Transcript DISCHARGE RATING VERIFICATION

WATER LEVEL
MEASUREMENT
Hydrography Skills Set Training
Course No. 27743
June 2012
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
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
Contact Gauges
Staff Gauge
Manual Measurement
The following example shows how depth of flow can be calculated by this method in a sewer.
DIP
ZERO
DEPTH
1.
2.
3.
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’
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 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
Ongoing datum checks
Minimal ongoing maintenance
Reliable
Accurate
High establishment costs
Environmental issues during installation phase
‘Time of Lag’
OH&S issues (working at heights, confined spaces)
Less expensive than float well
installation
Minimal maintenance
Siltation effects
Leakages not easily identified
Less expensive than float well
installation
Minimal maintenance
Proven and reliable system
Widely used
Siltation effects
Medium to high maintenance
Moisture and oil ingress
Leakage
Gas cylinder issues (safety consideration)
Float
Pressure - ‘Closed’
Pressure – ‘Open’
Comparison of Methods
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
Widely used
•Bed mounted systems in accessible
Radar
•Easily installed
•Range < 35 metres
Must be in a vertical position above water
Dead Zone
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
Questions ?