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Sea Level Measurement and Interpretation3. Instruments for theMeasurement of Sea Level3.1 IntroductionThis section contains information on the types of instrumentthat are presently available for the measurement ofsea level. The reason that so many different technologieshave evolved is connected with the difficulty of measuringa fluid that is in constant motion due to the processesdiscussed in section 2. In general, sea level measurementsare not concerned with the measurement ofsurface gravity waves which must be filtered out of thesystem. Waves can be appreciable in amplitude and cancause problems for most forms of tide gauge technology.Therefore, their potential effects on a ‘sea level’measurement must always be kept in mind. Anotherfactor that needs to be considered is that the propertiesof sea water (salinity, temperature and hence density)may change on a regular or irregular basis. How thisaffects an instrument depends much on the technologyused to acquire the observations. These are discussedalong with the merits of each tide gauge.There are fundamentally four types of measuring technologyin common use:• A stilling well and float: in which the filtering of thewaves is done through the mechanical design ofthe well.• Pressure systems: in which sub-surface pressureis monitored and converted to height based onknowledge of the water density and local accelerationdue to gravity. Such systems have additionalspecific application to ocean circulation studies inwhich pressure differences are more relevant thanheight differences.• Acoustic systems: in which the transit time of asonic pulse is used to compute distance to the seasurface.• Radar systems: similar to acoustic transmission, butusing radar frequencies.Within each of these four types, different technologieshave been employed, leading to different designs.In addition, there are direct measuring devices basedon resistance or capacitance rods, but these have foundless widespread use because of their lack of robustnessin hostile regions. Recent advances in technologies,such as Global Positioning System (GPS) reflectionmethods, have lead to other elaborate ways of measuringsea level which might be important in the future.At the present time, many of the above systems areundergoing tests and inter-comparisons by agenciesworldwide (IOC, 2004). It would appear that mostsystems for measuring sea level have a precisionapproaching 1 cm, given sufficient care and attention.This value is adequate for the measurement of mostof the hydrodynamic processes discussed in section 2.However, this precision does not necessarily imply anaccuracy for adequate measurement of the mean level.The determination of the mean level depends as muchon the long-term stability of the measuring system.There are practical constraints that govern the choiceof an instrument for a particular application. Theseinclude cost, degree of difficulty of installation, easeof maintenance and repair, support facilities etc. Forexample, the installation of a highly complex electronicinstrument with sophisticated software control wouldbe unwise without technical support staff who possess10IOC <strong>Manual</strong>s and Guides No 14 vol IV
Sea Level Measurement and Interpretationthe ability to maintain its operation. Another importantconsideration in the choice of an instrument is the siteat which it is to be located. This is discussed in the nextsection.Traditionally, permanent sea level stations around theworld have been mainly devoted to tide and mean sealevel applications, and this has been the main objectiveof GLOSS. This implies that not only wind wavesare filtered out from the records by mechanical ormathematical procedures, but any oscillation betweenwind waves and tides (e.g. seiches, tsunamis etc.) hasnot been considered a priority, and in fact not properlymonitored, owing to the standard sampling time ofmore than 5–6 minutes. If this range of the spectrumshould be covered from now on, it would be necessaryto consider this when choosing a new instrument anddesigning the sea level stations.3.1.1 The Choice of a Tide Gauge SiteIn many cases, the site for a tide gauge may be specified(e.g. it has to be located in a port area). However,in many instances, the choice of site will not be clearand can only be made by judging which of the constraintslisted below are more significant and whichshould be given greater emphasis. This emphasis maydepend on, for example, whether the gauge is intendedfor oceanographic research, in which case one clearlyrequires it to be located with maximum exposure to theopen ocean, and not situated in a river. Most GLOSSCore Network sites have been selected with this aspectin mind. For local programmes, where the processto be studied may be coastal erosion or storm surgeactivity, then clearly the gauge will have to be situatedoptimally for that purpose. In most cases, some of thefollowing constraints are still valid:• The installation must be capable of withstandingthe worst environmental conditions (winter ice,storms etc.) likely to be encountered. This is clearlyan issue relevant to the type of instrument and toits intended position. Positions exposed to environmentalextremes should clearly be avoided toenable the eventual accumulation of a long timeseriesof data.• The ground on which the installation is to be erectedshould be ‘stable’ as far as possible, not beingliable to subsidence because of underground workingsor land subsidence (e.g. due to the area beingreclaimed land). It must also not be liable to slippagein the event of heavy prolonged rain (i.e. thearea must be adequately drained) or being erodedby river or sea action. An installation on solid rockis the ideal.• River estuaries should, if possible, be avoided.Estuarine river water can mix with sea water tovarying extents during a tidal cycle and at differenttimes of the year, resulting in fluctuations inwater density. This may have important impactson float gauge measurements in stilling wellsbecause of ‘layering’ of water drawn into the wellat different times causing a difference in densityinside and outside the well. It will also impact onpressure measurements, as the density assumedfor the conversion of pressure to sea level will notbe constant. Currents associated with river flowcan also cause drawdown in stilling wells and inthe stilling tubes of acoustic gauges. Followingheavy rain-storms, debris floating down-rivercould damage a gauge.• Areas where impounding (isolation from the opensea) can occur at extreme low-tide levels should beavoided. Similarly, sandbars slightly below the surfacebetween the site and the open sea can result inuncharacteristic levels being measured. Monitoringacross long shallow sloping beaches should also beavoided for the same reasons.• Sharp headlands and sounds should be avoided,since these are places where high tidal currentsoccur which tend to result in unrepresentative tidalconstants and in a drop of MSL (Pugh, 1987).• Proximity to outfalls can result in turbulence, currents,dilution and deposits, and should be avoided.• Places where shipping passes or moors close to theproposed site, since there will be a risk of collisionand propeller turbulence causing silt movement; astudy should be made of this possible factor.• Places where construction work in the area at somefuture time may affect the tidal regime at the site(e.g. by construction of new quays or breakwaters);investigations should be made to determinewhether there is a possibility of this occurring. Thismight necessitate the relocation of the tide gauge,thus interrupting the sea level time-series. This issomething very difficult to avoid in some harbours.• A site should have continuous mains electricalpower (or adequate storage batteries/solar panelsor generator supply) and telephone or satelliteaccess for transmission of data to an analysiscentre.• There must be adequate access to the site for installationand maintenance and the site must be securefrom vandalism or theft.• The area of the site must be capable of containingthe benchmarks required for geodetic control ofthe sea level data. In particular, it must have goodTGBM and GPSBM marks, which must also besecure from accidental damage.• If stilling well or acoustic gauges are to beinstalled, then the stilling well or acoustic tubemust be tall enough to record the highest sealevels. This may require permission from portauthorities if, for example, the installation is on abusy quayside.• The water depth must extend at least two metresbeneath Lowest Astronomical Tide (LAT) for thesuccessful operation of a stilling well. The outlet ofthe stilling well should be clear of the sea bed andIOC <strong>Manual</strong>s and Guides No 14 vol IV11
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