Above average snowpack across Colorado in 2018–19 led to an unprecedented avalanche season, with the concomitant deposition of debris in streams creating flooding hazards across the state. This was especially true for Lake City in Hinsdale County, sitting at the confluence of Henson Creek and the Lake Fork of the Gunnison River, which together had more than 30 avalanches upstream of town. Compounding the threat to Lake City is the presence of two historic, unmaintained dams on Henson Creek built in the late 1800s. While both dams had been partially breached to ease flooding concerns in the past, emergency managers were concerned that the sheer quantity of debris in the streams could plug the breaches in each dam, filling the historic reservoirs with snowmelt.
The BLM National Operations Center (NOC) assisted the Gunnison Field Office and the State of Colorado with emergency mapping and analysis to 1) quantify the capacity of the upstream reservoirs should the historic dams become clogged, 2) document structures that would be threatened by flooding, and 3) provide timely catchment-wide imagery to monitor snowmelt. Given the need for both ultra-fine resolution and broad-scale remotely sensed products, the NOC leveraged unmanned aircraft systems (UAS) and multiple satellite imaging systems to accomplish these objectives.
Within 2 weeks of initial contact, UAS pilots from the NOC and the U.S. Geological Survey National UAS Office traveled to Lake City, conducting nearly 20 imagery and video acquisition flights over 2 days. By week’s end, the NOC team created a 2.5-centimeter resolution basemap and estimate of reservoir extent and capacity. Additional processing generated aesthetically pleasing three-dimensional models of important structures that will serve as a historical record should they be damaged by flooding. Sentinel-2 and WorldView-2 and -3 satellite imagery were used to provide multi-scale, multi-temporal environmental monitoring of the greater area. Taken together, these data allowed the BLM field office staff and state emergency managers to make more-informed decisions regarding mitigation efforts.
(1) Textured mesh of Ute Ulay Dam, (2) point cloud of water tower, and (3) aerial extent of historic reservoir.