Spinney Mountain Dam Seepage Mitigation Project How Aurora Water of the City of Aurora, Colorado Evaluated and Dealt with a Persistent Seepage Issue

Spinney Mountain Reservoir is a key storage/supply element for the City of Aurora, Colorado and was constructed in the early 1980’s for Aurora Water. Upon first filling, seepage was noted near the downstream of the dam near the left abutment and had been visually qualitatively monitored for almost 4 decades. In October 2016, the Colorado State Engineers Office recommended the dam be modified so that seepage from the seep could be collected and monitored quantitatively. Aurora Water contracted with HDR Engineering (HDR) to evaluate the source of the seepage and design a toe drain seepage collection system. HDR’s evaluation of the embankment, foundation and the seepage area determined that multiple plausible seepage paths could be responsible for the seep, but that a seepage collection system should be capable of intercepting and collecting the seepage regardless of the path of seepage through or below the embankment, if constructed at the embankment toe. Construction of the toe drain was performed in the fall of 2018 and maximum discharge flows from the seep on the order of about 2.5 gpm have been measured which is within the estimated design flow rate of 1.5 gpm to 3.5 gpm.