Manual on sea level measurement and ... - unesdoc - Unesco

Sea Level Measurement and InterpretationFloat GaugesEquipment A float in a stilling well is the tried and tested method of measuring sea level directly, rather than through an indirectparameter such as pressure or sound.Operation A stilling well filters out wave activity at periods shorter than the maximum tidal period, which might be 2 hours in shallowwater regions. In modern installations the float drives a shaft encoder or potentiometer the output of which is fed toan electronic data logger. In the past, chart recorders were extensively used, but are no longer acceptable as the principaldata-recording method, as they contain many sources of inaccuracy and require labour-intensive digitization.Installation Stilling well installations require heavy civil engineering work in areas of large tidal range. Many stilling wells exist throughoutthe world, as they are of robust construction, but new installations are less common, owing to the engineering cost.RequirementsA suitable building is required above the well to protect the well and its associated measuring equipment.Location Requires a sea wall or vertical structure for installation.Calibration Stilling wells can suffer from several defects which have been well documented. For example, density variations betweenthe inside and outside of the well in regions of stratification cause errors. Siltation and marine growth can cause changesto the dynamic response of the well. Absolute calibration usually involves dipping the well with a calibrated tape at periodicintervals.Accuracy Approximately 1 cm of sea level.Cost Band 1–2.Record of Use Used extensively in the United States and the United Kingdom for their national tide gauge networks.CommentsStilling wells have been used worldwide for a considerable period and are still used, both as the primary system and asbackup system for a modern tide gauge.Radar GaugesEquipmentOperationInstallationRequirementsLocationRadar tide gauges have so far been little used for GLOSS purposes, because it is a very recent technology. However, theyoffer a complete ready-to-go package which is relatively easy to install above the sea surface and seem to have advantageswith respect the acoustic sensors.Radar gauges measure the time of flight either from a pulsed radar or the phase change between a transmitted andreceived carrier wave, to determine the distance to the sea surface. They are much less affected by air temperature thanacoustic gauges.The installation requirements are relatively simple. The device requires a rigid structure to position it above the sea withsufficient clearance to avoid spurious reflections from any adjacent structures. As with many tide gauges, all ancillaryequipment (data logger, modem, satellite communications, battery backup), needs to be housed in an adjacent building.No need of a protective tube.Requires a site with vertical clearance sufficient to mount the device clear of the maximum sea surface, including wave action.CalibrationIn essence the device is self calibrating. However, for GLOSS purposes, a reflective target is mounted at a known distancebelow the radar transmitter.Accuracy Accuracy is expected to be approximately 1 cm of sea level.Cost Band 2–3.Record of Use So far these gauges have been used for relatively short periods experimentally by Spain and the United Kingdom.CommentsRadar tide gauges may consume excessive power if used in a continuous mode. In burst mode, they provide sufficientaccuracy for measuring most tidal parameters, but their use in a rapid sampling mode may be limited by this, althoughtests are being made in Spain for higher-frequency sampling.IOC <strong>Manual</strong>s and Guides No 14 vol IV27