HP-II Workshop on Real-Time Hydrological Information Systems

Mark Heggli, Meteorologist/HydrologistExpert Real-time Hydrology Information Systems

Innovative Hydrology, Inc.Consultant to the World Bank

Examples that refer to products are intended for illustrative purposes only, and do not imply an endorsement or

recommendation of any particular product

Instrumentation Solutions For:• Surface Water Level• Discharge• Ground Water• Rainfall• Data Logger

Surface Water Level:• Two categories of water level sensors

• Contact and Non-Contact Solutions• Contact

• Sensor has components that are in contact with the water being measured• Susceptible to debris and water pollution damage• Includes:

• Shaft Encoder/ Stilling Well• Bubbler• Submersible Pressure Transducer

• Non- Contact• Sensor components are not in contact with the water being measured• What flows in the water and any water pollution will not interfere or

otherwise foul the sensor• Includes:

• Ultrasonic Sensor• Radar

Stilling Well/Shaft Encoder• Concept

• Water comes into the stilling well through inlet pipes from the water source• As water level increases and decreases, a float moves up and down with the water level• A tape attached to the float turns a wheel which is connected to the shaft on the encoder.

As the wheel turns so does the shaft.• The shaft encoder will convert the shaft rotations to an electronic signal, which will be

measured by a data logger

• The components of this type of gauge include:• A stilling well• Inlet pipes from the water • Float and weight• Tape • Wheel• Shaft encoder Water Inlet

Float

Shaft Encoder Installation and Ideal Location

• Installation• The civil works for this type of station is among the most expensive,

while the sensor and associated equipment is among the least expensive sensor solutions

• Ideal Location• Stable river beds where the

channel does not change• Little sedimentation

Shaft Encoder Advantages/Disadvantages

• Advantages• Low cost of equipment• Never needs to be calibrated, but only checked and reset to an

outside staff gauge

• Disadvantages• Expensive civil works• Requires occasional flushing to remove sediments that may have

collected at the bottom of the stilling well. If left unchecked, the sediments could eventually block the inlet/outlet pipes

Bubbler• Concept

• The bubbler is a stage measurement device that has a non-submersible sensor

• Also known as a gas-purge system the system allows a small quantity of air or inert gas to bleed through a pipe or tubing to an orifice in the stream

• The pressure of the gas that displaces the liquid in the orifice is then measured by a pressure transducer

Non- Submersible Pressure Transducer

Bubbler Components• Compressor• Orifice Line• Pressure Transducer• Tank• Desiccant

Bubbler

Air Drying System

Bubbler Types and Location• Two Types

• Non- continuous• Only turns on at a set time

• This allows water to feed back up the orifice line so the line needs to be purged and stabilized

• Continuous• Keeps the orifice line under pressure by producing approximately 1 bubble

per second

• Ideal Location• Open Channels and Reservoirs• Stable river bed where channel

does not change• Not a lot of debris or

sedimentation that can cover the orifice inlet

• Great option if there is not a bridge or platform near by to put a radar or ultrasonic sensor

Bubbler Advantages/Disadvantages• Advantages

• It’s a Non-Submersible Pressure Transducer• Elimination of risk or damage of the most expensive part (pressure

transducer and compressor)• Orifice line (least expensive component) only part in the water

• Install is relatively inexpensive compared to the stilling well for the shaft encoder

• Disadvantages• Expensive compared to shaft encoder, ultrasonic sensor and

submersible pressure transducer• Desiccant replacement• Not suitable for changing channel and debris

Submersible Pressure Transducer

• Concept• The pressure exerted on

the sensor by the head of water above the sensor is converted to depth.

