Landslides triggered by Hurricane Mitch in Guatemala -- inventory and discussion

The torrential rains that accompanied
Hurricane Mitch in October and November
of 1998 triggered thousands of landslides in
the moderate to steep terrain bordering the
Motagua and Polochic Rivers in eastern
Guatemala. Using aerial photographs taken
between January and March 2000 we
mapped all visible landslides larger than
about 15 m in minimum dimension in a
study area of 10,000 km2 encompassing
twenty 1:50,000-scale topographic map
quadrangles.
Rainfall from Hurricane Mitch was
exceptional because it was geographically
widespread, prolonged over a period of
about a week, moderate to heavy in intensity,
and occurred at the end of the rainy season
when the ground already had a high
moisture content. As documented in this
report, this type of rainfall, on saturated or
nearly saturated ground, has the capability to
trigger both shallow and deep-seated landslides
over a large area.
We mapped about 11,500 landslides in
the study area. The mapped landslides were
of two general types: relatively small, translational
and rotational landslides that commonly
mobilized into debris flows and covered
less than several hectares in area (not
including flow paths), and large, commonly
translational, landslides that sometimes generated
debris flows and covered between
15 ha and 25 ha (not including flow paths).
The main concentrations of landslides are on
moderate-to-steep hillslopes underlain by
diverse geologic units. For the purpose of
describing the mapped landslides, we divided
the study area into five distinct regions
based on differing geologic and geomorphic
characteristics. These regions include the
upper Polochic valley and surrounding highlands,
the central Sierra de las Minas, the
hills surrounding La Union and Zacapa, the
eastern Sierra de las Minas, and the border
region with Honduras. All of these areas
received between 200 mm and 600 mm of
rain over a 13-day period between October
25 and November 6. The highest rainfall
amounts (400 mm to 600 mm) occurred in
the Upper Polochic valley and surrounding
highlands and in the central Sierra de las
Minas. The lower rainfall amounts (200 mm
to 400 mm) occurred in the hills surrounding
La Union, the eastern Sierra de las
Minas, and in the border region with
Honduras. In general, the rainfall received in
these areas is roughly equivalent to the average
precipitation received in a 1-year period.
We used 10-m digital elevation models
(DEMs) generated from contours on two
quadrangles in the central Sierra de las
Minas to create a map showing areas that
were susceptible to landslides during
Hurricane Mitch. To create the Hurricane
Mitch susceptibility map, we developed a
susceptibility threshold equation based on
elevation and gradient. The analysis indicates
that, at least on two quadrangles, gradients
less than 9? were not susceptible to
landslides during Hurricane Mitch. The
slope of the line defined by the threshold
equation indicates that less rainfall was
required to initiate landslides on steep gradients
than on shallow gradients. Ninety percent
of the mapped landslides that were triggered
by Hurricane Mitch are within the
susceptible zone shown on the map. Eightysix
percent of landslides that were mapped
as predating Hurricane Mitch, and all landslides
mapped as postdating Hurricane
Mitch, are within the susceptible zone.
We used LAHARZ software to model the
potential downstream area affected by debris
if a large landslide dam on the Rio La Lima
were to fail. The model shows that the area
affected would be similar to the area that
was affected by a debris flow that mobilized
from a large landslide along the Rio La
Lima during Hurricane Mitch.
The characteristics of rainfall-triggered
landslides described in this report can be
used as a partial guide to future landslide
activity triggered by rainstorms. On the
basis of existing data, hazardous areas
include: moderate to steep hillslopes and