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 was constructed to provide water for local farming communities in the arid plains of southeastern Colorado. It was a zoned earthfill (embankment) dam with a height of 115 feet and a normal storage capacity of more than 18,500 acre-feet at the spillway crest. The dam failed on the afternoon of August 22, 1923, when the reservoir reached a level two feet below the spillway crest and 12 feet below the dam crest. While no lives were lost, the resulting flood caused extensive damage downstream and limited water storage still impacts the area today. The dam experienced significant transverse cracking near each abutment during an initial partial filling of the reservoir in August of 1921. Attempts were made to repair the initial transverse cracks by sluicing material into each crack from the upstream slope of the dam. During the first full reservoir filling that resulted from a large storm event in the catchment basin above the dam in August of 1923, the embankment experienced an unusual sub-horizontal crack extending across the embankment from the right to the left abutment. Flow entering this crack at the reservoir surface on the upstream face near the right abutment and discharging from the downstream face near the lower left abutment caused a relatively rapid dam breach to develop. Remnants of the dam and spillway remain in place; the authors made a visit to the site, collected files from the state engineer’s office and the Engineering News Record (ENR), and coordinated with local historians. This paper reviews the state of practice for design and construction of embankment dams at the time Apishapa Dam was built; previous forensic investigations; discusses its place in the timeline of similar failure modes, construction materials and methods, lessons learned; and provides a look at consequences from both the floodwave and loss of water resources for the local farming community.