1. WATER LEVEL MEASUREMENT
Hydrography Skills Set Training
Course No
June 2012

2. AN EARLY HISTORY Originated 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

3. Options for Water Level Measurement
Manual Measurement
Float Operation
Submersible ‘Wet’ Pressure Sensor
Gas Purge Operation

4. Options for Water Level Measurement
Manual Measurement

5. Manual Measurement
Contact Gauges
Staff Gauge

6. Manual Measurement
The following example shows how depth of flow can be calculated by this method in a sewer.
ZERO
DIP
DEPTH
From a fixed point a measurement is taken to water level (ie the ‘dip’)
This reading is subtracted from a known measurement (ie the ‘Zero’)
Depth = ‘Zero’ – ‘Dip’ 6

7. Cantilevered Weight Gauge (Clear Ck at Golden, Colorado)
Manual Measurement
Cantilevered Weight Gauge
(Clear Ck at Golden, Colorado) 7

8. Maximum Height Indicator (Skokomish River, Washington).
Manual Measurement
Maximum Height Indicator (Skokomish River, Washington). 8

9. Options for Water Level Measurement
Float Operation

10. Float Operation 10

11. Typical Shaft Encoders

12. Options for Water Level Measurement
Submersible ‘Wet’ Pressure Sensor

13. Pressure Transducers – ‘Wet’
‘Gauge’ Type
(stainless steel)
‘Gauge’ Type
(delrin/brass)
‘Absolute’ Type 13

14. Options for Water Level Measurement
Gas Purge Operation

15. 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 15

16. Pressure Transducers – ‘Dry’16

17. 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 17

18. Bubble Unit Typical Installation18

19. 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) 19

20. Compressors – ‘Bubbler'20

21. Gas Purge (Open) System ‘Bubbler’
Single Orifice / Single Line 21

22. Gas Purge (Open) System ‘Bubbler’
Single Orifice / Dual Line 22

23. Gas Purge (Open) System ‘Bubbler’
Dual Orifice 23

24. Old Style (Closed) Gas System24

25. ‘Hydrostatic’ (Closed) Gas System25

26. Compressors – ‘Hydrostatic’26

27. 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 27

28. 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 28

29. 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 29

30. Laser Systems
Changes in water level based characterised by intensities of angular reflectance of light 30

31. Comparison of Methods Method Advantages Disadvantages Gauge
Low cost option
Easily installed
Need to engage a gauge reader
Manual data management
Ongoing datum checks
Weight Gauge
Alternative to staff gauges
Ongoing maintenance
High installation cost
Dipping
Easy to set up
Inaccurate in some cases
Cost of electric dip tapes
Float
Minimal ongoing maintenance
Reliable
Accurate
High establishment costs
Environmental issues during installation phase
‘Time of Lag’
OH&S issues (working at heights, confined spaces)
Pressure - ‘Closed’
Less expensive than float well installation
Minimal maintenance
Siltation effects
Leakages not easily identified
Pressure – ‘Open’
Proven and reliable system
Widely used
Medium to high maintenance
Moisture and oil ingress
Leakage
Gas cylinder issues (safety consideration) 31

32. Comparison of Methods 32 Method Advantages Disadvantages Compressor
Eliminates gas bottle transportation and storage (safety consideration)
Minimal ongoing maintenance
High establishment costs
Need to replace desiccant (in some types only)
Ultrasonic
Accuracy
Reliability
Widely used
Easily installed
Must be in a vertical position above water
Dead Zone
Effects of foam
Effects of air movement
Doppler
Positioned on surface or bed of stream
Proven reliability
Bed mounted systems in accessible
Radar
Range < 35 metres
False echoes
Not suited for turbulent streams
More suited to wide streams
Affected by electro-magnetic interference
Laser
Range > 150 metres
Accurate in turbulent water
Vertical alignment above stream NOT required
No false echoes
Works well on narrow and wide streams
Un-affected by electro-magnetic interference
Requires clear line of sight
Affected by fog 32

33. Questions ?