Hazard News: An Update on Utah’s Very Active Spring 2023 Landslide Season

by Greg McDonald and Ben Erickson

Utah’s 2023 spring landslide season was the most active since 2011, which the Utah Geological Survey (UGS) geo- logic hazards team anticipated given the record snowpack of the winter of 2022–23 across the state. Fortunately, landslide and flood-related damage was localized, partly due to infrastructure and development improvements that are increasingly being put in place as awareness of geologic hazards grows. Notable landslide damage included a house that was impacted by a landslide but was not structurally damaged, a hotel that was damaged by landslide debris breaking through part of a back wall, a cabin impacted by a debris flow, a shed that was demolished, two condominium units with damaged decks, and several roadways that were either covered by debris or were cracked/displaced by ground movement.

Aerial photo of a semi-mountainous landscape with pine trees with a landslide in the center that initiated as a debris slide and mobilized into a debris flow traveling about 1,200 feet.

Landslide in Duchesne County that initiated as a debris slide and mobilized into a debris flow traveling about 1,200 feet. Photo taken May 30, 2023.

Much of the UGS Geologic Hazards Program’s response included fulfilling requests for landslide evaluations from county and city officials and emergency managers in addition to field checking known landslides, which we visit every spring to document their yearly activity. All spring 2023 landslides were a result of elevated groundwater levels from melting of the record snowpack. Interestingly, many of the landslides we monitor annually, and that have been active in past wet years, showed no significant movement this year. Whether the lack of activity is a result of several previous years of drought, a transient or delayed response to groundwater recharge, or other factors is not clear at this time. Another factor may be that this year’s snowmelt occurred at a relatively moderate rate, in contrast to 1983 when temperatures rose dramatically causing rapid snowmelt that resulted in numerous debris slides and flows.

Our team documented several different types of landslides having a wide range of sizes, from small debris slides to debris flows with several hundred-foot runouts, to large complexes having partial or global reactivation. One of the first landslides we responded to was a relatively small earth slide in Fruit Heights that occurred on April 8th. The landslide was about 15 feet wide with a 200-foot runout and occurred on a scarp formed by the Wasatch fault displacing Lake Bonneville sands and silts. Although considered small, the landslide impacted a shed, injuring two occupants. A somewhat unusual landslide in downtown Park City resulted in damage to a several-story hotel. The base of the slope had been excavated, exposing in-place bedrock composed of Permian-age (about 250 to 300 million years old) Park City and Phosphoria Formations. The slide initiated as a bedrock dip-slope failure that moved a relatively large, intact block of rock nearly 10 feet high along with loose soil and sediment that impacted the hotel. One of the larger landslides we documented occurred in a remote part of Duchesne County. The landslide involved the partial reactivation of a landslide associated with the Eocene-age (about 34 to 56 million years old) Green River Formation. Our field observations indicated the landslide began as a debris slide from a roughly 450-foot-wide section of snowmelt-saturated soils that quickly mobilized into a debris flow having a runout of over 1,200 feet. A human-caused wildfire, the Church Camp fire, burned the forested area in 2013 and numerous fire-killed trees were incorporated into the debris flow.

Several of the landslides we documented this year are new slides that occurred in landslide-prone geologic materials and/or on slopes that have been modified. Some of the larger, damaging landslides involved landslide-prone slopes that had toes excavated or grading that did not account for proper drainage. We also documented numerous roadcut failures across the state. Many were relatively shallow, leaving thin debris deposits covering roads.

An aerial 3D model of a landslide in a neighborhood.

Image from a 3D model created using UAV-acquired photos of a landslide in the Cove Canyon subdivision, northwest of Park City. Photo taken May 8, 2023.

The UGS uses several different modern tools and techniques to monitor and gather data on landslides. To collect data on-site, including field observations and photos, we use a GIS (geographic information system) app on a cell phone or tablet. This method allows us to synchronize data to a working project, usually on the same day a landslide is visited, allowing other UGS geologists to see where landslides have been recently investigated and view information for any particular site. The data are then uploaded to the UGS website and an archive, making the information accessible to the public within a few days following an event (view the collection here). Another tool we use that has been extremely useful is a UAV (unmanned aerial vehicle, commonly called a “drone”). UGS geologists can deploy a UAV within minutes to assess the greater landslide area and collect videos and photos for documentation in a safe and effective manner. A UAV does not negate the need for on-the-ground field observations, but rather helps guide site visits and enhance our data collection capabilities. For example, images obtained from a UAV flight can be used to generate 3D models using structure-from-motion techniques to compare to models made from earlier years or to lidar (light detection and ranging) data to quantify landslide movement.

Investigating and documenting landslide activity, especially for wet, active years, plays an important part in better understanding landslide hazards in Utah. This information, in part, is used to determine and map landslide hazards and susceptibilities as part of the UGS’s hazards mapping initiative. Identifying and mapping geologic hazards is important for land-use planning and management and enables safer and more cost-effective development.

For more information and updates on the spring 2023 landslides, visit our Landslide Current Events page!