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Soil and Sedimentation Dynamics of a Widespread Shallow Landsliding Event in a Tropical Montane Setting

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Soil Erosion Research Under a Changing Climate, January 8-13, 2023, Aguadilla, Puerto Rico, USA  .(doi:10.13031/soil.23523)
Authors:   Edwin O Irizarry-Brugman, K. Stephen Hughes
Keywords:   Bathymetry, Erosion, Landslides, Principal Component Analysis, Sedimentation.

Abstract

High intensity rainfall from tropical cyclones is the predominant landslide triggering phenomenon in Puerto Rico. This study aims to refine our understanding of how specific soil characteristics relate to mass wasting vulnerability and to track the sedimentation record of thousands of landslides provoked by Hurricane María in an important watershed of the tropical island. By using quantitative modeling and field measurements, the impact of these extreme atmospheric events on weathering and sedimentation on the island can be better understood.

Principal Component Analysis (PCA) was used to explore quantitative soil characteristics that are common in soil map units that are known to be prone to rainfall-induced landslides. Chemical and physical soil properties of the 700+ soil map units on the island were evaluated in the analysis, obtained from the Natural Resource and Conservation Service SSURGO database. These properties were evaluated at different binned soil profile depths. Additionally, each soil map unit had a unique frequency-ratio (FR) value, determined by the correlation of slope-failures that occurred in a given soil unit. The goal of the PCA models was to find groupings of the soil map units based on their FR values and establish which soil properties influenced the groupings (Figure 1). The analysis results show that there is no unique or uniform combination of soil characteristics that correlate with a mass wasting vulnerable soil in Puerto Rico; this supports the conclusion that diverse soils and soil characteristics can combine with several other site-specific factors and conditions to yield a soil mantle vulnerable to mass wasting.

A bathymetric survey of the Lago Dos Bocas reservoir in central Puerto Rico was used to quantify the amount of sediment from Hurricane Maria triggered landslides that settled in the fluvial system. The reservoir is part of the island‘s fragile freshwater distribution network and receives sediment from the Rio Grande de Arecibo basin. The ~310 km2 basin experienced over 13,000 shallow landslides during Hurricane María in 2017. The 2021 bathymetric survey indicates that the lake‘s storage capacity is approximately 13.35 Mm3, a storage loss of 64% from its original capacity in 1942 (Figure 2). Based on USGS capacity loss estimates from 1999-2010, the current state of the important reservoir reflects a sediment “surplus” of about 2.1 Mm3 that could be associated with Hurricane María; this volume of material would represent the equivalent of around 15-20 years of background sedimentation. Results from the bathymetric survey indicate that widespread landslide triggering events are important generators of sedimentation. Additional bathymetric surveys are needed to reinforce the results of this preliminary conclusion and to monitor future sedimentation rates.

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