What causes mudslides?
The short answer is rainfall intensity. Eleven or more inches of late fall rain saturates the ground, priming it to wash away. During winter storms, when the rain’s coming down faster than it can drain out of the mountains, things start to move. Seepage pumps groundwater up to the surface, taking surrounding earth with it. Q: How can I tell if a rainstorm is likely to cause serious slides? A: The simplest way is to get a rain gauge, according to Hans Nielsen, a local engineering geologist. “It doesn’t matter what the total amount of rainfall is, or how long it rains; it’s the intensity of rainfall – how much rain falls within a certain time. It can rain at a tenth of an inch an hour for two days, and you’ll be fine,” he says. At a quarter-inch an hour, you start to see cut-slope slips – small slides that follow the paths of roads cut through the mountains, like the ones that make Highways 9 and 17 unreliable during the rainy season. A half-inch an hour starts larger debris flows – mixt
