STREAM GAUGING TECHNIQUES

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1 World Meteorological Organization -- Working together in weather, climate and water STREAM GAUGING TECHNIQUES Presentation based on the WMO Manual on streamgauging (2010), Volume II Chapter 2, with additional material provided by Alexandre HAUET (EDF DTG Grenoble, France) IAHR WMO IAHS Training Course on Stream Gauging Lyon -- September 2-4,

instream flow) / Punctual studies Discharge measurement by field hydrologist Knowing the discharge of a river continuously (and in

2 STREAM GAUGING WHAT FOR?? Different needs : Knowing the discharge of a river at a given place and a given time: Check environmental regulation (ecological instream flow) / Punctual studies Discharge measurement by field hydrologist Knowing the discharge of a river continuously (and in real-time): Use of a rating curve : Measurement of the water stage at a station, continuously and in real time Establishment of a stage-discharge relationship (rating curve), based on gaugings Computation of the discharge from the measured stage 2

3 RIVERS ARE COMPLEX Complexity at the spatial scale: Upstream Downstream Complexity at the time scale 17m 3

directly the discharge continuously rating curve Not a unique

4 RIVERS ARE COMPLEX Interaction with the environment: How the Mississippi has changed course No instrument for measuring directly the discharge continuously rating curve Not a unique gauging methods : Different methods / Field hydrologist toolbox 4

5 LET SPEAK THE SAME LANGUAGE Hydraulic description of a river cross-section: 5

6 LET SPEAK THE SAME LANGUAGE Hydraulic description of a river cross-section: Geometric characteristics River width Wetted area Water depth Bathymetry 6

7 LET SPEAK THE SAME LANGUAGE Hydraulic description of a river: Geometric characteristics Cinematic characteristics River width Flow velocity Wetted area Water depth Bathymetry 7

8 LET SPEAK THE SAME LANGUAGE Hydraulic description of a river: Geometric characteristics Cinematic characteristics Discharge : Q = wetted area * mean velocity [m 3 /s] [m 2 ] [m/s] Volume per unit time River width Flow velocity Wetted area Water depth Bathymetry 8

LET SPEAK THE SAME LANGUAGE Discharge is : a

the basin a characteristic quantity for water resource

cooling of power plants quantifying flood risk Rivers.

transport, biology, ecology, pollution, fishing But it

9 LET SPEAK THE SAME LANGUAGE Discharge is : a characteristic quantity of the state of the river and the basin a characteristic quantity for water resource management drinking water irrigation hydroelectricity / cooling of power plants quantifying flood risk Rivers.. It s not only a matter if discharge Sediment transport, biology, ecology, pollution, fishing But it is out of the scope of this course! Desman of Pyrénées Jiang Zi Gjiang Mékong 9

Index-velocity method (course of Jérôme Le Coz) Precalibrated measuring structures (course of Roberto Ranzi) Indirect determination of

10 STREAM GAUGING METHODS 3 classical methods for stream gauging: Volumetric method Tracer dilution method (slug and constant-rate injections) Velocity-area method (current meter and ADCP) Other ways for measuring stream discharge : Non-intrusive methods (course of Ichiro Fujita) Index-velocity method (course of Jérôme Le Coz) Precalibrated measuring structures (course of Roberto Ranzi) Indirect determination of flood peak (course of Jérôme Le Coz) WMO reference documents : WMO Manual on Streamgauging (2010) WMO Guide to Hydrological Practices, Volume I 10

PRELIMINARY OPERATIONS BEFORE GAUGING Prior site visits, especially for new sites Make sure that the whole stream discharge is measured Secondary branch during flood, for example Find the right

! Find the right gauging method To make the best discharge measurement safely Keep a constant discharge during the measurement Adapt the gauging method to the dynamics of the flow Avoid more than 10%

