The Montecito disaster did not come as a complete surprise. Indeed, the US Geological Survey warned of high potential for disastrous landslides if intense rain fell on mountainsides around Santa Barbara that had been scorched by the Thomas wildfire in December. When that perfect storm landed, it hit bare, baked soil that could not readily absorb water. So the rain ran off, picking up soil, boulders, and debris as it surged down canyons and streams. These debris-charged torrents slowed only where steep channels gave way to gentler slopes.

Most of the damage occurred along the run-out pathways of the debris flows-areas where material can flow after it starts sliding downhill. Yet landslide hazard maps generally don’t show predicted run-out zones. Instead, they typically show only the locations of the source areas where landslides are likely to start.

Better information for residents

There are reasons why people keep building homes in landslide-prone areas. Some decide it’s worth the risk. In Seattle, where I live, steep slide-prone slopes tend to offer the best, and most expensive, views. Conversely, in low-income regions such as Appalachia and many developing countries, the poor often are pushed up onto potentially unstable hillsides. Generally, however, I suspect that many Americans living in landslide country are simply unaware of potential hazards that the lay of the land presents to their homes, neighborhoods, and businesses.

Sometimes politics or greed plays a role. After Hurricane Frances hammered North Carolina in 2004, the state legislature approved a program to map landslide hazards. But once the first maps were produced, the program was canceled over concerns that the maps would affect land values and be used to regulate development.

Without this kind of information, residents are physically and financially exposed. “Earth movement,” such as landslides, generally is not covered by homeowner’s insurance policies. And by the time a landslide comes, developers are long gone, leaving homeowners holding the bag.

And, of course, different types of landslides pose different risks. In the slow-moving Rattlesnake Ridge landslide in central Washington state, a 20-acre parcel of land is sliding downhill about a foot and half per week. Residents have been moved out of the at-risk zone, and engineers and geologists are monitoring the site in real time to evaluate and update hazard assessments.

Upper part of the Oso, Washington landslide site, photographed April 8, 2014 (see small house just inside treeline at lower left for scale).
Upper part of the Oso, Washington landslide site, photographed April 8, 2014 (see small house just inside treeline at lower left for scale).
Image: USGS/Jonathan Godt

In contrast, the Oso landslide north of Seattle on March 22, 2014 was so large and fast-moving that even a real-time warning would not have prevented tragedy. This disaster killed 43 people in a couple of minutes when an entire hillside collapsed. In response, Washington state has started posting detailed topographical maps online for use in identifying areas at risk for generating landslides. But hazard maps still don’t identify potential downhill run-out zones.

It’s time to get serious about landslide zoning, in the way that the federal government maps areas at serious risk of flooding. Landslide hazard maps delineating potential run-out zones should be part of local land use planning. These maps could help guide zoning decisions and better inform homeowners, banks, and insurance companies of potential risks. Ultimately, the best way to reduce landslide risk is to avoid building things we value in places where run-out is likely. For when there’s no controlling nature, there’s only living with her.

This article was originally published on The Conversation. Read the original article.

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