the hydrometric station - whycos portal (@ wmo ... : the hydrometric station introduction 1. natural...

Module 1 : Hydrometry

4th- 9th December 20062iE / Groupe EIER-ETSHER,

Project Regional Center

IRD - Unité OBHI (Observatoires Hydrologiques et Ingénierie)

1st part : The hydrometric station

WMO / OMM

Volta-HYCOS Project


Introduction

1. Natural river beds

2. Hydrometric station

3. Station file

4. Acquisition, recording and transmission of hydrological data

• Hydrometry : methology and techniques for water level and flow measurements in rivers

• Hydrometric station : Cross section of a river bed where the following are measured:– Free water surface level:h (m)– Flow of the river: Q (l/s or m3/s)

• Difficulty in continuous flow measurement (technology exists but it’s expensive – eg ultrasonic flow meters)

Continuous water level measurements

Flow measurements to determine rating curves at station Q=f(h)

Introduction

• Principal geometric characteristics• The River bed, a dynamic and evolving

system• River bed’s roughness

1. Natural river beds

1. Natural river bed

The geometry of the river bed : the plan

Dentritic bed:Channels and islandsRiver has high slope and high sediment content(Mountaneous region)

Meandering bed:Sinusoïdal channelRiver has mild slope(Alluvial plain)


Major and minor beds

minor bed

major bed

The geometry of the river bed: longitudinal profile


Principal characteristics :•Slope of the bed : I•Water surface slope : J

Staff gaugeFree water surface

The geometry of the river bed: the cross section


Principal characteristics :•Flow area : A•Wetted perimeter : p•Hydraulic radius: R = A/p

Free water surface


•Lateral movement of channel (erosion of the banks)•Changes in longitudinal and cross section(siltation / scouring)

Example : changes in the Allier river channel (France)

Present river channel

Channel in 1946

The River bed, a dynamic and evolving system in space and time


Ex2 : Effect of siltation on the rating curve of a station during recession

Ex 1 : Effect of scouringon the rating curve of a station during recession

Rating curve Limnigraph

Rating curve Limnigraph


Variations in morphology of river bed can be caused by man :

Changes in sediment equilibrium

Consequences on the morphology of the river (Ex : Incision of the bed)

•Sand winning•Channelisation•Construction of dams•…

Changes in the rating curve at a station

The river bed, a dynamic and evolving system in space and time


Sandy river bed : Bittou station(river Nohao, Nakambe basin, Burkina)

Rocky river bed : Kara station(river Kara, Oti basin, Togo)

River bed’s roughness


20 à 100.050 à 0.100Natural river bed with vegetation

330.030Clean and rough natural river bed

500.020Clean and smooth natural river bed

n State of river bed

Manning’s formula (uniform flow):Q = A*V = A*(R2/3 * S1/2 * 1/n)

•A : Flow area •R : Hydraulic radius•S : Water surface slope (approximatively equal to slope of river bed in uniform flow)

n : Coefficient of ManningK = 1/n

River bed’s roughness

• Principles of operation(conditions of hydraulic controls)

• Criteria of d’implantation

2. The hydrometric station

Boromo station (Mouhoun, Burkina)

• The flow in a river section is said to be under the influence of a control when the flow depends only on the water level at a section (provided the geometry of the channel does not change)

• 2 types of controls: – Channel control (uniform flow)– Section control (critical flow)


Principles of operation(conditions of hydraulic controls)


Channel control (uniform flow) :

•Hydraulic and geometric conditions :•Regular channel reach•Flow sufficient to occupy the main channel•Slope sufficient to ensure a good transition of flow

•Under these conditions there exists uniform flow:•The flow characterisitcs are the same from one section to the other (same flow area, same water level, same velocity, …)•The slope of the water surface is about the same as the slope of the river bed

Channel control (uniform flow):

•Under these conditions, the flow is given by the Manning’s formula:

Q = A*V = A*(R2/3 * S1/2 * 1/n)

R : hydraulic radiusS : Water surface slope (approximatively equal to slope of river bed in uniform flow)1/n : Manning’s coefficient


K=1/n (Manning’s coefficient)V : Velocity S : SlopeR : Hydraulic radius

Monogram to calculate V

Section control (critical flow)

• Critical flow conditions occur in the channel section :

Q is only a function of hupstream : (no influence of downstream head)


