Geological and geotechnical land zoning for potential Hortonian overland flow in a basin in southern Brazil

Abstract

This paper presents a new procedure to perform geological and geotechnical land zoning at the 1:50,000 scale for potential Hortonian overland flow (Rpot values). The procedure was applied to the Ribeirão do Feijão Basin, São Paulo State, Brazil. The zoning was conducted using a procedure that combines rainfall data, engineering geological mapping, infiltration modelling, and anthropogenic and geomorphic aspects. The association between the geological-geotechnical units defined by the engineering geological mapping and the land use types generated 29 different infiltration condition categories, which were used to divide the basin into 94 territorial units. The in situ infiltration test and geotechnical data for each infiltration condition category combined with several rainfall event types were used as inputs to the Philip model to obtain the values of the Rpot and infiltration values. The Rpot and infiltration values estimated by the proposed procedure agreed well with the values estimated by the Chu model. The 94 territorial units were grouped into 4 classes based on the variability and magnitude of the potential Hortonian overland flow. The first class represents the areas where infiltration predominates (59 km2) for all selected rainfall events, and the second class represents areas of infiltration and flooding (11 km2). Infiltration is predominant in the third class; however, overland flow can be generated (133 km2) depending on the intensity and duration of the rainfall event. The fourth class represents the main source areas of overland flow generation and runoff (40 km2). The areas considered in the third and fourth classes are affected by erosion processes due to the potential generation of Hortonian overland flow. The results indicated that the proposed procedure is adequate for identifying, characterizing, classifying and dividing a region into units with distinct geological and geotechnical zones that are associated with potential Hortonian overland flow. Therefore, the procedure is an appropriate tool for regional planning and water management at the 1:50,000 scale because the results define land units with specific environmental characteristics. The application of the procedure is not expensive and uses common equipment but requires experienced or trained professionals to perform the field and laboratory work.