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Coastal Flood Mapping of Eastern Obolo and Ibeno LGAs of Akwa Ibom State of Nigeria, using SRTM and Satellite Imageries

Coastal flooding is one of the devastating noticeable hazards in Eastern Obolo and Ibeno LGAs of Akwa Ibom State, originating from incidence such as the rise in sea level, increase in precipitation, poor drainage network and some anthropogenic activities. Being the two coastal LGAs in the state directly bordering the Atlantic Ocean, there is coastal flood menace and its associated hazards such as waste deposits after the flood, inundation of people’s houses, covering of the surrounding environments with debris from water, rendering of agricultural lands unusable and alteration of the coastal ecosystem. To provide basic information that will help tackle these problems, this study mapped the coastal flooding of the area, using Shuttle Radar Topographic Mission (SRTM) and satellite imageries, with the objectives of determining the area at risk of coastal flooding, and to produce coastal flood map of the area. SRTM and satellite imagery (Landsat8 OLI 2017) were used to classify the area into coastal flood risk zones based on factors such as elevation and slope. Landcover/Landuse classification into Built-up area, Vegetation, Mangrove and Water bodies was equally done. The result of the study indicates that 60% and 47.12% of the land masses are at risk of coastal flooding in Eastern Obolo and Ibeno LGAs respectively. Base on the findings, the study recommended the establishment of a hydrometric gauging (water level monitoring) stations in the low-risk areas along the coastal lines to monitor the level of rising in coastal water which brings about the coastal flood.


INTRODUCTION
Coastal floods occur within the coastal environment. Coastal environment refer the ecotones between land and water bodies. Thus, the area extends some distances to the landward and seaward sides at the land-sea interface. When there is a rise in sea level, as a result of natural phenomenon, such as increase in precipitation etc., which leads to much run-off from the land, the resultant effect will be overflowing of the coastal areas causing coastal flooding. Excessive rainfall adds more water to surface runoff, leading to incessant, severe flood conditions [1].
Due to the undulating and sloppy topographic nature of most coastal areas and their low-lying elevations, coupled with a lot of anthropogenic activities such as coastal agriculture, human settlements, industrial and engineering activates, the zones experience more erosional and other hazardous effects during coastal flood menace. During coastal flooding, the low lying areas or the flood plain are mostly vulnerable to the negative effects. Coastal areas are zones rich in floral and faunal species, which are referred to as biological species or biodiversity. However, these diverse species are vulnerable to coastal flood and other coastal hazards. [2], observed that coastal areas are highly dynamic environments with many physical processes such as tidal flooding, sea level rise, land subsidence and erosion/sedimentation, and are faced with many human-induced environmental impacts. The study further observed that human influence on coastal change contributes to the accelerated trend in sea level rise that threatens coastal habitats.
the Atlantic Ocean. The area is affected by the coastal hazards associated with environmental degradation where the most noticeable one is coastal flooding. Being lower in elevation, there are also many creeks and water inlets which expose the area more to the coastal erosional activities. Ibeno LGA of Akwa Ibom state had bitter experience in August 2010, when six of its indigenes lost their lives, 17 missing, 40 fishing boats vanishing and many houses submerged due to coastal flood from the Atlantic Ocean [3]. Similar experience occurs in the adjacent LGA, the Eastern Obolo. [4], reported that rising sea level inundates low-lying wetlands and dry land, erodes shorelines, contributes to coastal flooding, and increases the flow of salt water into estuaries and nearby groundwater aquifers. [5], noted that flood is one of the most devastating natural hazards which can lead to the loss of lives, properties and resources. Thus, it becomes important to create easily read, rapidly accessible flood hazard map, to prioritize the mitigation effects. For proper coastal flood control and management, the availability of coastal flood information is essential. Unfortunately, such information are not available in Eastern-Obolo and Ibeno. Most of the available hydrographic data in the area are focused on the deeper off-shore locations, where oil and other minerals are found. This only provides information to the oil companies, state and the federal government on the means to tap the resources. There are no coastal flood maps to show the extent of the zone at risk to coastal flood during inundation. This study aims at producing coastal flood map of Eastern Obolo and Ibeno LGAs of Akwa Ibom state using SRTM and Satellite Imageries, with the objective of determining the areas and settlements at risk of coastal flooding. [6], applied an automatic urban structure type mapping approach on Landuse/Landcover classification generated from multispectral IKONOS data and LiDAR data in order to provide spatially detailed information about the building stock of the an area. The study shows that single-family houses suffer significantly higher damage of flood than urban structure types. Similarly, [7] found areas of various flood risks levels in Anambra River Basin using Satellite Remote Sensing.