Differents types of section controls : A) sil, B) contraction, C) change in slope


Natural control points

Boulders

Sill of an unstable control section

Sill of a stable control sectionLayers of calcareous materials

Level of 0 flowFlood level

Stable downstream control

Sections and controls

Upstream control

FlowDownstream control

Still of unstable control section

Artificial control structures (weirs)


1 : Submerged weir

2 : Unsubmerged weir1 & 2: Free Fall : the weir works

Hydraulic control

Summary:

•A flow in a channel reach is under hydraulic control when for each flow there are corresponding well defined hydraulic characteristics, in particular same water level

•The control can be localised at a section or occur in a channel reach (Channel- Control);

•The main quality attribute of a control is its permanence:In spaceIn time (stability of geometric and hydraulic characteristics)


Instability of the control Rating curve becomes invalid


ACCESSIBILITY TO SITESTABILITY OF THE BED (BOTTOM AND BANKS)HIGH SENSIVITYSTRAIGHT CHANNEL REACHSTABLE CONTROLSTATION ACCOMODATES ALL FLOW FROM THE CATCHMENTFAVORABLE LOCATIONS FOR INSTALLING STAFF GAUGESFLOW MEASUREMENT SECTION ACCESSIBLE IN FLOODS AND NOT TOO FAR

Criteria for installation of hydrometric stations: choice of site

• Stable river bed (composed of cohesive soils)

• Stable control

Stability of the station


Before flood

During flood

After flood

Control point

Control point washed away

Control point Lower than before flood

Section before bank erosion

Section after bank erosion

Sensitivity of the station


•The sensitivity of a station is better if a large change in gauge height produces a correspondingly small change in flow

•Sensitivity is expressed by the ratio or ΔΔ

Qh

ΔΔQ Q

h/

Shallow sectionNot very sensitive

Narrow and deep sectionVery sensitive

3. Station file

It consists of the following elements:

•Description of the station (name, code, coordinates, …)•Objectives of the station •Equipment installed at the station •Topographic survey of the station

•Longitudinal profile•Cross sectional profile•Staff gauges attached to a benchmark

•Management of the station•Reports of station visits •History of repair works•Changes in gauge reader, …

•Rating curves and flow measurements carried out•…

Description de la stationNom de la station GonséCode OMM 19315200Cours d'eau MassiliBassin (Nakambé / Mouhoun / Nazinon / Oti) NakambéPays Burkina FasoSuperficie bassin versantLongitude (dd°mm'ss") 12°28' NLatitude (dd°mm'ss") 01°19' WAltitudeDate de mise en service 06-mai-75Gestionnaire (Brigade régionale ou nationale) DGRE (Ouagadougou)Accessibilité de la station Bonne

Objectif de la station

- Contrôle des débits déversés par les barrages de Lumbila et de Ouagadougou n°3 (qui servent pour l'AEP de Ouagadougou) - Station exploitée par le réseau OMS -Onchocercose jusqu'en 19

Plan de situation

Données morphologiques du site

Géométrie du cours d'eauLargeur du cours d'eau à l'étiage 3 mLargeur du cours d'eau en crue 150 mProf moyenne 2 mProf maximale 4 mNature du cours d'eauNature des berges Argile/TerreNature du lit Argile/TerreSection de jaugeageSensibilité de la section SensibleStabilité de la section StableStabilité du seuil

Profils en travers et en large

Projet Volta-HYCOS

DOSSIER DE STATION (prototype)

Station de Gonsé (Massili, bassin du Nakambé) BURKINA FASO

St at ion de Gonsé (Massili) : prof il en t ravers d'après jaugeage du 7- 09- 86 ( Pont , côt é aval)

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

EchellesNombre d'élèments d'échelle 4Elèments en place 1-2, 2-3, 3-4Eléments manquants 0-1Elèments âbimés 1-2, 2-3Borne Oui -Type Borne hydrologique -Localisation Trottoir - côté aval -Côte 0 -Zéro échelle / borne -5.633

Capteur limnimétrique Existence d'un limnigraphe Oui -Type OTT X -Etat Limnigraphe OK Gaine OK -Date d'installation 06/05/1975Autre type de capteur limnimérique Non

Travaux de réhabilitation à prévoir

ObservateurPrésence d'un observateurNomVillageTéléphone

Projet Volta-HYCOS

DOSSIER DE STATION (prototype)

