Contribution of Shallow Subsurface Geology to Flood Hazard in the Sokoto Basin Using Geophysical Mapping

This study investigates the contribution of shallow subsurface geology to flood hazard in the Sokoto Basin, North West Nigeria, using geophysical mapping to characterise near-surface hydraulic properties relevant to surface runoff generation and floodwater infiltration. Severe seasonal flooding affecting Sokoto and Kebbi States has increased in frequency and magnitude over the past decade, causing loss of life and agricultural damage. Understanding the role of shallow geology, particularly the distribution and hydraulic properties of impermeable clay layers and permeable sandy deposits, can improve flood hazard zonation beyond what surface topographic analysis alone provides. This study deploys ERT and frequency-domain EM surveying along 12 transects totalling 14.4 kilometres across floodplain and flood-prone terraces in Sokoto, Wamakko, and Argungu local government areas. ERT data are inverted and interpreted for subsurface lithological variation. EM34 apparent conductivity profiles at two effective depths map clay distribution. SRTM topographic data are integrated with geophysical maps. Water infiltration rate tests at 30 sites correlate geophysical parameters with hydraulic conductivity. Findings reveal that areas of high flood frequency consistently overlie shallow clay sequences with low infiltration capacity (less than 5 millimetres per hour at 65 percent of sites in high-flood-frequency zones), while infrequently flooded areas above coincide with sandy surface soils (infiltration greater than 30 millimetres per hour). ERT mapping delineates the lateral extent of flood-promoting clay layers as 3,400 hectares, significantly larger than topographic flood zone estimates. The study recommends incorporation of geophysical shallow geology data into Sokoto Basin flood management planning.

Keywords: flood hazard, shallow geology, ERT, Sokoto Basin, infiltration capacity.