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

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Sea Level Measurement and Interpretation2. The Nature of Sea LevelVariations2.1 IntroductionThe study of sea level has many different facets. It is notsimply the measurement of the sea level that requirestechnical expertise. The data must be carefully calibrated,checked and evaluated. The measurements should be tiedto local benchmarks that in turn are fixed into a country’snational levelling network and further fixed into the globalnetwork using modern geodetic techniques. The recordeddata need to be archived, documented and protectedfor future studies. Only then is it of benefit as a valuableresource and can be used for studies ranging from localengineering projects to long-term global climate change.Variations in sea level contain contributions from differentphysical sources that are usually distinguished bytheir period. Components range from surface gravitywaves with periods of 1 to 20 seconds; seiches andtsunamis with periods of minutes to over an hour; tidescentred around 1/2 and 1 day; meteorological effectsof several days to 1 year; interannual and decadalvariability; and long-term trends in the mean levelcaused by geological and climatological effects. Themagnitudes of these components vary enormously.Surface gravity waves can have amplitudes up to 30 m.Tsumanis tend to be less than 1 m in the deep oceanbut may be several metres near the coast. Tides arerelatively small in the ocean but may be 10 metres nearthe coast. Storm surges may be of the order of a fewmetres in shallow seas. Within this mix one is tryingto estimate long-term trends in the mean level of theorder of 1 mm per year. The fact that this is possible,and has been for over 100 years, is testimony to theexpertise and dedication of the engineers and scientistswho are involved in sea level research.The majority of historical sea level data were collected fromfloat and stilling-well tide gauges with analogue charts,many of which are still in existence, but superseded by themodern trend to the digital systems described below. Withdigital technology it is possible to improve the accuracyand reliability of the data and make the data available tothe user in real time.In analogue form the charts were always available forre-analysis and errors could be rectified by reappraisal ofthe chart and re-sampling of the pen-trace, if necessary.In digital form a corresponding re-analysis is not alwayspossible. The decision has to be made in advance as towhat is a reasonable sampling (or averaging) interval. Onecannot return and re-sample the data at a more frequentinterval. In the past, the generally accepted sampling (oraveraging) rate was 1 hour, since this allowed the studyof all processes, from tides to mean sea level (IOC, 1990).Waves were, by their nature, considered a different scientificprovince and were filtered out of the data. Morerecently, the sampling frequency has been increased to 15minutes, 6 minutes and even higher rates.The disastrous tsunami of 26 December 2004 in theIndian Ocean made it clear that the normal tide gaugesampling would be inadequate and that it would be necessaryto increase it to 1 minute or ideally to 15 seconds.This places constraints on the tide gauge technology andincreases the demand on the storage and transmissionrequirements of a tide gauge network. There is a balanceto be struck between the need to capture the essence ofthe data and the need to store and perhaps transmit largevolumes of data.2IOC <strong>Manual</strong>s and Guides No 14 vol IV
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
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