• Components• Submersible Pressure

Transducer• Pipe

Submersible Pressure Transducer Installation and Setup

• Installation• The transducer is installed in a pipe below the minimum water line

• Ideal Setup• A submersible transducer should be avoided in open channels• It is more appropriately used in wells to measure ground water, or in lakes where the

occurrence of damaging debris or toxic water is not an issue

Submersible Pressure Transducer Advantages/Disadvantages

• Advantages• Relatively easy to install

• Sensor only needs to be run down a pipe to some level that is lower than the expected minimum water level

• One of the lowest cost sensors (ranging 50,000-80,000 INR)

• Disadvantages• Contact solution is in contact with the water where it is vulnerable to

debris during high water flows and water pollution• More susceptible to damage caused by lightning striking the water • Require calibration which must be performed at the factory

Non-Contact Solutions:

• Ultrasonic Sensor• Radar

Ultrasonic Sensor• Concept

• The ultrasonic sensor measures the distance of target objects (water surface) by sending pulsed ultrasound waves at the object and then measuring the time for the sound echo to return. Knowing the speed of sound, the sensor can determine the distance to the object

• Components• Sensor• Mounting Platform• Pipe

Ultrasonic Sensor Installation and Location• Installation

• Requires some sort of bridge or platform to mount the sensor• Sensor must not have any object within the area of signal projection• Should be mounted where it can easily be accessed to repair if necessary

• Ideal Location• Need a Bridge or Platform

directly over water• Not practical in reservoirs or

rivers with shallow slopes• More ideally suited for canals• Measurement distance up to

10m

Ultrasonic Sensor

Signal Projection

Data Logger

Ultrasonic Sensor Advantages/Disadvantages

• Disadvantages• Need structure to mount the

sensor (bridge railing or boom) • Measurement is not as accurate

or precise as other measurement techniques, usually being within .25 to .4 % of the measured distance

• Advantages• The advantage to the ultrasonic sensor is the price (around 75,000 INR)• Non-contact method of measurement. The measurement is generally

unaffected by the transparency, reflectivity, opacity or color of the target

Radar• Concept

• The radar works by releasing electromagnetic energy, which is reflected by objects with high dielectric properties (ex: metal and water)

• The distance reflected by the microwave energy is determined by the measurement of the time of flight divided by the speed of light, known as Time-Domain Reflectometry (TDR)

• Various frequencies from 1 Ghz to 30 Ghz are typically used, with the higher the frequency, the more accurate and more costly the device

• Components• Radar• Mounting Platform• Pipe

Radar Installation and Location• Installation

• Requires some sort of bridge or platform to mount the sensor• Sensor must not have any object within the area of signal projection• Should be mounted where it can easily be accessed to repair if

necessary• Ideal Location

• Ideal for use in moist, vaporous, and dusty environments as well as in applications in which temperatures vary

• Requires some structure (bridge railing or boom) to be mounted on

Radar Advantages/Disadvantages• Advantages

• Non-Contact Solution• Radar will penetrate temperature and vapour layers that may cause

problems for other techniques, such as ultrasonic • Compared to the ultrasonic, radar provides a solution with a greater

range of detection between the sensor and the water body (> 30 m)• The radar is one of the more accurate sensors for water level

• Disadvantages• Need a bridge or platform to mount• More expensive than Ultrasonic

Discharge• Contact• Non- Contact• Mechanical Manual

Radar Discharge• Concept

• Two sensors are incorporated in the device, one sensor measures the Doppler velocities of the water at the surface of the water, and the other sensor determines the depth of water

• Measures velocity over one area and makes assumptions on the velocity profile, in both horizontal and vertical aspects. A profile of the channel must be made and provided to the Doppler radar discharge sensor’s algorithm so that the discharge approximation can be calculated

Radar Discharge Installation and Location• Installation

• Simple and Inexpensive• Mounted to existing bridge or platform

• Ideal Location• Works optimally in well-defined

symmetric channels, such as canals• Works well on creeks and smaller

rivers, where the area where the radar velocity measurement is made represents the flow in the channel

• Challenges• Wide channels may require more

than one instrument along the cross section

• Bifurcated channels will require additional instruments in each bifurcated channel

