1915 Failure of Lyman Dam Upstream of St. Johns in Apache County, Arizona

Abstract Only - The Lyman Dam stores up to 30,600 acre-feet of water for irrigation use on the upper mainstream of the Little Colorado River about 11 miles upstream from the City of St. Johns in Apache County, Arizona. The original Lyman Dam was designed in 1910 by the Hinderlider Engineering Company and constructed between the years 1911 and 1913 under supervision of the irrigation company. The 800 foot-long, 65 foot-high dam was built as a homogenous embankment of clayey materials borrowed from the reservoir area, without foundation cutoffs or drainage. The downstream slope was on the order of 2:1 (H:V) and the upstream slope nearly 2.2:1. The crest width was approximately 10 feet and the base width of the dam about 270 feet. Lyman Dam is founded on alluvial deposits consisting of fine montmorillonite-rich soft clays derived from weathering of the “Chinle” geologic formation, underlain by interbedded sand and gravel deposits. The dam impounded a depth of water of about 35 feet for nearly two years before filling rapidly to a depth of nearly 60 feet in the spring of 1915. Lyman Dam failed suddenly at 10:45 pm on April 14, 1915, due to piping of the embankment near its maximum section. Observers reported, “The first evidence of the break was a sudden rush of water coming out at the base of the dike in the center of the channel, which was instantly followed by a volume of water rising to a depth of 30 feet on the lower side of the dam. The water immediately cut out a section forming an arch with an estimated span of 75 feet, which collapsed as cutting increased, leaving a gap 350 feet in length.” The failure claimed the lives of two women and six young children, and resulted in $500,000 of property damage (unadjusted for inflation). Post-failure investigations suggest the nearly instantaneous failure of this dam having a base width four times the reservoir head, was due to piping initiated by reservoir pressures being transmitted to the downstream toe area through coarse deposits in the foundation and rapid erosion of the soft clay soils. The presence of soft soils in the dam foundation are further supported by investigations performed during the 1916-1922 reconstruction, repairs made in 1931 and 1948, and the 1999-2001 major rehabilitation to meet current dam safety requirements. Lyman Dam emphasizes the importance of (1) detailed geotechnical investigations when building dams on foundations of complex alluvial deposits and (2) learning lessons from investigations of locations where previous dam failures have occurred.