title: National Park Service - Park Geology
arrow bulletRockfall
Geo Hazards Outline
Rockfall


1. Terms and Definitions

a. Geohazards

The changing of landscapes through natural and human induced processes of erosion involving rockfalls, landslides, debris flows, slumping ground, swelling soils, and dissolving bedrock that impacts people’s lives and property.

Like a tree falling in the forest with nobody to hear it, a rockfall without impact on humans is not a geohazard.


b. Types or forms of hazards expressed

i. Rockfalls
ii. Rock and debris avalanches
iii. Rockslides
iv. Mud and debris flows
v. Landslides
vi. Earthflows
vii. Slumps (rotational)
viii. Ground settlement

c. NPS Management Policy 2001

Geologic Hazards 4.8.1.3 – Although the magnitude and timing of future geologic hazards are difficult to forecast, park managers will strive to understand future hazards and, once the hazards are understood, minimize their potential impact on visitors, staff, and developed areas. Before interfering with natural processes that are potentially hazardous, superintendents will consider alternatives such as closing an area to visitors or relocating facilities.

2. Cause


a. What factors (geologic, climatic, human) cause a hazard to occur?

Geologic – fractures, joints, bedding planes, type of rock (shear strength, compression strength, tensile strength), chemistry (oxidation, hydration, carbonation), seismic activity (earthquakes), ground-water movement, crystal growth, topography.

Climatic – temperature (freeze and thaw cycles, thermal expansion and contraction), moisture (rain and snow, rapid variations, wave action, flooding).

Human – activity location, drainage diversion, irrigation, hillside excavation (change in slope gradient), artificial fill failure, retaining wall failure, blasting (vibration, air blast), transportation vibration (auto, truck, train, heavy equipment, etc.), increasing the load that the ground must bear, changing vegetative cover, and pumping.

Miscellaneous – root penetration, burrowing animals, forest fire and range fire changing vegetative cover.

b. How often does the hazard occur?

While people think of them as isolated, sporadic events, a major rainstorm or earthquake event can trigger tens of thousands of landslides and rockfalls over a broad area in a brief period of time.


c. What is the typical setting or circumstance in which the hazard occurs?

Steep slopes, weak or fractured rock, high moisture event, freeze and thaw cycles (springtime).

d. What are the dangers associated with the hazard?

• Loss of life and injury
• Property damage
• Blocked highways, roads, and trails.
• Broken gas and oil lines, water mains, power lines.


3. Safety


a. What can be done in case of an emergency?

• Call park incident command (rangers, etc.) in a park unit.

• Contact local emergency personnel (911).

• The National Landslide Information Center located in Golden Colorado is part of the U.S. Geological Survey and can be reached at 1-800-654-4966, web address: http://gldage.cr.usgs.gov.

• Also the various states have a State Geological Survey that can offer assistance. All of these state contacts can be found at: http://geology.er.usgs.gov/eastern/stgeomap.html
• Contact the Geologic Resources Division (303-969-2090) for assistance.

b. What can be done in terms of prevention?

• Geohazard planning prior to activity in the area.
• Engineering solutions following proper codes (e.g., remove the hazard, stabilize the hazard [rock bolts, mesh, steel, retaining walls, gabions, tension cables, grout, drains, etc.], stay away from the area if possible [avoidance], and minimize or control the rockfall run-out area with ditches, specially designed tension cable nets, talus wall, for small rock falls, proper drainage.
• Proper maintenance and inspection [visual inspection, measure movement, monitor and record falls, scaling, sounding, etc.].
• Good land use planning using appropriate geologic maps.
• Education and understanding. Interpreters work in partnership with the scientific community to ensure that complex information can be conveyed accurately, and in a form that is comprehensive and relevant to non-specialists.

4. Loss


a. What damages are associated with the hazard?