11 PRELIMINARY OPERATIONS BEFORE GAUGING Prior site visits, especially for new sites Make sure that the whole stream discharge is measured Secondary branch during flood, for example Find the right gauging site Not necessary at the exact location of the stage gauge, but not too far! Depending on the gauging method used Depending on the river morphology / vegetation Allowing to work safely!! Find the right gauging method To make the best discharge measurement safely Keep a constant discharge during the measurement Adapt the gauging method to the dynamics of the flow Avoid more than 10% of discharge variation during the gauging Monitor the stage variation during the gauging Computation of mean gauge height H H = σ t h t q t Q Have equipment well maintained and in working order Course Assessment of the performance of flow measurement instruments on Wednesday And trained staff knowing how to use it!! 11

12 VOLUMETRIC METHOD Q = Volume per unit time Measuring the time (t) to fill of a container of known capacity (Vol) Q = V / t Equipment required calibrated container and stop-watch Only applicable to small discharges Q < 10 L/s..but it is the most accurate method of measuring such flows! 12

TRACER DILUTION METHOD Based on the mass conservation Determining the degree of dilution by the flowing water of an added tracer solution Tracers used : salt conductivity or

stream (or present only in negligible quantities); not decomposed in the stream s water and not retained or absorbed by sediment, plants or organisms; concentration can be

13 TRACER DILUTION METHOD Based on the mass conservation Determining the degree of dilution by the flowing water of an added tracer solution Tracers used : salt conductivity or Fluorescent tracers (Uranine, Rhodamine) fluorescence Requirements for the tracer : dissolves readily in the stream s water at ordinary temperatures; absent in the water of the stream (or present only in negligible quantities); not decomposed in the stream s water and not retained or absorbed by sediment, plants or organisms; concentration can be measured accurately by simple methods; harmless to humans, animals and vegetation in the concentration it assumes in the stream. 2 injections methods : Slug injection and Constant rate injection 13

TRACER DILUTION METHOD Principle of the slug injection method Injection of a known mass M of tracer Tracer is dilute onto a cloud Mass conservation M = Volume of the cloud V mean Concentration of the

14 TRACER DILUTION METHOD Principle of the slug injection method Injection of a known mass M of tracer Tracer is dilute onto a cloud Mass conservation M = Volume of the cloud V mean Concentration of the cloud C When mixing length is reached, V = stream discharge Q duration of the cloud t M M = V C = Q t C Q = M t C = C ) t C 0 )dt M = Q t C M = V C M C C 14 IAHR / WMO / IAHS 0 International Streamgauging course t

TRACER DILUTION METHOD Principle of the constant rate injection method Injection of a tracer of concentration c 1 at a constant discharge of q during a long time Container equipped with a Marriott

15 TRACER DILUTION METHOD Principle of the constant rate injection method Injection of a tracer of concentration c 1 at a constant discharge of q during a long time Container equipped with a Marriott vessel Tracer is dilute onto the river discharge Q When mixing length is reached, concentration is homogeneous on the cross-section C 2 Mass conservation c 1 q = C 2 (Q + q). As q Q, Q = q c 1 C 2 C 2 C 15 t

TRACER DILUTION METHOD Recommendations / limitations Adequate mixing of the tracer with the stream water in a short length of channel Adapted for

cross-sectional area cannot be accurately measured or is changing during the measurement.

16 TRACER DILUTION METHOD Recommendations / limitations Adequate mixing of the tracer with the stream water in a short length of channel Adapted for turbulent streams Mountainous streams, bends or abrupt constrictions Supercritical flows Flood flow (if not too much suspended sediments) If cross-sectional area cannot be accurately measured or is changing during the measurement. Fish passage Slug injection : fast method Adapted for unsteady flow measurement Not adapted for : Fluvial flow with long mixing length High sediment concentration flows Adsorption and masking Avoid large dead-water zones 16

17 TRACER DILUTION METHOD Prehistoric first dilution gauging 17

18 VELOCITY-AREA METHOD USING CURRENT METERS Q = Cross sectional area X Average velocity Measure cross sectional area Streambed bathymetry Sufficient sampling to catch the shape of the wetted area River width Flow velocity Wetted area Water depth Bathymetry 18

