test Flashcards
(21 cards)
What are hydrometric measurements and how are they collected?
Water level and tidal ranges are measured using:
- instrumental methods (float, pressure, acoustic, radar)
- visual methods (tide poles, high water marks).
What are key considerations for selecting a tide gauge site?
- environmental durability
- stable ground
- be open to a main water body
- avoid disturbances (currents, outflow)
- ensure power supply
- accessibility
- a benchmark linked to a reference system, - avoid estuaries (density sensitivity).
What is a tide pole and its characteristics?
A vertical pole fixed to a structure used for quick visual water level readings.
- low accuracy, is affected by waves and currents
- used for cross-checking instruments
- must be firmly fixed and leveled.
How does a stilling well and float gauge system work?
measures long-period water level changes via a float connected to a drum. It filters out short-period waves while
Accuracy: 2 cm in water level, 2 min in time.
How do pressure gauges measure water level?
They measure hydrostatic pressure using P = ρ × g × (h + a). Calibration must be done in controlled settings to match field conditions.
What is a pneumatic bubbler system and where is it used?
Compressed air is pushed through a small tube to a submerged orifice. The pressure needed to bubble equals water pressure. It’s used with a compressor, sensor, air tube—suitable for rivers and estuaries.
How do pressure sensor gauges function?
They use strain or quartz sensors to detect pressure changes, convert outputs to physical units, and often include temperature compensation.
What are bottom-mounted pressure gauges used for?
They are placed on the river or seabed, powered by long-life batteries, and used for tidal and seasonal studies.
How do acoustic gauges work and what are their dependencies?
They use sound pulses reflected from the water surface. Installed in open air or tubes; accuracy depends on temperature and humidity and requires compensation.
How does radar measure water level?
Radar uses time-of-flight or phase shift of signals. It’s low maintenance, sensitive to environmental conditions, and mounted on fixed structures for sea level monitoring.
How is GPS used on buoys and what is its application?
High-accuracy GPS with motion sensors is used. It provides a few cm accuracy and is useful for tsunami detection and early warnings.
What is GNSS reflectometry and how does it work?
It reflects GPS signals off water surfaces and detects time delay between direct and reflected signals using passive sensors.
What are two simple techniques to estimate highest water levels?
1) Bottle method: bottles at heights mark the peak level; 2) Painted stick method: paint washed off by flood indicates peak.
Why is datum control and levelling important?
It allows accurate separation of land movement (tectonics, isostatic adjustment) from true sea level change due to ocean dynamics and climate change.
What are levelling benchmarks and their types?
Primary: stable land reference point. Local: 3–5 markers connected to national system. TGBM: tide gauge benchmark used for reference.
What are GPS benchmarks and how are they linked to tide gauges?
GPS benchmarks (GCP) are physical parts of tide stations used for vertical referencing, linked to zero reference level (TCZ).
What are examples of national levelling networks?
TAW in Belgium, NAP in Netherlands, ODN in UK.
What are tidal datums and examples?
LAT: lowest astronomical tide, HAT: highest expected tide, MHWS/MHWN: mean high water during spring/neap tides.
How is levelling performed and what accuracy does it achieve?
Annually done. Local accuracy <1 mm, extended levelling <1 cm (1–10 km), using absolute gravity based on free-fall acceleration.
What are the two modes of data transmission and their purposes?
Real-time: for alerts via GSM/Satellite. Delayed mode: saves energy, used in research.
What is data validation and how is it done?
Through quality control (value ranges, spikes), real-time use, and human oversight.
