Modelling and Prediction of Gully Initiation in the University of Benin Using the Gultem Dynamic Model

Abstract

For a very long period of time there was environmental equilibrium between rainfall and soil erosion in the University of Benin until man's intervention caused a disruption in the equilibrium by the improper termination of the external drainage structures in the University of Benin, Benin City Nigeria. This led to the initiation of gully erosion which has caused the University a lot of environmental damages and if left unchecked, the effect will escalate and become very devastating and disastrous. The study was to evaluate and analyze the gully erosion problem that is developing in the western end of the University of Benin with a view to providing useful information for future planning, land conservation and control. Topographical Survey of the gully site were carried out using Differential Global Positioning System (GPS) Survey for controls and Total Station  instrument for mapping of gully bed, gully walls and bank. This was to acquire morphological data of the gully site and generate geospatial data needed for monitoring the progressive growth of the gully. Using the generated 3D co-ordinates, spot heights, contour and Triangular Irregular Network models were generated in ARC-GIS environment. Soil samples were collected from the gully site for laboratory analysis and tests carried out included Specific gravity test, Particle size analysis, Compaction test and Shear Strength test in order to ascertain the overall contribution of the soils to the erosion problem. The data obtained from the surveys and investigations were inputed into the GULTEM Dynamic Erosion Model, for the evaluation of the rate of gully channel initiation. From the results the area affected by the gully erosion in this site is 11,100 m².  The geotechnical investigation carried out, revealed that the clay content of the soil in the area is only about 18%. This makes the soil highly susceptible to erosion as soils with less than 30% clay content are easily erodible. It also showed that the soil is finely graded, fairly cohesive and does not compact well. Information from the geospatial data of the gully site, revealed that the University of Benin Gully became steeper between the years 2005 to 2012 and thereafter the slope began to flatten out. The result of the model showed that the computed rate of gully channel initiation increases initially and then began to decrease steadily with the longitudinal distance of the gully for the period under study and also correlates well with the physical observation of the gully at various time interval monitored..  These models were validated using the data on gully morphology and dynamics from University of Benin Gully Erosion site.

Keywords: environmental equilibrium, Gully erosion, Network models, GULTEM Dynamic Model.