not a question of planning to use one method or another but to use the dilution method to expand the limits of the other.

This method requires special training of personnel, in the field as well as in the lab. The time needed to make the measurements in the field is the same overall since the time needed to collect data is rather constrained (physico-chemical dosage, etc.)

This method also requires the acquisition of laboratory equipment (for fluorimetry), tracer chemicals (rhodamine WT, sulferodamine G, pyranine, Eosine, etc.) and the use of equipment that has been adapted for injection (most often custom made by the team itself. Specific training should be given to personnel before this method is employed and should then be maintained with regular use or training sessions.
 

CONCLUSIONS

As far gauging goes, there are no fixed, unchangeable rules. The main objective is to get as close as possible to the existing realities of the field.

The choice of measuring methods and the means of implementing them is a function of the configuration of site, available resources, both human and material, and the degree of accuracy expected.

In difficult conditions, such as equipment breakdowns or dangerous situations, estimations of speed, photographs taken, observing floodwater marks on permanent structures, etc., are actions that can mitigate the absence of a true measurement and establish coherent limits on further evaluation of a site.

The selection and adaptation of gauging sites, competence of personnel, equipment to be used for a job should all serve to advance a project toward optimum measurement precision.

The most important elements for good measurements are motivated, well trained and properly equipped personnel. Not only the must the measuring equipment itself be in good condition, but also the annex equipment must be in good working order. In other words, for good measurements, you also need good vehicles.

Harmonization of practices comes with the profound motivation of men, with skilful development of collected data and a grasp of the basic notions of hydraulics.