HYDRAULICS OF SLOPE EROSION BY OVERLAND FLOW (A Case Study of Nnobi-Onitsha Hillslope Site, Anambra State S.E. Nigeria.)

The erosion problem at Nnobi was investigated with a view to determining the types and major factors responsible for accelerated erosion in the area in other to develop a simple prediction model for erosion rate. Severe erosion of ground surface such as the 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 beset with challenge of producing adequate drainage works to check the menace of erosion by overland flow. Six flume 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 intensities, with intensities of up to 80mm/hr being recorded for a duration of 30minutes. The slope of the land is generally steep with slope gradient varying from 15% to 22% and the 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 flow for Nnobi experimental watershed was obtained, using Saburo [1] equation to predict erosion rate,