National Preparedness Month 2020: Landslides and Sinkholes

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Natural hazards have the potential to impact a majority of Americans every year. USGS science provides part of the foundation for emergency preparedness whenever and wherever disaster strikes.

USGS:  Start with Science

The USGS works with many partners to monitor, assess, and conduct research on a wide range of natural hazards. USGS science provides policymakers, emergency managers, and the public the understanding needed to enhance family and community preparedness, response, and resilience.

By identifying potential hazards and using USGS hazards science, federal, state, and local agencies can mitigate risk. In addition, USGS science can inform planning for major infrastructure investments and strengthen private property standards and materials, which help make homes and communities more resilient to natural hazards. While everyone should be aware of the hazards that are most prevalent in their community, the annual National Preparedness Month is a great time to learn about all hazards.  Two of the lesser-known hazards for most Americans – but which can occur almost anywhere – are landslides and sinkholes.

Landslides

Landslides occur in all fifty states and every U.S. territory, and cause the loss of life and billions of dollars in damage to public and private property annually. The USGS Landslide Hazard program (LHP) is dedicated to understanding how and why these events occur and how best to make informed assessments of the hazard to inform communities that may be at risk, ultimately helping to save lives and property, and to support the economic well-being of American communities.

Diagram of deep-seated landslide, from USGS Fact Sheet 3004–3072, “Landslide Types and Processes.”

Diagram of deep-seated landslide, from USGS Fact Sheet 3004–3072, “Landslide Types and Processes.”

(Public domain.)

Landslide processes and characteristics, such as size, distance travelled, trigger, and speed can vary tremendously, and these differences make understanding landslide events challenging. USGS scientists work to assess where, when, and how often landslides occur and how fast and far they might travel.

The following examples of recent landslide research by the USGS LHP show how our scientists provide reliable scientific information to minimize the loss of life, infrastructure, and property.

United States Landslide Inventory Map – Our understanding of landslide hazards at the national scale is limited because landslide information across the U.S. is incomplete, varies in quality, and is not accessible in a single location. In order to fix these obstacles, USGS scientists produced a website that marks an important step toward mapping areas that could be at higher risk for future landslides. In collaboration with state geological surveys and other Federal agencies, USGS has compiled much of the existing landslide data into a searchable, web-based interactive map called the U.S. Landslide Inventory Map. This database is an important first step to helping assess where, when, and how often landslides occur in the United States.

Color graded topo map showing landslide

Oso Landslide 3D Elevation example screenshot from the USGS 3D Elevation Program (3DEP).

(Public domain.)

 

Barry Arm Landslide, Prince William Sound, Alaska – In May of this year, the Alaska Division of Geological & Geophysical Surveys (ADGGS) alerted nearby communities and businesses about the possibility of a large landslide at the tip of the Barry Glacier in the Prince William Sound of Alaska that could enter the fiord and cause a potentially significant tsunami in the region.

Photo of flowing glacier

Annotated photo showing landslide areas of Barry Arm Fjord, Alaska. Subaerial landslides at the head of Barry Arm Fjord in southern Alaska could generate tsunamis (if they rapidly failed into the Fjord) and are therefore a potential threat to people, marine interests, and infrastructure throughout the Prince William Sound region.

(Public domain.)

USGS landslide geologists and remote-sensing experts coordinated with the Civil Applications Center and collected radar and optical imagery over the landslide, which revealed less than a few centimeters of landslide movement from late June to August of 2020. To complement these data, ADGGS collected high-resolution light detection and ranging (lidar) data and optical ortho-imagery (or imagery collected by remote sensors and then enhanced with geometric methods) of the landslide to get a more comprehensive picture of what is taking place in the area around the glacier. The interagency group continues to regularly meet and release data and information for the public.

 

Post-Wildfire Debris-Flow Hazard Assessments:  Glenwood Canyon, Colorado – In August of 2020, the Grizzly Creek Fire burned through steep terrain in Glenwood Canyon and closed Interstate 70, which is the main east-west transportation route through Colorado. This is near the location of the deadly 1994 South Canyon Fire and Storm King mountain, where debris flows in September 1994 swept vehicles off the road.

