Three dimensional gully mapping and erosion quantification within a geoinformatics framework

The contribution of gully erosion to total soil loss has been much debated amongst geomorphologists and hydrologists. However, there is growing evidence that gullies are a major contributing source of sediment in river systems. The study area for this thesis, the Fergusson River catchment; a sub-catchment of the Daly River, Northern Australia, is no exception. This thesis presents and uses a Geoinformatics based framework for three dimensional (3D) gully mapping that focuses on the spatio-temporal analysis and quantification of gully erosion. The framework incorporates traditional and cutting edge spatial data and technology, namely Cartosat-1 stereo imagery, topographic maps, a 1_second Shuttle Radar Topographic Mission, Digital Elevation Model, historical stereo aerial photos, and Differential Global Positioning System measurements and thematic layers representing the physical and human environment. The framework is used to create Digital Surface Models (DSM) from the 2008 Cartosat-1 stereo imagery and 1948 stereo aerial photographs. Gullies are identified based on 3D visualization of these images and their 3D properties mapped from the DSMs. Four classes of gullies were identified in the study area based upon their spatial distribution in the catchment, morphology and inferred underlying erosion processes; amphitheatre gullies, gullied slumps, bank gullies and ‘other gullies’. Sapping is the primary erosion process for gullying. Volumetric changes in gully erosion between 2008 and 1948 have been measured and the specific yield of gully erosion is determined to be 6.1x103 t km-2 yr-1. A regional and global comparison of the estimated specific sediment yield indicates that it is globally among the highest. Gully control factors are examined and causes of gully initiation and development have been described. The results show that alluvial soils are most prone to gullying but heterogeneity in gully distribution and form indicates that other factors are likely to affect gully formation and development in the catchment.