19 VELOCITY-AREA METHOD USING CURRENT METERS Q = Cross sectional area X Average velocity Measure cross sectional area Streambed bathymetry Sufficient bathymetric sampling to catch the shape of the wetted area Determine average velocity over the cross-section Stream velocity varies through the stream profile Sufficient sampling to determine the average velocity Velocity contour : Low vel. High vel. highest velocity at the top and the middle energy lost due to friction along the stream channel lowest velocity close to the bottom and the banks 19

20 VELOCITY-AREA METHOD USING CURRENT METERS Selection of the gauging cross-section A stream reach as simple as possible subcritical flow uniform reach upstream and downstream / No singularity (bridge, weir, dam, gorges ) A cross section perpendicular to the flow General recommendations: velocities at all points are parallel to one another and at right angles to the cross-section curves of distribution of velocity in the section are regular in the vertical and horizontal planes; velocities greater than m/s; depth of flow greater than 0.3 m; regular and stable streambed; no aquatic growth, minimal formation of slush or frazil ice 20

VELOCITY-AREA METHOD USING CURRENT METERS Selection of the gauging cross-section Small modifications of the cross section are possible Small dikes

21 VELOCITY-AREA METHOD USING CURRENT METERS Selection of the gauging cross-section Small modifications of the cross section are possible Small dikes to concentrate the flow Removing stones from the bed Stream modification must be limited and reversible Not disturbing the fishes (spawning areas) 21

EQUIPMENT Different supports depending on

Section fully accessible by foot, distance

Gauging truck Retractable arm mounted on a

22 EQUIPMENT Different supports depending on the accessibility of the river Wading rod : Section fully accessible by foot, distance across the section measured with a tape Gauging truck Retractable arm mounted on a truck sounding weights Cableways Carrier cable permanently stretched across a section Equipped with a sounding weight or a cable car 22

MECHANICAL CURRENT METERS Velocity by counting revolutions of rotor during a short-time

cup type with a vertical shaft (Price AA) propeller type with a horizontal shaft (Ott

normal to the cross-section Larger section Lower velocity component Same discharge

23 MECHANICAL CURRENT METERS Velocity by counting revolutions of rotor during a short-time period V flow = f V rotor current meter calibration Two types of current meter rotors cup type with a vertical shaft (Price AA) propeller type with a horizontal shaft (Ott C2) Contact to generate an electric pulse for indicating the revolutions of the rotor Can measure from 0.05 to 5 m/s, depending on the rotor type Mind measuring the component of velocity normal to the cross-section Larger section Lower velocity component Same discharge Advantages : Mechanical : one can see when it is not working properly or not Drawbacks : Need care and periodic verification of the moving parts Susceptible to vegetation 23

ELECTROMAGNETIC CURRENT METERS Principle : Water (conductor) moving

principle) Velocity of the water is proportional to the electric

2cm/s Advantages : Can measure low velocities No moving part less

24 ELECTROMAGNETIC CURRENT METERS Principle : Water (conductor) moving through a magnetic field produce an electric current (Faraday principle) Velocity of the water is proportional to the electric current produced Velocity range : 0 to 6 m/s; accuracy of 2% +/- 2cm/s Advantages : Can measure low velocities No moving part less maintenance Can measure with vegetation Drawbacks : Susceptible to electrical interference 24

ACOUSTIC CURRENT METERS Principle Transmit acoustic signals into a water column with a

frequency of the backscatter signals f Computation of radial velocity on each beam f f V cos α

25 cm/s Advantages : No moving part less maintenance Measurement of 2D or 3D velocity

25 ACOUSTIC CURRENT METERS Principle Transmit acoustic signals into a water column with a frequency f Signal is backscattered by particles moving in the water Doppler effect change of frequency of the backscatter signals f Computation of radial velocity on each beam f f V cos α = f Computation of flow velocity Velocity range -0,2 m/s à 2,4 m/s; accuracy : 1% ± 0.25 cm/s Advantages : No moving part less maintenance Measurement of 2D or 3D velocity components (Vx, Vy, Vz) and backward flow Can measure very low velocities (2 cm/s) Drawback : Susceptible to vegetation 25