Radar Discharge Advantages/Disadvantages• Advantages

• The radar discharge sensor is a non-contact sensor

• Disadvantages• A bridge or some other support structure is required to mount the

equipment• Measures only surface velocity and water level, and calculates

discharge based approximations and empirical theory• The operator of these devices must understand the assumptions in

order to determine if the sensor will provide usable measurements• The Doppler radar discharge instrument can be expected to cost

between 500,000 to 750,000 INR

Acoustic Doppler Current Profiler (ADCP)• Concept

• ADCP relies on SONAR which uses sound waves to determine the distance to targets

• Doppler Effect is used to resolve the speed of the targets • ADCP operates by determining the velocity of water across the

cross section

• Types• Up Looking• Side Looking• Down Looking

ADCP General Information• Optimally suited for canals, estuaries, river deltas, and

river reaches that experience backwater• Especially effective in tidal areas or in the ocean to

measure current, where current changes even though the water level stays the same

• Not a suitable solution for streams where debris such as logs and moving rocks can dislodge and possibly damage this expensive sensor

• An increasing number of Acoustic Doppler systems are being installed in open channels since it removes the effort associated with obtaining the stage-discharge points

Up Looking ADCP • ADCP is placed at the bottom of the

channel. Channel shape needs to be programmed in order for the discharge calculations to be accurate

• Ideal Location• Good application in a canal where the water is

generally free from sediment and debris• Good application in artificial channels, such as

culverts

Side Looking ADCP• The side looking ADCP is placed on

one of the banks of a channel• The shape of the channel will need to

be determined for flow calculations to be accurate

• Ideal Location• Typically deployed in an open channel

application• Reduces the chance for sediment covering

the sensor• Sensor out of the way of the quickest current

which also carries debris

Down Looking ADCP (Profiler)

• Non-permanent ADCP• Usually attached to some small floating

device that can be pulled across the cross-section

• Ideal Location• Generally used for stream gauging

measurement replacing standard mechanical cups

ADCP Advantages/Disadvantages• Advantages

• Measures discharge in a wide variety of situations and where stage-discharge relationships have problems

• Ideally suited for estuaries and canals• Instantaneous Discharge!

• Disadvantages• Contact solution• Requires a stable channel or continuous channel surveys if

channel bed/banks are changing• Very expensive

Ground Water• Concept

• Ground water is most commonly measured with a submersible pressure transducer

• The submersible pressure transducer measures pressure to calculate the water depth

• Commonly come equipped with data logger, internal battery (readily available)

• Internal battery last beyond a year with GPRS transmissions

• Fits within well casing. No other enclosure needed

Ground Water Applications• External Data Logger

• Suited towards adding additional equipment to data loggers such as rainfall, evaporation, or other hydromet measurements

• Internal Data Logger• Standalone system for just ground water level recording• Internal batteries• No solar panels required• Additional features such as GPRS and water quality

Ground Water Continued• Two General Versions

• Data logger in transducer body• Data logger at the top of the pipe with GPRS radio

Rainfall Measurement• Two Primary Methods of Measurement and Operation

• Tipping Bucket Rain Gauge• Usually used in warmer climates

• Storage Rain Gauge• Preferred choice with frozen precipitation

Tipping Bucket Rain Gauge• Concept

• The tipping bucket is comprised of two buckets which rotate in collecting rainfall

• As one bucket is filled, it empties, and moves the other bucket in place.

• The tipping action is sensed by a magnetic switch which is recorded by the Data Logger

• Buckets size usually holds between 0.2 and 1 mm of equivalent rainfall depth

• Options• Siphon and non- siphon

• Non-Siphon• Siphon

Tipping Bucket Rain Gauge General Information• General

• Not all tipping bucket rain gauges are created equal. Better gauges employ highly corrosion resistant powder coated metal or stainless steel, better engineered bearings, etc.

