The role of land use and land cover changes and gis in flood risk mapping in Kilifi County, Kenya
Mwangi, Maina Paul
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An increase in the size of population leads to changes in land use and land cover as the growing community seeks more land for agriculture, settlements and infrastructural development. Land use and land cover change (LULCC) alter natural drainage systems, impact on surface runoff and affects infiltration capacities of an area; factors which contribute to flooding. Management of floods begins by mapping flood prone areas and understanding the vulnerability factors. The main objective of this study was to identify areas in Kilifi County that are vulnerable to flooding and to assess the cause of floods using GIS - based flood risk mapping. The specific objectives were to determine the extent and nature of land use and land cover changes occurring in Kilifi County in the period between 1990 and 2014; to establish the effects of land use and land cover change on surface runoff and infiltration capacities and to generate a flood risk map for Kilifi County. Landsat images for 1990, 2000 and 2014 were used to classify the area into forestlands, grasslands, croplands, settlements, wetlands and shrublands. The mapped data from satellite images of 1990 indicated a forest cover of 1042.9km², a 26.3km² cover for settlements and a 5142.0km² cover for croplands. In 2000 there was a forest cover of 940.4km², 27.8km² for settlements and 4693.0km² for croplands. In 2014, there was 825.8km², 46.5km² and 5123.8km² cover for forestlands, settlements and croplands respectively. Between 1990 and 2014, forest cover reduced by 580.3Km², croplands increased by 1170Km² while settlements increased by 93.3Km² respectively. These changes alter surface runoff, river discharge and affect soil infiltration capacities. Infiltration experiments conducted in the different land cover classes using a Double Ring Infiltrometer established that infiltration rates were highest in the sandy soils and lowest in the clay soils. It took an average of 5.5min and 29min for water to percolate into loamy soils in the forestlands and settlements respectively; an average of 30min and 21min for infiltration under clay soils in the grasslands and shrublands respectively, while under sandy soils; it took 21.5min for infiltration in the settlement areas. Analysis of trends in stream flow data for Sabaki River available for the period between 1990 and 2012 indicated a change in the river discharge over this period albeit not significant. This data did not adequately cover the study period but covered 95% of the period between 2001 and 2012. Different thematic maps on land use and land cover, slope, rainfall, soil and drainage were generated. Different weightage values were assigned depending on their importance to flood risk and overlaid in the spatial analyst tool in ArcGIS 10.1 to generate a flood risk map. A flood risk map was developed identifying five categories of risk zones; the very high, high, moderate, low and very low risk zones. At 6735.3km², Kilifi County generally has a high risk of flooding. The low risk areas cover only 122.45km² of the total area. Zoning of flood risk areas is important for planning development in the area. The document provides base information for the national government, county governments, NGOs and the community on flood risk areas in order to intervene during flood preparedness, response, mitigation and recovery processes respectively.