A comparative study of gully erosion contributing factors in two tertiary catchments in Mpumalanga South Africa

Abstract

Gully erosion is one of the most important forms of soil erosion due to the loss of agricultural production land, sediment generation that leads to siltation of reservoirs, and damage to infrastructure such as roads, bridges, buildings and water pipes. Gully erosion has attracted the interest of many scientists worldwide, but some key priority research areas have not yet been widely reported on. A review of gully erosion research in South Africa presented in this study shows that contributing factors have not been addressed sufficiently and that most of the previous research has been confined to the Eastern Cape and KwaZulu Natal regions. This study quantifies, compares and contrasts the influence of various biophysical and land use factors controlling gully erosion, including land use history, from the two tertiary catchments (X12 and W55) of the Komati and Usutu Rivers in the Mpumalanga province. A desktop-based research approach was followed, which includes the extraction of topographic variables, including the slope, upslope contributing area, planform curvature, Topographic Wetness Index and Stream Power Index from contours derived Digital Elevation Models. The study used existing contributing factor datasets including soil, geology, rainfall erosivity and vegetation cover and type. Gullies were digitised manually from a background 0.5m spatial resolution aerial photographs. Land use history and gully dynamics were studied from historical aerial photographs within two selected gully systems. The influence of a contributing factor class on gullies was assessed based on the proportion of gullies in each class, calculated using a zonal approach in ArcGIS®. Zonal results were analysed further using Information Value statistic to assign the weight for each class. Numerous gully erosion features were observed in tertiary catchment X12 (n = 5 397) compared to W55 (n = 1 654). An Information Value statistic in tertiary catchment X12 shows that gullies are influenced by soils with varied depths belonging to oxidic, vertic, duplex, lithic and melanic soil groups. These soil groups overlie ultramafic rocks, colluvium and alluvium deposits and granite rock of Swazian age. Topographic variables influencing gully location include low to moderately steep slopes, concave curvature, higher upslope contributing area, high Topographic Wetness Index and both low and high Stream Power Index. Other contributing factor classes influencing gullies include lower erosivity values, Swaziland sour bushveld, Barberton serpentine sourveld, moderate Normalised Difference Vegetation Index, moderate to high Normalised Difference Vegetation Index and cultivation. In tertiary catchment W55, gullying significantly affects the very deep Hutton dominated soil form with texture varying from loamy sand, sand and very sandy, underlain by granitic rock of Randian age. A topographic influence was observed on moderate to steep slopes on an area where flow accumulates and is characterised by concave and convex planform curvatures, low and high Topographic Wetness Index and high Stream Power Index. Both moderate and moderate to high Normalised Difference Vegetation Index classes have a significant influence on gully location, as does indigenous forest and thicket/ bushland. An historical land use analysis showed that cultivation was responsible for the initiation and expansion of gullies at two selected sites. The findings confirm that factors important in one area are not necessarily important in the other due to factors spatial variability. However, results can be improved by applying and comparing further statistics, deriving new datasets for the study and detailed historical land use and gully dynamics analyses.