FLOOD RISK ASSESSMENT OF OFU RIVER CATCHMENT IN NIGERIA
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ABSTRACT
The aim of this study was to carry out a flood risk assessment for Ofu River Catchment in Nigeria. Shuttle Radar Topographic Mission Digital Elevation Model (SRTM DEM) and River Map of Africa were used to delineate the catchment boundaries in ArcGIS 10.2.2 coupled with ArcHydro and HEC-GeoHMS extensions while the stream ordering was done using the spatial analyst hydrology tool in ArcGIS 10.2.2. Stage and discharge measured from February, 2016 to January, 2017 were used to develop the rating curve at Oforachi Hydrometric Station. Key Informant Interviews and household surveys were carried out to ascertain the opinion of the people on flood occurrence and their perceived causes. Assessment of the rainfall characteristics of the catchment was also carried out using the 35 years radar data corrected with data obtained from the Nigeria Meteorological Agency (NiMet). The soil map of the study area was extracted from the digital soil map of the world while Atterberg limits, sieve analysis and infiltration tests were carried out at Oforachi as a confirmatory test. The percentage loss in volume and surface area and loss in flow depth of Ofu River at Oforachi between 2000 and 2011 were also carried out using GIS tools in ArcGIS 10.2.2 while the loss rate was used to estimate the flow depth for 2016. The annual sediment load of Ofu River at Oforachi Hydrometric Station was also estimated. The terrain elevation, slope and proximity were also measured via field measurement and GIS analysis. The landuse/ landcover (LULC) changes between 1987 and 2016 were also examined using the LULC map for 1987, 2001 and 2016. The runoff curve numbers for these years were also estimated. Synthetic stream flow for 1974 to 2016 was generated using the modified Thomas-Fierring’s model. Peak discharge for the catchment was also estimated using the Natural Resource Conservation Services Curve Number method and the rational method. The average values for the three were compared with that from field measurement. Flood frequency viii analysis was carried out on the 62 years synthetic discharge values (1955-2016) and peak discharge values for 2, 5, 10, 25, 50, 100 and 200 years return periods were estimated. The extent of flood inundation was estimated first by Multi-criteria evaluation in ArcGIS and secondly by hydraulic modelling in HEC-RAS and HECGeoRAS/ArcGIS using hydrologically generated stream flow. Flood Hazard, vulnerability and risk analysis were carried out in ArcGIS 10.2.2. The resulted show that Ofu River Catchment covers a total drainage area of 1604.56 km2 covering 27.02 % of Dekina, 23.48 % of Ofu, 14.06 % of Igalamela/Odolu, 9.25 % of Idah and 14.04 % of Ibaji Local Government Areas in Kogi State and 0.80 % of Uzo-Uwani Local Government Area in Enugu State, Nigeria. A rating curve equation, 𝑄 = 15.54096(𝐻 − 55.43192) 0.69051was developed. Ofu River has lost 6.90 m of its flow depth at Oforachi between 2000 and 2016 at a rate of 0.431 m per year which is the major cause of flood within the catchment. The annual sediment load of the river at Oforachi station is 66,824.73 x 103 kg. The runoff curve numbers for 1987, 2001 and 2016 were 61.8, 63.3 and 62.8, respectively showing no significant change. The modified Thomas-Fierring Model was effectively used to generate 12 months synthtic stream flow data for Ofu river from 1974 to 2016. The peak discharge values for 2, 5, 10, 25, 50, 100 and 200 years return periods were 444.57 m3 /s, 499.23 m3 /s, 530.35 m3 /s, 565.61 m3 /s, 589.59 m3 /s, 611.96 m3 /s and 633.25 m3 /s, respectively while the peak discharge values for 1995 and 2000 flood scenarios were 448.89 m3 /s and 446.46 m3 /s, respectively. Three hazard, vulnerability and risk zones-High, moderate and low were identified which have put several elements at varying degrees of risk in 1995 and 2000 flood scenarios and other flood events of 100 years and 200-year return periods. An assessment of the open defecation status of Oforachi, the most developed of the communities within Ofu River Floodplain shows that 50.81 % of the population still ix defecate in the open field which will pose a serious health risk to the populace in the event of flood, since Ofu River is a major source of household drinking water in the community. The study demonstrated that Modified Thomas-Fierring’s model, Remote Sensing, Geographic Information System, HEC-RAS and HEC-GeoRAS could be effective tools for flood risk assessment. Efforts should be made by the government to urgently dredge Ofu River to provide more discharge-carrying capacity.
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