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Halligan Water Supply Project

Preliminary Design and Construction#

Overview#

Halligan Dam

Once permitted, construction on the dam could take up to four years.

Construction will expand the existing reservoir from its current capacity of approximately 6,400 acre-feet to an enlarged capacity of over 14,500 acre-feet, providing Fort Collins Utilities with approximately 8,200 acre-feet of additional water storage. Achieving this capacity will create roughly 130 acres of new inundation at the reservoir.

We have contracted with AECOM as our engineering consultant and Dragados-Pulice as our construction contractor for the future work on the Halligan dam. 

Design Concept#

Workers extracting cores from the existing Halligan dam.

Extracting cores from the existing Halligan dam.

The existing Halligan Dam is a concrete arch dam that was constructed in 1909. The current design concept for the enlargement of Halligan Reservoir is to increase the height of the existing dam by 25 feet to provide additional water storage needed for the resiliency of future Fort Collins Utilities’ water supply. Preliminary design work began in 2020 and included several analyses in and around the existing dam.

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

  1. Existing Dam Concept: 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 was identified, which would entail building a replacement dam immediately downstream of and adjacent to the current dam.

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

Preliminary analyses indicate that the Adjacent Dam Concept could provide several benefits over the Existing 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.

To ensure we have the best project not only for Fort Collins Utilities customers but also for the environment, staff are performing due diligence activities to evaluate the Adjacent Dam Concept for feasibility, effectiveness, and environmental impacts. More information will be shared as additional data are gathered and a dam concept is chosen.

Construction Mitigation#

The reservoir will be maintained at a low elevation during the construction period. Fort Collins Utilities will use best practices to minimize disturbances during construction and is developing a Conceptual Mitigation Plan that will describe general mitigation approaches that will be used to offset temporary construction impacts.

View Draft Conceptual Mitigation Plan

Future Dam Rehabilitation#

The current dam undergoes routine inspections and is structurally sound, but it is more than 110 years old and will need repairs in the coming years regardless of if the Halligan Water Supply Project is approved. If the project is not approved, the burden of repairing the dam will fall to the North Poudre Irrigation Company (NPIC) and subsequently its shareholders, which includes the City of Fort Collins.