Oceanography

3.3.2.Tide prediction

3.3.2.Tide Measurement

Tide gauges

Tides are waves of long wavelength and known period, so the major properties of interest for measurement are wave height, or tidalrange, and wave induced current. The latter is measured with currentmeters;any type is suitable. Two types of tide gauges are use to measure the tidal range(1). The stilling-well gauge consists of a cylinder with anconnection to the sea at the bottom. This connection acts as a lowpass filter: it is so restricted that the backward and forward motion ofthe water associated with wind waves and other waves of short period cannot pass through; only the slow change of water level associatedwith the tide can enter the well. This change of water level  is pickedup by a float and recorded. Stilling-well gauges allow the direct reading of the water level at anytime but require a somewhat laborious installation and are impracticable away from the shore(2). In offshore and remotelocations it is often easier to use a pressure gauge. Such an instrument is placed on the sea floor and measures the pressureof the water column above it, which is proportional to the heightof water above it. The data are recorded internally and notaccessible until the gauge is recovered.Tide gauge are increasinglyused to monitor possible long term changes in sea level linked with climate variability and climate change. The expected rate of sea levelchange is only a few millimeters per year at most, so very highaccuracy is required to verify such changes. Most tide gauges are not suitable for such a task, for a number of reasons.For example, a long term trend in observed sea level can alsobe produced by a rise or fall of the land on which the tide gaugeis built. (This is known as benchmark drift). The wire in a stilling-wellgauge that connects the float with the recording unit stretches and shrinks as the air temperature rises and falls. Such effects are insignificant when the gauge is used to verify the depth of water for shipping purposes but not when it comes to assessing trends of millimeters per year. A new generation of tide gauges is being installed worldwide which gives water level recordings to absolute accuracies of a few millimeters with long term bench mark stability. In theseinstruments the flaot and wire arrangement of stilling-well gauge is replaced by a laser distance measurement, and the data are transmitted ia satellite to a world sea level centre which monitors the performance of every gauge continuously.