26 VELOCITY-AREA METHOD USING CURRENT METERS Measurement protocol Temporal sampling of the velocity Streams : turbulent flow average turbulent velocities Exposure time of at least 30s to get average velocity And at least 100 rotations for a mechanical current meter Optimal exposure time should be evaluated for each measurement (increase exposure time when velocity decrease) Standard Iso748 : 26

VELOCITY-AREA METHOD USING CURRENT METERS Measurement protocol : measuring the stream bathymetry Spatial sampling of the cross section bathymetry at n verticals depth of each vertical d i

27 VELOCITY-AREA METHOD USING CURRENT METERS Measurement protocol : measuring the stream bathymetry Spatial sampling of the cross section bathymetry at n verticals depth of each vertical d i Spatial sampling of the cross section velocity distribution at the n verticals Velocity distribution method Reduced point method Integration method depth-averaged velocity of each vertcal Vi 27

28 VELOCITY-AREA METHOD USING CURRENT METERS Measurement protocol : computing depth-averaged velocity at vertical i Velocity distribution method Important number of velocity measurement along each vertical between surface and bed 28

VELOCITY-AREA METHOD USING CURRENT METERS Measurement protocol : computing depth-averaged velocity at vertical i Velocity distribution method

bed and the surface (log- or power-law) V i = d i 0 V measured d i number and spacing of the points are chosen as to define accurately the

29 VELOCITY-AREA METHOD USING CURRENT METERS Measurement protocol : computing depth-averaged velocity at vertical i Velocity distribution method Important number of velocity measurement along each vertical between surface and bed Interpolation between measured velocity Extrapolation to the bed and the surface (log- or power-law) V i = d i 0 V measured d i number and spacing of the points are chosen as to define accurately the velocity distribution in each vertical with a difference in readings between two adjacent points of not more than 20 % with respect to the higher value 29

VELOCITY-AREA METHOD USING CURRENT METERS Measurement protocol : computing depth-averaged velocity at vertical i Reduced point method 1 to 6 velocity

5 (V 0.2di + V 0.8di ) 3 pts method : V i = 0.25 (V 0.2di + 2 V 0.6di + V 0.8di ) 5 pts method : V i = 0.1 (V surface + 3 V 0.2di + 3 V 0.6di + 2 V 0.

30 VELOCITY-AREA METHOD USING CURRENT METERS Measurement protocol : computing depth-averaged velocity at vertical i Reduced point method 1 to 6 velocity measurements per vertical At a relative depth below the free surface Computation of V i with algebraic formula 1 pt method : V i = V 0.6di 2 pts method : V i = 0.5 (V 0.2di + V 0.8di ) 3 pts method : V i = 0.25 (V 0.2di + 2 V 0.6di + V 0.8di ) 5 pts method : V i = 0.1 (V surface + 3 V 0.2di + 3 V 0.6di + 2 V 0.8di + V bed ) 6 pts method : V i = 0.1 (V surface + 2 V 0.2di + 2 V 0.4di + 2 V 0.6di + 2 V 0.8di + V bed ) Surface velocity method : V i = 0.85 V surface Example with 2 pts method : 30

VELOCITY-AREA METHOD USING CURRENT METERS Measurement protocol : computing depth-averaged velocity at vertical i Integration method using mechanical current meters current meter is lowered and raised

31 VELOCITY-AREA METHOD USING CURRENT METERS Measurement protocol : computing depth-averaged velocity at vertical i Integration method using mechanical current meters current meter is lowered and raised through the entire depth at each vertical at a uniform rate. average number of revolutions per second is determined depth-averaged velocity the speed at which the meter is lowered < 5% of the flow velocity and between 0.04 and 0.10 m/s two complete cycles are made in each vertical if the results differ by more than 10 per cent, the measurement is repeated. restriction of use : depth > 1 m velocities > 1 m/s the integration method should not be used with a vertical axis current meter 31

32 COMPUTATION OF DISCHARGE WITH CURRENT METERS Mid-section method A cross-section with n verticals i of known depth d i and depth-averaged velocity V i 32

