THE AUGUST 8, 2019, DEBRIS FLOWS IN LITTLE COTTONWOOD CANYON AND DAMAGE TO STATE ROUTE 210, SALT LAKE COUNTY, UTAH

On the evening of August 8, 2019, intense thunderstorm rainfall triggered debris flows and floods that prompted closure of State Route 210 (SR-210) in Little Cottonwood Canyon. Debris flows deposited sediment up to 15 feet deep on the roadway. Sediment from both debris flows and flood waters stranded 35 vehicles along a 4-mile length of highway. Visitors were also stranded in the upper canyon at Snowbird Resort and the Town of Alta. In addition to highway and culvert damage, erosion exposed subsurface utilities adjacent to the highway. A “State of Emergency” was declared by the Governor of Utah to assist with rescue, cleanup, recovery, and repair efforts that involved numerous state, county, and city agencies.

Little Cottonwood Canyon is a U-shaped glacially-carved canyon and SR-210 crosses several steep, active alluvial fans that lie below steep bedrock drainage basins. The Lisa Falls and Tanner Gulch drainage basins produced the largest volume debris flows and have relief ratios of 0.49 and 0.46, respectively. The alluvial fans below the drainage basins have steep gradients to the highway: 13º at Lisa Falls and 11º at Tanner Gulch. Both alluvial fans have deeply incised channels that maintain debris-flow velocity and convey sediment directly to the highway, causing culverts to plug and sediment to be deposited on the roadway. We used a small unmanned aerial system (sUAS) at the Tanner Gulch alluvial fan to collect high-resolution imagery to create a digital elevation model (DEM) and orthomosaic image of the deposit. The DEM was topographically differenced with a 2006 lidar-derived DEM yielding an estimated deposit volume of 15,800 cubic yards. The sUAS was also used for reconnaissance of other smaller volume debris-flow deposits.

The 2019 event illustrates how vulnerable SR-210 is to rainfall-triggered debris flows. Field observations indicate that ample, readily-erodible sediment remains in the Lisa Falls and Tanner Gulch drainage basin channels to potentially form future debris flows of similar magnitude as the 2019 flows. These steep, high-relief bedrock drainage basins create optimal conditions for rapid rainfall runoff to erode and entrain channel sediment forming debris flows and posing a significant debris-flow risk to SR-210.