It should come as no surprise that landslides are dangerous. Massive and tragic events like the 2014 Oso landslide are exceedingly rare, but even so, landslides in the United States kill about 25 people each year and are responsible for $1-2 billion of property damage. In most cases, homeowner’s insurance doesn’t cover property damage due to landsliding, which can be devastating to survivors and the people around them.

Cities and counties have an interest in regulating development around landslide hazards in order to enhance public health, safety, and welfare. Indeed, it is not an exaggeration to say that local governments have a moral obligation to protect their citizens from naturally occurring hazards that can be deadly and are largely invisible to the average person. Given recent technological advances, local governments also have the ability to characterize and map hazards and can use these data to guide development and reduce risk.

What causes a landslide?

Despite the fact that most people have never experienced one, small and medium-sized landslides are quite common.

Landslides are grouped by geologists into a larger category of rapid erosion called “mass wasting.” Mass wasting is a process that occurs anywhere a slope is steep enough to not hold its own against gravity, causing surficial materials to slide downhill. Slope failures occur in several manners, differing in the type of materials that fail (rock versus soil), the depth of the slide plane, how much water is involved, and the rapidity of debris movement. Adding water to any steep slope can trigger a landslide (as commonly happens during winter storm events), as can shaking from a seismic event, but even without rain or earthquakes rock and soil will slide downhill if on too steep a slope or if there are natural planes of weakness.

Although the forces that drive landslides are natural, most landslides affecting humans are actually initiated by human activity, such as discharging storm runoff incorrectly, clearing vegetation, or grading a slope in such a way that it steepens or undercuts the slope. The Rattlesnake Ridge landslide near Yakima, recently in the news, may be an excellent example of a human-caused failure. There is a rock quarry at the toe of the slope and the quarrying of rock and soil may have removed a natural buttress, triggering a failure above. As of this writing, it is not known whether the quarry is connected to the landslide, but it is being investigated.

How can we determine where landslides are located?

Geologists interested in mass wasting have been mapping and describing landslides for decades, usually by direct observation. Forensic studies of past events have yielded insight into the likelihood of future events and allowed scientists and engineers to design hazard mitigation. This has undoubtedly saved many people but the most important thing is to know is where landsliding might occur.

Fortunately, geologists now have access to LiDAR imagery. LiDAR is an airborne scanner that can be used to generate detailed maps of surface topography. The LiDAR imagery can reveal evidence of many past landslides, typically the largest ones, but proper interpretation is important. Small errors can make a big difference at the individual property level so it’s important that the people doing the mapping are well qualified.

Within Washington State, for a geologist to be qualified requires adequate experience in the mapping of surficial geology, as well as licensing by the state. Like engineers and some other professions, geologists must be licensed in order to perform the “practice of geology for others,” as regulated under the Revised Code of Washington and the Washington Administrative Code, and city and county employees are not exempt from that statutory requirement. When an unlicensed person performs geologic work or alleges the ability to perform geologic work, regardless of education or experience, that person is in violation of state law. It is for this reason that city and county agencies are wise to confirm that those individuals who need licensing do indeed have it.

How landslide hazards are regulated

Today, potential hazards to development from landslides are minimized principally through regulations in local building and zoning codes.

If a hazard is suspected, code provisions typically require a developer to complete a geotechnical assessment of the property and to include landslide mitigation, if needed, in the development proposal. This works well for new construction, but because there are many built-up areas that predate the newer codes, there are plenty of places where development has occurred without taking hazards into account.

Having the authority to regulate development near landslides and within other geological hazard areas comes with the responsibility of knowing where such hazards exist, hence the reason for mapping them. The act of mapping potential hazards addresses the moral obligation to protect citizens. But even in the absence of such data, when a public agency becomes aware of a hazard to people or property, especially one already regulated, it is their responsibility to act on that knowledge. Proper steps must be taken to ensure public safety, either by confirming a hazard is not present or by taking actions to prevent injuries or property damage. The regulatory agency can call for more studies, including geologic studies and mapping, and they can go beyond that (if deemed prudent or necessary) with detailed engineering studies and evacuations (if needed). The issue is one of good government, combined with potential liability, best engineering/scientific practices, and who’s responsible for regulating and/or mitigating the hazard.

Things to remember

Landslides are common throughout the Pacific Northwest and in many other parts of the United States. We’ll be dealing with them as potential hazards until the Law of Gravity is repealed. Until then, regulatory agencies and local governments must do their best to understand and document the hazard potential in their jurisdictions by employing trained and qualified geologists who have the proper experience.

Beyond the qualifications of the investigators, regulatory agencies should have clear code authority to regulate the hazards that are present. Specific and understandable criteria for identifying hazards should be in place: Definitions are important and should be broad enough to include everything you need to regulate.

It is also important that the public understands what is being done and why, partly because the public can alert you to problems your entity did not know existed. Keeping the public informed through press releases, information on public websites, and community meetings will help minimize future conflict.

For all of these reasons, whatever you do should be scientifically valid, reasonable, and responsible, as well as reflective of good government principles. Small investments of time and money now can save a lot of headaches in the future.

 

This blog post was originally posted 2/20/18 on the website of the Municipal Research and Services Center (MRSC) at http://mrsc.org/Home/Stay-Informed/MRSC-Insight/February-2018/Regulating-Landslide-Hazards-Can-Be-Hazardous.aspx.