Cornforth Consultants
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Iron Gate Dam Crest Raise​

Hornbrook, California
Iron Gate Dam is one of five hydroelectric dams operated by PacifiCorp on the Klamath River in Northern California. The dam is a zoned earth and rockfill embankment 189 feet high, with a crest length of 740 feet. The spillway is located in bedrock adjacent to the right abutment of the dam and contains near-vertical rock cuts up to 120 feet high.

FERC Part 12 study concluded that the existing dam would not pass the revised Probable Maximum Flood (PMF) without being overtopped. PacifiCorp initially considered enlarging the existing spillway to pass the PMF flow without overtopping the dam crest. Cornforth Consultants (CCI) was retained by PacifiCorp to evaluate the potential impacts of the PMF level on the dam crest. CCI determined that the impermeable core would be resistant to erosion and the crest would remain stable during the PMF.

​Initial cost estimates indicated that enlarging the spillway to accommodate the PMF would be very expensive. As an alternative, CCI suggested that raising the dam crest to retain the PMF would be less costly than enlarging the spillway. At PacifiCorp’s request, the firm developed several dam raise alternatives. A cantilever sheetpile wall along the crest was selected as the most cost effective option. The abutments were raised using concrete stem walls and embankment fill. Hydraulic scale modeling performed by others provided estimates of flow velocity and elevations within the spillway during the PMF. The spillway was modified with fiber-reinforced shotcrete to protect the weak, potentially erodible rock in the spillway wall. In addition, low areas along the spillway were raised using conventional concrete cantilever and gravity retaining walls. CCI prepared technical plans and specifications, an engineer’s cost estimate, and assisted PacifiCorp with construction control, including vertical tiedown anchors bonded in bedrock.
Installation of Rock Bolts above the Flume
Iron Gate Dam, Spillway, and Outlet Works

Services Provided:

  • Site Reconnaissance
  • Dam Raises and Modifications
  • Spillway Design and Inspection
  • Home
  • Services
    • Earth & Rockfill Dams
    • Reservoirs, Canals, and Levees
    • Spillway Safety Inspections
    • Landslides and Rockfall
    • Earthquake Engineering
    • Geologic Hazards
    • Geotechnical Site Characterization
    • Hydrogeology
    • Expert Witness
    • Concrete Dams and Locks
    • Geotechnical Instrumentation
    • Deep and Shallow Foundations
    • Geotechnical Manuals
  • Industries
    • Water Resources
    • Energy
    • Natural Resources
    • Industrial
    • Transportation
    • Ports and Airports
    • Expert Witness
  • Projects
    • Earth and Rockfill Dams >
      • Barney Reservoir
      • Frog Lake Reservoir
      • Iron Gate Dam Raise
      • McGuire Dam Raise
      • Bonneville Cofferdam
      • Bull Run No. 2 Seismic Stability Analysis
    • Levees and Canals >
      • Libby Dike Rehab.
      • St. Johns Dike Stabilization
      • Cowlitz River Levees
      • Swift Canal Failure
    • Earthquake Engineering >
      • Barney Dam Seismic Analysis
      • Fern Hill WTP Liquefacation Study
      • Bull Run No. 2 Seismic Stability Analysis
      • Mt. Tabor Seismic Stability Analysis
      • Port Westward Power Plant Seismic Design
    • Rock Slopes and Tunnels >
      • Libby Rockfall Mitigation
      • Sandy River Pipeline Tunnels
      • Ketchikan Third Avenue Extension
      • Sain Creek Water Supply Tunnel
      • Montana Rockfall Hazard Rating System
    • Foundations >
      • Port Westward Power Plant
      • Cascade Locks Casino
      • Air National Guard Hangar
      • Biglow Canyon Windfarm
      • Oakridge Water System Improvements
    • Concrete Dams and Locks >
      • John Day Lock & Dam
      • Marmot Dam Removal
      • The Dalles Dam
      • Willamette Falls Dam
      • Bonneville Dam
    • Transportation >
      • OTIA Cracked Bridge Geotechnical Baselines
      • Newburg Dundee Bypass Design DEIS
      • I-84 Sandy River Bridge Investigation
      • Tanana River Bridge Foundation, Tok, Alaska
      • Montana RHRS Implementation
      • Ketchikan Third Avenue Extension
    • Ports and Airports >
      • Navy CVN Drydock
      • Depoe Bay Seawall Distress
      • Willamette Falls Flow Control Structure
      • Air National Guard Hangar
    • Water and Wastewater >
      • Fern Hill Water Treatment Plant
      • Tualatin Valley North Transmission Line
      • Evaluation of 28 Water Storage Facilities
      • City of Sandy Water Treatment Plant
      • Mt. Tabor Reservoir No. 5 Cap Study
      • Oakridge Water System Improvements
  • Staff
  • Resources
  • Careers
    • Geotechnical Engineer
  • Contact Us