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Colorado. Case Study. PL566 Remedial Assistance Project. Frank Riggle Colorado Assistant State Conservationist Water Resources. The Dam. Built with PL-566 in 1971 Flood Control Crest to reservoir: 30’ Structural Height: 70’ Structure length: 490’. Holes. Intelligent Rodents???.

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Frank RiggleColorado Assistant State ConservationistWater Resources

the dam
The Dam
  • Built with PL-566 in 1971
  • Flood Control
  • Crest to reservoir: 30’
  • Structural Height: 70’
  • Structure length: 490’
accelerating problem
Accelerating Problem
  • Holes along crest increased in the past 4-5 years
  • Colorado State Engineer’s inspection started in 1991
  • NRCS State Engineer’s review mid 1990s
  • Northern Plains Engineering Team Review 1/2004
breach order
Because of the worsening condition of the dam and a concern with dam safety:

The Colorado State Engineer issued the sponsors a breach order in March 2004

Breach Order
  • Met with sponsors and State Engineer
  • Discussed needs and strategy
  • Initiated Formal Engineering Investigation
why is the dam cracking
Why is the dam cracking?
  • Extended drought?
  • Poor operation and maintenance?
  • Poor construction?
  • Engineering and design oversight?


  • Investigation Engineering Team included members from Colorado, Texas, and Nebraska
    • Site Visit - Visual Examination
    • Excavated Exploration Trenches
    • Interviews with NRCS State Engineer, former NRCS District Conservationist, and Colorado State Dam Safety Engineer about involvement during construction
    • Examination of construction drawings, job diaries, soil engineering report, geology report, drill logs, profile and cross section drawings,

½” – 3” diameter holes became evident in early 1980’s, and are considerably larger today

View of embankment with sink hole in foreground, shows developing pattern of holes along crest.


Site Survey/Excavation

  • Holes, cavities, and cracks of significant size observed
  • Located in a line along the crest
  • Lined up in three distinct areas perpendicular to axis of dam
  • Lines of holes were on either side of principal spillway, and the third close to auxiliary spillway
  • Virtually all holes located on upstream half of embankment

Test pit dug over hole shows crack at depth

  • Bucket width approximately two-feet
  • Yellow soil is paint used to reveal crack while digging.
emergency action plan
Emergency Action Plan
  • Breach Inundation Study
  • NOAA
  • Monitor precipitation
  • Monitor dam when rainfall >2”
  • Reverse 9-1-1
  • Brief County and City Emergency Staff

Engineer’s Report

  • Cause of cracking/cavities differential settlement of upstream embankment and at the steep abutments and principal spillway along upstream edge of cutoff trench
  • Holes and cavities are surface expressions of cracks in compacted earthfill
  • Transverse cracks represent the most serious concern regarding safety of dam
  • Responsibility for problem can be attributed primarily oversights and/or misjudgments during design, and construction of original embankment
  • Underestimating collapse potential of sandy alluvium
    • Decision to locate cutoff trench on dam centerline instead of near the upstream toe
current activities
Current Activities
  • Draft environmental assessment
  • Interagency review
  • Copies of draft to affected landowners
  • Final draft designs to Fort Worth for review