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

Sea Level Measurement and InterpretationFigure 2.1 Spectrum of Sea Level Variations. The long-period variations and mean sea level changes are part of theenhanced energy at low frequencies.A second important issue is that, historically, a tide gaugewas attended continuously by a trained observer whocollected ancillary tide-pole information, and height anddatum corrections were appended to the chart weekly.This produced a very stable reference and of course meantthat faults were quickly identified. In modern systems thedatum and calibrations tend to be checked less frequently.Thus greater reliance is placed on the accuracy and stabilityof the measuring equipment. Fortunately, modern technologicalimprovements have allowed this, not only throughbetter equipment, but with two-way communication thesea level station can be interrogated and its operationalcharacteristics adjusted as necessary.The need for an operator to be permanently at the tidegauge has been removed. Perhaps one can speculate thatit is time to withdraw all manual intervention. Certainly,with the growing requirement for real-time data, manualintervention will not always be possible. In the future, theonly viable approach might be to check and authenticatethe data automatically at source before transmission. It canthen be passed to the end user and be placed in a formthat can be entered directly into the global sea level databanks without intervention.2.2 Surface WavesSurface waves are probably the most noticeable variationof the sea surface to a casual observer. They have beenrelatively little discussed in previous editions of this manual,as most tide gauges are designed to filter out such waves.However a brief description of their characteristics is worthincluding, as the design of a tide gauge relies on an understandingof their general characteristics.Waves are characterized as wind-waves or swell. Windwavesare generated by the effect of the wind on the localsea surface and have a relatively broad spectrum. Swell isproduced when the waves propagate out of a storm area.They occupy a narrower part of the spectrum. In general,wind waves have periods from 1 to 15 seconds, and swell,from 12 to 25 seconds, although this definition is notexclusive. Outside this range of periods, wave amplitudesare small. Wave period is usually calculated via the timebetween successive zero up-crosses of the wave (T z ).Wave heights are usually defined in terms of theirpeak-to-trough range in height, although wave amplitudeis sometimes calculated as the height above amean level. Significant wave height (Hs) is the usuallyquoted parameter which closely approximatesthe height of the highest one-third of the waves in agiven period of time. Traditionally, a wave record hasa duration of 20 minutes and is re-sampled every 3hours, choices which were derived originally from thestochastic properties of storm duration. It is difficultto give an overall figure for maximum wave height,as it depends critically on location. Waves are subjectto amplification, dispersion, refraction and focusing.In general, significant wave heights of several metresare common during a storm, but individual waves upto 30 metres have been measured.IOC <strong>Manual</strong>s and Guides No 14 vol IV3