Tide Gauge Instrumentation
Humans have been measuring water depths for many centuries, using many different techniques. The oldest are still very common, even irreplaceable, like the tide staff. The instrumentation has naturally evolved from the simple tide staff, which still serves as a reference for modern technologies such as radar tide gauges, float type tide gauges, pressure gauges and ultrasonic tide gauges. Each measurement system is detailed in the following pages.
The Nilometer: the ancestor of the tide staff
The Nilometer is a way to measure changes in water level of the Nile. First used in Egypt in the 4th century BC, the Nilometer consists of a graduated scale used to quantify the flood levels of the Nile. The taxation of grains depended on the quality of the flood: the better the peak level (not too high or too low), the more silt enriched soil was deposited, so taxes were raised in anticipation of expected good harvests.
We do not know how the Egyptians determined the zeros for Nilometers, but the tide gauges used in France in the early 18th century were generally referenced to the base of the raft foundations of dry docks. This was true in Brest, where routine measurements began in 1711.
Float tide gauge and stilling well
A stilling well is needed for permanent tide stations near a coast. Most permanent gauges are located in port areas or along navigable estuaries. The use of float type tide gauges is decreasing, because of the analogue chart records, and especially because of systematic errors caused by the mechanical parts (friction of the float in the stilling well, mechanical wear, etc.). These mechanical gauges are being replaced by automatic digital data logging systems.
Digital tide gauges: radar tide gauge, pressure tide gauge, ultrasonic tide gauge
There are few disciplines in the field of physics that offer such a wide choice of measurement systems, many of which have been developed and used for tide gauging.
The configuration of the installation site is an important factor. The sensors and recorders are not the same for coastal sites, where tide stations can be set up (ports, estuaries, shallow areas), and offshore sites.
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The GNSS Buoy: the tide gauge of the future?
With the developments in data processing, the Global Navigation Satellite System, better known by its acronym GNSS, can now determine the ellipsoidal height of a point, in this case an autonomous oceanographic buoy, with high accuracy. The GNSS receiver on the buoy can measure the water depth at all times.
Which tide gauge to choose?
There are numerous manufacturers specialized in hydrography or industrial instrumentation that offer off-the-shelf radar sensors suitable for sea level measurement. Sensors are chosen according to the site on which they are to be installed (tidal range, sensor in open air, type of stilling well), financial aspects and the quality of measurement required.
To find out more:
References
- Simon B. (2007). La Marée - La marée océanique et côtière. Edition Institut océanographique, 434pp.
- Pouvreau N. (2008). Trois cents ans de mesures marégraphiques en France : outils, méthodes et tendances des composantes du niveau de la mer au port de Brest. Thèse de doctorat de l'Université de La Rochelle, soutenue le 26 septembre 2008, 466 pp.
- Holgate S., T. Aarup (2003). Workshop on new technical developments in sea and land level observing systems. COI, Workshop Report N°193, 196pp.
Last updated: 12/12/2012