Hydrometry

The hydrometry is the branch of the Métrologie which relates to measurements of water run-off.

Concept of instantaneous flow

The instantaneous Débit is the volume of water passing through the section of a River during a unit of time

Q = \ frac {V} {T}

Q flow (in m ³ /s)
V volume (in m ³)
T time (in S)

The low flows can be expressed in Liter S a second (l/s) or in Mètre S cubes per hour (m ³ /h)

If volume is seen like the product of a surface (S in square meters) by a length (L in meters) that is to say $Q = S \ times L$ , then the flow is given by the relation:

Q = \ frac {S \ times L} {T} = \ frac {L} {T} \ times S = V_ {moy} \ times S

where $V_ {moy}$ is the mean velocity in the section.

The precision of a measurement of flow depends on many factors: method employed, care taken during measurements, rigor in the examination, influences ground, etc

Various methods of instantaneous measurement of flow

There does not exist method, nor of universal equipment for the measurement of the flow , the choice of a method is conditioned by various factors, one can quote in particular:

configuration of the site and conditions of flow;
the material of measurement and serviceable time;
the number of people which can take part in measurement;
finally the precision which one wishes to obtain.

The measurement of the flow of a liquid running out with the free air rests on three big families of methods (classified according to the physical principle on which they rest) which are:

the methods kinematics
dynamic methods
physical methods

the methods kinematics

They consider primarily the rate of the flow. Two types of methods presented hereafter, are attached to this category:

capacitive method

This method consists in measuring the time of filling of a volume given using a stop watch. This method applies well to the small flows (a few liters a second) and to the small sections. The method must be repeated at least three times. If one obtains the same order of magnitude the three times, one makes the average of three measurements and one uses the formula Q = V/T average. This method is simple, fast and inexpensive;

method by exploration of the field speeds

The rate of the flow is not uniform in the cross section of the river.

It is thus necessary of " to explore the field of vitesses" by carrying out measurements in several points of the section, generally located along verticals judiciously distributed over the width of the river.

Starting from these specific statements, one determines on each vertical, a mean velocity which is regarded as representative the rate of the flow on an element of wet surface.

This one being thus cut out in several juxtaposed elements, the total flow is obtained by making the sum, over all the width of the river, the products mean velocity of the flow by the surface of the corresponding element of section. The velocity measurements of the flow can be realized with various types of materials, most usually used being the Moulinets and the Flotteurs. In the Années 1990 appeared the measurement technique by Débitmètre Doppler used in oceanography to study the Courants sailors.

measure using a traditional winch

Measurements are generally made starting from propeller winches, with horizontal axis. The winch is composed of an element called “body of the winch” comprising a horizontal axis on which a Hélice turns. The rotation of the propeller produces impulses which are detected and entered by an electronic device called meter, connected to the body of the winch. This meter indicates the number of revolutions carried out during preselected time. The winch is assembled on a support fixed on a graduated, round or oval pole.

The winch is immersed in the river vis-a-vis the current, the number of revolutions of the propeller is bound, by a relation, at the local speed of flow. A propeller is characterized by its step and its diameter. The step is the distance covered by water to generate a turn of propeller. The relation between the rate of flow and the number of revolutions of the propeller is called “calibration curve “of the propeller.

$V = has \ times N + b$

V current velocity in m/s
has not propeller in m
N number of revolutions of propeller a second
B speed of friction or speed of starting

propeller Dumas n°1-71605:

$N < 1,8 \ qquad \ qquad \ quad v = 0,0615 \ times N + 0,012$
$1,8 < N < 6,67 \ qquad v = 0,0572 \ times N + 0,02$

Each propeller must be used in the speed range for which it was damaged.

measure using an electromagnetic courantometer

Another type of apparatus can be employed to measure the rate of the flow. It is about the electromagnetic courantometer (electromagnetic probe associated with an electronic indicator speed).

Water, while moving in the magnetic field generated by the probe, produces an induced electromotive force proportional to the rate of the flow.

This type of material has many advantages: no part moving (weak risk of deterioration and reduced maintenance), direct indication rate of the flow, wide range measurable speeds -0,1 to + 6 m/s

This type of material is very interesting because it makes it possible to measure very low speeds and it is not obstructed by watery grasses.

The examination, carried out formerly manually, from now on is made automatically thanks to the computer tools;

measure using floats

This process is to be used:

* if it is impossible to use a winch (excessive speeds or depth, presence of materials in suspension or speeds too low, etc);

* if one wants to obtain a fast estimate of the flow.

