MODELLING SEASONAL FLOOD IN THE DOWNSTREAM OF SHIRORO DAM, NIGER STATE, NIGERIA

Abstract

The highest occurring natural disasters in Nigeria are attributed to flood with its attendant consequences on lives and properties. Nigeria witnessed an increased number of dam construction between 1970 and 1995 as an aftermath of the Sahelian drought of 1970-1972. Communities located downstream of these dams, are traditional areas of special importance for rural communities. The resulting population growth and degradation of natural resources in these areas, aggravates flooding and vulnerability. This study assessed the vulnerability and level of risk through flood modelling of these communities downstream of Shiroro dam, Niger State, Nigeria. The specific objectives include extraction of hydrologic layers from Digital elevation model (DEM), estimation of surface runoff and analysis of satellite imageries of the area from (1990-2016) and flood simulation of the study area. Meteorologic data (rainfall, temperature, relative humidity and evaporation), hydrologic data (inflow, reservoir elevation, average discharge and storage) satellite images for the period of 26 years (1990 – 2016), DEM were acquired. The Hydrologic Engineering Center (HEC) Hydrologic Modelling System (HMS) and River Analysis System (RAS), ARCGIS 10.3, ERDAS IIMAGINE and IDRIS Selva were employed for data analysis. Results revealed variation in hight in the analysis of the DEM of the study area from 48 m to 723 m, where 44.21% of the area is on lower elevation, 32.19% are moderate risk areas while 23.60% are low risk areas based on elevation. The surface runoff analysis reveals that the sub-basins actually aggravate the flooding in the study area with a discharge of 2459.3 (m3/s). The analysis of satellite images revealed a trade-off between vegetation and agricultural land. As the agricultural land increases vegetation decreases. Between 1990 - 2016 vegetation drastically decreased from 62.84% in 1990 to 28.34%, on the contrary, agricultural land shows significant increase during the said period from 22.43% to 50.98%. Built-up areas and bare ground /rock outcrops shows a slight increase from 3.77% to 6.37% and 8.93% to 12.34% respectively. Water body on the contrary continue to shrink from 2.03% to 1.87%. The analysis of the flood simulation revealed that at an altitude of 50 meter above sea level 1.787616 km2 (0.01%) of land area was inundated, similarly, at 100 meter above sea level 2729.162657 km2 (14.47%) of land area was inundated, furthermore, at 150m 6647.622904 km2 (35%) was inundated while, at 200m 9486.40261km2 (50.29%) was inundated. Generally, the result indicates the study area is vulnerable and at risk of seasonal flood. It is recommended that these findings should influence rational decision making with respect to resettlement and compensation and facilitate realistic flood sensitisation campaign on sustainable flood plain management for disaster risk reduction in the study area. The result should serve as a further knowledge that improve our understanding of the flood plain and its attendant advantages and disadvantages to the communities.