Home Sustainable Land Development Today February 2007

Landslides: Catastrophic Fall Without a Net

Written by Rob Kundert

Thursday, 01 February 2007

With prime real estate declining and prices rising, development professionals should be wary of landslide risks. That old adage that lightning won’t strike twice in the same place doesn’t necessarily hold with landslides. The earth can move more than once in the same location and that should be a litigious red flag for developers. “Generally where there have been landslides in the past there’s a tendency for landslides in the future,” said Paula Gori, Associate Coordinator of the Landslide Hazards Program with the U.S. Geological Survey. Landslides occur in their various forms in every part in the nation. Regions outside the contiguous states — Alaska, Hawaii, Puerto Rico and the Virgin Island — are particularly susceptible due to their mountainous terrain and volcanic activity. Landslides cause approximately $3.5 billion in damage and kill between 25-50 people per year. Globally, the casualty numbers escalate exponentially. The most susceptible regions of the country are the coastal areas of California, Oregon, and Washington, mountainous regions further inland, from Idaho to New Mexico and their counterparts in the East, along the Appalachians and Blue Ridge Mountains from Maine to Georgia. Least susceptible are low-topographical relief states like Florida, however, they can be found in scenic river valleys, and even in plain states like Nebraska. A severe concern for developers should be the liability exposure. With limited insurance coverage opportunities available, that leaves the governing body that permitted the development and the developer to sit on the bull’s eye when the earth moves under the feet of property owners. Still, Gori said that the attraction for what the land can offer sometimes outweighs the risks involved. “There is more marginal land being developed that generally wasn’t developed before on steep slopes, like in coastal areas and in areas that might be developed for recreation or for good vistas,” she said. “Those areas are now being developed so we think there are more problems with landslides than in the past.” Know the Opponent Landslides are going to happen. As long as the earth’s internal geologic engine continues to cause the continents to move and force the faulting and folding that shape its surface, nature’s forces of wind, rain, ice, and seismic activity seek to flatten it out. In the process, sections of land will loosen and seek the equilibrium of the evenly-distributed horizontal. Freezing and thawing, saturated soils, volcanic activity and human excavation are some of the root causes of a landslide, which is a general term applied to a range of events, from avalanche-fast debris flows and mudslides, to glacial-speed events that slump down at a barely-discernable pace. “A debris flow or mudslide can come down very quickly, picking up debris as they move along and become much larger at the bottom than what it they were at the top,” Gori said. “Then you have deep-seated landslides. They can be in existence but have not completed their sliding.” Gori co-authored a study for the American Planning Association, “Landslide Hazards and Planning.” The publication is a go-to source of information about the causes and effects of landslides. “The obvious driving force behind a landslide is gravity, but a landslide occurs only when the force of gravity overcomes the inertial forces of friction that hold a slope together,” it states. All that is needed is something to trigger the landslide. The conditions that permit this to happen include high moisture levels, freezing of ice in joined soil or rock, seismic activity or destabilizing human activities such as grading, digging or removal or vegetation. Hurricanes can be a great producer of landslide triggers because of the torrential rains that they carry inland. They saturate the soils, loosening the friction-bonds that hold the landmass in place. Massive snow melt can also cause soils to be soaked and loosened. Mt. St. Helens provided a graphic example of landslides when it erupted in 1980. Rapid snow melts from the heat caused rivers of mud and forest debris to wash down its sides while a large section of its mountainous flank collapsed in another huge mass. “Exacerbating these triggers would be human causes such as excavation of a slope or the toe of a landslide, loading the slope or loading the crest of the slope, deforestation, heavy irrigation, mining, leakage from utilities, such as skeptics,” Gori said. Road building can also be a culprit. “One of the most important ways to mitigate landslides would be to build roads that would not cut the slope that cause a landslide. The way the grading and the placement of the road are very important in areas that are somewhat prone to a landslide,” she said. Road cuts can cause rock falls and rock tumbles, which are other types of landslides. “Sometimes, with a road cut, you don’t leave anything else. Then you have a rock topple because it is too weak to stay,” she said. The Fall Guy More than ever, when it comes to the threat of landslides, developers need to evaluate the risks of the terrain when deciding to develop a site. They would be among the first on the liability target in case their project slides down a slippery slope, according to Gori. Property owners have little recourse. “It’s a catastrophic loss if you have a landslide. You lose not only the house, but you can’t rebuild it. The land has no value,” she said. “They generally go after the jurisdiction that permitted the development or the developer who put the houses on the landslide because they don’t have anyone else to go after.” Meanwhile, she said insurance companies are reluctant to insure against landslides. The Federal Emergency Management Agency may not necessarily be there either. “Sometimes they cover through flood insurance, but few jurisdictions have been able to get FEMA to reimburse people for loss. It depends on the definition of mudslide and how much water was in the slide,” she said. Upfront Study When eyeing that prime hillside location for a subdivision, resort or building complex, Gori said it is wise to take a three step approach: local, regional and then the private engineer. “A developer aught to first go to their local authority, a planning commission or public works official to see if there are landslides in their area,” she said. Next, check out the local university for a geology department, or seek out the state’s geological survey and its state geologist. “They sometimes will have landslide susceptibility or hazard maps,” she said.   The final step would be to work with a geo-technical engineer. “They could do the research and do that site-specific analysis of the land to be developed,” she said. Regulatory statutes differ from state to state, depending on their history with landslides. California and Idaho for instance, which have had extensive histories, might have more on the books than other states. On the other end, North Carolina, is just now establishing more regulation due to its landslides caused by recent hurricanes. “Some states, with areas that have had landslides before, require a study prior to permitting any development,” she said. Mitigation Option “There are methods to mitigate landslides—such as a myriad array of retaining walls, planned excavation and other stop-gap measures – but they are not used in this country as much as in places like Japan and Canada, according to Gori. “In the United States we don’t do a lot of this because we have a lot more land available for development. We haven’t been forced into these kinds of mitigations,” she said. The reality is that the earth does move, sometimes naturally, sometimes inadvertently spurred on by development. A passage in the text Landslides Hazards and Planning succinctly summarizes landside circumstances: “Landslides are an essential part of the earth’s natural geological processes. Like other hazardous occurrences, they become disasters only when humans have placed the built environment in the likely path of those processes. A landslide with no development or human presence in its past is not a disaster, but simply a natural event.” LDT