The rate of the flow is given by measuring the rate of travel of floats released in the river. One can use natural or artificial floats: stoppers of fishing, water bubble, pieces of wood, tubes PVC stopped, bottles, table tennis balls, rubber ball, etc

The floats can be used for the determination the rates of the flow on the surface, in-depth, or average on a vertical

Q = K \ times V_m \ times S

K coefficient of discharge lower or equal to 1 (without dimension);
$V_m$ mean velocity (in m/s) data by the average time put by the floats to traverse a distance;
S wetted cross section in m ².

dynamic methods

hydraulic method

These methods can be implemented when the flow occurs on works of well defined structure for which the flow can be obtained to leave, in particular, the height of water measured with the upstream of the work.

The relation used to obtain the flow “Q” according to the height of water upstream “H” comes from results of tests carried out in laboratory or on the site, constituting the” calibration” (or the taring) of the work. The relation “Q=f (H)” can be appeared as a curve or a table giving the correspondence height-flow directly, or of a hydraulic formula comprising one or more coefficients resulting from the calibration.

The level of precision which one can await from these methods depends obviously on the care taken in the realization of measurements, but also:

of the quality of the preliminary calibration and, in the case of use of formulas, the choice of the adapted coefficient;
of the importance of the difference between the conditions which prevailed during the calibration and the conditions really met during measurement (in particular, characteristic of the work and conditions of flow).

On a River, one can meet several kinds of hydraulic works: outfalls, or thresholds, Valve S, openings, Scale with fish, etc One can gather them in two main categories:

calibrated work or certified copy of calibrated work;
work not calibrated.

The relation height of water-flow can be calculated without calibration for the regular thresholds.

Currently, the works on which the flow must be controlled in accordance with the article “L 232-5”, for the majority, are not calibrated. However, this situation should develop since the regulation imposes to the owners, during all new construction or of any renewal of concession, the installation of a calibrated device allowing the permanent control of the flow maintained in the river.

physical methods

method by dilution

The method by dilution is included in the category of the physical methods, since it is based on the taking into account of the variations of the concentration (physical property) of a tracer in water. However, the tracers employed are often chemical bodies which one proportions by chemical processes; from where name “chemical method” or “chemical gauging” to also qualify the method of dilution.

The method is here exposed only in a summarized and simplified way: one gives the general principle of one of the processes of measurement by this method (proceeded by instantaneous injection). From the start should be announced that this method, more than the others, requires much technicality and for implementation an effective, it is necessary to have followed a preliminary hands-on training or to call upon one or more people experienced to lead the operations.

These recommendations being given, the method of dilution present of the interesting potentialities, and when the conditions of measurement are optimal, the precision obtained are very satisfactory (about 5%). Moreover, it is particularly adapted to the small flows and represents, under certain conditions of site (in particular for certain sections of torrents of mountain), the only means of determination of the flow. It is also complementary method by exploration of the field speeds: the characteristics of flow being appropriate for the use of the method of dilution are precisely, for the majority, those which make disadvise the use of the method by exploration of the field speeds.

The method consists in injecting a tracer in solution (of known concentration) in a point of the river and following the evolution of its concentration to the level of a section located at the downstream. The distance between the point of injection and the selected section downstream must be sufficient so that the mixture of the tracer with water can be completely carried out. The minimal length of the section of river necessary to ensure this mixture is commonly called “length of good mixture”.

The method is particularly adapted to the torrent S of mountain on which mixing is important (swirls, sinuous beds, strong Rugosité), but it can also be employed on calmer Rivers, on the condition of taking a “larger Length of mixture”.

The injection is carried out in the form of a solution concentrated, either in a quasi instantaneous way, or continuously with constant flow.

Only the method by instantaneous injection (also called global method or method by integration) is given here, because:

its implementation remains simple (in particular, there does not need particular equipment of injection);
it requires the injection only one small quantity of tracer, from where less risks for water quality and a moderate cost.

Q = K \ left (\ frac {C_1} {C_2} \ right)

K coefficient of discharge lower or equal to 1 (without dimension)
$C_1$ concentration of the tracer to the point of injection
$C_2$ concentration of the tracer to the downstream of the point of injection

Various permanent stations of flow

Traditional stations

In fact stations make it possible to calculate the flow thanks to a univocal relation between the height and the flow. For a height H given, there are only one and one flow.

These stations shelter a power station of acquisition which measures the level with step of time fixes or variable and stores information in the form of file height-time

The teams of ground regularly take specific measurements of flow, thus describing all the hydrological modes (high, average and low waters).

