A Digital Oil Spill Sensitivity Atlas for Mauritius using GIS 2. Method of study Gunlach et al. [7], Mosbech et al. [8] and Anderson et al. [9] describe various techniques for building sensitivity maps for oil spill response. Using the Environmental Sensitivity Index (ESI), shoreline ranking, biological resources and human-use resources were delineated on ArcMAP workspace by colour coding, symbols and other markings. The ESI method compiles the data on shoreline sensitivity, biological resources, exposure to wave and tidal energy and human-use resources into standard and comprehensible formats. The shoreline habitats of Mauritius are delineated and presented in order of increasing sensitivity to spilled oil as listed in Table 1. Factors such as their vulnerability to shoreline type, exposures to wave and tidal energy, biological productivity and weakness and ease of cleanup of an intertidal habitat have determined their relative sensitivity. A ranking of "1" represents shorelines least susceptible to be damaged by oiling, and "10" represents the locations most likely to be damaged. Examples of shorelines ranked as "1" include steep, exposed rocky cliffs, where oil cannot penetrate into the rock and will quickly be washed off by the action of waves and tides. Shorelines ranked as "10" include protected, vegetated wetlands, such as mangrove swamps and saltwater marshes. Oil in these areas will remain for a long period of time, penetrate deeply into the substrate, and inflict damage to plants and animals. 
The intertidal habitats of Mauritius, which cover a shoreline of about 173 km, were identified and mapped during ground surveys conducted from June 2003 to January 2004. The readings were taken starting from 09 hr 00 min to 14 hr 30 min daily. These intertidal habitats were delineated directly onto 1:25000 scale Mauritian geological topographic maps (CAD format). Data were collected using a handheld Global Position System (GPS) receiver which has an accuracy of 3 meters. The geodetic reference (datum) used for GPS is the World Geodetic System 1984 (WGS84). Since the base map is on National Grid Coordinates with origin Le Pouce (20011’42.25 S, 57031’18.58 E), false coordinates (1000000.000 mE, 1000000.000 mN), conversions from WGS84 to Lambert Conical (-one parallel) projection were processed. Additional information through the use of historical sources [6 and 10], maps and aerial photo interpretation were applied. Accuracy of all descriptive and spatial attributes were tested by visual comparison of hard copy check plots to the source materials and verifying the location of the data on screen relative to other data layers in the same geographic area. The ArcToolbox application in ArcGISTM 9.0 provides a powerful set of geoprocessing functions, one of which is used to import the CAD layers into ArcMap application. Each of the different shoreline types is imported as different layers. In the geographic data view, geographic layers representing these shoreline types are compiled into GIS data sets. A table of contents interface organizes and controls the drawing properties of the GIS data layers in the data frame. The shoreline types are color coded in a ranked format on a scale from 1 to 10 as described before. ArcMap also enables the use different inbuilt symbols to represent some important environmental and human resources that could be affected by an oil spill. These map elements include birds (sea birds, shore birds and herons), public beaches, hotels and special areas designated by legislation such as fishing resources and nature reserves. These areas are indicated specifically to aid and direct the response effort. An example of the legend showing the different color codes is shown in Fig 2. In the page layout view, we can modify the layout to improve the design and visual balance of the composition by adding new map elements and changing the properties of the existing map elements. ArcMap also allows the inclusion of several key components when producing a map. These include the title, scale bar, legend and north arrow. ArcMap also integrates attributes tables, text files, digital photographs and video imagery in the digital maps. 3. Test case: Description of Map 18 Although a greater number of maps with greater accuracy can be produced, we have restricted ourselves to 19 maps as shown in Fig 1. As a result, rapid search will be possible and implementation on the model will be more structured. With all the necessary information gathered, the sensitivity maps can now be produced. Oil spill countermeasure considerations are described for each of the 19 operational maps. In this section, we give an overview of their basis and content. As an example, we present below the case of Map 18 (North-West part of Mauritius) in Fig 2.  Fig 1: Map index for oil spill sensitivity atlas  Fig 2: Oil spill sensitivity Map 18 Map 18 covers the coastal area from Albion to Baie du Tombeau. The shoreline consists mainly of structures and coastal developments at Port-Louis. The coastline of the harbour comprises mostly of metal or concrete walls and sheltered rocky shores. Natural shorelines to the north and west are composed of sand or sand mixed with coarser material. Mangroves and marshes are not very common; they are found only along the sheltered areas of upper Grand-Rivière Bay. The reef platform is several hundred meters wide to the north and west of Port-Louis, and nonexistent within the port area. Resources at risk The waters off Port-Louis are designated as a fishing reserve. The shoreline environment needing protection is the marsh and mangrove area in upper Grand-Rivère Bay. A small public beach is present at mare Samson and there are two coastal hotels in Baie du Tombeau area (Corotel and Hotel Les Cocotiers). Precautions need to be taken during the application of dispersant or other chemical because of the fishing reserve present in the area. An electricity-generating facility, which uses seawater for cooling, is located at Bain des Dames. A number of small boats are moored in Grande Rivière Bay. Response strategies The port facilities are a possible source of an oil spill. In such an event, the response strategy is to contain the oil within the industrialized port area and not allow it to impact adjacent, more sensitive environments. The calm waters of the port area enable a full response using booms, skimmers and sorbents to be undertaken. The electical station at Bain des Dames must be notified of a spill to determine if intakes should be protected, monitored or closed. Booms and sorbents should also be used to protect the mangrove and marsh area in Grand-Rivière Bay. Oil that impacts adjacent shorelines should be cleared up with the minimal removal of sand to avoid potential erosion problems. |