Station de Gonsé (Massili, bassin du Nakambé) BURKINA FASO

Ex : Gonsé station file (Massili, White Volta Basin - Burkina Faso)

Module 1 : hydrometry

IRD - Unité OBHI (Observatoires Hydrologiques et Ingénierie)

Acquisition, recording and transmission of hydrological data

WMO / OMM

Projet Volta-HYCOS


4th- 9th December 20062iE / Groupe EIER-ETSHER,

Project Regional Center

Acquisition, recording and transmission of hydrological data

1. Hydrometric sensors2. Data acquisition and recording3. Transmission of hydrologic data4. Flow measurements

Staff Gauges:

• These are installed along a section of the river• One gauge starts where the other ends• They are referenced to a TBM or a survey pillar

1. Water level measurement

• Float devices– Automatic water level recorder (chart)– Eletronic shaft encoder (ex : Thalimedes from OTT)

• Pressure sensors– Hydrostatic pressure sensors– Sensors based on the bubble in principle

• Ultra-sonic sensors• Radar sensors

Different Types of Water Level recorders

1. Water level sensors

• Principle :– The variation in water level causes a float to move along with the

water level– A pulley and cable arrangement converts the movement into electrical

signal proportional to the change in water level (ex : Thalymedes)• Precision : in mm

• Advantages : simple, highly robust• Disadvantages :

– Important civil engineering works required (stilling well)– Scaling of sensor

Shaft encoder with float

Electronic shaft encoder (Thalymèdes-OTT) associated with a traditionnal limnigraph


Daboya Station (White Volta, Ghana)


Bittou Station (Nohao, Burkina)

• Principle: measures the hydrostatic pressure of a column of water– Deformation of a membrane under the effect of hydrostatic

pressure– Transformation of the deformation mechanical into electric

signal by a pressure transducer– Transducers more frequently used: ceramic

• Precision : from 0.5 mm to 1 cm depending upon the range of measurement

Hydrostatic pressure sensors


• Advantages : – Simple installation – High precision

• Disadvantages : – Depends on deformation of the membrane

• Necessity to recalibrate the sensor periodically– Scaling of sensor– Exposed for extended periods in ephemeral rivers



• Principle :– Compressor sends a bubble in an immersed tube– The resultant pressure is proportional to the water level

above the sensor• Precision: from 0.5 mm to 1 cm depending upon the range of

measurement• Avantages :

– Simple installation– Good sensitivity (1 cm for 10 m)

• Disadvantages : – Heavy maintenance required– Difficult to measure in turbulent flows


Sensor based on bubble-in principle

• Principle: – Measures the time of travel of a signal which is emitted from the sensor,

reflected and detected by the sensor again.

– Water level (h) is deduced from the time of propagation of the wave by the water as h = c*T/2 where c of the signal in air and T the time required for the signal to return

• 2 types:– Immersed sensors– Air borne sensors

• Advantages:– Can be used to measure flows with high concentration of sediments (Air

borne sensors)• Disadvantages:

– Factors that influence the readings are difficult to take into account (T°, salinity, wind, …) : necessary to smoothen and recalibrate the values


Ultra-sonic sensors


Hydrometric sensors : summary

Precision Influence by fatigue

Robustness Installation Cost


A few mm no Robust Tidious and cosly (civil engineering works)

~600 €


A few mm yes Sensitive to scaling and exposure (ephemeral rivers)

Simple 500 € –4000 €

Bubble in sensors

A few mm yes Sensitive to scaling Simple ~1000 €

Ultra-sonic sensors

A few mm no Simple > 10 000 €

Radar sensors

A few mm no 1500 €

2. Data acquisition and recording

• Data logger : – Memory for data storage– Programming of sensors :

• For data recording at predetermined time intervals• For data recording as a function of water level

• Number of sensors connected• Memory : up to several Mb (several months of autonomy)• Independent or integrated to sensor

Infra Red linkage

Connection to sensors

GSM Modem

Data logger

Cable for connecting water level sensor« bubble-in principle »

Hydrological station with GSM tele-transmission

2. Data acquisition and recording

• Objective:– Real time (flood fore casting, management of

water systems in real time, …)– In quasi-real time for the day to day management

or surveillance of hydrometric network • Different systems of data transmission:

– Telephone: Line telephone/GSM– Satellite– Radio

3. Hydrological data transmission

• 2 families of satellites in use:– Geostationnary Satellites (ex: Meteosat) – Orbiting satellites (ex: Argos, Inmarsat)


