Groundwater-induced seasonal slumps in gullies of the Bação Complex, Southeastern Brazil

Areas with a high density of large-scale gullies are frequent in regions of the crystalline basement of Southeastern Brazil, such as the Bação Complex. These gullies pose a high hazard for people and properties, and cause loss of agricultural land, among other impacts. Gullies originating as erosion by channelized runoff can be controlled in their first stages by ordinary containment practices. However, when erosive channels reach the groundwater, erosive processes conditioned by subsurface flows start acting, causing mass movements and their control becomes more difficult. Field monitoring shows that these mass movements occur not only in the rainy season, as expected, but also in the dry season. To understand the dynamics of mass movement in the evolution of these features, a representative unstable gully in this Complex was selected. As it is common in this region, this gully presents unstable slopes, especially due to slumps. Numerical simulations of saturated and unsaturated flow have shown that, in this region of high seasonality, the aquifer is anomalously recharged in two stages. Safety factor analysis by limit-equilibrium method indicates a condition near to instability, with slumps occurring during the rainy season, when the aquifer at the toe of the slope is recharged, and in the dry season, when the upper slope is recharged after a few months' lags due to thicker unsaturated zone. As these gullies are very unstable and difficult to access, a viable means of stabilization could be the sediment retention with gabion walls, backfilled with soil or inert materials.