Controlling factors of the location of large gully complexes: an analysis based on topographic reconstruction and logistic regression

Abstract

The Tertiary sedimentary formations of the Ulme and Vale do Casal Velho river basins, on the left margin of the Tagus basin, are affected by hundreds of gully complexes, varying in size from somewhat less than 200 m2 to more than 3 ha. These landforms currently show infilling bottom sectors covered in vegetation. Evidence of present activity, in the form of mass movements, is constrained to the walls and headcuts. In this paper we investigate the factors determining the location of these gully systems in their respective basins. A set of 90 gully complexes were selected and vectorized from aerial ortophotos. Logistic regression was then applied in order to assess the role of 8 potential control variables, obtained through reconstruction of the pre-erosion topography. Results indicate that hillslope topography had a significant effect in determining the places of initiation of the present gully systems. This initiation seems to have been triggered by a downcutting drainage network, originating lateral instability along channel reaches. In places relatively steep and prone to the accumulation of sub-surface drainage, this instability will have ca used the initiation of bank gullies, precurso r to the large gully complexes that presently evolve on the hillslopes.

DOI: http://dx.doi.org/10.14195/0871-1623_34_8