Loss of capital assets (homes, buildings, structures, transportation networks, utility interruptions to water, gas, and electric supplies), injuries, deaths, and interruption to economic activities.

b. What monetary costs are associated with the hazard?

In the United States landslide related losses are estimated to cost $1-2 billion per year with 25 to 50 associated deaths. The 1983 landslide at Thistle, Utah blocked the Spanish Fork River, formed a lake, inundated the town and cost an estimated $400 million.


5. Natural Resource Connections


a. What are the long-term effects of these natural processes?

Mother Nature’s ongoing process to shape the landscape through uplift and erosion and challenge the patience and intelligence of humanity.


b. Are any environmental benefits associated with the hazard?

• Create off-limit zones to humankind so that nature may flourish.
• Give us something to think about besides ourselves.
• Continues Nature’s recycling program.
• Habitat in talus slopes for a variety of wildlife.

6. Links about Hazard

See above and add to them from web search.

7. Parks at Risk

Every park unit with high topographic features is at risk if facilities or trails pass within the “fall zone” of geohazards. The natural process of erosion is only a risk to parks that interfere with the process.

8. Quick Facts and Trivia About Hazard

• Landslides, flows, collapses, and other forms of ground failure are a hazard that virtually all states have in common.

• In 1998 the El Nino-related landslide damage in the 10 San Francisco area counties was assessed at more than $140 million.

• Worldwide, landslides are responsible for at least 600 deaths annually.

• In the 20th century, catastrophic landslides have destroyed entire villages, killing upwards of 20,000 people at a time.

• During historical time more than 400 rock falls have been documented in Yosemite National Park.

• The eruption of Mount St. Helens in 1980 triggered a 2.8-km rockslide/debris avalanche that rushed 24 kilometers down the valley of the North Fork of the Toutle River, destroying nine highway bridges, miles of roads and highways, and many public and private buildings.

• Flash floods can erode more than 25 centimeters (5 feet) of soil in only a few hours.

• The airblast (atmospheric pressure wave) generated by the Happy Isles rockfall in 1996 in Yosemite NP created a wind comparable in velocity to that of a tornado or hurricane (110 meters/second = about 250mph) which snapped off about a thousand trees with the area.

9. Stories from Parks Threatened by Hazard

• Ranger Report 98-11 Blue Ridge Parkway (NC/VA) - Within about a 12-hour period on January 7th, portions of the parkway received between seven and ten inches of rain, resulting in two major rock and mud slides blocking the road a milepost 449 and at milepost 358, totally closing public access to Mount Mitchell State Park. Federal Highway Administration engineers have identified a massive potential rockslide at milepost 428 which poses an extreme and immediate hazard to travelers.

• Ranger Report 99-266 Yosemite NP (CA) – Rockslide with Fatality, Injuries, Evacuation: At 7:35 p.m. on Sunday, June 13th, a 525-ton rockslide occurred above Curry Village in Yosemite Valley. The release point was on the Glacier Point Apron approximately 2,000 feet above the valley. One person was killed by the rock; his two climbing companions and a bystander received minor injuries. After the slide, 1,363 guests and Yosemite Concession Services employees were evacuated from their tent cabins.

• Ranger Report 96-354 Yosemite NP (CA) – Major Slide; Multiple Fatalities, Injuries; A 200-foot portion of the sheer granite cliff at Glacier Point broke loose just after 7:00 p.m., sending a half-mile-wide section of rocks down the cliff. A portion of the nature center was buried. Much of the damage apparently was caused by the huge blast of air pushed by the rockslide.

• Ranger Report 99-271 Blue Ridge Parkway (NC/VA) – A rockslide covered both lanes of the parkway near milepost 339 around 10:30 a.m. on June 15th. It blocked traffic from passing through a five-mile section of the road. The slide is comprised primarily of a single boulder with an estimated size of 3,000 square feet and an estimated weight of 18 tons.

