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Sediment dispersion from offshore TIN dredging along Andaman sea coast using satellite data

Resamee Suwanwerakamtorn, Anong Paijitprapaporn,
Darasri Downreasng, Prakmard Suwanasing
National Research Council Department of Mineral Resources
Bangkok, Thailand.



Abstract
The study on sediment dispersion from offshore tin dredging along the Andaman sea coast was performed by the use of three time-series Landsat-5 TM data acquired on Feb 04, Feb 20 and May 11, 1989. The main purpose was to study the effect of the sadiment dispersion resulting from tin dredging on the coral reef along Ko Ka--- Island and Bang Sak beach, Phangnga province, North and East of the dredging area . The data were analysed on the Meredian Image Analysis System by using contrast stretch enhancement technique of bands 1, 2 and 3 incorporation with mining concession map, number of dredges and oceanographic data.

The results of the study showed that during the North East Moonsoon which is the dredging period, the current circulation along the Andaman seashore in the Northern hemisphere moved from North to South and then inclined southwest somewhat parallel to the Equator. This caused the sediment from tin dredging to disperse from the shoreline in elingate pattern when the diffuser was not installed. In the contrary, after installation, the sediment dispersion was changed to become patchy or circular pattern. However, in both cases, it was found that the tin dredging has only slight effect on the coral reef environment around Ko Ka Island and Bang Sak beach and vicinity area. The study will be continued in order to monitor such effect on a long term basis to obtain a better understanding of the sediment dispersion process.

Intdoduction
With the proposal from the tin company to increase the dredging activities to about 200 dredges in the mine concession area in Bang Muang - Ko Khor Khao, Takua Pa district, Phanganga Province, the Department of Mineral Resources (DMR) with its responsibility in the control of off shore tin dredging has put more efforts in finding appropriate means to protect the sea environment. This is because the unefficient dredging may cause damages to the marine ecology due to the sediments that are drained directly from jig to sea and could have effect on teh coral reefs along Ko Ka Island and Bang Sak beach which are the tourism spots. These two areas are about 5 and 7 kilometers (km.) lying in the North and East of dredging area. Basides that reason, the protection of mineral resources with marginal return is also important. Therefore DMR has set out regulations that every dredge must have a specific dredging system and a diffuser.

The activities began with the first four dredges that had been inspected and approved to dredge in February, 1989. So far, 120 dredges have been approved to dredge in the Andaman seashore.

In order to control the dredge to follow the government specifications, to verify the efficiency of diffusers as well as to protect and to reduce the impacts resulting from tin dredging, the detection and monitoring scheme has been programmed by the use of satellite data. The work has been carried out by the Division of Environment and Mineral Resource of DMR with support from the National Research Council for the satellite data and the digital image analysis facilities.

Objectives
  1. To study the pattern and direction of sediment dispersion from tin dredging sites.

  2. To detect and monitor the bounderies of sediment dispersion before and after diffuser installation.

  3. To study the impacts of sediment dispersion on the coral reef environment along Ko Ka Island , Bang Sak beach and vicinity area.
The Study Area
The study area is located in the Andaman sea along the coast about 35 kilmeters long from Ban Name Khem to Ban Bang Niang, Takua Padistrict, Phangnaga Province in the South of Thailand. (as shown in Figure 1)




Data used
  1. Satellite data : Three dates of Landsat-5, TM digital data, path 129 row 54, were used to detect the sediment dispersion. One which was acquired 11 days before diffuser installation was of February 04, 1989 while the other two were after installation of February 20, 1989 and May 11, 1989.

  2. Topographic maps at a scale of 1:50,000 were used to facilitate the registration of the TM scene to the map projection.

  3. Mining concession maps were used to locate the boundaries of tin dredging sites.

  4. Mining concession and dredge: information such as shown in Table 1 were also used to relate with the satellite data of different dates in order to obtain background information for satellite data interpretation .

    Table 1 The mining concession and dredge information in the study area.
    Concession Sites Number of dredges
    Feb 04, 1989 Feb 20, 1989 May 11, 1989
    A 1 dredge 10 dredges 1 dredge
    B - - Temporary close
    C 11 dredges 15 dredges Temporary close
    D Temporary close Temporary close Temporary close
    E Temporary close Temporary close Temporary close

  5. Tidal data.: The dredging period is in the NE Moonsoon when the wind is quite calm during November to May, The tidal heights used were those at Pak Nam Ranong station, Ranong province and Khao TaphaoNoi station, Phuket province in the North and South of the study area respectively, The two stations are located at about 100 kilometers away from Ban Nam Khem. The tidal data of both stations at the same period as Satellite over passing between 9.00 a.m. are shown in Tables 2 and 3.

