An integrated assessment of erosion drivers facilitating gully expansion rates—A near century multi-temporal analysis from South Africa

Abstract

Rural areas in the Eastern Cape Province of South Africa are severely affected by gully erosion, yet little is known about the rates at which these features are expanding. This study explores the areal extent, physical mechanisms, rates, and drivers of gully expansion with the aim of investigating how erosion rates fluctuate in response to temporal variations of drivers over the last century. Investigations involved the creation of an erosion inventory geo-database, the identification of area-specific physical expansion mechanisms, an assessment of static and dynamic drivers, and a multi-temporal study of 25 gullies in the Mthatha area. Results show gully erosion affects 2.3% of the study area, with gullies exhibiting an average annual areal increase of 2.08%, a sidewall retreat rate of 0.2 m/y, and a headcut retreat of 1.03 m/y over an 82-year period between 1938 and 2020. A multi-temporal case study of the Ngwevana Gully showed average annual areal growth rates ranging from 3.9% between 1938 and 1948 to 0.7% between 2017 and 2020. Findings indicate that although gullies consistently expand their areal footprint, they do so at fluctuating rates. An assessment of erosion drivers reveals that temporal promotion or suppression of these erosion rates occurs in response to the complex and dynamic interactions of both natural mechanisms and anthropogenic activities. In Mthatha, the periods of increased rates of erosion are linked to large-scale flooding events during drier climate cycles, which were further exacerbated by invasive dryland agricultural practices, inappropriate land use, haphazard infrastructure development, and rapid population increases facilitated by past Apartheid settlement laws.