Satellite transmission

DCP (Hydrological station )

Internet, WMO Global Telecommunication System …

Centre for data analysis

Users (Hydrological National services, …)

• Frequency of emission can be changed• Alarm channel• Centre for data Analysis:

European Spatial Agency (ESA) Germanywww.eumetsat.de

• Free Annual subscription in the framework of WMO WHYCOS programme


Meteosat transmission

http://www.eumetsat.de/


Meteosat transmetter

Power supply and antenna

• Sends out data every 200 second• Reception is a function of the visibility of the emitter

to the satellites• 2 centers for data analysis:

– CLS (Toulouse, France)– Argos Services Inc. (Landover, USA)

• Annual subscription: ~1000 Euros/perstation (for one transmission per day)


Argos transmission system

HydroArgos transmetter

• Data logger equiped with a modem and a SIM card• Data mode or SMS mode• 2 communication ports for data mode:

– User calls the data logger– The data logger calls the user (can be programmed for alarm)


GSM transmission

• WHO-Onchocerciasis (Argos)• HydroNiger Hydrological network (Argos)• Flood warning system for Bagre dam

(Inmarsat satellite)• …. Volta-HYCOS Stations (Meteosat)


Some examples of tele-transmission in the sub-region

4. Flow measurements

• Different techniques of flow measurement: – Flow measurements with current meter– Flow measurements using ultra-sonic sensors– ADCP– …

• Flow measurements with current meter:

Current meter C31 (OTT) Flow measurements by wading

Flow measurements using ultra-sonic sensors :• Principle : it measures the difference in time of propagation of

an ultrasonic wave that moves in and against the direction of flow

• Measurement of water level with an independant sensor is necessary to calculate wetted flow area

Céramiques ultrasonores

4. Flow Measurements

• Conditions for use :– Straight channel reach– Flow less loaded (sediment, vegetation, bubbles,…cause

attenuation of acoustic signal– Stability of river bed (determination of the wetted area with

the help of cross section)– Price : about 15 000 Euros


http://www.ott-hydrometry.de/web/ott_fr.nsf/id/pa_sonicflow_fr.html

http://www.ott-hydrometry.de/web/ott_fr.nsf/id/pa_sonicflow_fr.html

• Flow measurements with ADCP (Acoustic Doppler Current Profiler):– A recent technology (about 15 yrs) – Principle: speed of water measured from the particles in

suspension by Doppler effect– Price : about 25 000 Euros


ADCP (WorkHorse model from RDI)

Transducer

Acousticpulse

Magnified viewof scatterers

Flow measurements with ADCP : principle

•Measure the relative velocity with respect to the ADCP

•Water speed measured based on the speed of suspended matter in the water column

•Mesure de la vitesse de l’eau et du déplacement de l’ADCP

•Measures the speed of ADCP with respect to the bottom (reflection of the acoustic waves from the bottom)

•GPS


cell 1

cell 2

cell 3

cell 4

echo echo echo echo

Transmitpulse

start endGate 1 Gate 2 Gate 3 Gate 4

TIME

Blank

Bin 1

Bin 2

Bin 3

Bin 4

RAN

GE

FRO

M A

DC

P4. Flow Measurements

Flow measurements with ADCP : principle

Some Internet links…

• Hydrometry :– http://wwwrcamnl.wr.usgs.gov/sws/SWTraining/FlashFandR/Index.html : a

good on-line training course on the l’USGS ‘U.S. Geological Survey’ internet site– http://www.usobhi.net : IRD-OBHI internet site– http://hydram.epfl.ch/e-drologie/ Ecole Polytechnique Fédérale de Lausane (EPFL)– http://www.enpc.fr/cereve/HomePages/gaume/courshydro/courshydro.html#4

(E.N.Pont et Chaussées-CEREVE)

• Some manufacturers of hydrological equipment (on the Net…) :– OTT (German)– SEBA (German)– Eijkelkamp– Sutron (USA)– RD Instruments (ADCP) – …

http://wwwrcamnl.wr.usgs.gov/sws/SWTraining/FlashFandR/Index.html

http://www.usobhi.net/

http://hydram.epfl.ch/e-drologie/

http://www.enpc.fr/cereve/HomePages/gaume/courshydro/courshydro.html#4

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