GIS as a Tool to Identify a Coastal Sediment Management Option using Remote Sensing - A Study of Songkla Bay in Southern Thailand 2 Methodology Songkla bay in southern Thailand is located approximately between 100° 32’E - 100° 37’E and 7° 10’N - 7° 17’N. Digital Bathymetric information for the study area in 1950, 1977 and 1990 were available for the study. The coastlines for each year were extracted from the same nautical charts that were used to extract bathymetric data. Computations were done by considering the Kinematic Element Model (One-Line Model) on coastal sediment transport. The One-Line Model that ensures the conservation of volume of sediment in a shoreline cell indicates the possible existence of a direct relationship when coastal sediment transport is considered in the long term. The availability of bathymetric information for Songkla beach enables the computation of sediment volume change using GIS. The same information can also be used to identify the coastline shift. In the present study, the volume of sediment moved in pre-determined beach cells were compared with the coastline shifts during the periods of 1950 - 1990 and 1977 - 1990. In Huq (1990), it has been identified that in the study area, the ocean depth that demarcates the surf boundary could be taken as 3.2 meters. A depth of 4 meters was taken to compute the limits of the dynamic zone for each year under consideration. This depth was used to identify the surf boundary limits of each year under consideration. Taken together, the surf boundaries for the three years indicated a common direction, which was taken as the long-shore direction. The seaward extent of the beach cells was defined by the outer limits of the surf boundaries from all 3 years. The beach cells were selected perpendicular to the identified general direction of sediment transport and a cell width of 100 meters was taken for initial computations. The landward boundary of the cells was taken as the boundary defined by the outer limits (inland) of all 3 coastlines from 1950, 1977 and 1990. The GIS used both vector and raster models for computations and employed standard functions to draw perpendiculars, perform densification of lines etc, to make the shoreline cell delineation accurate. The bathymetric surfaces were constructed using a grid cell resolution of 20 meters. Using these surfaces, the volume of water in each cell was computed for each year under consideration. The difference of the volumes of water between each set of years was taken as the volume of sediment change during that period. Computation of beach profile shift was done by taking the average shift of the beach for two parallel sections of each shoreline cell. In an attempt to ensure whether the entire dynamic zone in the cross-shore direction was captured while computing volumes for each cell, the surf boundary was extended seawards by another 500 meters and calculations were repeated. This selection was made, taking an island formation near the bay into consideration to ensure that the assumptions made in the one line model were maintained. 3 Results Surf zones for each year with bathymetry data, the respective coast lines, the common direction of long-shore from surf zone computations and the beach cells at 100m apart are shown in Fig. 2. The surf boundaries for the years 1977 and 1990 are in close proximity to each other, where as the 1950 surf boundary appeared further seaward, thereby contributing significantly to the overall surf boundary. The volumetric change within each beach cell for the three concerned periods were computed and plotted against the respective linear shifts in coastal extent (Figs. 3 & 4). It was observed that changes in sediment volume varied between the profile lines taken to the 4m surf boundary and the profile lines that extended a further 500 seaward. This suggests that the delineation at 4m is insufficient to characterise the dynamic zone for along shore sediment transport. The present work limited the investigation to this boundary, 500m beyond the 4m depth line, and the study of sediment movement was performed on the data set for the profile lines extended to this limit.  Fig2. Illustration of the GIS model used to compute shifts in extent of the coastline and corresponding changes in sediment volume in each of the 100m wide “shoreline cells”. Orange shading shows change in bathymetry |