Relative Vulnerability to Gully Erosion of Three Geological Sediments: A Texture-Based Assessment.

Abstract

This study was carried out to evaluate the particle size distributions and erodibility indices at two soil depth ranges, 0 – 60 cm and 60 – 90 cm, of the three geological sediments underlying 100% of the landscape of the Idah-Ankpa Plateau of the Anambra Basin, Nigeria.  The aim was to explain why the three geological sediments, namely, the Upper Coal Measures (UCM, 36%), the Ajalli Sandstones (AS, 44%) and the Lower Coal Measures (LCM, 20%) are exposed to varying degrees of vulnerability to gully (soil) erosion.  The depth range 0 – 60 cm was considered the topsoil most prone to rill erosion, the range 60 – 90 cm (and above) was the subsoil, the normal depth of a formed gully.  The particle size distributions were determined by the hydrometer method plus NAOH for dispersion, and the erodibility indices were computed in decimal units using the clay ratio method.  Data were analyzed using descriptive statistics.  In the top and subsoils, the LCM was observed to have the highest mean clay contents (39% and 41% respectively). This was followed by the UCM (20% and 36%), while the As had the least (5% and 18%). Nevertheless, the mean erodibility indices of the AS in both the top- and sub-soils were respectively the highest (0.244 and 0.084), followed by the UCM (0.041 and 0.022), whereas the LCM was the least (0.016 and 0.016). The results show that the AS is the most erodible both in the top and subsoils, followed by the UCM. The LCM is the least. The relative vulnerability to soil (gully) erosion of the three sediments can, therefore, be ranked as AS>UCM>LCM. The ranking agrees with the observed order of proliferation of gullies on the IAP. Expenditure on gully erosion control on the IAP should be concentrated on the areas covered by the underlying sediments in the descending order of their vulnerability to gully formation: AS > UCM > LCM.

Keywords: Vulnerability, Rill Formation, Gully Formation, Soil Erodibility.