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

Sea Level Measurement and InterpretationOptimization of tide-staff measurementsA bench-mark-levelled tide-staff can be found inevery harbour. The Intergovernmental OceanographicCommission (IOC) of UNESCO has recommendedthe application of tide-staff measurements for qualitycontrol of sea level measurements made fromautonomous instrumentation (IOC, 2002). However,no reports are available on the optimal quantumof tide-staff measurements to be made, nor themost appropriate sequence of such measurementsin relation to spring/neap cycles for their use inoptimizing quality control. Thus, at present, lack ofclarity exists as to what constitutes a ‘sufficientlylarge’ and ‘representative’ data set to be used forthe construction of the model. In an attempt to findan answer to these questions, we constructed severalmodel equations of the type mentioned above,wherein data sets (acquired at 15-minute intervals)corresponding to differing tidal cycles during neapand spring tides were used. We then examined theoptimal number of tidal cycles and their neap/springrelationships in achieving stabilization of the modelparameters G and O corresponding to each of thesemodel equations (Figure 2).In this study, it was observed that, in the case ofneap tide measurements (indicated by ¥ on thegraphs), even after several tidal cycles, the valuesof G and O never stabilized (i.e. the values did notlie on a line parallel to the x-axis). However, unlikeneap tide, spring tide measurements (indicated by_ on the graphs) provided significantly improvedresults. In this case, G and O stabilized with the useof the data set corresponding to a 3-tidal cycle (oneand half days), and the incorporation of more tidalcycles for the construction of the model did notprovide any added benefits in terms of stabilizationof G and O. We then examined whether centeringthe data set on spring tide peak could reduce thesize of the data set for construction of the model.With reference to Figure 2; points a, b, c, d, e, f, andg represent, respectively, the results achieved withthe use of quarter-hourly data sets acquired at/nearspring tide between points 4–5 (1 cycle), 5–6 (1cycle), 4–6 (2 cycles), 3–7 (4 cycles), 2–8 (6 cycles),1–9 (8 cycles), and 0–10 (10 cycles). It is seen that,in relation to several of the G and O values obtainedfrom the models, the use of data sets correspondingto a single cycle that encompasses the largesttidal range (bounded by points 4–5 at spring tide)provided stabilized G and O values (indicated by aon the Gain and Offset graphs). However, data setscorresponding to another adjacent single cycle thatrepresented a much smaller tidal range (bounded bypoints 5–6 at spring tide) provided G and O values(indicated by b on the Gain and Offset graphs) thatdiffered significantly from the stabilized G and Ovalues. In many cases a single tidal cycle at springtide may not represent the maximum range of thewater column height. This explains why sometimes adata set corresponding to three or more tidal cyclesduring a given spring tide becomes necessary toachieve stabilization of the model parameters G andO. It may therefore be concluded that the crucialfactor, which determines stabilization of the modelparameters G and O is the incorporation of data setsfrom that (those) tidal cycle(s) which encompassesthe largest range of water column height during agiven spring tide. Figure 3 provides the chart-datumreferencedsea level measurements obtained fromTakoradi harbour during the period 1 July 2004 to30 June 2005 based on the above scheme.ConclusionsThe primary objective of our present studies, basedon the GLOSS sea level data from Takoradi harbour,was to examine a method to improve the accuracy ofbottom-pressure-based coastal and estuarine sea levelmeasurements. In this method, time-series bottompressure measurements are translated to chart-datumreferencedsea level elevation through post-processingusing a statistically derived mathematical model. Thismodel is constructed from a set of successive periodic(15-minute intervals) bottom pressure measurementsover a few tidal cycles during the spring tide, whichencompasses the maximum span of water columnFigure 3. Chart-datum-referenced sea level measurements from Takoradi harbour, Ghana.70IOC <strong>Manual</strong>s and Guides No 14 vol IV