ERS-1 for mapping jetties effects on shoreline change
Methodology
Study Area
The study area is located in the South China Sea between 5° 14' N to 5° 18' N and 103° 10' E to 103° 12' E. This area lies in an equatorial region dominated by two monsoon seasons Rosnan [5] and Maged [6]. The southwest monsoon lasts from May to September while the northeast monsoon lasts from October to March. The monsoon winds affect the direction and magnitude of the waves. Strong waves are prevalent during the northeast monsoon when the prevailing wave direction is from the north from December to February, while during the southwest monsoon (May to September), the wave direction from the south Wong [9]. The rate of longshore drift based on wave effects is about 40,000 to 50,000 cubic meters per year Stanely et al., [7].
Methods
Wave Spectra Model
Wave spectra are derived from the C-band ERS-1 by applying two-dimensional Fourier Transform. The wave spectra derived from ERS-1 Cvv band was acquired on 8 August 1993. The quasi-linear model developed by Vachon et al., [8] is examined. According to Vachon et al., [8] the quasi-linear model is forward -mapping heave wave buoy onto SAR image. The width measurements are correlated with observed values for significant wave height or the azimuith shift and the local wind speed. This allowed definition of a quasi-linear transform that includes the velocity bunching deceleration effects and wind speed dependent coherence time effects. The general equation introduced by Vachon et al., [8] is given below
S(Q)=H(Kx;Kc)S(L)S(K) (1)
where S(Q) is a quasi- linear transform function,
K
x is wave number along the azimuth direction; K
c is the cut-off wave number, which function of wind speed (U).
S(K) is an ERS-1 wave spectrum while S(L) is real wave spectra measured in situ.
Equation 1 used to model significant wave height H
5 along the jetty as
H(Kc=F(Kc,U) (2)
The significant wave height then used to model the wave spectra energy (E) as given
E=F(H5,U) (3)
The modulation spectra were used to model the wave diffraction feature along the jetty located on Chendering port. The method of Huygens was used to plot wave ray diffraction.
Shoreline Change Model
The governing equation for shoreline position y is given by
¶Y/¶t
+ 1/D (¶Q/ ¶x ± q) =0 (4)
where x is the longshore coordinate, t is the time, D is the depth of closure (beyond which the profile is assumed not to be move), and Q is the longshore sand transport rate. The predictive expression for the longshore transport rate is taken as
Q= Hb2 Cgb / 16
(rs/¶-1)
(1-p) * (K1 sin 2qbs
- 2 k2 ¶
Hb/¶x cot b
cos qbs) (5)
where C
gb is the wave group velocity at the jetty line,
(
rs(
r) is the sand water density, p is the sand porosity,
qbs is the angle of the breaking wave crests to the shoreline and
tan
b is the beach slope.
The coefficient K
1 and K
2 are treated as the parameters in the calibration of the model Kraus et al., [1].
Finally, the shoreline change was detected by using the vectors layers of aerial photography during 1970 to 1980 with vector layer of shoreline extracted from ERS-1 (1993). This method will be compared with shoreline change model from volume change of sediment transport.