A “debris flow” is a type of landslide that typically consists of a fast-moving mass of water, rock, soil, vegetation, and even boulders and trees, and can be very hazardous to infrastructure and public safety. USGS investigations into the 1994 event marked the beginning of coordinated efforts to provide what are called “post-fire debris-flow” hazard assessments. Emphasis on this specific type of landslide was the result of realizing that after a fire, certain conditions including burn severity, vegetation type, slope steepness, soil type, and, most significantly, the amount of post-fire rainfall can contribute to these highly destructive debris flows. Debris flows are one of the most dangerous hazards after a wildfire, and community awareness is critical.

Map of burn area showing different colors to indicate levels of debris flow hazards

Grizzly Creek Fire (Colorado) Post-fire Debris-flow Hazard Map displays estimates of the likelihood of debris flow (in %), potential volume of debris flow (in m3), and combined relative debris flow hazard. These predictions are made at the scale of the drainage basin, and at the scale of the individual stream segment. Estimates of probability, volume, and combined hazard are based upon a design storm with a peak 15-minute rainfall intensity of 24 millimeters per hour (mm/h). Predictions may be viewed interactively by clicking on the button at the top right corner of the map displayed above.

(Public domain.)

Typically, after a forest fire is contained or nearly contained, the USGS LHP provides rapid assessments of post-fire debris-flow potential and size, in relation to estimates of triggering rainfall. These assessments support Federal, state, and local land and emergency response managers, homeowners, natural resource agencies, and other government agencies in identifying and potentially mitigating post-wildfire debris flows. Landslide hazard mapping and data are posted at the USGS  Emergency Assessment of Post-Fire Debris-Flow Hazards website along with more than a dozen additional post-wildfire debris flow assessments for large fires across the country.

For the Glenwood Canyon Fire, at the request of the U.S. Forest Service, and while the fire was still rapidly growing, the USGS delivered a “pre-fire” debris-flow hazard assessment for all of Glenwood Canyon, including portions that had not burned. USGS scientists collaborated with an interagency Burned Area Emergency Response (BAER) team, including the Natural Resources Conservation Service, USFS, and state agencies to produce these assessments.

The maps were then used to inform fire operations staff of locations at potential risk for post-wildfire debris flows and to stage fire-fighting equipment and personnel. The USGS will also deliver a final post-fire debris flow assessment after the BAER team finalize their soil burn severity map, which is a key input to the overall hazard model.

 

Sinkholes

Every year, land subsidence and collapse – or “sinkholes” – causes significant damage to personal property and public infrastructure. By one estimate, the cost of sinkhole damage to the country is several hundred million dollars annually; however, sinkhole occurrences are often not reported, and the true cost is likely much higher.

Image: Winter Park Florida Sinkhole of 1981

Photo 14 of 15: Remnants of community pool in sinkhole. View to east across the sinkhole.

(Credit: Anthony S. Navoy, USGS. Public domain.)

One of the more famous sinkhole events occurred on February 12th, 2014. A large sinkhole opened beneath the National Corvette Museum in Bowling Green, Kentucky and damaged or destroyed eight cars. Although the property damage was substantial, the collapse occurred in the early morning while the museum was closed and fortunately no one was hurt.

A much more tragic event occurred a year earlier. A man in Seffner, Florida, lost his life when a sinkhole opened beneath his house and swallowed him while he was in bed.

What exactly is a sinkhole, and why do they occur?

Sinkholes form when the land surface slowly subsides or collapses into pre-existing voids underground – essentially “air pockets” underneath the perceptibly “solid” ground we walk on every day. Such voids are often the result of movement and removal of sediments by water flow, a process known as piping. A sinkhole can occur as a result of natural processes or can be induced by human activities.

Diagram showing how cover-collapse types of sinkholes develop.

Cover-collapse sinkholes

Cover-collapse sinkholes may develop abruptly (over a period of hours) and cause catastrophic damages. They occur where the covering sediments contain a significant amount of clay. Over time, surface drainage, erosion, and deposition of sinkhole into a shallower bowl-shaped depression.

Learn more about sinkholes

(Public domain.)

