Construction Overview

Spillway Design

The Municipal Subdistrict is seeking a construction contractor for a major civil works project to build the new Chimney Hollow Dam and reservoir appurtenances located in Larimer County. The reservoir is the primary component of the Windy Gap Firming Project. Although the design is not complete, we recognize that this will be a large, complex project. The estimated construction period is four years and the estimated construction cost is approximately $425 million. Primary components for Chimney Hollow Dam and Reservoir include:

  • Chimney Hollow Dam will be a rockfill structurewith a hydraulic asphaltic concrete (HAC) core, approximately 350 feet tall.  The main dam will contain approximately 12 million cubic yards of rockfill and 75,000 cubic yards of asphaltic concrete. Rockfill and aggregates will be sourced from within the reservoir footprint. A two-row grout curtain and consolidation grouting will be required for foundation preparation.
  • A combined reservoir inlet and outlet consisting of a submerged single-level inlet, a 2,000-foot tunnel under the right (east) abutment of the dam with a 72-inch diameter steel-lined, concrete-encased conduit with access to the centrally located valve chamber from downstream.
  • A-5,000 ft long concrete spillway on the left (west) abutment with stilling basin to convey a peak discharge of about 850 cubic feet per second peak. The spillway contains approximately 7,000 cubic yards of concrete.
  • A 36-foot- high saddle dam to close the southern end of the reservoir. It will be a clay core rockfill dam.  The saddle dam contains approximately 100,000 cubic yards of rockfill and 30,000 cubic yards of clay.
  • Water conveyance facilities include new buried pipelines connecting the Chimney Hollow Reservoir's inlet/outlet to the Bald Mountain Tunnel and Carter Lake Pressure Conduit (approximately 7,500 feet of 72-inch steel pipe) and a valve house consisting of energy dissipation facilities to control flow and pressure into and out of the reservoir.