Hydrographic Mapping of Coastal Area of Ibeno LGA of Akwa Ibom State of Nigeria using Remote Sensing Method

Abstract

One of the most accurate and reliable methods of carrying out a hydrographic mapping of a larger coastal area is by remote sensing technique because it gives a wider coverage than the ground surveying method. Remote sensing is the method of gathering an information of a remote object from a distance position when there is no physical contact with the object in question. Hydrographic mapping deals with mapping of water related environments. It includes bathymetric mapping for determination of water depth, mapping of the coast line for change detection and mapping of coastal waters for resource exploitation and for sitting of engineering structures. Ibeno LGA is one of the LGAs found in the coastal region of Akwa Ibom state of Nigeria. The area is highly vulnerable to coastal hazards like coastal erosion, deforestation of the swampy forest, flooding and biodiversity destruction. There are much the activities of crude oil exploration and exploitation within the area. The occupants also are seen building residential structures in the low-lying areas and in the flood plains within the creeks and water ways resulting in casualties during high flooding. The study aimed at applying a remote sensing-based approach for hydrographic mapping of the area. The objectives of the study were to; acquire and process a medium resolution satellite imagery of the study area, using empirical modelling of the processed images to determine the depth and topography of the seabed of the coastal waters of the area, determine the available water surface area and the Landuse/Landcover (LULC) pattern of the area by performing its LULC classification, and to generate a detailed hydrographic or a composite map of the area by integrating its bathymetric data and LULC pattern. The methodology adopted for the study involved acquisition and processing of satellite imagery (Landsat 8 OLI).  The software used to acquire LULC classification and bathymetric analysis included ArcGIS Pro and ENVI 5.3. The result of the study revealed that in the area, Water bodies covered the largest area with an extent of 601.85 km², Forested areas covered 175.46 km², Croplands covered 6.844 km², Built-up areas covered 12.013 km², and Mangroves covered 0.283 km² while Open spaces covered an area of 3.39 km². The bathymetry result of the area showed that the shallow zone covered an area of 132.76 km² along the shoreline, with depth values ranging from 10.04 meters to 25.42 meters, the moderate zone covered an area of 146.02 km² along the shoreline, with depth values ranging from 25.42 meters to 40.79 meters, the deep zone covered an area of 136.79 km² along the shoreline, with depth values ranging from 40.79 meters to 56.17 meters, while the very deep zone covered an area of 139.04 km² along the shoreline, with depth values ranging from 56.17 meters to 71.54 meters. Data validation with that gotten from Shell Petroleum Development Company gave a correlation coefficient of 0.68, indicating a moderate positive correlation between the two datasets. It was then deduced that the bathymetric results of the processed imagery obtained using log ratio algorithm from Landsat 8 OLI are moderately accurate, meaning that it can be used with a good degree of confidence for decision-making processes. Some recommendations were made including the need for application of other remote sensing and GIS techniques for bathymetry of the area in order to acquire a result with a higher validation, accuracy and reliability.