Mass movement processes in the Sierra Norte de Puebla, october 1999: Cause and effects
A mountainous zone of approximately 5,000 km2 was affected during October 1999 by thousands of mass movement processes mainly of rock falls, slides, ﬂows, and combined types. These processes involved earth volumes from few to hundreds of thousands cubic meters and the formation of landslides scars with longitudes ranging from 10 to 100 m, and in quite a few cases of hundreds of meters. The mass movements were triggered by a four day long rainfall event, equaling 50–60% of the total annual rainfall in some localities.
As a ﬁrst approximation to the understanding of these hazards, the involved relief was classiﬁed in morphological units of three orders associated to six geologic units which encompass several Paleozoic to Quaternary formations. The phenomenon affected especially the upper slopes of the Sierra Madre Oriental, mainly the Triassic–Jurassic sedimentary rocks and the upper layer of volcanic rocks and weathered materials. Different landforms and processes were produced by this event, among them a new lake formed, which remains after ﬁve years; more than three meters of alluvium were accumulated in many mountain rivers; new gullies formed and others were sourced more than 20 m. The human inﬂuence was signiﬁcant given the fact that many of these slope movements occurred on deforested hillslopes and roads (with averages of even two processes per km). A map of head of gullies frequency was developed. The total measured number of head of gullies was about 19,000 for an area of 4,000 km2. Mean values are in the order of 5 head gullies heads per km2, with a maximum of 13 stream heads per km2. This indicates a strong relationship between mass movement processes and headward erosion in Holocen times.
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