FLOOD RISK ASSESSMENT OF OFU RIVER CATCHMENT IN NIGERIA
FLOOD RISK ASSESSMENT OF OFU RIVER CATCHMENT IN NIGERIA
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Date
2018-06
Authors
ALFA, Meshach Ileanwa
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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
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
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.
Description
A DISSERTATION SUBMITTED TO THE SCHOOL OF POSTGRADUATE STUDIES,
AHMADU BELLO UNIVERSITY, ZARIA NIGERIA
IN PARTIAL FULFILMENT OF THE REQUIRMENTS FOR THE AWARD
OF DOCTOR OF PHILOSOPHY DEGREE IN WATER RESOURCES AND
ENVIRONMENTAL ENGINEERING
DEPARTMENT OF WATER RESOURCES AND ENVIRONMENTAL
ENGINEERING,
AHMADU BELLO UNIVERSITY, ZARIA
NIGERIA
Keywords
FLOOD RISK ASSESSMENT,, OFU RIVER CATCHMENT,, NIGERIA