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Sea Level Measurement and Interpretationthe world (Gill et al., 1993). These systems wereoperated alongside existing (float or bubbler) tidegauges at many stations for a minimum period ofone year to provide datum ties and data continuity.Dual systems were maintained at a few stations forseveral years to provide a long-term comparison.Tide gauges using the same technology have beendeployed in a number of other countries, such asAustralia, where they are known as SEAFRAME systems(Lennon et al., 1993).The NGWLMS tide gauge uses an Aquatrak waterlevel sensor developed by Bartex Inc. and acquired byAquatrak Corporation, together with a Sutron dataprocessingand transmission system. The Aquatraksensor sends a shock wave of acoustic energy downa 1/2-inch-diameter PVC sounding tube and measuresthe travel time for the reflected signals froma calibration reference point and from the watersurface. Two temperature sensors give an indicationof temperature gradients down the tube. Thecalibration reference allows the controller to adjustthe measurements for variations in sound velocitydue to changes in temperature and humidity. Thesensor controller performs the necessary calculationsto determine the distance to the water surface. Thesounding tube is mounted inside a 6-inch-diameterPVC protective well which has a symmetrical2-inch-diameter double cone orifice to provide somedegree of stilling. The protective well is more opento the local dynamics than the traditional stillingwell and does not filter waves entirely. In areas ofhigh-velocity tidal currents and high-energy sea swelland waves, parallel plates are mounted below theorifice to reduce the pull-down effects (Shih and Baer,1991). Figure 3.5 is a schematic of a typical NGWLMSinstallation. To obtain the best accuracy, the acousticsensor is calibrated by reference to a stainless steeltube of certified length, from which the zero offset isdetermined.The NGWLMS gauges have the capability of handlingup to 11 different ancillary oceanographic and meteorologicalsensors. The field units are programmed totake measurements at 6-minute intervals with eachmeasurement consisting of 181 one-second-intervalwater level samples centred on each tenth of anhour. Software in the instrument rejects outliers etc.which can occur as a result of spurious reflections.Measurements have a typical resolution of 3 mm.The instrument contains the necessary hardware fortelephone and satellite communications.Papers by Gill et al. (1993) describe the operationalperformance of the NGWLMS instrumentation.Lennon et al. (1993) and Vassie et al. (1993) presentcomparisons between NGWLMS and conventionalstilling well or bubbler systems in Australia and theUK. Most comparisons show small differences, of theorder of a few millimetres, for the various tidal anddatum parameters, which are generally within theuncertainty of the instrumentation. Such differencesare very small when compared to typical tidal rangesand even seasonal and interannual sea level variations.NGWLMS systems are considered sufficientlyaccurate for mean sea level studies.Figure 3.5 NGWLMS tide gauge.18IOC <strong>Manual</strong>s and Guides No 14 vol IV
Sea Level Measurement and InterpretationA modern version of the NGWLMS is called a SeaRanger which is claimed to have a number of advantagesover the earlier technology including self calibration(IOC, 2004)3.4.2 Acoustic Gauges without Sounding TubesSeveral acoustic instruments have been producedthat are operated without a sounding tube, normallylocated inside an existing stilling well or inside aplastic tube some 25 cm in diameter. Some of themmay operate in the open air, but are not normallyemployed for high-quality sea level measurements(see Table 3.1 in section 3.6). These acoustic instrumentsoperate at a frequency of 40–50 kHz and havea relatively narrow beam width of 5°. Their measurementrange is approximately 15 m and an overallaccuracy of 0.05% is claimed by the manufacturers(see websites below).Contradictory experiences can be found with this typeof acoustic sensor, from some problems in achievingthe stated accuracy under all environmental conditions(e.g. see presentation by Ruth Farre, in IOC, 2003), tothe high-quality and continuous operation of 15 tidegauges in the REDMAR network (Spain), most of theminstalled in 1992 and still in operation (e.g. see presentationby Begoña Pérez in IOC, 2003).A crucial aspect of this type of sensor is the dependenceof the velocity of sound on the environmentalconditions, such as the air temperature. On the otherhand, tubes tend to increase the temperature-gradientbetween the instrument and the sea surfaceunless special precautions are taken to ensure thatthe air is well mixed in the tube. A complementaryand necessary method is to compensate for soundvelocity variations using a reflector mounted at asuitable distance below the transmitter, as is the casefor the SRD gauges employed in the REDMAR network.A careful design of the installation, avoidingdifferent ambient conditions along the tube and followingthe maker’s requirements about the minimumdistance to the water surface, become crucial for thefinal accuracy of the data.The performance of one of these sensors (SRD) overan existing stilling well inside a hut or small buildingin Santander (Spain), has been incredibly good (nearlyperfect and continuous during 15 years). The conditionsof this installation are probably perfect, perhapsbecause the temperature inside the building is ratherhomogeneous. Data from this acoustic sensor have infact helped to correct malfunctions of the float gaugethat operates inside the same stilling well.Studies of mean sea levels from 12 years of data inSpain, comparing this type of acoustic sensor (SRD)with the traditional float gauges, has shown theirhigh quality and has even helped to identify referencejumps in the older float gauges. This is, again,a contradictory experience to the one in South Africa(see article by Farre in Appendix V of this volume).Nevertheless, it seems that radar gauges will replacethis type of acoustic sensor everywhere, in the nearfuture.3.5 Radar GaugesRadar tide gauges have become available during thelast few years from several manufacturers. Although thistechnology is relatively new, radar gauges are being purchasedand installed by a number of agencies as replacementsfor older instruments or for completely newnetworks. The reason is that they are as easy to operateand maintain as acoustic sensors, without their maindisadvantage: their high dependence on the air temperature.Radar gauges have a relatively low cost and theengineering work necessary to install them is relativelysimple compared to other systems. The instruments aresupplied with the necessary hardware and software toconvert the radar measurements into a sea-level height.In addition, the output signals are often compatible withexisting data loggers or can be interfaced to a communicationnetwork. Like many modern systems they can beset up using a portable computer.The active part of the gauge is located above thewater surface and measures the distance from thispoint to the air–sea interface. A diagram is given inFigure 3.6. The gauge has to be mounted in such away that there are no restrictions or reflectors in thepath of the radar beam, between the gauge mountingand the sea surface. It has to be positioned abovethe highest expected sea level and preferably abovethe highest expected wave height, so as to preventphysical damage.It has many advantages over traditional systems inthat it makes a direct measurement of sea level.The effects of density and temperature variations,even in the atmosphere, are unimportant. The mainconstraint is that the power consumption may berelatively large in radar systems if used on a continuousbasis in a rapid sampling mode. Averages aretypically taken over periods of minutes. This maylimit its use in some applications (e.g. tsunami warning)where observations are required on a continuoushigh-frequency (e.g. 15-second) basis. In such areas,pressure gauges may be more appropriate, althoughwork and research is still being done concerning thisparticular application.Radar gauges fall into two categories. Those thattransmit a continuous frequency and use the phaseshift between transmitted and received signal to determinesea level height (frequency-modulated continuousIOC <strong>Manual</strong>s and Guides No 14 vol IV19
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