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

Sea Level Measurement and Interpretationwaves: FMCW). The OTT Kalesto, Miros and Radac instrumentsuse this method. The VEGA and SEBA systems usepulsed transmissions and time-of-flight measurement.All these gauges have undergone initial tests and intercomparisonsby various agencies in different countries.Details of these tests can be found in IOC WorkshopReport No 193. Details of the individual instruments canbe found on the websites shown below.In principle, the instruments are self calibrating,as far as a datum value is concerned. However, toprovide confidence that the datum remains constantover long time periods, alternative means are beinginvestigated. These take the form of a reflector thatcan be placed in the radar beam at appropriateintervals. The reflector is placed at a known distancebelow the contact point of the installation for a shortperiod. Over a period of a year or more the datumvalue can be verified and used to adjust the measurements,if necessary.Initial indications that these instruments can provideacceptable measurements for the purposes ofGLOSS are promising. As with all tide gauges, practicalconsiderations related to a particular applicationoften dominate other considerations. For example,they have very limited application in polar regions.They have not yet been used in extremely hostileenvironments, for example on remote islands, whereextreme waves may overtop the gauge by severalmetres. However, for a normal application in which astilling well or bubbler gauge is presently in use, theyappear to operate satisfactorily.3.6 Summary of the Merits of DifferentTechnologiesIn this section, we summarize the relative meritsof different tide gauge technologies for scientificresearch, operational oceanography and for localizedpractical purposes, such as harbour operations.The GLOSS programme has scientific research asits raison d’être, although it is intended that thedevelopment of the GLOSS networks should serve toimprove standards overall (see IOC, 1997). We canuse the designation ‘GLOSS’ to indicate the mostdemanding requirement of scientific-quality performanceof a gauge (Appendix I).There are also sea level requirements from operationalusers of oceanographic data in such topicsas marine infrastructure (e.g. offshore industry,transport, coastal recreation) and coastal defences(e.g. flood protection from surges, and studies ofcoastal erosion or sea level rise impacts). Many ofthese applications overlap GLOSS interests, thestudy of secular changes in sea level being an obviousexample. However, the particular applicationswill vary from country to country. Therefore, suchgauges will be capable of deployment for extendedperiods, but perhaps not to the same high standardsas those intended for GLOSS, and will be affordablefor use in larger numbers than for GLOSS, especiallyby developing countries.Finally, there will be applications which require acheap instrument capable of showing the state ofthe tide at any moment but certainly not accurateenough for GLOSS.Table 3.1 presents a summary of the main conclusionson the relative merits of each gauge technologybased on the previous sections of this <strong>Manual</strong>.The Table also includes an estimate of the likely costof a basic system with gauge, data transmission (e.g.modem) and meteorological package, although thisis an extremely difficult item to quote given the largenumber of manufacturers, monetary exchange ratesetc. For example, the cost of a pressure transducerwill vary by a factor of 3 depending on the quality.With these reservations in mind, Cost Band 3has been set as the highest cost, which might be12,000–20,000 US$ (at the time of writing andwithin a large band, say 30%); Band 2 might be8,000–12,000 US$; and Band 1, 5,000–8,000 US$.However, in our experience, the real costs of any tidegauge station are those of installation (e.g. some kindof engineering support will be needed for installationof a stilling well, acoustic sounding tube gauge, or‘B’ gauge; diver support will be needed for pressuregauge installations etc.), ongoing maintenance anddata analysis (with implications for staff resources).Anyone planning a gauge installation has, therefore,to take into account all the local costs as well as theup-front costs of gauge hardware. Agencies participatingin GLOSS which require the input of expertisemay wish to explore the possibilities of collaborationwith other GLOSS participants.Our recommendations are:• If one is planning a new GLOSS tide gauge stationin a mid- or low-latitude location, one should probablyopt for:- an acoustic gauge with sounding tube or- a radar tide gauge or- a ‘B’ pressure gaugeIf low tidal range or other factors preclude the use ofa ‘B’ gauge, then a single transducer pressure gauge,a bubbler pressure gauge or a pressure transducer ina stilling well would be options. In addition, in mostcases, the main tide gauge should be accompaniedby a pressure sensor installed in the sea and capableIOC <strong>Manual</strong>s and Guides No 14 vol IV21