PL566 Remedial Assistance ProjectColorado

Case Study

Frank RiggleColorado Assistant State ConservationistWater Resources

The Dam

Built with PL-566 in 1971

Flood Control Crest to reservoir:

30’ Structural Height:

70’ Structure length:

490’

Holes

Intelligent Rodents???

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

OPTIONS ???

Met with sponsors and State Engineer

Discussed needs and strategy

Initiated Formal Engineering Investigation

Why is the dam cracking?

Extended drought? Poor operation and

maintenance? Poor construction? Engineering and

design oversight?

Investigation 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

Breach Inundation Study NOAA Monitor precipitation Monitor dam when rainfall >2” Reverse 9-1-1 Brief County and City Emergency Staff

How Many Engineers Does it Take……??

EMBANKMENT AS RESERVOIR FILLED

Horizontal Crack

Transverse Cracks

Transverse Crack

Transverse Crack

The filter has not been contaminated with piped soil

material.

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

RESTORED EMBANKMENT

Current Activities

Draft environmental assessment Interagency review Copies of draft to affected landowners Final draft designs to Fort Worth for review

Questions ???