10th International Conference on Geomorphology
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Monitoring hillslope hydrology associated with rocky outcrops in an abrupt contact with talus deposits

Jenifer da Silva Roméro1, Maria Maria Rocha Lamas2, Nelson Ferreira Fernandes2, and Eurípedes do Amaral Vargas Junior3
Jenifer da Silva Roméro et al.
  • 1Department of Geology, Federal University of Rio de Janeiro, UFRJ
  • 2Department of Geography, Federal University of Rio de Janeiro, UFRJ
  • 3Pontifical Catholic University of Rio de Janeiro, PUC-Rio

Hillslope hydrology is a major factor controlling the spatial and temporal distribution of landslides in the mountainous landscape of southeastern Brazil. Previous studies have attested that the movement of water through preferential soil paths and bedrock fractures contributes to the development of hydrologic conditions required to trigger landslides in the region. In Rio de Janeiro, the upper portions of many hillslopes are characterized by rocky outcrops of unweathered granites and gneisses, with thick talus deposits on the lower parts. Although most hydrological studies carried out in the region consider the surficial effects of the impermeable rocky surfaces, the effects of fractures on infiltration and exfiltration have not been extensively considered, as well as lateral flows at the soil-bedrock boundary. In this study, we monitor an experimental hillslope in the city of Petrópolis (Rio de Janeiro) to understand how the rocky outcrops, the fractures present in the bedrock and the talus deposit stratigraphy interact and define the hydrological behavior of this environment. Three matric potential nests were installed in a transect along the hillslope (upper, middle and lower portions), each one with 7 sensors installed from the soil surface to the soil-bedrock boundary (between 4.0 and 5.0 m depths), with readings every one hour. The main soil physical properties were characterized on the samples retrieved from the drillings and two geophysical methods were integrated, the GPR (Ground Penetrating Radar) to investigate soil depth and the VLF (Very Low Frequency) to map bedrock fractures. Our results suggest the occurrence of preferential subsurface flows conditioned by soil stratigraphy and the presence of fractures in the bedrock. The rocky outcrop causes an increase in soil moisture in the upper soil layers in the upper and middle positions of the hillslope, favoring the entrance of water into the upper parts and the generation of lateral and ascending subsurface flows along the soil-rock interface zone during heavy rains. Geophysical data suggests that the talus deposit has a higher concentration of boulders in the middle and upper portions of the hillslope and the presence of bedrock discontinuities that were interpreted as fractures that have the same NE-SW dip direction of the main regional trend. Our results suggest that both the rocky outcrop and the bedrock fractures control the development of subsurface flows and the generation of high positive pore-pressures along the hillslope profile.

How to cite: da Silva Roméro, J., Rocha Lamas, M. M., Ferreira Fernandes, N., and do Amaral Vargas Junior, E.: Monitoring hillslope hydrology associated with rocky outcrops in an abrupt contact with talus deposits, 10th International Conference on Geomorphology, Coimbra, Portugal, 12–16 Sep 2022, ICG2022-714, https://doi.org/10.5194/icg2022-714, 2022.