    Table 2 The tidal data at Pak Nam Ranong Station
    Date Water level (Decimeters)
    9.00 10.00
    Feb 04, 89 33.0 31.0
    Feb 20, 89 33.5 36.6
    May 11, 89 18.5 20.5


    Table 3 The tidal data at Khao Taphao Noi station
    Date Water level (Decimeters)
    9.00 10.00 11.00
    Feb 04, 89 24.5 24.5 _
    Feb 20, 89 23.5 27.0 28.5
    May 11, 89 18.0 20.0 _
Image Enhancement
The Landsat digital images that were obtained from the Thai Ground Station were already rectified and geo-coded. These images were enhanced for suspended sediments pattern on the Meridian image analysis system. The contrast stretch technique was applied to the individual bands 1, 2, and 3 and combination of the three bands were used for interpretation in term of the pattern and direction of sediment dispersion with respect to mine concession boundary, number of dredges and tidal data.

Results
The enhanced TM images of bands 1, 2, and 3, show clearly the pattern and , direction of sediment dispersion. The interpretation of images incorporation with the supplementary data mentioned above has indicated that:
  1. The image acquired on Feb 04, 1989 shows that there was one dredge carried out in concession area A which agreed with the dredge data shown in Table 1. The sediment dispersion from tin dredging was elongate pattern around 10 kilometers in length. The width of dispersion patch was about 200 meters at the source and expended to 1 kilometer at its end. The dispersion direction was observed to be from North to Southwest following the current along the Andaman coast which was from North to South during the ebb period. ALthoug the sediment dispersed to a much further distance than expected, if its pattern and direction remained unchanged during dredging period Ko Ka Island and Bang Sak beach would be affected only slightly from tin dredging.

    In the concession area C, at least 3 dredges could be detected on the image to be out of concession boundary. The sediment dispersion width and length were about 2-3 km. and 8 km. respectively. The plume first moved from South to North due to the reversed coastal current which changed from North-South to South-North when passing Kran Yai cape. The South-North dispersion trend of the sediment then changed to North-South direction at the further end of the plume where offshore North-South currents prevailed. (Figure 2)

    In the other concessions, there was no dredge appeared on the image which corresponded to the ground truth data because these consessions have been temporarily closed.

  2. The image acquired on Feb 20, 1989 is partly covered with some cloud and caused a little in problem the interpretation. However, the image shows that there were a lot of dredges in the concession area A. The sediment patch was relatively small with the width and length of about 2 km. and 3 km. respectively . The dispersion shape was not so elongate as that shown on Feb 04, 1989 image. This is because the dredge grouping and diffuser have already been installed.

    In the concession area C, the image reveals a group of dredges further North of dredging boundary. The dispersion pattern of sediment was not clear because of the cloud cover as shown in Figure 3.

  3. The image acquired on May 11, 1989 is covered with thin layer of cloud along the shoreline such that the dredges and sediment dispersion could not be observed clearly. However, in a circular pattern under the thin cloud. No defined pattern could be observed for other concession areas because of cloud as shown in Figure 4.

Figure2:The colour composite image of TM bands 1, 2 and 3 which was acquired on Feb 04, 1989 illustrates the dispersion pattern of sediment 11 days before diffuser installation


Figure 3:The TM image acquired on Feb 20, 1989 shows the dispersion
characteristic of sediment 5 days after diffuser installation



Figure 3: The TM image acquired on May 11, 1989.

Conclusion
This study illustrated the use of Landsat TM data to detect and monitor the sediment dispersion from tin dredging areas with an objective to protect the natural environment as may be affected by tin dredging. Although there were few images available and some of them were covered with cloud, the results have indicated the potential of using satellite data for monitoring purpose on a long term basis so that a better understanding of the sediment dispersion process could be obtained.

References
  1. Hydrographic Dept., Royal Thai Navy, 1989. Tide Tables, Thai Water, Mae Nam Chao Phraya - Gulf of Thailand and Andaman Sea. 348 pp.

  2. Wyrtiki, K., 1961. Physical Oceanography of the Southeast Asian Waters, Naga Report, Volume 2, Univ. Calif., Scripps Inst. Oceanog., La Jolla, California. 195 pp.