This paper presents the results of a research conducted in Gouveia with the objective of characterizing the phenomenology of gully processes under the geotecnical point of view. It emphasizes the influence of different materials related with the dynamics of the mass movements involved in the regressive retreat of gully head. The study area is localized at approximately 8 km of the city of Gouveia on the slopes limited by Padre and Tamanduá streams. On these slopes there were 11 gullies observed, 8 of which are active. The gully process in this area shows an intimate relationship with the local geology and with the anthropical activity which has been taking place in that area over the decades. The influence of geology in the gully evolution begins when planes of rock weakness (foliation, fractures, etc.) are intercepted by channel incision in the initial stages of erosion. Both regressive headwater erosion and lateral enlargement transversal to the gully axis are conditioned by the direction and dip of structural rock features. Also the different resistance to the erosion is imposed by different rock lithology, being the diabase the most prone one in comparison to the schist and granit-gnaiss. When the depth of the erosion reaches the phreatic level, the process is abruptly increased with development of piping, digging out the foot of the gullywall and collapse of blocks of the saprolite leaving verticals high walls. The results of the hydrogeotecnial analysis, including discharge measurements, allow the following conclusion: the stream flow average inside the sampled gullies reached its maximum 7 months after the beginning of the rainy period. During May the stream flow is 10 to 15 times bigger than that measured in October, period of the beginning of the rain; the role of piping in the headwater regression is more accentuated in those gullies situated at the upper streams basin.
*This research was financially supported by FAPEMIG.