The Study Area
Eastern Obolo and Ibeno LGAs (  Anambra  LGAs of the state. In the south, they border with Atlantic. West has Ikot Abasi and Atlantic Ocean while East has Mbo and Atlantic Ocean as the borders.

MATERIALS AND METHODS
Data used include Landsat 8 OLI 2017 imagery from www.earthexplorer.usgs.gov, SRTM imagery, shape file of the administrative boundary of Akwa Ibom LGA, the coordinates of sample points of features used for accuracy assessment obtained using handheld GPS and non-spatial data describing the characteristics of features of interest which was collected on ground.
In processing the data, Image Sub-Mapping was carried out using ArcGIS 10.0 in order to cut out the area of interest from the satellite image using the shape file of Eastern Obolo and Ibeno LGA extracted from the administrative boundary of Akwa Ibom local government area. This was followed by identification and definition of various class features. Thus, the following class features in Eastern Obolo and Ibeno L.G.A were identified and defined according to level II classification scheme: built up, mangrove, swamp and water body. This scheme was adopted because of the resolution of the image sets and to ensure that the features are discriminated adequately following the field visits to the study area. Ground truthing was carried out after definition of class categories in order to identify the features on the ground and collect sample points for accuracy assessment. Afterwards, the images were classified using the supervised classification method. The SRTM image was sub-mapped and filled for sinks in the elevation dataset and was then reclassified into risk elevation class. These categories of risk were created based on the elevation and ground information obtained within the catchment during the ground truthing. The SRTM was also used to create the slope gradient. Then, the slope gradient obtained from the SRTM was reclassified using the following five classes based on Food and Agricultural Organization (FAO) classification of slopes (www.fao.org). These categories of risk were used to produce the slope map of the area. In order to delineate the different risk level of flooding in the area, four maps were generated by overlaying different categories of slope class and the reclassified elevation class. Through these combinations, the risk zones obtained were:

(a) Slope class (VHR) + Elevation class (VHR) = Very High Risk (b) Slope class (HR) + Elevation class (HR) = High Risk (c) Slope class (MR) + Elevation class (MR) = Moderate Risk (d) Slope class (LR) + Elevation class (LR) = Low Risk
The resultant maps produced from a, b, c, and d were overlaid to produce the map showing areas with very high risk, high risk, moderate and low risk. The spatial data sets were vectorized as polygons while the attribute records were structured into tabular database before producing the final map.  Figure 3.0.  Mapping flood risk class distribution in both Eastern Obolo and Ibeno L.G.A as presented in Table 2 shows that Very high risk covers a total area of 7137.16 hectares (22.83%). High risk also takes up a total coverage of 7414.12 hectares (23.71%) in the area. Moderate risk covers 8016.31 hectares (25.64%) while Low risk covers a total area of 8698.86 hectares (27.82%).

CONCLUSION AND RECOMMENDATIONS
This study has demonstrated the potential of Remote Sensing, SRTM, Satellite Imageries and GIS technologies in identifying areas with very high, high, moderate and low risk of coastal flooding. Landsat-8OLI image and SRTM data were acquired before performing image classification, reclassification and accuracy assessment, delineating flood risk zones, delineating areas at risk of coastal flooding and producing a coastal flood map of the area. Landsat-8OLI image of 2017 was used to classify the area and a combination of elevation and slope from SRTM data was used to delineate coastal flood risk zones. Consequently the results indicate that a total area of 6038.96 and 9996.74 hectares are at high risk of coastal flooding in Eastern Obolo and Ibeno LGAs respectively, meaning that 60% and 47.12% of the land mass are at risk of coastal flooding in Eastern Obolo and Ibeno LGAs respectively.
Based on the results and analysis obtained, it is recommended that; i. There should be an establishment of hydrometric gauging (water level monitoring) stations in the low coastal flood risk areas along the coastal lines to monitor the level of rise in coastal waters which brings about coastal flood.. ii. Suitability of sites for siting evacuation centers should be assessed and established in the coastal areas of the state to house people displaced by hazard in case of emergency of coastal flood occurrence. iii. The state government should mount an intensive effort in organizing a sensitization workshop to the inhabitants of the coastal communities on the effects of building residential houses within the coastal floodplain, and the need to always be alert, to relocated to a saver area, should there be a prediction or a forecast of a coastal flood, less they have the bitter experience of the negative effects of coastal flooding..