33 COMPUTATION OF DISCHARGE WITH CURRENT METERS Mid-section method A cross-section with n verticals i of known depth d i and depth-averaged velocity V i The relative location of each vertical b i is measured 33

COMPUTATION OF DISCHARGE WITH CURRENT METERS Mid-section method A cross-section with n verticals i of known depth d i and depth-averaged velocity V i The relative location of each vertical b i is

34 COMPUTATION OF DISCHARGE WITH CURRENT METERS Mid-section method A cross-section with n verticals i of known depth d i and depth-averaged velocity V i The relative location of each vertical b i is measured For vertical i, a subsection is defined with : A width W i of (b i+1 -b i-1 )/2; a depth of d i, a mean velocity of V i The discharge q i of the subsection centered on i is q i = W i d i V i 34

35 COMPUTATION OF DISCHARGE WITH CURRENT METERS Mid-section method A cross-section with n verticals i of known depth d i and depth-averaged velocity V i The relative location of each vertical b i is measured For vertical i, a subsection is defined with : A width W i of (b i+1 -b i-1 )/2; a depth of d i, a mean velocity of V i The discharge q i of the subsection centered on i is q i = W i d i V i Total discharge is the sum of the q i : Q = σ q i With an interpolation of the velocity between the banks and the first and last verticals 35

36 COMPUTATION OF DISCHARGE WITH CURRENT METERS About the sampling Enough verticals to have a good sampling of the bed bathymetry Enough verticals and points per vertical to have a good sampling of the velocity but not too long to be in a steady flow.. find a compromise between sampling and duration Some Rules of thumb : No general rules! Try to detail areas with strong gradients of bathymetry or velocity, especially if they contribute significantly to the total flow The interval between any two verticals should not be greater than 1/20 of the total width The discharge of any subsection should not be more than 10% of the total discharge. 36

ADCP mounted on a float, generally pointing down Sending an ultrasonic acoustic wave in the water Backscatter by

37 STREAM GAUGING WITH AN ADCP ADCP : Acoustic Doppler Current Profiler Ultrasonic measurement (300 à 3000 khz) Sonar principle to measure the river bathymetry wetted area Doppler shift to measure flow velocity Q Profiler : ADCP mounted on a float, generally pointing down Sending an ultrasonic acoustic wave in the water Backscatter by particles in suspension in the water Analyze of the Doppler shift between the transmitted and the backscatter signals 37

38 HOW AN ADCP MEASURES THE WATER DEPTH Sonar principle : Peak of retuned intensity when the echo hit the river bed 38

39 HOW AN ADCP MEASURES THE WATER VELOCITY Let postulate that the ADCP is not moving ADCP transmits an ultrasonic pulse in the water Pulse is backscattered by particles in the water ADCP received backscattered echo Analysis of Doppler shift between transmitted and backscattered pulses velocity of the particles Basic hypothesis: particles are advected by the water velocity of the particle = velocity of the water 39

40 BACKSCATTERED PARTICLES Maximum backscatter for d=0,4mm if F=1200kHz d=0,2mm if F=2400kHz zooplankton and small particles 40

41 HOW AN ADCP MEASURES THE WATER VELOCITY Doppler shift radial velocity Component of velocity in the beam axis V cos α = C f / f 0 with f = f 1 f 0 f 0 : frequency of the transmitted pulse f 1 : frequency of the backscattered pulse C : speed of the sound. C depends on the water temperature it is crucial to measure accurately the water temperature ADCP transducer 41

pulse f 1 : frequency of the backscattered pulse C : speed of the sound.

