#Sciencedeskdigs: Rockfall Fragments

small rock fragments on desk with yellow helmet in background

NPS Photo/ M.Reed

It's not easy to predict landslides and rockfalls, but they can be terribly dangerous and cause a lot of damage! That's why geomorphologists like Eric Bilderback use every available tool to be as accurate as possible.

What are these?
These are pieces of sedimentary rock debris from different rockfalls at national parks. Assessing rockfalls, landslides, or any sort of geologic risk or hazard in parks is one part of my job. For example, this winter in Zion National Park there was a rockfall that hit the Zion Lodge and blocked the employee walkway. After something like this happens, we need to ask a few questions to assess the future risk: What caused the rockfall? Is it over? Is there potential for a more destructive event? What would be the risk if no mitigation is done?

The NPS policy is to allow natural processes (like rockfalls and landslides) to proceed, but we’re also concerned with visitor safety. To assess risk, I make models of slopes in parks. I do that by taking measurements of the slope using Light Detection and Ranging (LIDAR), which is a remote sensing method used to examine the surface of the Earth. From the LIDAR data, I can make a topographical map to get a slope profile, which I then use to create a model of the slope.

Once I have all the information in the modeling software I can virtually roll boulders and rocks of various sizes and weights down the slope and calculate what size of boulder would cause the most damage. Then I take this information back to the real slope. If there are unstable boulders of a destructive size on the real slope then mitigation should be considered. Luckily, in the case of Zion Lodge, a rock that could have been destructive hit a tree on its way down the slope, slowing its velocity and preventing it from putting a hole in the wall. My job is fulfilling in that I can assist parks in understanding natural processes and, in some cases, risks from natural processes.
large boulder next to Zion lodge with facility men standing by it
The largest boulder from the rockfall that hit Zion Lodge in Zion National Park

NPS Photo/ M.Reed