The rains of the past few days have Santa Cruz Mountains residents looking over their shoulders and keeping their radios tuned in. Winter rains mean mudslides, road closings, and threats to mountain homes.
Earlier this week, a slide forced a Felton couple from their house. A slide closed Highland Way three miles from Spanish Ranch on the way to Rattlesnake Gulch, and Monday, Empire Grade was blocked less than a mile from the UC Santa Cruz campus by a slide.
Indeed, mudslides are a fact of life. Can they be predicted? What sets them in motion? Here's a quick look at the whys and wherefores of the situation:
A: 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.
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 - mixtures of mud and earth that drip like thick syrup down the sides of the mountains. Before the massive mudslides in Santa Cruz County in 1982, it had rained an inch an hour for eight hours straight, and whole mountainsides came down.
A: In a half-inch of rain per hour, says Nielsen, you'd have your car's windshield wipers on full steam, but you'd still have trouble seeing. In an inch an hour, visibility would be zero; you'd be a fool to drive at all.
A: Last year's storms didn't even come close to causing serious slides. The worst storms last year ranged from a half-inch an hour to three-quarters of an inch; and didn't stay that intense for long. There was certainly more rain than normal, but it was well within established limits.
A: So far, none of this year's storms has topped one-third inch an hour for more than a few minutes. These past few winters really haven't been all that unusual, Nielsen says.
A: Engineering geologists study land formations in the area and how mudslides occur. They're probably your best choice of consultant, especially if they're local and familiar with conditions in the Santa Cruz Mountains.