Neither the Evergreen Dam nor Genesee Dam No. 2 is remotely capable of failing under the pressure of the kind of storm that happens only once in a hundred years, according to state dam safety engineers.
The Friends of Bear Creek, a local watershed protection group, organized a meeting Nov. 29 at the Morrison Town Hall to address members’ concerns about the safety of the new Genessee Dam No. 2, which went into service in September.
Experts also answered questions about the 79-year-old Evergreen Dam. Both dams accept and store water that flows from the mountains down Bear Creek, the main water source for Evergreen and a source for other Front Range cities.
Both dams are classified in the state’s dam safety program as high-hazard dams, meaning if they failed, there would be loss of human life, said Jack Byers, deputy state engineer.
Byers assured the audience of about 15 people that dams are built to survive 100-year storms and have relief valves, or spillways as well as drains, that allow excess water to be unloaded safely during a catastrophic event.
All dams are regularly inspected. The Genesee Dam is inspected weekly by the owner of the dam, Genesee Water and Sanitation District, as well as monthly by the design engineer and at least annually by the state’s Dam Safety Branch.
In an emergency, a dam owner initiates the emergency action plan and contacts the Office of the State Engineer. If failure is imminent, the sheriff is contacted first.
Water flowing over the dam top is not necessarily dangerous because dams are designed to spill a certain amount of water. The overflow that one can see flowing over the top daily at the Evergreen dam is in fact the spillway and is designed for that purpose.
When overflow is out of control, it becomes “overtopping,” which means the flood flow exceeds the capacity of the emergency spillway and the outflow washes over the crest of the dam.
Overtopping is more serious on an earthen embankment dam than a structural concrete or roller compacted concrete dam, which are designed to be stable when overtopped. Both the Evergreen and Genesee dams are concrete structures.
The Genesee Dam is engineered to be able to spill more water than would flow into it in a 100-year storm or greater.
In a catastrophic rain event, Genesee Dam can discharge as much as 5,000 cubic feet per second compared to the maximum discharge expected, which is 3,698 cubic feet per second. Maximum probable precipitation would be much greater than a 100-year event, said P. Paul Perri, design review engineer, Dam Safety Program.
“That is well beyond the size of any historical flood,” Byers said. “I would have no problem building my house at the bottom. The construction was thoroughly evaluated, inspected and monitored.”
The Evergreen Water District, which owns the Evergreen Dam, inspects the structure periodically. The state inspects the Evergreen dam once a year.
“If we see any problem at all, the dam will be brought down and released. We aren’t aware of any issues,” Byers said.
“We have highly trained individuals looking at dams on a daily basis,” Byers said.
To put things in perspective, the last significant dam that failed in Colorado was the Lawn Lake earthen embankment dam in Rocky Mountain National Park.
That dam failed July 15, 1982, due to internal erosion of the embankment materials (known as piping), which is not a factor in concrete dams.
The Genesee Dam is a roller-compacted concrete, or RCC, dam. Such a dam has never been known to fail in the state’s history, Byers said.
A lot of people are under the impression that the 1976 flood of the Big Thompson River was due to dam failure, when in fact it was a result of a rainstorm, as was the flooding of West Creek in Douglas County in 2006, which washed out part of a road.
Earthquakes are not much of an issue in Colorado, which is classified as Zone 1, or very low risk, compared to California, which is a 5 or a 6.
Dams are constructed so they can leak a little or sometimes a lot without failing. So leaking is nothing to worry about.
“Dams are designed to crack and be stable,” Byers said.
A dredging operation is not related to flood control but is basically to replace lost storage capacity due to silt buildup as a result of erosion.
Colorado has 1,928 dams and 345 high-hazard dams. Five new dams are constructed every year on average.