water level measurement hydrography skills set training course no. 27743 june 2012

WATER LEVEL MEASUREMENT

Hydrography Skills Set Training

Course No. 27743June 2012

AN EARLY HISTORYOriginated in Ancient Egypt (3000 year s ago)• - monitoring of Nile River water level for flood prediction• - approximately 20 recording stations (‘Nilometers’) were

located - ‘Nilometer’ attached to temple walls, quays and the inside walls of temple annexes

• - units of measurement were ‘cubits’ (the measurement from the elbow to the tip of the middle finger, approximately 450mm to 530mm)

• - modern day methods of measurement have evolved from this primitive method of water level measurement

Options for Water Level Measurement

Manual Measurement

Float Operation

Submersible ‘Wet’ Pressure Sensor

Gas Purge Operation

Options for Water Level Measurement

Manual Measurement

Manual Measurement

Staff Gauge

Contact Gauges

Manual Measurement

ZERO

DIP

DEPTH

The following example shows how depth of flow can be calculated by this method in a sewer.

1. From a fixed point a measurement is taken to water level (ie the ‘dip’)

2. This reading is subtracted from a known measurement (ie the ‘Zero’)

3. Depth = ‘Zero’ – ‘Dip’

Manual Measurement

Cantilevered Weight Gauge

(Clear Ck at Golden, Colorado)

Manual Measurement

Maximum Height Indicator (Skokomish River, Washington).

Options for Water Level Measurement

Float Operation

Float Operation

Typical Shaft Encoders

Options for Water Level Measurement

Submersible ‘Wet’ Pressure Sensor

Pressure Transducers – ‘Wet’

‘Gauge’ Type(stainless steel)

‘Gauge’ Type(delrin/brass) ‘Absolute’ Type

Options for Water Level Measurement

Gas Purge Operation

Gas Purge (Open) System ‘Bubbler’

Basic Operating Principle

• dry nitrogen (or air) bubbled into the stream via a small diameter tube

• bubbles escape from the end of the tube (orifice)

• pressure transducer monitors changes in pressure within the tubing

• available as ‘single’ or ‘dual’ orifice/riverline options

System pressure is proportional to water level

Pressure Transducers – ‘Dry’

Bubble Unit

Operating Principle• Uses ‘dry nitrogen’ cylinder• Provides a constant ‘differential’ of 3-5 PSI (21-35 kPa) above the pressure head at the orifice• Differential maintains a constant stream of bubbles at the orifice• HS23 bubble rate ‘pre-set’ at factory• Eliminates risk of silicon oil entering ‘riverline’• ‘Quick Connect’ fitting available for check of bubble rate

Bubble Unit Typical Installation

Compressors• Alternative to gas cylinders (Safety Issues)

• Effective air drying system essential - moisture ingress - aquatic growth

• Some limitations – ‘riverline’ length < 200 metres - maximum head of 30 metres

• Low powered types available (12V: 38aH with solar)

Compressors – ‘Bubbler'

Gas Purge (Open) System ‘Bubbler’

Single Orifice / Single Line

Gas Purge (Open) System‘Bubbler’

Single Orifice / Dual Line

Gas Purge (Open) System ‘Bubbler’

Dual Orifice

Old Style (Closed) Gas System

‘Hydrostatic’ (Closed) Gas System

Compressors – ‘Hydrostatic’

Ultrasonic Systems • Ultrasonic pulses emitted by the transducer

• Pulses are reflected by the water surface and reflected back to the transducer

• Time from emission to receipt of the signals is proportional to the level in the vessel

• Mounted vertically above stream

Doppler Systems • If the distance between the transducer and the reflecting

object is decreasing, frequency increases

• If the distance between the transducer and the reflecting object is increasing, frequency decreases

• Water level measured by ‘vertical’ acoustic beam

• Systems also capable of flow measurement

Radar Systems • Extremely short microwave impulses are emitted by the

antennae system to the water surface

• These impulses are reflected by the water surface and received again by the antenna system

• Time from emission to reception of the signals is proportional to water level in the stream

Laser Systems • Changes in water level based characterised by intensities of

angular reflectance of light

Comparison of Methods Method Advantages Disadvantages

Gauge •Low cost optionEasily installed

•Need to engage a gauge readerManual data managementOngoing datum checks

Weight Gauge Alternative to staff gauges Ongoing maintenanceHigh installation cost

Dipping •Easy to set up •Inaccurate in some casesCost of electric dip tapesOngoing datum checks

Float Minimal ongoing maintenance ReliableAccurate

High establishment costsEnvironmental issues during installation phase‘Time of Lag’OH&S issues (working at heights, confined spaces)

Pressure - ‘Closed’Less expensive than float well installationMinimal maintenance

Siltation effectsLeakages not easily identified

Pressure – ‘Open’Less expensive than float well installationMinimal maintenanceProven and reliable systemWidely used

Siltation effectsMedium to high maintenanceMoisture and oil ingressLeakageGas cylinder issues (safety consideration)

Comparison of Methods Method Advantages Disadvantages

Compressor •Eliminates gas bottle transportation and storage (safety consideration)Minimal ongoing maintenance

•High establishment costsNeed to replace desiccant (in some types only)

Ultrasonic •AccuracyReliabilityWidely usedEasily installed

•Must be in a vertical position above waterDead ZoneEffects of foamEffects of air movement

Doppler •Positioned on surface or bed of streamProven reliabilityWidely used

•Bed mounted systems in accessible

Radar •Easily installed •Range < 35 metresMust be in a vertical position above waterDead ZoneFalse echoesNot suited for turbulent streamsMore suited to wide streamsAffected by electro-magnetic interference

Laser •Range > 150 metresAccurate in turbulent waterVertical alignment above stream NOT requiredNo false echoesWorks well on narrow and wide streamsUn-affected by electro-magnetic interference

•Requires clear line of sight•Affected by fog

Questions ?