Flood Prediction and Hazard Mapping Using QGIS and HEC-RAS for the Lower Niger River at Lokoja, Nigeria

Flooding is one of the deadliest climate hazard faced in Nigeria and recent times flood frequency and intensity has increased drastically.

Lower Niger River at Lokoja-Kogi State tends to experience flood due to its location at the confluence of Niger and Benue Rivers.

This research combined QGIS and HEC-RAS analysis to forecast the characteristics and spatial delineation of flood hazard zones using the hydrological data spanning 1995-2020.

The Flood frequency analysis was conducted using Gumbel Extreme Value Distribution and the Return periods of 5, 10, 20, 30, 50, 100, 200 and 300 years were estimated.

The corresponding peak discharges were 24,325 m³/s, 27,184 m³/s, 29,926 m³/s, 31,504 m³/s, 33,476 m³/s, 36,137 m³/s, 38,787 m³/s, and 40,335 m³/s, with associated water levels of 10.8 m, 11.6 m, 12.4 m, 12.7 m, 13.5 m, 14.3 m, 15.0 m, and 15.5 m, respectively.

The model was calibrated with the 2012 flood and the simulated water level of 12.59 m compared closely with the observed level of 12.8 m.

Simulation of the flood hazard assessment shows that with an increase in the return period there is a gradual increase in the discharge and flood extent as well.

When compared to the historic flood event of 2012 (31,692 m³/s; 12.8 m) there is roughly a 14% increase in discharge for the 100-year return period and more than a 27% increase for the 300-year return period.

In conclusion, the combination of QGIS and HEC-RAS offers an effective data-driven methodology for flood prediction and hazard mapping to inform better flood risk management, spatial planning, and climate adaptation strategies in the Lower Niger Basin.