42 HOW AN ADCP MEASURES THE WATER VELOCITY Doppler shift radial velocity Component of velocity in the beam axis V cos α = C f / f 0 with f = f 1 f 0 f 0 : frequency of the transmitted pulse f 1 : frequency of the backscattered pulse C : speed of the sound. C depends on the water temperature de l eau it is crutial to measure accurately the water temperature ADCP transducer 42

HOW AN ADCP MEASURES THE WATER VELOCITY Doppler shift radial velocity Component of velocity in the beam axis How measuring 3D velocity components (North / East / Vertical components) Geometric

43 HOW AN ADCP MEASURES THE WATER VELOCITY Doppler shift radial velocity Component of velocity in the beam axis How measuring 3D velocity components (North / East / Vertical components) Geometric configuration : 2, 3, 4 divergent beams Measurement of radial velocity on each beam trigonometric calculation to obtain 3D velocity under the assumption that the velocity is homogeneous on the 3 beams 43

44 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Measurement of the radial velocity on each beam 44

45 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Measurement of the radial velocity on each beam 45

46 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Measurement of the radial velocity on each beam Beam A Beam B 46

47 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Plan view : in the plan of the beams

48 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Plan view : in the plan of the beams 3-4 Beam 4 Beam 3 V water V water 48

49 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Plan view : in the plan of the beams 3-4 V water = V Vz Beam 4 Beam 3 Vz Vz V 3-4 V

50 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Plan view : in the plan of the beams 3-4 Beam 4 Beam 3 V water = V Vz On Beam 3: V3 = V 3-4 *sin + Vz*cos Vz Vz V 3-4 V

51 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Plan view : in the plan of the beams 3-4 Beam 4 Beam 3 V water = V Vz On Beam 3: V3 = V 3-4 *sin + Vz*cos On Beam 4 : V4 = -V 3-4 *sin + Vz*cos Vz Vz V 3-4 V

52 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Plan view : in the plan of the beams 3-4 Beam 4 Beam 3 V water = V Vz On Beam 3: V3 = V 3-4 *sin + Vz*cos On Beam 4 : V4 = -V 3-4 *sin + Vz*cos V 3-4 = (V3-V4)/(2*sin ) Vz=(V3+V4)/(2*cos ) Vz Vz V 3-4 V

53 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Plan view : in the plan of the beams y 53

54 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Plan view : in the plan of the beams 1-2 Beam 1 Beam 2 V water = V Vz On Beam 1: V1 = V 1-2 *sin + Vz*cos On Beam 2 : V2 = -V 1-2 *sin + Vz*cos V 1-2 = (V1-V2)/(2*sin ) Vz=(V1+V2)/(2*cos ) Vz Vz V 1-2 V

55 HOW AN ADCP MEASURES THE WATER VELOCITY Geometric configuration : Stationary ADCP Plan view: With Beams 1 and 2, one can compute V 1-2 and Vz With Beams 3 and 4, one can compute V 3-4 and Vz One can compute 3 components of V water 2 measurements of Vz Difference between those measurements «error velocity» If strong error velocity, no homogeneity invalidated measurement

56 HOW AN ADCP MEASURES THE WATER VELOCITY ADCP = Profiler : Ability to measure a profile of the water currents throughout the water column range-gating the backscattered signal in time : ADCP assigns discrete sections of the echo record to distinct sections of the water column depth cells or bins ADCP assigns separate measurements of the three components of water currents to different depth cells and generates a water-current profile 56

UNMEASURED AREAS ADCP measurement range: when frequency : TRDI StreamPro 2400

ADCP: Transducer depth Distance (time) the emitted sound travels while internal

the transmission and become quiescent enough to accurately record the

57 UNMEASURED AREAS ADCP measurement range: when frequency : TRDI StreamPro 2400 khz : 6m maxi TRDI RioGrande 600 khz : 60m maxi when sediment load Close to the ADCP: Transducer depth Distance (time) the emitted sound travels while internal electronics prepare for data reception and the transducers stop vibrating from the transmission and become quiescent enough to accurately record the backscattered acoustic energy Flow disturbance: Hydraulic of the flow around the ADCP 57

58 UNMEASURED AREAS Close to the river bed «side-lobes» hit the river bed before the main lobe Shortest distance 58

59 UNMEASURED AREAS Close to the river bed «side-lobes» hit the river bed before the main lobe α Side-lobe blanking d = P (1 cos ) 6% for = 20 14% for = 30 59

60 UNMEASURED AREAS Close to the river bed «side-lobes» hit the river bed before the main lobe Side-lobe blanking d = P (1 cos ) 6% for = 20 14% for = 30 Mesured, but filtered 60