Some minerals, such as salt, limestone, or gypsum, in bedrock can dissolve slowly over time and leave open voids within the rock where groundwater flows. These areas are called karst and have characteristic landforms such as caves, sinking streams, and springs in addition to sinkholes.

About twenty percent of the nation has the potential to host karst landscapes. In karst areas, the sediments overlying the bedrock are piped down into the bedrock voids and ultimately carried away by moving groundwater. The surface landforms are the result of voids in the bedrock formed by this process over long periods of geologic time, and as those sediments covering the bedrock are removed, subsidence occurs. The USGS produces geologic and subsurface maps that help managers and others to understand karst regions and identify local areas that may be susceptible to sinkholes.

Where do sinkholes occur?

Although there is not yet an effective method to predict where an individual sinkhole may occur, the USGS produces geologic maps that help to identify regions that may be susceptible to sinkhole formation. However, sinkholes can occur just about anywhere. It all depends on the subsurface geological composition and the characteristics of the area, i.e., type of unconsolidated and consolidated soils, infrastructure, and void dynamics.

Karst Map of the Conterminous United States - 2020

Map shows karst areas of the continental United States having sinkholes in soluble rocks (carbonates and evaporites), as well as insoluble volcanic rocks that contain sinkholes. The volcanic bedrock areas contain lava tubes that are voids left behind by the subsurface flow of lava, rather than from the dissolution of the bedrock.  Hot spots of sinkhole activity are also shown in areas of greater susceptibility. Source: Progress toward a preliminary karst depression density map for the conterminous United States https://doi.org/10.5038/9781733375313.1003 

(Public domain.)

What to do if you think you have found a sinkhole?

It is recommended that people constantly observe their property for signs of “subsidence,” (aka, “sinking”) such as tilted floors, misaligned door frames, or cracking and small holes in and around structural foundations. Water that flows on the surface and sinks into a depression or directly into a hole within the ground surface may indicate a sinkhole. In areas historically susceptible to sinkholes, surface streams can disappear entirely into active sinkholes and may be a concern for groundwater quality.  Additionally, information on the locations of areas susceptible to sinkholes can be obtained from county offices, local or state geological surveys, or maps produced by the USGS.

Sinkhole at Top of the Rock

Excavated sinkhole at a golf course at Top of the Rock Ozarks Heritage Preserve in Missouri that occurred in May of 2015. Photo taken in February of 2018.

(Credit: David Weary, USGS. )

I have (or think I have) a sinkhole on my property. What should I do? 

First, rule out human causes for a possible sinkhole. Some sinkholes are the result of leaky underground pipes (this would be an issue for your utility company), poor drainage control around building foundations, or old construction pits or buried materials that have settled. While the USGS studies the areas that can potentially form natural sinkholes, the agency does not investigate individual sinkholes on private property. 

Image: Sinkhole in Frederick, Maryland

Cover-collapse sinkhole in limestone near Frederick, Maryland (September 2003). Many sinkholes occur along highways where rainwater runoff is concentrated into storm drains and ditches increasing the rate of sinkhole development (note the sewer drain pipe beneath roadway).

(Credit: Randall Orndorff, U.S. Geological Survey. Public domain.)

If you are confident that a possible sinkhole is a result of natural causes, you can: 

  • check your homeowner's insurance policy to see if you might be covered (depending upon which state you live in, insurance policies may not cover damage due to natural sinkholes).
  • contact the appropriate utility company if you are concerned about damage to gas, electric, water, or sewer lines.
  • contact your state geological survey. They are the experts in your area’s geology, and they might be able to explain why a sinkhole is forming. Some states have extensive online information about sinkholes, and they often have a mechanism to report them. 

If you do confirm that there is a sinkhole in your area, ultimately a professional geologist or geotechnical engineer should be consulted either by you or state authorities to determine what is happening and how the impacts of a sinkhole might be mitigated.

 

More Information on Landslides and Sinkholes

Landslide-Generated Tsunami Risk in Prince William Sound webpage

USGS Barry Arm Facebook post, August 28th, 2020

The Cost of Sinkholes

USGS Water Science School – Sinkholes