161-4 Discerning the relative importance of surface erosion and shallow landsliding in driving sediment yields of steep tropical settings

Session: Dynamics of Natural and Built Environments

Presenting Author:

Carlos Ramos Scharron

Authors:

Ramos Scharron, Carlos E¹, Arima, Eugenio Y², Bhatt, Rushi³, Hughes, Kenneth Stephen⁴, Matos Llavona, Pedro⁵, Ohr, Sewon⁶, Tavakoli, Keivan⁷

(1) Department of Geography and the Environment, The University of Texas at Austin, Austin, Texas, USA, (2) Department of Geography and the Environment, The University of Texas at Austin, Austin, Texas, USA, (3) Environmental Science Institute, The University of Texas at Austin, Austin, Texas, USA, (4) Department of Geology, University of Puerto Rico at Mayaguez, Mayaguez, PR, Puerto Rico, (5) Department of Geology, University of Puerto Rico at Mayaguez, Mayaguez, Puerto Rico, Puerto Rico, (6) Department of Geography and the Environment, The University of Texas at Austin, Austin, Texas, USA, (7) Department of Geography and the Environment, The University of Texas at Austin, Austin, Texas, USA,

Abstract:

Sedimentation signifies a key risk to the world’s water reservoirs on which many countries depend upon to satisfy their water demand. Therefore, improving our understanding of sediment dynamics is crucial, particularly in small, densely populated, high-standing tropical islands characterized by high sediment yields. This study compares potential contributions from surface erosion on active cropland and rain-driven shallow landsliding to sediment delivery into several reservoirs in central and western Puerto Rico from the mid-20^th century onwards. Our approach relies on data sources including land cover, road, and landslide inventory maps, high resolution digital elevation models, long-term rainfall observations, sediment cores, streamflow data, and bathymetric surveys to document the sediment budget of four 22 – 125 km² watersheds spanning different lithologies (i.e., metamorphic, non-calcareous sedimentary, and intrusive), climate zones, levels of exposure to rainfall associated to tropical cyclones, topographies, and both historic and modern land uses. Preliminary results from the Loco, Lucchetti, Yahuecas, and Caonillas watersheds suggest the importance of the interrelation of these driving factors in exacerbating sediment yields, which have remained above 1,000 Mg km^-2 yr^-1 and several times higher than baseline levels even though the watersheds presently are dominated by secondary forests. In all cases, the importance of surface erosion in driving sediment yields has decreased to minimal levels as farmland became abandoned. However, the relative and net importance of landsliding as a sediment source is believed to have increased due to a greater frequency of both tropical cyclone and non-cyclone related rainstorms capable of transporting sediment through the fluvial network and triggering landslides particularly on 35- to 60-degree hillslopes with a high density of roads. While road densities can be high in volcaniclastic sedimentary and granodiorite terranes (20 – 30 km km^-2), they can be quite low in serpentinite covered areas (~7 km km^-2) due to the poor agricultural productivity of its soils. Many roads are farm access roads representing a legacy of past agricultural practices that presently still influence the occurrence of landslides and maintain high sediment yields.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10271

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