61 UNMEASURED AREAS Bad bins / bad ensembles error velocity > threshold correlation < threshold 61

62 MEASUREMENT OF THE ADCP DISPLACEMENT The ADCP moves across the river cross-section V water/bed = V water/adcp + V adcp/bed It measures the section area (m²) Discharge (m 3 /s) It measures the velocity distribution (m/s) The displacement of the ADCP must be measured 62

is not moving, the measured Doppler shift is directly related to the velocity of the boat

63 MEASUREMENT OF THE ADCP DISPLACEMENT Bottom Tracking: Based on the Doppler-shift Measures the Doppler shift of the acoustic pulses reflected from the streambed Assuming the streambed is not moving, the measured Doppler shift is directly related to the velocity of the boat GPS: Bottom-tracking pulses are sent before the water measurement pulses Supplementary equipment 63

64 DISCHARGE COMPUTATION To compute the discharge : only the angle between the water-velocity and the boat velocity vectors is needed. where T is the total time for which data were collected; D is the total depth; V f is the mean water-velocity vector; V b is the mean boat-velocity vector; θ is the angle between the water-velocity vector and the boat-tracking vector; dz is the vertical differential depth; and dt is differential time. 64

65 DISCHARGE COMPUTATION With Bottom Tracking, the ADCP : measures the water velocity in the beams reference system Measures it s displacement in the beams reference system Same reference system: direct computation Compass of the ADCP not used for the discharge computation 65

DISCHARGE COMPUTATION With GPS, the ADCP Measures the water velocity in the beams reference system Measures it s displacement in a geographical reference system (North / East) Need to link the 2

66 DISCHARGE COMPUTATION With GPS, the ADCP Measures the water velocity in the beams reference system Measures it s displacement in a geographical reference system (North / East) Need to link the 2 coordinates systems: Compass of ADCP used to orient the beams on the magnetic North Magnetic declination to find the True North Prefer Bottom Tracking positioning except : If the quality of the BT is bad (aquatic vegetation, for example) If the bed of the river moves (bed load) we'll discuss it again later 66

67 UNMEASURED DISCHARGE Areas without measured velocity Top and bottom blankings Close to the banks Missing cells or ensembles Estimation of the unmeasured areas Unmeasured areas must be as small as possible Importance of the choice of the measurement section! 67

68 UNMEASURED DISCHARGE Invalid cells: interpolation from neighboring cells Invalid ensembles: interpolation from neighboring ensembles 68

69 UNMEASURED DISCHARGE Top and bottom blankings Extrapolation of vertical velocity profiles At the top: Fitted Power law Linear slope Constant Fond : Fitted Power law «No slip» power law Power Linear slope Constant ADCP measurements Power Power No slip 69

70 UNMEASURED DISCHARGE Select the best extrapolation: Fitted on the measured velocities Software «Qrev» of the USGS Measure carefully the transducer depth Depends on the instrument, the float, etc 70

71 UNMEASURED DISCHARGE Extrapolation of the discharge close to the banks Q bank = C. V m. L. d m C bank coefficient (triangle : 0,35, rectangle : 0,91) V m closest measured velocity (average on n shore ensembles) d m closest measured velocity depth L distance to the bank River bed River bed 71

72 UNMEASURED DISCHARGE Give it some attention or not! Depends on the relative weight of each components of computed discharge Key point for uncertainty estimation Q measured / Q total > 80% uncertainty 5-10% Q measured / Q total > 60% uncertainty 10-15%; give attention to the computed discharge Q measured / Q total < 50% find an other measurement site (if possible) ADCP is well adapted for subcritical, quite deep (> 0.6 m) flows 72

MOVING BED PROBLEM Bottom-tracking assumes that the bed is not moving If bedload, the bed moves: The moving bed bias introduces an apparent upstream boat velocity, which reduces

73 MOVING BED PROBLEM Bottom-tracking assumes that the bed is not moving If bedload, the bed moves: The moving bed bias introduces an apparent upstream boat velocity, which reduces the calculated downstream water velocity and the corresponding discharge will be biased low. Moving bed error Solution: integration of a GPS to measure the velocity of the ADCP : 73

