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

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Sea Level Measurement and Interpretationbe set deep enough to allow the float to operateabout one metre below LAT.Finally, it is clear that tide gauge datum control is anessential issue for any installation. Consequently, evenif the station is equipped with the most modern equipment,it is common sense to provide confirmation ofthe datum from time to time by means of an inexpensivetide ‘pole’ or ‘staff’ to guard against gross errorsin the datum.3.2 The Stilling WellA stilling well gauge is probably the most common ofall sea level recording systems on a worldwide basis.These gauges were at one time employed at every portinstallation and were the primary technology by whichsea level records were compiled. Recent stilling wellinstallations are less common, since they require a considerableamount of costly engineering work, so thatthey have often been superseded by one of the othertechnologies discussed below. In some circumstances itmay not be possible to install a well, e.g. on a shelvingbeach, and other methods have to be adopted.The function of a well is to filter out, ‘to still’, the waveactivity, so that the tides and longer-period processescan be recorded accurately. It is most commonly associatedwith having a float gauge in the well driving apen and chart recorder or, in more recent years, a shaftencoder such that the readings of sea level height canbe digitized automatically. It is not uncommon for othertypes of instrument, e.g. a pressure sensor, to also beplaced in the well.The well itself is a vertical tube about 1 m in diameterconstructed of concrete, coated steel or plastic, with ahole or, less frequently, a pipe connection to the sea.The ratio of the hole diameter or pipe length and diameterto that of the well gives it the characteristics of alow pass filter (Noye,1974a, b, c). In other words, it actsas a mechanical filter. Care has to be exercised in tryingto measure processes such as tsunami waves, as thefrequency response is not 100% for periods ≤4 hours.The stilling well suffers from amplitude attenuationand a phase lag at shorter periods which are criticallydependent on the design of the well and sometimesdifficult to change.The characteristics, installation and use of a stilling wellwere covered in substantial detail in Volume 1 of the<strong>Manual</strong> of Sea-Level Measurement and Interpretation(IOC, 1985). The reader is advised to refer to that publication,and for additional information on the characteristicsof the stilling well, to Noye (1974). Lennon (1971)dealt in detail with errors that arise in the operation ofsuch a system.A schematic diagram of a float gauge in a stilling wellis shown in Figure 3.1. The float wheel is shown drivinga pen recorder, but the same pulley could equally drivea digital shaft encoder or a potentiometer, which canthen be recorded by a local data logger or interfacedto a telemetry system. The well is shown with a conicalinlet at its base, since this is the most common configurationand is to some extent self-cleaning. Manyother configurations of the inlet are acceptable, andalthough the conical orifice does restrict the inflowrelative to the outflow, this does not appear to have asignificant effect on the records even in the presenceof waves.Figure 3.1 Stilling well tide gauge.3.2.1 Datum SwitchesIn common with all other types of sea level recordingsystems, the setting and control of datums is of crucialimportance. This topic is dealt with in section 4.Stilling well tide gauge installations were, at one time,attended on a continuous basis. Under these circumstancesvisual comparisons were made with a fixed tidegauge staff on a regular basis and appropriate time anddatum corrections were applied to the data. Withoutthis, alternative means of fixing the datum have to befound. One alternative is to site a level switching deviceas part of the installation at approximately mean sealevel. The switch indicates the instant at which the seacrosses the level of the switch, a level that is known rel-12IOC <strong>Manual</strong>s and Guides No 14 vol IV
Sea Level Measurement and Interpretationative to all other datums of the tide gauge. Ideally theswitch, which can be mechanical, optical or acoustic,should be sited outside the well in its own mini-stillingwell. The switch provides a correction for any datumshift that previously would have been manually recordedby an operator. Although the switch will not workcorrectly under all conditions, e.g. when high waves ora seiche is present, there will usually be sufficient daysof calm to obtain an accurate datum check.3.3 Pressure GaugesInstruments that measure subsurface pressure insteadof sea level directly have found widespread use. Aknowledge of seawater density and gravitational accelerationis required to make the conversion from pressureto sea level, but in spite of this, the instrumentshave many practical advantages as sea level recorders.The most commonly used types are the pneumaticbubbler gauges and pressure sensor gauges in whichsensors are mounted directly in the sea. The two typeshave much in common and a choice of which type issuitable is usually based on practical considerations ata proposed site.3.3.1 Pneumatic Bubbler GaugesThe pneumatic bubbler tide gauge has been successfullyused worldwide for several decades. Itreplaced many of the float-operated harbour gaugesas the primary standard for sea level measurement incountries such as the United States and the UnitedKingdom, although in the USA they have since beensuperseded by acoustic gauges (section 3.4). TheUK still operates its National Tide Gauge Network(44 stations) based on this technology. It has beenshown to be robust, both in terms of accuracy anddatum stability. It has demonstrated its value insituations where there are no vertical structures onwhich to attach the equipment, e.g. on coral atolls(Pugh, 1978), as the part of the equipment installedin the sea and on land can be several hundred metresapart, which is not the case with many other typesof instrument.Figure 3.2 shows the basic essentials of a bubbler system.Air is passed at a metered rate along a small-boretube to a pressure point fixed underwater well belowthe lowest expected sea level. The pressure pointnormally takes the form of a short vertical cylinderwith a closed top face and open at the bottom. Asmall ‘bleed hole’ is drilled about half way down itslength and metered air is entered through a connectionon the top surface. As air from the tube entersthe pressure point it becomes compressed and pushesthe water down inside the chamber until the levelof the bleed hole is reached at which time the airbubbles out through the hole and back to the surface.Provided that the rate of air flow is low and the airsupply tube is not unduly long, the pressure of air inthe system will equal that of the pressure due to thedepth of the sea water above the bleed hole coupledwith atmospheric pressure. A pressure-recordingFigure 3.2 Bubbler pressure gauge.IOC <strong>Manual</strong>s and Guides No 14 vol IV13
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