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When a landslide occurs, how long does the ordeal typically last?

Obviously the answer is that they vary a lot, however, I'm talking about some specific examples such as the one in Sierra Leone in 2017, or even the landslides in the Philippines going on right now. How quickly did these (or anything similar) landslides move, or how long does it take for them to occur from start to finish?

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  • $\begingroup$ I don't think this is really answerable, as landslide duration can last from seconds to years. $\endgroup$ – BillDOe Jan 2 '19 at 20:14
  • $\begingroup$ You're right. However, do you know how long those specific examples lasted? Or is there any source you can point me to? $\endgroup$ – F16Falcon Jan 2 '19 at 23:16
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A historic yet well observed landslide in the East Kootenays of British Columbia took approximately 100 seconds to settle, Frank Slide. The slides at Sierra Leone and the Philippines that you mentioned are a different type of slide made of mud and other unconsolidated materials. I cannot find any information on them but I have witness a few slides in my previous forestry work. From my observations a slide can occur in 10s of seconds, quick slump flowing fast enough that one cannot escape if you were to be in the path, the one I witnessed took a truck off the road and about 150 meters down a shallow bank the people in the truck escaped but we lost the truck. They also can occur very slowly, we had an area that due to changes in the drainage up slope started slumping. The slumping started in March but didn't stop until September. In the case of the slow slide/slump it took a hectare of forest and slowly moved it down slope and across a road very slowly. Often the speed of the slide seems to be affected by the amount of water in the soil. The fastest being the closest to be categorized as a debris flow rather than a slide, the other factor is the steepness of the slope, the steeper falling faster.

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  • $\begingroup$ Interesting! Thanks for the reply. Just a little further clarification, in general, do larger, more powerful (and deadly) landslides typically take longer to happen (ex: 100 sec, as you cited), while smaller ones take less time (like 10's of seconds, as you observed)? Or were the ones you observed large and deadly, too (like Frank Slide)? $\endgroup$ – F16Falcon Jan 3 '19 at 1:50
  • $\begingroup$ The Frank Slide is east of the divide in Alberta (about 1 km east of me). The height was ~1000 metres above the river, so assuming a 45 degree slope and neglecting friction the main mass of rock would hit the bottom in 10 seconds at 500 km/hr. Friction probably slowed it somewhat, as in this video (150-200 ft. height). $\endgroup$ – Keith McClary Jan 3 '19 at 3:46
  • $\begingroup$ Just to be clear the slide at Frank was a rock slide, a portion of Turtle Mtn. near the top just broke free and slid down a nearly vertical face until it reached the base of the mountain. The biggest debate about the Frank slide is how the material managed to "drift" so far from the base of the mountain. The 100 seconds is based on observer reports, I expect there was a lot of dust but that the main collapse occurred much faster. $\endgroup$ – user824 Jan 3 '19 at 22:37
  • $\begingroup$ In regards to the slide speed in general. I believe the main difference between the death dealing slides is the state of the unconsolidated materials. Once the slide starts does it trigger the adjacent materials to slide as well. In general I would say the larger slides tend to be faster larger than the slow ones but this generalization is simply based on watching slide areas develop as I drove to work each day. $\endgroup$ – user824 Jan 3 '19 at 22:44

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