Tidal, meteorological, and oceanographic forces cause variations in seabed depth relative to chart datum
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Reference materials and hydrodynamic studies confirm that tidal, meteorological, and oceanographic forces influence water levels and locations of chart datums, thereby causing effective variations in observed depths relative to chart datum.
Abstract:
Investigations of the role of sea level in producing coastal erosion have focused mainly on the long-term rise due to melting of glaciers and thermal expansion of sea water. There are, however, additional shorter term changes in the local sea level produced by a variety of ocean processes. Variations in the coastal currents, for example, can alter the water level at the shoreline due to the geostrophic balance between the current and the offshore sea-surface slope. Other factors which may alter local sea level include changes in atmospheric pressure, winds blowing either in the longshore or cross-shore directions, and the occurrence of upwelling. Because the inclined continental shelf and slope act as a wave guide, the fluctuations often become trapped and propagate over longshore distances beyond where they are actually generated. In that many of these processes are typically seasonal, the responding sea level also has a pronounced seasonal cycle, but frequently there can be significant fluctuations at periodicities of several days to a few weeks. The magnitudes of such changes vary considerably with coastal location but are typically on the order of 10 to 30 cm, achieving a maximum of about 100 cm in the Bay of Bengal.
The occurrence of an El Niño in the equatorial Pacific is known to have considerable impact on the erosion of the coasts of California and Oregon. This occurs because associated with an El Niño are shifts in the storm paths and a temporary rise in sea level. An El Niño is a breakdown of the normal equatorial wind and current patterns. This breakdown releases water which is normally set up in the western Pacific by the trade winds. The release creates a “wave” of sea-level rise, which first propagates eastward along the equator and then poleward along the eastern ocean margin. Such “waves” have been measured in the tide records of the western United States, amounting to some 20 to 60 cm and lasting for several months. Such transient sea-level changes have likely played an important role in coastal erosion.
Estuaries are transitional environments with ideal conditions for port construction and navigation. They represent a challenge to hydrographic services due to the dynamics of the seabed and the tidal wave deformation. The bottom slope, the convergence of the channels, and the nonlinear effects produced by the bottom friction produce variation in both the tidal range and the location of the chart datum (CD). In this study, sea level data series obtained from the nodes of the mesh of a hydrodynamic model (virtual tide gauges) were used to calculate the harmonic constituents, form factor, asymmetry, and estuary type. The final chart datum surface, obtained from the hydrodynamic model, was used to determine the separation values between zones and also the number of tidal zones in an estuarine system. It was found that in a complex hydrodynamics scenario, the use of the ellipsoidal referenced surveying (ERS) method is more convenient than traditional tidal zoning survey. In the ERS method, once the CD model is complete, it must be attached to the ellipsoid directly. Finally, the variation of the CD in different scenarios (due to anthropogenic action) was assessed.
depths are relative to a "chart datum", which is typically the water level at the lowest possible astronomical tide (although other datums are commonly
Tides are the periodic rise and fall of sea level resulting from the differential gravitational forces exerted primarily by the Moon and the Sun, combined with inertial effects associated with the Earth–Moon system’s orbital motion and the Earth's rotation.
While these astronomical forcings generate the fundamental tidal potential, actual observed tides are strongly modified by terrestrial factors
Nautical charts display the water's "charted depth" at specific locations with "soundings" and the use of bathymetric contour lines to depict the submerged surface's shape. These depths are relative to a "chart datum", which is typically the water level at the lowest possible astronomical tide (although other datums are commonly used, especially historically, and tides may be lower or higher for meteorological reasons) and are therefore the minimum possible water depth during the tidal cycle. "Drying heights" may also be shown on the chart, which are the heights of the exposed seabed at the lowest astronomical tide.
Tide tables list each day's high and low water heights and times. To calculate the actual water depth, add the charted depth to the published tide height. Depth for other times can be derived from tidal curves published for major ports. The rule of twelfths can suffice if an accurate curve is not available. This approximation presumes that the increase in depth in the six hours between low and high water is: first hour — 1/12, second — 2/12, third — 3/12, fourth — 3/12, fifth — 2/12, sixth — 1/12.
Batimetri adalah ukuran dari tinggi rendahnya dasar laut yang merupakan sumber informasi utama mengenai dasar laut. Dalam pengolahan data batimetri diperlukan beberapa koreksi, salah satunya adalah koreksi pasang surut. Data pasang surut selama ini diperoleh dari pengamatan pasang surut yang dilakukan di stasiun pasang surut selama dilaksanakannya pemeruman. Perkembangan teknologi memberikan metode perekaman data pasang surut menggunakan Global Navigation Satelltie System (GNSS). Dalam tugas akhir ini dilaksanakan perekaman data GNSS yang dipasang pada sounding boat dengan metode Post-Processing Kinematic (PPK). Data yang diperoleh merupakan data tinggi permukaan air dari elipsoid yang kemudian dikurangkan dengan tinggi chart datum terhadap elipsoid. Hasil yang diperoleh kemudian digunakan untuk mereduksi kedalaman pada pengolahan data Multibeam Echosounder (MBES). Hasil dari penelitian ini berupa grafik perbandingan pasang surut GNSS tide dengan tide gauge dan perbandingan angka kedalaman pengolahan data MBES dengan koreksi pasang surut GNSS tide dan tide gauge.
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