Each measurement of flow makes it possible to obtain a couple (average height, flow). The whole of the couples (H, Q) makes it possible to plot a curve passing closest to the points, curved which is called the curve of taring. The curve is defined in the form of mathematical, parabolic or more unspecified functions in the shape of broken lines.

These curves evolve/move in time, and of specific measurements of flow can deviate from the ideal curve, one speaks then about détarage as for example during the period when the watery vegetation pushes. The work completed on a river, like a clearing out, can also make a curve of taring null and void.

A curve of taring is thus valid of a date on another date. Exploitation of files height - time depends on the quality of the curve: if a level is exceeded and that one does not know his flow corresponding, one speaks height except curve. In this case, one extrapolates the curve and one waits sometimes years before having the confirmation of this extrapolation

Average costs of an automatic station (H, T) télétransmise: (7500 € - 50.000 F)

Certain natural or channeled rivers do not have a curve of taring: the whole of the points (H, Q) is a group of dots. Certain debitmetric stations measure an instantaneous flow without needing curves of taring.

ultrasonic stations

The technique used is based on the measurement of the journey time of ultrasonic impulses crossing the river according to an angle compared to the current. Time according to the speed of water and the flow in is deduced.

This system is not adapted for measurements of flow in the broad and not very deep rivers (risk of reflection on surface or the bottom.
the river must be rectilinear over a length at least equal to 10 times its width. Its geometry must be stable in time. Water must be most homogeneous possible from the temperature point of view of.
the presence of bubbles of air caused by water falls must be avoided

Description of measurement

Several pairs of Transducteur S are assembled in opposition on banks, with various well defined depths, each transducer behaving alternatively like transmitter then like receiver of ultrasonic waves.

These waves are received by the opposite transducers of each pair after a time depend on the length of the way, speed C of the sound on this way and the component speed of water.

If water is calm, the two waves arrive simultaneously, but if it is agitated, one of the two impulses will arrive before the other. The moments of transit relating to the way considered are called $t_1$ and $t_2$ . As from these moments, the mean velocity of the water section can be calculated.

For an ultrasonic way, indeed, the component speed of average water along the way can be formulated in the following way: $t_1= \ frac {L} {C + V_p}$ and $t_2 = \ frac {L} {C - V_p}$ with:

L length of the way,
C speed of sound in water,
$V_p$ component mean velocity,
$t_1$ and $t_2$ time of way

$V_p$ is independent of C and can be expressed according to $t_1$ and $t_2$ :

V_p = L \ times \ frac {t_2 - t_1} {2t_1 \ times t_2}

The mean velocity of water

V_1

is then of:

V_1 = \ frac {V_p} {\ cos \ theta}

where $\ cos \ theta$ is the cosine of the angle between the direction of the flow and the way of the ultrasonic waves

The total flow is obtained by adding the results of these various measurements

$Q = \ sum_ {i=0} ^ {N} Q_i \ times S_i$

N many ultrasonic cords plus a
$Q_i$ partial flow
$S_i$ surface between cords

cost of an ultrasonic station: 40.000 € with 150.000 €

electromagnetic stations

This technique is founded on electromagnetic induction. Electrodes posed in the banks of the river measure a voltage generated by the drainage duct through a vertical magnetic field creates by an electromagnetic rolling up.

The river must be papered of an insulating heavy membrane. The Faraday's law $e = - \ frac {dF} {dt}$ makes it possible to write:

$Q = F (F)$

F magnetic flux

The electromagnetic technique is usable when water is not homogeneous because of a bad mixture of two sources, or when the channel is blocked by mud, grasses, deposits or remains

Electromagnetic measurement is not affected by the passage of barges, the modification of the deposits and the instability of the funds. It insensitive with is pushed watery vegetation, with the bubbles of air and the suspended matter. It is not affected by variable temperatures, nor by oblique currents.

On the other hand this measurement can be disturbed by electric lines or radio transmissions, sources of interference

The river must be rectilinear at a distance equal to 3 times its width and make less than 20 meters broad because of the cost price.

cost of an electromagnetic station: equal to or higher than 150.000 €

Finality of hydrometry

national Bank of the flows: bank HYDRO
general Connaissance: management of water (dryness, Flood S)
Modelings of the Raw S
Dimensioning of works of art
Measurement of pollution
Networks of Cleansing

See too

Internal bonds

External bond

Charter quality of hydrometry - Code of good practices synthetic document of September 1998 (format pdf).
hydro Bank, flows filed and current on all the French areas

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