CS1B - Concurrent Session 01B: 100th anniversary of the 1923 Apishapa Dam failure, Colorado: an analysis of an underappreciated embankment dam settlement/cracking case history and the local impacts of a lost resource

Apishapa Dam failed on the afternoon of August 22, 1923, during first filling. The earthen embankment was 115 feet tall and failed with the reservoir two feet below the spillway crest, and 13 feet below the dam crest. Owned by Apishapa Consolidated Irrigation Company, the dam was being constructed to provide water for local farming communities in the arid plains of southeastern Colorado. While no lives were lost, the resulting flood caused extensive damage downstream and limited water storage still impacts the area today.

The dam was constructed with local and unsuitable fill material, light sandy soils that were poorly compacted, placed by wagons in one foot layers, sprinkled and rolled with a smooth concrete roller. Leaks attributed to settlement cracks had been observed during construction. The reservoir filled quickly due to thunderstorms in the headwater region of the 700 sq mile basin, without excessive rains observed at the dam. The dam failure released about 18,000 acre-feet along Apishapa Creek, then east along the Arkansas River, observed as far as La Junta, Colorado, about 40 miles downstream.

Remnants of the dam and spillway remain in place today; the authors made a visit to the site, collected files from the state engineer’s office and ENR, and coordinated with local historians. This paper/presentation will review previous forensic investigations and the postulated settlement/cracking mechanism and concentrated leak internal erosion failure mode; discuss its place in the timeline of similar failure modes, construction materials and methods, lessons learned and advancement of design and construction technology during the early to mid-1900s; and provide a qualitative look at consequences from both the floodwave and loss of water resources for the local farming community.