Thresholds of gully erosion in the coastal plains sands of southeastern Nigeria

Abstract

The concept of geomorphic threshold as applied in gully erosion studies assumes that water erosion occurs when the combined power of the rainfall energy and overland flow exceeds the resistance of surface materials to detachment and entrainment. This line of reasoning presupposes that certain environmental factors that affect runoff generation and erodibility status of soil can be used in estimating thresholds for gully initiation and sustenance. The entire 413. Km2 Ikpa River basin was covered with grids 1km2 and random number table was used to select 15% of the target population in the basin. The pair-wise correlation technique was employed to establish the threshold values of vegetation cover, organic matter content, relief, slope gradient, litter cover and bulk density for gully incision and development in the fifth order drainage basin underlain by Coastal Plains Sands in eastern Nigeria. The results of the analyses indicate that the threshold value of vegetation cover was 72.5%, while the thresholds value of depth of litter cover was 3.2cm. Other threshold values for gully initiation in the catchment area were 1 degree for slope gradient; 2.70% for organic matter; and 10metres a.s.l for relief. These threshold values were exceeded in almost all the sampled gully sites. The results of the research have implications for cropping in the study area. The convectional rain-storms that is associated with the beginning of the planting season in April/May cannot be controlled by man. Its effects can be reduced to desirable level by increasing the surface contact cover. Mulching, intercropping and the planting of cover crops can ensure adequate vegetation cover of >72.5% on valley –side slopes. Also, crop residue, dry stalks and stovers should be used to encourage the build-up of organic matter above the critical level of 2.70% during the planting season. KEYWORDS: Gully erosion, threshold, Ikpa river, coastal plains, morphometry, intrinsic factors