Flood Risk Assessment Of Ofu River Catchment In Nigeria

The aim of this study was to carry out a flood risk assessment for Ofu River Catchmentin Nigeria. Shuttle Radar Topographic Mission Digital Elevation Model (SRTM DEM)and River Map of Africa were used to delineate the catchment boundaries in ArcGIS10.2.2 coupled with ArcHydro and HEC-GeoHMS extensions while the stream orderingwas done using the spatial analyst hydrology tool in ArcGIS 10.2.2. Stage and dischargemeasured from February, 2016 to January, 2017 were used to develop the rating curveat Oforachi Hydrometric Station. Key Informant Interviews and household surveys werecarried out to ascertain the opinion of the people on flood occurrence and their perceivedcauses. Assessment of the rainfall characteristics of the catchment was also carried outusing the 35 years radar data corrected with data obtained from the NigeriaMeteorological Agency (NiMet). The soil map of the study area was extracted from thedigital soil map of the world while Atterberg limits, sieve analysis and infiltration testswere carried out at Oforachi as a confirmatory test. The percentage loss in volume andsurface area and loss in flow depth of Ofu River at Oforachi between 2000 and 2011were also carried out using GIS tools in ArcGIS 10.2.2 while the loss rate was used toestimate the flow depth for 2016. The annual sediment load of Ofu River at OforachiHydrometric Station was also estimated. The terrain elevation, slope and proximity werealso 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. Syntheticstream flow for 1974 to 2016 was generated using the modified Thomas-Fierring’smodel. Peak discharge for the catchment was also estimated using the Natural ResourceConservation Services Curve Number method and the rational method. The averagevalues for the three were compared with that from field measurement. Flood frequencyanalysis was carried out on the 62 years synthetic discharge values (1955-2016) andpeak discharge values for 2, 5, 10, 25, 50, 100 and 200 years return periods wereestimated. The extent of flood inundation was estimated first by Multi-criteriaevaluation 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 showthat 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 LocalGovernment 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 flowdepth at Oforachi between 2000 and 2016 at a rate of 0.431 m per year which is themajor cause of flood within the catchment. The annual sediment load of the river atOforachi station is 66,824.73 x 103 kg. The runoff curve numbers for 1987, 2001 and2016 were 61.8, 63.3 and 62.8, respectively showing no significant change. Themodified Thomas-Fierring Model was effectively used to generate 12 months synthticstream 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, 530.35m3/s, 565.61 m3/s, 589.59 m3/s, 611.96 m3/s and 633.25 m3/s, respectively while thepeak discharge values for 1995 and 2000 flood scenarios were 448.89 m3/s and 446.46m3/s, respectively. Three hazard, vulnerability and risk zones-High, moderate and lowwere identified which have put several elements at varying degrees of risk in 1995 and2000 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 thecommunities within Ofu River Floodplain shows that 50.81 % of the population stilldefecate in the open field which will pose a serious health risk to the populace in theevent of flood, since Ofu River is a major source of household drinking water in thecommunity. The study demonstrated that Modified Thomas-Fierring’s model, RemoteSensing, Geographic Information System, HEC-RAS and HEC-GeoRAS could beeffective tools for flood risk assessment. Efforts should be made by the government tourgently dredge Ofu River to provide more discharge-carrying capacity.

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