Examination of the Soil Mass Movement Risk Rating in Puerto Rico
K. Stephen Hughes, Associate Professor, Department of Geology, University of Puerto Rico 2021 NCSS National Conference
The soil mass movement risk rating is an interpretation scheme designed to estimate a soil’s propensity to move downslope due to gravity. In 2017, more than 70,000 mostly shallow mass wasting sites were generated by high intensity – long duration rainfall associated with Hurricane María. The areas most affected by these landslides were not correlated to the zones identified as most vulnerable in the soil mass movement interpretation. The discrepancy of the model prediction and the effects of the hurricane may be a result of the distinct tropical chemical weathering climate in Puerto Rico, compared to the continental United States. A comprehensive event inventory of landslide sites is an important tool to refine our understanding of soil units and combinations of soil characteristics that are more likely to result in mass movement activity. A complementary goal of the study is to better understand the area-volume scaling of mass wasting sites in target vulnerable soils. Approximately 700 SSURGO soil map units on the island of Puerto Rico were analyzed with the Hurricane María landslide inventory and a frequency-ratio (FR) score was assigned to each unique “mukey.” The FR scores and diverse soil unit data mined via Soil Data Access allow for multivariate analysis approaches in order to gauge the important distinctions between soil units that combine to produce vulnerable soils vs. those that are less susceptible to mass movements. Results from the study are intended to guide a mass movement interpretation specific for Puerto Rico, given its particular geographical and climatic concerns. USDA is an equal opportunity provider, employer, and lender.