How Large Can Gullies Be?

Abstract Gullies can expand into large networks that contribute significantly to soil loss, causing an increase in catchment sediment connectivity, loss of arable land and sedimentation of reservoirs. To improve gully management strategies, it is not only critical to understand the factors that lead to erosion, but also to identify the extent of in-channel erosional and depositional features. Although several studies include evaluations of gully dimensions, few studies consider in-channel patterns of erosion and deposition within gully systems. In this context, the aim of the study is to determine the dimensions of a large gully network in South Africa and to map the position and extent of erosional and depositional features where sediment is generated and deposited within the gully, respectively. The expansive gully network is located near the town Tsolo in the Tsitsa River Catchment in the Eastern Cape Province of South Africa (Figure 1). This study utilized a fixed-wing UAV (Trimble UX-5HP) and SfM photogrammetric software (Agisoft Photoscan 0.9) to create an orthomosaic and high-resolution (10 cm) DSM. Using the DSM, a longitudinal profile was drawn from head to toe of the main channel, as well as 10 cross-sectional profiles. The last, more difficult task was to map the extent of source areas where sediment is generated within the network, as well as the spatial extent of storages where connectivity is reduced. Results are presented as a series of maps illustrating the dimensions/profiles and location of various source areas that are actively eroding, as well as temporary sediment sinks. The studied example is one of the largest known gully networks in the world (its surface area is approximately 0.5 million m² and volume 5 million m³), more than twice as large compared to the largest gullies described in peer-reviewed literature (Imwangana et al., 2015; Fuller et al., 2020; Le Roux et al. 2022). Formation of such a large gully in relation to its small catchment size (8 Km²) is very unusual. The main reason is soil from the mudstone parent material in the catchment are associated with duplex and dispersive soils that are highly erodible. Longitudinal and cross-sectional profiles indicate the gully network consists of three channel types (i.e. V-shaped, U-shaped and trapezoidal), each with a unique combination of erosional and depositional features. The most prominent source features are flutes (126,185 m²), followed by headcuts with plunge pools (7,331 m²), undercutting with mass failure (2,778 m²) and erosion on outside meander bends (1,139 m²). Most of these features occur at ‘knickpoints‘ with structural control, forced by renewed channel bed incision into softer mudstones after barriers/walls of resistant dolerite or sandstone. Sediment storages where connectivity is reduced, occur at the lower half of the gully network, including flat areas and/or depressions in the channel bed (13,634 m²), vegetated areas (10,554 m²), as well as deposition inside meander bends (1,357 m²). This study demonstrates the value of identifying and mapping of intra-gully features to better understand sediment transfer within the gully network to implement appropriate restoration strategies, which in this instance should focus on depositional zones to enhance channel decoupling.