• The funnel must be kept clear of debris, and insects need to be kept out of the gauge body

• Calibration should be performed just prior to the rainy season

• Not well suited for the measurement of mixed phase or frozen precipitation

• Proper clearance required (as for any precipitation gauge)

Tipping Bucket Rain Gauge Advantages/Disadvantages• Advantages

• Tipping bucket rain gauges are relatively inexpensive, usually being less than 50,000 INR

• Continuous Operation• Less maintenance then the storage gauge

• Disadvantages• Tipping bucket concept causes errors with varying intensities of

rain

Storage Type Rain Gauge• Concept

• Rainfall is collected in a large collection pipe or a bucket

• The pipe/bucket is weighed, or;• The pipe/bucket is retrofitted with a

pressure transducer that measures the depth of water

• In freezing conditions, antifreeze (propylene glycol) is placed into the pipe/bucket to keep water in the liquid state

• The pipe/bucket must be drained every 1000 mm of rainfall

More Storage Gauges

Storage Rain Gauge Advantages/Disadvantages• Advantages

• Measures total rainfall and rainfall rate equally well• Well suited for mixed phase and frozen precipitation

• Disadvantages• Gauges are more expensive than tipping bucket rain gauges (5x)• Liquid needs to be removed during the season (more maintenance)

Data Logger• Concept

• A Data logger provides a hydrometric station with the capability of making measurements from electronic sensors, storing the data, transmitting data from a variety of radio solutions

• Data loggers can nominally trigger and record measurements at any set time• This includes all digital and analog hydrological and meteorological

sensors • This includes battery power, charge state, and charging power, which is

used primarily by maintenance technicians

Common Features of Advanced Data Loggers• Screen to view/alter data• Very low power consumption• Multiple communication ports for simultaneous data relay

(GPRS, INSAT, ALERT)• Adaptable to Portable Storage Modules• Analog, Digital, RS-232, SDI-12 Data Inputs• Digital, RS-232 outputs• Frequency of collection/logging can be different for each

sensor• Logging/Transmissions can be triggered based on a

measured event

Common Features of Advanced Data Loggers• Ability to switch on/off power to sensors (power saving

concept)• No data loss upon power loss• Automatic start up when power is restored• Program load feature• Multiple year warranty• Others??

Sensor Communication Challenges• Sensor very far from data logger• Multiple sensors in the area (surface water, ground water,

climate)• Other areas where using cable is prohibitive

• River Crossing• Reservoir Structure (Pool elevation, river release, etc.)

Wireless Sensor Links

Ground Water Sensor

Climate StationWater Level

Master Station

Data Communication to Data Center

Advanced Sensor Communications• Wireless Sensor Communication

• Effective in combining sensors• Saves cost of data logger/transmitter• Effective in reducing long wire runs, or providing data

collection where running wire is not feasible or possible• Utilizes low power spread spectrum transmission

frequency (similar to that used by portable phones)• Operates on very low power at 12/24/48 Volts

Cost Table (Approximate)

Item Cost (USD) Cost (INR)Shaft Encoder 1,000 55,000 Bubbler with sensor 3,000 165,000 Radar Discharge 12,000 660,000 ADCP 15,000 825,000 Ultrasonic 1,500 (495) 82,500 Radar Sensor 3,000 165,000 Submersible Transducers 1,000 55,000 Data Logger 2,000 110,000 Tipping Bucket Rain Gauge 1,000 55,000 Storage Rain Gauge 4,500 247,500

a. Shaft Encoder

b. Radar

c. Bubbler

d. Ultrasonic Sensor

e. Side Looking ADCP

Which of the following sensor types are the best for measuring water level for the location below?

An ADCP is an Ideal Solution for the following

a. Reservoir

b. Tidal River

c. Mountain River

d. Canal

e. A river reach with rapidly changing morphology

When might you use an ultrasonic level instrument over a radar level instrument?

a. If there is a temperature gradient

b. When you don’t have enough funding

c. If the water is smooth

d. When the measurement distance is less than 3 m

e. When power is not available

When would you use a ground water level sensor with an internal data logger?

a. When you want to use GPRS for data telemetry

b. When there are no other sensors to connect

c. When the well is extremely deep

d. When a minimum instrument presence is necessary

e. When a more accurate sensor is desired

End of Module 2 - Part 1