Hydraulics of slope erosion by overland ow A case study: Nnobi-Onitsha hillslope site, Anambra state S.E. Nigeria

The erosion problem at Nnobi was investigated with a view to determine the types and major factors responsible for accelerated erosion in the area, in order to develop a simple prediction model for the erosion rate. The severe erosion of ground surfaces, such as unpaved road shoulders and bare soil surfaces, in high density areas of South Eastern Nigeria is a common feature. For many years, hydrologist and engineers have been faced with the challenge of producing adequate drainage programs to check the menace of erosion by overland ow. Six ume experiments were conducted where erosion rates were measured. The rainfall data for the past ten years were obtained and analyzed, a survey of the catchment area for each of the gully sites was carried out and soil samples were obtained from the eroding layers of the gullies. Analysis of the results show that the area experiences rainfall of high intensity, with intensities of up to 80 mm/hr being recorded for a duration of 30 minutes. The slope of the land is generally steep, with slope gradient varying from 15% to 22%, and soil particles with very low organic and clay content, ranging between 0.2 to 0.4 to 1 and 6%, respectively. A mathematical model of hill slope overland ow for the Nnobi experimental watershed, obtained using the Saburo [1] equation, to predict the erosion rate, is presented as: E = 0:0158 D C:ACE(fI)15=8:L3=8:S3=2: