Rito Hondo Dam, owned and operated by Colorado Parks and Wildlife (CPW), is an embankment dam originally constructed in 1957 with a crest elevation of 10,252 feet. Multiple repairs to the dam were completed during the post-construction era, with an outlet rehabilitation project executed in the summer and fall of 2019. During and after refilling the reservoir to the normal operating level in 2020, the dam exhibited widespread, unusual seepage on the abutments and downstream slope. The 2020 seepage incident led to draining of the reservoir and a zero-storage restriction being placed on the dam.
CPW contracted with AECOM to complete a geotechnical investigation of the dam, abutments, and reservoir rim. AECOM also performed supporting studies including flood hydrology in mountainous terrain, semi-quantitative risk analysis, alternatives evaluation for the seepage remediation and spillway rehabilitation, final design of the selected alternative, and on-site engineering support during construction. The ultimate goal of the project was to restore the safe operating condition of the dam by providing seepage remediation at the embankment and abutments and providing a new spillway which is capable of conveying the inflow design flood. The project was contracted through the construction manager/general contractor (CM/GC) method following the 30% design phase. AECOM worked alongside CPW and Zak Dirt (the selected CM/GC contractor) in partnership to prepare the construction documents.
This paper will present the alternatives considered, the geotechnical/geologic properties of the embankment and foundation, the selected alternative for seepage remediation (a combination of soil bentonite cut-off walls and a downstream filter buttress), the spillway replacement, the advantages of designing through the CM/GC method, and design and construction considerations for dams at high altitude. The paper will also present a few challenges faced during the design and construction phases and how those challenges were managed by the collaborative group.
