Assessing the short-term inter-annual growth of the largest documented gully network in South Africa using UAV and SfM methodology

ABSTRACT

This research explores intra-gully sediment volume changes of a massive gully network (42 ha) located in the Eastern Cape, RSA, in response to high rainfall events during a three-year study period (2017–2020). Unmanned aerial vehicles (UAVs) and photogrammetric processing were used to generate high resolution digital surface models (DSMs). Volumetric calculations were run to estimate the extent of gully expansion within the system to determine intra-gully changes and sediment erosion over the study period. The gully was categorised into four zones based on predominant structure and processes. These are the Extension Zone, Expansion Zone, Transport/Deposition Zone and the Side Branches. Rainfall data from a rainfall station (10 km away) was used to investigate the correlation between rainfall depth, rainfall intensity, and flow accumulation over two wetter years and one drier year with gully expansion experienced during these periods. The gully network encountered a total gully sediment volume erosion of 104,489 m³ (6.01%) over the study period. Nineteen high to extreme intensity (>20 mm per day) rainfall events produced the runoff necessary for gully expansion during the 2017 and 2018 hydrological years. Notably, lower gully growth was recorded in the Expansion and Transport/Deposition Zones during the subsequent below average hydrological year in 2019–2020.

KEYWORDS:

• Gully erosion
• rainfall intensity
• flow accumulation
• UAV
• DSM
• South Africa

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. Additional funding was provided by the Water Research Commission of South Africa project K5/2448. B. v.d. Waal was supported in part by the National Research Foundation of South Africa (Grant number: 126382). Additional student support and availability for postgraduate studies was provided through the Unisa AQIP program. Our thanks also go to N.H. Huchzermeyer and the Tsitsa Project for collection and organization of the rainfall data used in the project. Technical assistance with drone work was provided by Andreas Keyser (Regional Council for Climate Change, Energy and Sustainable Development, Siegen-Wittgenstein). Additional field assistance from Lefa Morake, Bradley Vorster, Jessica Tavener and Lerato Boihang is appreciated. Assistance from Ms Nosiseko Mtati at Rhodes University for receiving permission from the local authorities for fieldwork in the community.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Due to the large size of the datasets, the data that support the findings of this study are available from the corresponding author, RL Anderson, upon reasonable request. These datasets include the DSMs and orthophotos for 2017, 2019 and 2020.