Tidal Currents near DongSha - a Local Model
Date Issued
2007
Date
2007
Author(s)
Wu, Shu-Fen
DOI
zh-TW
Abstract
The purpose of this thesis is using data analysis to study the relationship between sea level and barotropic tidal current near DongSha Island, and developing a local model to examine the tidal dynamics in a shallow water environment. Right at the coast of the DongSha Island, a tide gauge was deployed from November, 2004 to March, 2006, 100 km offshore in a water depth of 350 m, an ADCP (Acoustic Doppler Current Profiler) was moored to record the current velocity from July 28 to November 1, 2005. There have been simultaneous measurements of both sea level and current for 3 months. Harmonic analysis of the DongSha Island sea level data shows that the most significant tidal constituents in this region are the K1 (amplitude: 27 cm) and O1 (24 cm) diurnal tides and the M2 (14 cm) and S2 (5 cm) semidiurnal tides. The barotropic tidal current was computed using the depth-averaged velocity. Harmonic analysis reveals that the tidal current velocity of K1, O1 and M2 are about 4 ~ 5 cm s-1, and S2 is about 2 cm s-1.
The relationship between barotropic tidal currents and sea level were established for the first 45-day period using the transfer functions and harmonic analysis, and then carry the prediction for the second 45-day period. There is very good agreement between the observed and predicted semidiurnal tidal current, the rms (root mean square) error is less than 3 cm s-1. However, prediction on diurnal tidal current is rather disappointing, the rms error is about 19 cm s-1, suggesting strong baroclinic tidal component (internal tides) may have a significant contribution to the tidal current, which is not registered in the sea level records.
Knowing the tides propagate in the northwestward direction (Beardsley, 2004) toward the coast of mainland China, the barotropic tidal current can be computed through a simple one-dimensional, barotropic tidal model (Clarke and Battisti 1981; Battisti and Clarke 1982 a,b; Clarke 1991). Giving the M2 sea level at the DongSha Island and topography, the local model successfully computes the tidal current near the observational site with a bottom friction coefficient r = 3×10-2 m s-1. This friction coefficient is rather large compared with that of most known coastal waters, but closed to what have been found near Great Barrier Reef, Australia, where both have similar coral reef environment. Computed K1 and O1 tidal currents are within acceptable range, but not as good as the M2 component.
In essence, both the data analysis and model results confirmed that the M2 semidiurnal tidal current is largely barotropic, and can be predicted through the coastal sea level. However, the baroclinic component of O1 and K1 diurnal tidal current are rather obvious and prohibited us from establishing a tight relationship, and hence, a prediction scheme from the coastal tidal data.
Key Words: DongSha, tidal current, internal tides,
barotropic, baroclinic.
Subjects
東沙島
潮流
內潮
正壓
斜壓
DongSha
tidal current
internal tides
barotropic
baroclinic
Type
thesis
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