Gully erosion susceptibility modelling to support avoided degradation planning

ABSTRACT

Restoration resources are usually limited and strategic information on and erosion susceptible areas are required to avoid further degradation. This study has potential in the Mzimvubu River Catchment, South Africa, where two large reservoirs are planned on the Tsitsa tributary. The Tsitsa River Catchment, however, consists of highly erodible soils with widespread gully erosion evident. It is important to prevent further gully erosion in the catchment due to the presence of duplex and dispersive soils. Therefore, this study modelled areas that are susceptible to gully development in the Tsitsa River Catchment, as well as estimated the sediment yield potential from the susceptible areas if gully development occurs. This was achieved by mapping gully-free areas in a GIS that have the same DEM-derived topographical variables, soil associations and land cover as gullied areas, followed by scenario analysis of the potential sediment yield. More than 30 000 ha (7%) of the catchment is intrinsically susceptible to further gully development, consisting of drainage paths with a large contributing area and erodible duplex soils. If not protected, these susceptible areas could contribute an additional 300 million m³ of sediment to the river network, reducing the volumes of both reservoirs by more than 50%.

KEYWORDS:

• Gully erosion
• susceptibility
• soil erodibility
• GIS
• catchment scale
• avoided degradation

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Publisher’s Note

Acknowledgments

The authors are thankful for research funding from the Department of Environment, Forestry and Fisheries: Environmental Programmes – Natural Resource Management Operational Support and Planning, as well as the National Research Foundation of South Africa (grant number 126382). Numerous colleagues at Rhodes University and part of the Tsitsa Project have assisted the research efforts. A special word of gratitude to Mrs. E. van den Berg at North-West University for providing the terrain unit map. The paper benefited greatly from reviewer comments on an earlier version.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Department of Environment, Forestry and Fisheries: Environmental Programmes – Natural Resource Management Operational Support and Planning.