Urban development and informal drainage patterns: Gully dynamics in Southwestern Nigeria

This study evaluated the attributes of gullies in selected urban towns in part of Southwestern Nigeria. It identified the gully type and characterized them in terms of dimension and volume of eroded material. It also determined the relationship among their morphometric attributes and the factors of gully development in the study area. This was done with a view to modeling their development process. Twenty (20) towns were randomly selected for this study from where thirty (30) gully systems comprising forty (40) 1st order and five (5) 2nd order gullies were measured using Total Station (TS) and Global Positioning System (GPS) receivers for the transfer of coordinates (xyz values) to gully catchment. Google Earth images (2.5 m spatial resolution) were used to extract catchment characteristics such as area, drainage lengths, number of houses in the catchment and the impervious surface area. Soil samples taken from gully shoulders (soil tops) and floor were analyzed for textural characteristics and bulk density. Data collected was analyzed using descriptive statistics, analysis of variance, multiple regression (stepwise) and geospatial techniques. The result showed that impervious surface (X1) and bulk density (X2) contributed 89.8% (R2 = 0.898; β = 0.557; P < 0.05) to the soil loss variance in the study area. The study showed that urban gully development was largely caused by large volume of runoff on mainly steep earth roads and unpaved drains coupled with poor engineering work and drains maintenance.