Page 183 - Fundamentals of Geomorphology

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HILLSLOPES

Hillslopes are an almost universal landform, occupying some 90 per cent of the land surface. This chapter will
explore:

The form of hillslopes
Hillslope transport processes and hillslope development
Humans and hillslopes

Hazardous hillslopes

Any geomorphic process of sufficient magnitude that occurs suddenly and without warning is a danger to humans.
Landslides, debris flows, rockfalls, and many other mass movements associated with hillslopes take their toll on
human life. Most textbooks on geomorphology catalogue such disasters. A typical case is the Mount Huascarán
debris avalanches. At 6,768 m, Mount Huascarán is Peru’s highest mountain. Its peaks are snow- and ice-covered.
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In 1962, some 2,000,000 m of ice avalanched from the mountain slopes and mixed with mud and water. The
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resulting debris avalanche, estimated to have had a volume of 10,000,000 m , rushed down the Rio Shacsha valley at
100 km/hr carrying boulders weighing up to 2,000 tonnes. It killed 4,000 people, mainly in the town of Ranrahirca.
Eight years later, on 31 May 1970, an earthquake of about magnitude 7.7 on the Richter scale, whose epicentre lay
30 km off the Peruvian coast where the Nazca plate is being subducted, released another massive debris avalanche
that started as a sliding mass about 1 km wide and 1.5 km long. The avalanche swept about 18 km to the village of
Yungay at up to 320 km/hr, picking up glacial deposits en route where it crossed a glacial moraine. It bore boulders the
size of houses. By the time it reached Yungay, it had picked up enough fine sediment and water to become a mudflow
consisting of 50–100 million tonnes of water, mud, and rocks with a 1-km-wide front. Yungay and Ranrahirca were
buried. Some 1,800 people died in Yungay and 17,000 in Ranrahirca.