Mapping The Susceptibility of Akwa South Local Government Area to Gully Erosion Using Geographic Information System

ABSTRACT      

The mapping of areas in Awka South L.G.A according to their susceptibility to gully erosion was carried out using GIS, with the aid of data derived from satellite imageries, Global Positioning System (GPS), Shutter Radar Topographic Mission (SRTM), as well as direct field work. The sites established were Amawbia, Umuawulu, Isiagu, Okpuno, Awka, Nibo, Ifite- Awka. The integration of Geographic Information System (GIS) with Global Position System (GPS), ArcGIS 10.2, Digital Elevation Model (DEM), the Sentinel 2 imageries were used to estimate the area that are susceptible to erosion. The results of the study show that Awka South LGA covers an area of about 170 km2, a larger extent of the area is moderately prone to erosion as moderate vulnerability zones cover up to 114.2 km2 (67.2%) of the study area. This is seconded by high susceptibility zones with area coverage of 51.8 km2 (30.4%) of the area. Low susceptibility zones were the least in terms of area with 4 km2 which covers to 2.4% of the area. With reference to the major factors (Rainfall, Slope variation, Landuse landcover, Elevation,Soil, Drainage density) used to access the vulnerability of Awka South LGA to erosion, Rainfall contributes the highest with 31.7%, Slope ranks second with 22.5%, LULC 13.2%, Soil characteristics 12.9%, Elevation 12.6% while Drainage density contributes the least which is 7.5%. Using a GIS approach, the soil erodibility factor K ranged between 0.03- 0.073 (MghMJ-1mm-1) were mapped (masked and unmasked) using Surfer 11. The soil erodibility factor (K) value 0.073 MghMJ-1mm-1 is at Nibo while the lowest value is 0.031 MghMJ-1mm-1 is at Isiagu respectively. The susceptibility map of Awka South delineat Awka, Amawbia and Ifite-Awka fell within low susceptibility zones to erosion. Isiagu, Umuawulu and Okpuno towns are in moderately vulnerable to soil erosion while Nibo, Nise, Mbaukwu towns all fell within high susceptibility zones. The geotechnical and chemical parameters of the soil sample obtained from the various sites were Bulk density (proctor test), Moisture content, Dry density, Grain size/Particle size distribution, Organic matter content, pH, Atterberg Limit (Liquid limit, Plastic limit, and plasticity Index) and Shear Strength (Cohesion and angle of internal friction). The findings from the study show that the Bulk density ranges from 1.26 – 1.83 mg/m3 with mean average of 1.45mg/m3, the plastic limit obtained ranges from 15.60 – 20.82 %, liquid limit ranges from 21.00- 46.00 % and plasticity index from 2.8 – 26.97. The soils reaction (pH) is extremely alkaline ranging from 7.6 – 9.5, Shear strength values are generally low with an average value of 92.91kN/m2, having average cohesion of 8.00 KN/m2and angle of internal friction of 25.38o. The Organic Matter Content of Awka South LGA is low, ranges from 0.35 – 21.3% with a mean value of 6.19%. The average moisture content analysis shows a value of 10.79 % and the average Dry density of 1.31mg/m3. The low values of optimum moisture content and Dry density in Awka South predispose the soil to water erosion. The amount of soil loss is obtained as 943,857 m3 which is 2,274,695.37 tonnes in Awka South Local Government Area of Anambra State, this amounts to 5 % of the total land mass of the study area. In comparison, it is necessary for all the factors affecting gully erosion to be considered, not one single erosion parameter factor should be used in determining the erosion risk of any location.