Gully fan morphodynamics in a small catchment in the Eastern Cape, South Africa

Fans, in association with hillslope gullies and badlands and their impact on the catchment sediment cascade, slope-channel connectivity and local landscape evolution, have been reported in diverse geomorphic environments. However, research on the development and morphodynamics of gully fans is still in its infancy in South Africa. An investigation was undertaken in mid-slope discontinuous gullies in the Eastern Cape Province of South Africa using field surveys and sediment source fingerprinting techniques to establish the dynamics of fan systems and their impacts on-slope sediments. Findings showed that a sediment core collected from the upper region of the fan was composed mainly of sediment mobilised from subsurface sources, whereas that from the lower region of the fan consisted mostly of sediment from surface sources. 137Cs data were consistent with the sediment source fingerprinting results, confirming the respective surface and subsurface origins of the sediment in the sampled cores. Additionally, 137Cs pointed to the recent formation of the fan. Geomorphic reconstruction of the fan did not support a cut and fill mode of gully fan development. Rather, the fan formed as a consequence of discontinuous gullies in the mid-slope leading to on-slope sediment deposition and slope-channel decoupling. It was also observed that hillslope sediment storage in the gully fan is temporally variable. Changes in the balance of morphodynamic processes may lead to sediment flushing and slope-channel coupling.