The changes in tide around the Irish coast means that INFOMAR surveys operate in areas and bays where tidal ranges vary from around 1.75 metres on the south east coast of Ireland and average 4.5 metres on the west coast. Without accurate tide height data, sonar measurements would have a large error range. To overcome this problem tide gauges are positioned both along the coast and within the survey area to gain an accurate representation of the tide dynamics operating in the area.
Zonation of lichens, barnacles and sea algae illustrate the range in tide height experienced in Killary Harbour, Connemara (top) and a tidal curve showing variations in tide over the duration of a survey off the Cork and Waterford coasts from Ballycotton, Co. Cork (bottom).
Tide data is available for INFOMAR surveys through the Irish National Tide Gauge Network or by installing semi permanent gauges that are installed in advance of the survey and removed after the survey has been completed with regular maintenance including regular data downloads, instrument calibration and battery changes through the survey period. INFOMAR uses the following tide height measuring devices;
OTT Nimbus Bubbler Gauge
HOBO Water level logger Pressure Gauge
These gauges use a battery powered air compressor which pumps air down a thin plastic tube (protected by galvanised pipe down along the pier edge or ladder) to the underwater bubble pot. To force the air from this bubble pot, a measured pressure is delivered from the pump depending on the depth of water above the bubble pot (based on the Archimedes' principle). This recorded pressure can then be converted to a measurement of the height of water above the bubble pot.
Data being downloaded from an OTT Nimbus gauge at Rossaveal, Co. Galway during the survey of Galway Bay during the summer of 2008. The battery, data logger and air pump are all housed in a water proof box on the pier
Offshore gauges are deployed from the survey vessel during the survey and can be relocated to the most suitable location as the survey progresses. The gauges are placed inside a protective cage which is lowered onto the seabed and marked with a buoy, flag, light beacon and radar reflector. Their positions are also reported to the coast radio station to warn other vessels operating in the area. The primary gauge used offshore is the Valeport Midas WLR with HOBO Water level Logger Pressure gauges used as back up. The data from these gauges are later matched with onshore data so both datasets can be referenced to an onshore datum and then used to correct the MBES data.
Image of the Midas tide logging instrument that is used to measure changes in offshore tide height during the survey.
|Geological Survey of Ireland
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