• Natural Resources Report NPS/NRPO/NRR-93/10 – Between 1983 and 1991 there have been five large landslides within Hagerman Fossil Beds National Monument (ID). Human use of the landscape adjacent to the fossil beds now threatens their existence. Water from irrigation projects saturates the loosely cemented fossil beds, causing decreased shear strength, increased pore water pressure, and excess weight with the sediments. Solutions being considered include additional canal lining, piping of canal water, de-watering of perched aquifers, and other possible long-term solutions.


• USGS Fact Sheet-020-98 – Popular Beach Disappears Underwater in Huge Coastal Landslide at Sleeping Bear Dunes National Lakeshore (MI) – On an unusually warm February mourning in 1995, local resident George Weeks was walking his dog along the shore in the park and was shocked to find that where there had only recently been a beautiful beach was now a steep; 100-foot drop into Lake Michigan. The 35-millions of cubic feet of sand that made up the beach and part of the high bluff above it had disappeared beneath the water of the lake in a huge coastal landslide.

• Zion National Park Rockslide on Narrows Trail – On August 1, 1968, 4000-tons of rock broke way from a vertical face of the cliff and buried the trail for 250 –feet. Rangers rescued a family trapped on the far side of the fall and noticed a 40-ton rock precariously hanging 300-ft above the trail. After failure to dislodge it with dynamite placed by rock climbers, staff called in the Marines who use a 3.5 anti-tank rocket (four shots) to knock the hazard from the cliff. A dangerous situation was handled by a method new to the Park Service in such a way that all publicity was totally lacking.

• Lake Mead National Recreation Area Rock Fall – On August 5, 2000, a landslide from a cliff along the shoreline of Lake Mohave resulted in numerous injuries to 5 people. These injuries included broken ribs, broken femur, broken back, broken wrist, broken knuckle, lacerations and bruises. Luckily nobody was on the beach where the cliff fell, although chairs, coolers, etc. were buried on the beach by the fallen material. A number of people in Bullhead and at Katherine reported an earthquake the night before this incident.

• Ranger Report 00-317 Follow-up on Rockfall Carlsbad Caverns NP (NM) – Rangers discovered a significant rock fall in the vicinity of Iceberg Rock on the morning of June 26th. Enough material – about three cubic yards of rock – had fallen to almost entirely block the trail to visitors and it was accordingly closed. Removal of the rock is now underway. (It was later decided that the fallen rock had been hand-placed during blasting of the trail in the 1930’s by the Civilian Conservation Corps.)

• Ranger Report 00-645 – Scotts Bluff NM (NE) Rockslide – For the third time in three years, a major rockslide has closed the Saddle Rock Trail. The recent wet snow and rain that fell on the park evidently contributed to the rockslide, which occurred on the evenings of October 6th and 7th. A large rock about the size of four large dump trucks fell at a pivotal point on the trail; several tons of rock [later estimated at 1000-1500 tons] and sand also fell and blocked the trail about 700 feet above the plain. The large rock is extremely unstable, which will make clearing the slide very difficult. The location is inaccessible to large equipment. Blasting is only a limited option due to the risk of causing serious additional slides.

• GRD Trip Report Rock Stability at South Kaibab Trailhead GRCA (AZ) – The extreme risk to hiker health and safety, and high probability of failure occurring at anytime necessitate immediate action to secure the area and remove the threat from the 380-ton geohazard in a time-controlled manner. The upside-down “L” shaped rock with prominent overhang and smaller undercut, coupled with weathered jointing creates the perfect mass wasting environment and is located above 8 switchbacks of trail. Field observation showed that the adjacent 40-foot of cliff face had already failed and fallen in the recent past (estimated to be 100-400 years). The trail was closed and the geohazard removed by explosives.


 

Search | Site Map | Email | Privacy/Disclaimer


Go to Nature Net Home Page . Go to Park Net Home Page

Last Updated: February, 3, 2003
National Park Service, Geologic Resources Division
http://www.nature.nps.gov/grd/index.htm