Town Hall Notes: The Slopes Above Kestrel Falls
1Kestrel Falls is a mountain town of about 6,000 people in a narrow river valley. The only road into town, Switchback Road, zigzags up a steep hillside, and landslides have closed it several times. Hillside School sits at the foot of another slope on the east side of town. This fall, the town's hazard team, led by engineer Ms. Ilona Brzezinski, invited seventh graders Joaquín Teller and Saoirse Mbatha-Klein from the school's science club to help plan for the next landslide season.
2A landslide is a mass of soil and rock that slides downhill. On a slope, gravity pulls the soil downhill, while friction between grains and the roots of plants help hold it in place. Steep slopes slide more easily than gentle ones. When heavy rain soaks into the ground, water fills the spaces between grains, adds weight and pushes the grains apart, so they grip each other less firmly. Strong shaking during an earthquake can also jar soil and rock loose. Small earthquakes are common in this region, and a large one strikes every few decades.
3The team began with the ten largest storms of the past 20 years (Table 1). For each storm, it recorded the total rain in 48 hours and whether the ground was already wet from rain during the week before.
4Next, the team studied a magnitude 6.3 earthquake that shook the region in 2009. Geologists had checked 160 slopes after it and sorted them by how steep they are (Table 2).
5Last, the team looked at a crack across the top of the slope above Hillside School. In August, the crew strung a crack meter, a wire that measures how much the crack opens, across it. Before a slope slides, it often creeps slowly at first, and the creeping speeds up as the slide gets close.