Home Articles ERS-1 for mapping jetties effects on shoreline change

ERS-1 for mapping jetties effects on shoreline change

ERS-1 for mapping jetties effects on shoreline change

Maged Marghany
Faculty of Science and Technology
College University Science and Technology Malaysia
21030 Kuala Terengganu, Malaysia
[email protected],
[email protected]


Abstract

Coastal hazards such as erosion are induced by the effects of wave force or the effects of the coastal engineering structures. Coastal engineering structures such as jetties could trap a sediment transport along the coastline. This could induce erosion in the down stream. The aim of this study is to model the jetties effects on shoreline change along the coastal water of Chendering, Malaysia. The numerical model will be based on the change of wave spectra extracted from ERS-1 data. For this purpose two-dimensional Fourier Transform was applied on window size of 200 x 200 pixels. The quasi-linear model was employed to model volume change of sediment transport function on ERS-1 significant wave height. The result shows that the erosion occurred in the south of Chendering with rate of change of 4 m/month. It can be said that ERS-1 data are able to predict shoreline evaluation along the coastal structures. The jetty induced a dominant erosion on the south of Chendering. This is due to that jetties-trap sediment in the north of Chendering, which lead to erosion in the south of Chendering


Introduction

Coastal engineering studies by remote sensing have not been yet established. This is because of the fact that scientists do not used the full operational ca-abilities of remote sensing to coastal and environmental studies. There are plenty of problem raised up along the coastal waters. Most important problem is poorly understanding the interaction between wave dynamic and coastal structures. This leads to shoreline change. Scientists could not establish an excellent solution to erosion due to coastal structures. This could be due to the unrelated laboratory experiments to the nature. These experiments are unable to cover many factors of interest and are unable to investigate the wave interaction with coastal structures on a large scale. For instance, laboratory experiments can not be established any broad concept relating wave spectra and their effects on shoreline change.

It is will known that hard coastal structures (jetty, breakerwaters, etc.) placed amidst a sedimentary coastline with obliquely incident waves can induce erosion of the downdrift shoreline. According to Kraus [1] this effect can occur via impoundment of the ambient littoral drift against the structure. Coastal erosion occurs due to the effects of the coastal structures such as jetty, which is an interesting topic between scientists. Kraus et al., [1] and Hanson et al.,[2] used a numerical model of waves in order to predict shoreline change due to the present of breakwater. Kraus et al., [1] found that the accumulation of the sand was began to take place as the breakwater progressed and by the time the latter were completed, nearly 370,000 cu. yds. of sand were found deposited in the harbor.

Microwave remote sensing such as the Synthetic Aperture Radar (SAR) has been proven accurate for recording wave spectra image over the ocean. SAR data assimilation in real time could be a major tool for wave modeling and forecasting. The sediment transport due coastal structures could be modeled by SAR. This is because the fact that SAR data are able to investigate the interaction of ocean wave with coastal structures in real time. This is because that SAR allows a nearly instantaneous coverage of large areas with a fairly high resolution.

Here we address the question of whether SAR can investigate the shoreline change due to jetties effects. The main objective of this study to model the jetty effects on shoreline change to along coastal water of Chendering, Malaysia by using SAR data.

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,
Kx is wave number along the azimuth direction; Kc 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 H5 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 Cgb 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 K1 and K2 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.

ERS-1 for mapping jetties effects on shoreline change


Results and Discussion

Fig. 1a shows that the wave peaks propagated from southeast direction. When the wave spectra approached the jetties, the wave spectra turn to propagate towards the southwest direction and tend to diffract along the jetties. The wavelength along the jetties ranged from 25 m to 100 m (Fig. 1b). It obvious that the wavelength decreased when the wave spectra changed their directions (Fig. 1b). Fig. 2 shows that the wave orthogonal tended to curve along the jetties. The wavelength decreased near the jetties and inside the port. This indicates that the wave diffraction patterns. A similar finding is in Fig. 1. The wave spectra energy along the diffraction orthogonal have a maximum peaks of 0.6 m2 s (Fig. 3). The wave spectra peaks are shifted in azimuth direction due to the effect of Doppler shift frequency Vachon et al., [8] . Fig. 4 shows that the longshore current vectors moved with velocity ranged from 0.4 m/s to 1.3 m/s towards the northwest direction. The maximum velocity was observed in the south jetty while the minimum velocity was observed in the north jetty. This means that the jetty could reduce the longshore current velocity. The longshore current curved inside the port and moved out the port with minimum velocity towards the north of Chendering.



Fig.1 (a) Wave spectra Modeled from ERS-1 data (b) Wave spectra Direction and Length



Fig. 2. Wave Diffraction pattern along Port Chendering



Fig. 3. Spectra Energy of Wave Diffraction



Fig. 4. Longshore Current Modeled from Wave Spectra Derived from Quasi-
linear Model

ERS-1 for mapping jetties effects on shoreline change

Fig. 5 shows that the erosion occurred in the south of Chendering and north of Chendering with approximately 4 m/month. The sedimentation occurred near to the south jetty with -2.3 m/ month. The lower rate of shoreline change is investigated from aerial photography data during 1970 to 1980. The rate of shoreline change increased to 3.8 m/year during 1996. The maximum peaks of erosion model from quasi-linear model and ERS-1 vector layer are coincided each other. In addition Fig. 6 shows a good correlation between ERS-1 vector layer (1996) with historical vector layers derived from aerial photogrpahy and quasi-linear model with of 0.82 and p less than 0.05. This result indicates a high accuracy of the quasi-linear model for the shoreline detection.



Fig. 5. Shoreline Change Model



Fig. 6. Regression Model of Shoreline Change

It can be explained that the mechanism of the shoreline change along the Chendering as: the southerly wave when approached the jetty diffracted and induced strong longshore current at the southerly jetty. This southerly jetty trapped all the sediment transport and deposited beside the jetty. This action caused erosion in the south of Chendering. Inside the Chendering port the current meander and carried the sediment out the port causing erosion inside the port. The sediment carried out from the Chendering deposited along the north jetty. Due to the weak littoral drift the shoreline extends along the north of Chendering exposed to erosion by less than 3 m/month. These results are not similar to study of Stenely et al., [7] and Salleh et al.,[6] . This is because that Stenely et al., (1985) did not consider that diffraction could induce erosion and sedimentation. It obvious that most studies have been along the east coast of Peninsular Malaysia have not applied any numerical model to detect shoreline change due to coastal structures.

In spite of that, August represents a southwest monsoon season in which characterized by less wave energy Maged and Mansor [4] in coastal water of Kuala Terengganu, erosion occurred along the Chendering shoreline. This suggests a future study, which includes the modeling of the shoreline change along the Chendering by using the integration of Kapas Island and jetty in shoreline configuration.


Conclusion

In conclusion, ERS-1 data could be used to investigate the jetty effects on shoreline configuration. This is because that the ERS-1 data can be used to model wave information compared to classical methods over large scale. The shoreline change simulated from ERS-1 data suggested that jetty caused erosion along the coastal water of Chendering.


References

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  • H. M.C. Hanson, Kraus, M. ASCE “Shoreline Response to a Single Transmissive Detached Breakwater”. Proceeding of 22 coastal Engineering Conference.July 2-6, 1990,Delft, Netherlands. pp. 2034-2046, 1990.
  • M.M, Maged “Coastal Water Circulation off Kuala Terengganu”. M.Sc. Thesis, Universiti Pertanian Malaysia, 1994
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  • P..P.Wong. “Beach changes on a monsoon Coast Peninsular Malaysia”.Geol. Soc. Malaysia. Bull. 14: 59-47,1981.