Construction will expand the existing reservoir from its current capacity of approximately 6,400 acre-feet to an enlarged capacity of roughly 14,600 acre-feet, providing Fort Collins Utilities with approximately 8,200 acre-feet of additional water storage. Achieving this capacity will create roughly 138 acres of new inundation at the reservoir.

Utilities has contracted with AECOM as the engineering consultant for the future work on the Halligan Dam. 

The existing Halligan Dam is a concrete arch dam that was constructed in 1909. After extensive research and analyses in and around the existing dam, Fort Collins Utilities is pursuing building a replacement dam immediately downstream of and adjacent to the current dam to provide the additional water storage needed for our water supply. 

The preliminary design efforts revealed that two viable dam concepts exist for enlarging Halligan Reservoir:

1. Initial Dam Concept: Previously, we had planned to rehabilitate the existing dam and raise it by 25 feet. Upon further research, the existing dam would need to be significantly modified to meet current regulations and hold up to the increased scrutiny given to dam design following the 2017 Oroville Dam spillway failure. Modifying the more than 110-year-old dam will present unique challenges during design and construction. 
2. Adjacent Dam Concept: An additional dam concept has been identified, which would entail building a replacement dam immediately downstream of and adjacent to the current dam. Utilities is moving forward with this concept.

The two dam concepts were similar in cost given the information available at the early stage of design. The environmental impacts of the two dam concepts were also similar.

Preliminary analyses indicated that the Adjacent Dam Concept could provide several benefits over the Initial Dam Concept, including:

• Easier incorporation of modern construction techniques and materials.
• Improved long-term functionality and increased efficiency for operations.
• Improved maintenance access and possible reduction in long-term maintenance costs.
• Reduced construction risk by:
  • Allowing better management of water and sediment during construction.
  • Reducing uncertainties and design/construction challenges related to enlarging a dam more than 110 years old.
• Reduced impacts to reservoir operations during construction.

Moving forward with the Adjacent Dam Concept will ensure we have the best project not only for Utilities customers but also for the environment.