Subsurface soil erosion phenomena in Transkei and southern KwaZulu- Natal, South Africa.

Abstract

Subsurface erosion forms has been regarded as a unique exception to the more common surficial erosion forms such as rills and gullies, and have therefore been viewed as being of little consequence consequence for the total annual soil loss within any given region. A total of 148 subsurface erosion system occurring at 66 sites in Southern KwaZulu-Natal and Transkei were analysed morphologically to determine the significance of subsurface erosion within this region, to assess the extent to which the observed phenomena may be explained by current theories. Based on morphological criteria related to the dimensions of the subsurface erosion phenomena, it has been shown that there are five distinct -subsurface erosion systems namely scree slope systems; gully- sidewall systems; anthropogenically induced systems; system associated dispersive soils, seepage systems. It was further found that, under certain circumstances, the sediment lost through surficial erosion can be increased 77% by subsurface erosion and the subsurface erosion is spacially to particular slope units which are defined on the basis of the dominant geomorphic processes. Although soil chemistry, in particular dispersion related to the exchangeable Sodium percentage and the Sodium Absorption Ratio, is an important factor in facilitating subsurface erosion, other factors are also important as scree slope systems for example occur in soils which are completely non-dispersive. It has been possible to demonstrate that there is a statistically' significant correspondence between the spatial orientation of inter-ped surfaces the orientation of bedrock joints. This correspondence has enabled the explanation of how the well documented phenomenon of structurally controlled drainage basins may develop._ This correspondence has enabled the explanation of how well documented phenomenon of structurally controlled drainage basins may develop.