74 MOVING BED PROBLEM If you don t have a GPS you can correct the moving bed error! Stationary moving bed analysis (SMBA) Keep the ADCP at a fixed location during 5 minute minimum Moving bed BT course goes upstream, travelling an upstream distance D Moving bed velocity = D / duration of the test Repeat the analysis for different location across the section Computation of the averaged moving bed velocity over the cross section D 74

course distorted upstream Distance D between departure and arrival Moving bed velocity = D / duration of the

75 MOVING BED PROBLEM If you don t have a GPS you can correct the moving bed error! Loop analysis: Round trip across the section Departure and arrival exactly at the same location Moving bed BT course distorted upstream Distance D between departure and arrival Moving bed velocity = D / duration of the test Averaged moving bed over the cross-section The compass must be well calibrated! Moving bed corrections are included in the ADCP softwares 75

76 MOST USED INSTRUMENTS 76

77 MOST USED INSTRUMENTS TRDI StreamPro 2400 khz / maximum depth: 6 m For shallow and slow flows TRDI RioPro 1200kHz for the 4 velocity beams 600kHz for the vertical bathymetric beam Maximul depth : 25m Automatic adaptation of the cells size SonTek S5 and M9 1 vertical bathymetric beam at 0.5 Mhz 4 velocity beams at 3.0Mhz 4 velocity beams at 1.0Mhz Automatic adaptation of the cells size and of the frequency 77

78 ADCP: EXAMPLES OF DEPLOYMENT Floats Remote controlled boats Attached to a holder 78

79 ADCP: RULES OF THUMB 1. Before the measurement: Choose the right measurement section ADCP system tests and compass calibration Put the ADCP in the river (check the measured temperature) 2. Evaluation of the cross-section: round trip without recording Is the section OK for and ADCP gauging? 3. Measurement of the moving bed: Stationary or Loop analysis No Moving Bed use BT Moving Bed use GPS / correct moving bed using SMBA or Loop if no GPS 4. Gauging (finally!) Go slow and smooth, over a straight transect Do not get too close to the banks: keep at least two valid cells Repeat at least 4 transects (WMO) France : at least 6 reciprocal transects USGS : at least 2 reciprocal transects and a combined duration of all transects of at least 720 s 79

80 ADCP: SOFTWARES TRDI : WinRiver II For collecting and postprocessing data SonTek : RiverSurveyor Live For collecting and postprocessing data USGS s Qrev For quality analysis and postprocessing 80

81 ADCP: SOFTWARES USGS s Qrev Quality Analysis / Quality Control Green OK / Yellow take care / Red problem 81

82 ADCP: SOFTWARES USGS s Qrev Quality Analysis / Quality Control Green OK / Yellow take care / Red problem Estimation of the uncertainty 82

83 STREAM GAUGING METHODS What should you remember? No single solution: a toolbox of methods for the field hydrologist Volumetric method very small discharges (< 10L/s) Dilution method supercritical, very turbulent flows Velocity-area using current meters subcritical shallow flows Velocity-area using ADCP subcritical deeper flows Choice of the method to use depends on : River hydraulic The site configuration Hazardousness, accessibility of the gauging site Material available: Different prices : mechanical current meter = ; ADCP = 30 k 83

84 STREAM GAUGING METHODS What should you remember? No single solution: a toolbox of methods for the field hydrologist Volumetric method very small discharges (< 10L/s) Dilution method supercritical, very turbulent flows Velocity-area using current meters subcritical shallow flows Velocity-area using ADCP subcritical deeper flows Choice of the method to use depends on : River hydraulic The site configuration Hazardousness, accessibility of the gauging site Material available: Different prices : mechanical current meter = ; ADCP = 30 k 84

Stream flow measurements are fundamental in water management and. Different streams and rivers require

Stream Flow Measurements Introduction Stream flow measurements are fundamental in water management and projects Different streams and rivers require appropriate flow measurement methods Weirs and Flumes