Can hillslopes keep pace with landslide frequency in the humid tropics
of Puerto Rico (USA)?
Abstract
Bedrock weathering and soil production set the rate that hillslope
colluvium is produced, while material properties determine whether
sediment can fail under a given set of conditions. Together these
factors generally control both the frequency and magnitude of
landslides. In 2017, Puerto Rico (USA) experienced widespread
landsliding across a range of lithologies due to Hurricane Maria, making
it an ideal setting to explore the role of sediment generation in
landslide response to storms. Based on an inventory of
>70,000 landslides island-wide and detailed field mapping
from a subset of source areas, we estimate that 0.01-0.1 km3 of material
was evacuated from the hillslopes (approximately 1-10 mm of lowering).
Focusing on the high-density landslide area of Utuado, we estimate an
average lowering of 5-50 mm. From past inventories and records of storm
events, a watershed is impacted by a hurricane every ~25
years with enough rain for widespread landslides every
~5 years. Assuming a similar density, the landslide
contribution to hillslope lowering could be on the order of 1-10 mm/yr.
In a humid-tropical environment, where weathering rates are likely high,
can hillslopes continue to produce material at this pace? Elsewhere on
the island, soil production rates are on the order of 0.1 mm/yr leading
to soil residence times of approximately 10 ky. However, to keep pace
with landslide events like Hurricane Maria, soil production likely needs
to be at least an order of magnitude faster to maintain soil-mantled
hillslopes in this study area. For our study area, we ask: has the
large-magnitude rainfall from Hurricane Maria caused an abnormally high
density of landsliding, resetting the clock on the material availability
for areas like Utuado? Here we relate measures of material properties,
bedrock weathering intensity, denudation, and land-use history to begin
answering this question.