Assessment of gully erosion using photogrammetric techniques. A case study of upper Mbuluzi river, Swaziland

Abstract

Swaziland is severely affected by gully erosion contributing to a sediment budget up to 250,000 m3 y-1 (WMS Associates, 1988). This type is more important than inter-rill and rill erosion. Severe gully erosion is mainly in the Middleveld especially on communal land highly populated (43.65 inhabitants km-2) and with high livestock concentrations. Here, the calculated carrying capacities are 0.27 LSU ha-1 (Livestock Units) vs. stocking rates 0.87 LSU ha-1. In general, to assess gully erosion, numerous investigators have made use of aerial photos and GIS to predict the morphometric conditions that favoured gulling (Nachtergaele and Poesen, 1999). In Swaziland, from 1947 to 1987, the WMS Associates (1988) established gully erosion rates from aerial stereo photos. Subsequently, Mushala et al., 1994 analyzed the gullies distribution and their relationship to lithology and land tenure. In the present research long terms rates of gully erosion have been measured by aerial photos taken from 1947 to 1996. Particular attention was focused on two dendritic gully systems. One of them is dynamically evolving, whereas the other one is in a static phase (Sidorchuk, 1999). The objectives of this research were to calculate the historical development of the morphology of gully as input data to the gully erosion model (Sidorchuk et al., 2001) and to predict hillslope area susceptible to gulling. A High Digital Terrain Models (HDTMs) with 1-m resolution were devised for this purpose.