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ABSTRACT
The aim of this study is to carry out a comprehensive evaluation of sediment yield and river bank erosion modelling of the Nun River Basin using the popular Revised Universal Soil Loss Equation (RUSLE) integrated in Geographic Information Systems (GIS) and the Radionuclide (Pb-210) sediment dating technique. Core sediment samples were collected from relatively undisturbed areas (nine different stations) using Uwitec Triple sediment cutter. Water samples were also collected and the in-situ parameters were determined immediately after collection. Core sediment samples were processed in the lab for sediment particle size analysis (PSA) and Pb-210 atmospheric deposition with age using alpha spectrophotometer. Spatial rainfall data of the study location were employed to generate the rainfall erosivity map in order to determine the erosivity factor (R). Particle size distribution analysis validated using Scanning Electron Micrograph (SEM) and integrated with Geographic Information Systems (GIS) was employed to create the soil erodibility map which was subsequently used to determine the erodibility factor (K) while the cover management factor (P) was determined using the land use land cover map. The resulting data which consist of rainfall erosivity (R), soil erodibility (K), cover management factor (P) were then employed to model the river bank erosion around the study area and also reconstruct the sediment accretion and mass accumulation rate over a given period of time (1990 to 2018). Finally, spatial distribution of heavy metals around the study area was studied using geostatistics base on Kriging interpolation method. Result of the in-situ analysis of water sample shows an average pH value in the range of 6.05 to 6.40 which are slightly below stipulated regulatory limits. For particle size distribution analysis, it was observed that the wet soil passing the 0.075 mm sieve ranges from 42 to 89.8% while for the dry sieve, the range is 31.3 to 75.6% which implies that more than 35% passed the 0.075mm sieve an indicates that the soil is composed of mainly silty clay materials. Result of the RUSLE parameter estimation revealed that; annual average R value ranges from 249.968 – 265.103, soil erodibility (K) factor ranges from 0 to 0.06, slope length and steepness (LS) factor ranges from 0 to 0.20. The cover management factor (C) range from 0 to 0.99 while the support practice factor (P) ranges from 0 to 0.80. Overlay of these factors using GIS technique resulted in an annual soil loss of 0.66 tons/ha. /year. Spatial distribution of heavy metals around the study area revealed some level of zinc pollution in both the underlying sediments, groundwater and surface water. Pb-210 sediment dating model revealed that; the relationship between excess Pb-210 activities at different depth followed the same trend and the dynamics of the trend shows a decreasing concentration of excess Pb-210 with increasing depth. It was also observed from the results of the study that; sediment accumulation rate was constant around the Nun River until 1990 when it increased drastically from 0.023 g/cm2 .y-1 in 1970s to 0.16 g/cm2 .y-1 in 2002 probably capturing the flood events of 1987, 1991 and 1994. Using the model, it was predicted that a constant 40% increase per year of sedimentation rate in the Nun River will result to a sedimentation rate of 4.23 cm y-1 by the year 2030. The implication of this is the likelihood of flood around the neighbourhood of the Nun River which if not curb by government and other concerned agency may lead to loss of lives and properties
