Aging Breckenridge dam can handle 100-year flood, official says | SummitDaily.com
Eli Pace
epace@summitdaily.com

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Aging Breckenridge dam can handle 100-year flood, official says

A state official familiar with the eroding spillway at Goose Pasture Tarn Dam believes that, with mitigation efforts currently in place, the aging barrier made of earth and concrete should hold, even in the event of a 100-year flood.

About 1.5 miles south of Breckenridge, the dam was built by the Theobald family in 1965 to create a recreation area.

Now under town ownership, it stands 57 feet tall and stretches 550 feet long, holding back a 770-acre-foot reservoir that serves as the primary source of water for the town, with homes in the Blue River community directly below the dam.

The reservoir collects runoff from approximately 42 square miles of the Upper Blue River watershed and is fed primarily by two sources: the Blue River, which drains about 85 percent of the watershed upstream of the dam, and Indian Creek, which accounts for the remaining 15 percent.

Because of snowmelt, water flows feeding the reservoir ramp up in May and last through September, with the peak discharge in early June through July.

State regulators have given the Tarn Dam a high hazard rating and issued a report in which it is said to be in unsatisfactory condition, meaning a problem has been recognized that requires immediate action.

Spillway erosion

In this case, the problem is blamed on frost heave — an upward swelling of soil during freezing conditions — that allowed gaps to form underneath some of the concrete slabs that make up the dam spillway.

The problem was discussed during a June 27 work session of the town council, in which comparisons to Oroville Dam in California were made. Built in 1961, a failure of the main and emergency spillways at Oroville Dam in February 2017 forced the evacuation of some 188,000 people living near the dam.

In 2004, seepage was discovered at Tarn Dam coming out of a slab joint when the reservoir was about a foot below the spillway crest. The joint was sealed to stop the leak, but more seepage was found in an emergency spillway slab, according to the report produced by Kumar and Associates, the design firm working to repair the dam.

Also, cored holes through slabs at several locations and "extensive" 0.5-inch to 3-inch voids were discovered beneath some of the slabs.

Repairs to the spillway were made in 2006. However, in 2015 town officials recorded higher-than-normal groundwater pressure readings at the dam and alerted the state, prompting another round of repairs.

Again the concern was that the groundwater pressure could exceed the downward force on the spillway slabs, or flows over the spillway, could break up the spillway, "potentially causing dam failure," according to the report.

Mitigation

The good news is state and town officials have both expressed supreme confidence that by reinforcing the spillway and lowering the reservoir level — both actions taken in 2016 as a response to the above-average groundwater pressure readings in addition to other measures — the dam will hold for the foreseeable future.

Adjacent to one another, the dam actually has two spillways, a service spillway and an emergency spillway.

The service spillway — sometimes called a surface spillway — has been regularly used to release heavy flows from spring runoff, as those flows often exceed the amount of water that can be released through the dam, said Bill McCormick, dam safety chief at the Colorado Division of Water Resources, who's been working with the town to assess the risk of a failure at Tarn Dam.

On the other hand, the emergency spillway was built decades after the initial construction of the dam 4 feet higher than the service spillway for emergency drainage scenarios.

The emergency spillway had never come into use since its construction, McCormick explained, until recently, when mitigation efforts required engineers dig out the emergency spillway and reduce its height to below that of the service spillway.

The excavation work effectively turned the emergency spillway into a new service spillway and the service spillway into an emergency spillway.

Coupled with reducing the water level of the reservoir to 4 feet below the normal spillway crest, McCormick called it a "semi-permanent fix" and said he's confident the dam won't fail, even in a 1 percent event or 100-year flood.

For the dams with the highest potential to kill people and destroy property in the event of a failure, most are required to plan for such an unlikely event.

Asked if the Tarn Dam, as it currently sits, could handle it, McCormick responded in a word: "Yes."

He added that, in the event of major rainfall or runoff, the old service spillway could be used as an emergency spillway "for a short period of time" without giving way.

Dam regs

Contrary to what some may believe, there is no nationwide federal program for regulating dams, and the vast majority of the nation's more than 90,500 dams fall under the scope of state regulators, according to the Association of State Dam Safety Officials, a national group dedicated to dam safety.

The average age of all those dams in the U.S. is 56 years old.

With 12 full-time state employees imposing state regulations on more than 1,850 dams in Colorado alone, it might seem like state regulators are woefully understaffed and ill-equipped to handle their mission.

However, Colorado actually stands as one of the best states in terms of dam safety spending, according to the dam safety group, and has a more robust state dam regulating agency than most others. McCormick agreed with the ASDSO's assessment. However, regulations don't always prevent problems, as California too is seen as a national leader in regulating dams, with a ratio of one inspector per every 20 dams, according to the ASDSO.

As for the dams, each is given one of four hazard ratings — high, significant, low or no hazard — but that classification isn't meant to say anything about the condition of the dam, nor does it suggest the dam is in any danger of failing.

Rather, the hazard rating is based on how catastrophic it would be should the dam fail, with the highest rating reserved for dams that, if breached, would likely result in a loss of life, and significant hazard ratings given to dams that would likely lead to major property losses if they were to fail.

High and significant hazard dams are the most stringently regulated by the state, with Colorado's regulators performing annual inspections on high hazard dams and inspections every other year on significant hazard dams, McCormick said.

In Summit County, Dillon Dam, built in 1963, is also rated as a high hazard dam, as a failure there would inundate Silverthorne. However, according to the state, Dillon Dam is in satisfactory condition.

Other nearby dams that also constitute a high or significant hazard but are in good condition include Reynolds Dam on Soda Creek near Keystone, built in 1938; the Sawmill Dam in Breckenridge, built in 1890; and the Clinton Gulch Dam on Tenmile Creek, built in 1977.

An expensive fix

Complicating the problem at Goose Pasture Tarn Dam is the construction of a new $50 million water plant that's directly tied to the dam fix.

Councilman Mike Dudick previously explained the town can't start working on Tarn Dam until the current water plant, which is near the dam, can be shut off.

Without another water source in place, the town can't turn the existing plant off until the new one is built, which is tentatively expected to happen in 2020.

While the cost of fixing the dam was expected to be somewhere between $16 million and $18 million, the reality is that estimate was based on old figures, and nobody expects it to be that cheap by the time the town can begin work in the next three to four years.