Morpho-lithostructural analysis of Ala River basin for flood risk assessment: geospatial techniques intervention

Abstract

An integrated approach of morphometric and litho-structural studies for flood risk assessment of Ala River Basin (ARB) in Akure city, southwestern Nigeria aided by geospatial techniques was carried out. The Ala River Basin comprises of four sub-basins covering a total area of 427km2 with fourth stream order of dendritic and sub-trellis drainage patterns. The values obtained for the morphometric features such as Form Factor, Circularity Ratio, Elongation Ratio, Stream Frequency, and Relief Ratio and Compactness Coefficient for ARB revealed an elongated basin type, posing a great flood threat to the urban community. Drainage density for all the sub-basins is very low, typically a coarse type that supports ample time for overland flow. All the lithological units are characteristically impermeable with low lineaments density. Thinness of the soil profile, and frequency of stream flow showed good relation to low infiltration with attendant high runoff leading to flood events in the upstream section. Flood sub-basin wise prioritization was carried out by employing the result obtained from morphometric classification and structural analysis. Different weightages were assigned to the different morphometric and structural parameters for the four sub-basins based on flood potentials. Ala River sub-basins have been classified into three namely; high, moderate and low based on flood potentials. The sub-basin I and II located at the upstream section are highly vulnerable to floods, while sub-basin III and IV located at the downstream section are not presently vulnerable to flooding respectively. The flood water in the flood threatened sub-basins (I and II) can also be harvested through siting and construction of natural groundwater recharge structures such as check dams and contour bunds.