Experts warn: Act now or Cherry Creek Dam will fail in a big storm

*editor note, the following article was presented in the Denver Post April 2001. It is presented as a companion piece to the Disaster Scenario for Cherry Creek Dam Weighted printed in the Denver Post Jan23, 2015*

By Penelope Purdy
Denver Post Columnist

April 11 2001 – A nightmarish scenario is fueling a controversy over how or whether to shore up the Cherry Creek Dam.

Experts say an extreme downpour could burst the Cherry Creek Dam, sending a 52-foot-high wall of water roaring into a metropolis of 2 million people. The flood’s force would uproot trees, topple power lines and tear buildings from their foundations. Cars on Interstate 225 would be swept away. Much of Glendale would be engulfed.

The crest would undulate with the shape of the landscape and creek bed, but remain big, fast and deadly for a long distance.

It would stand four stories high and be traveling a frightening 16 feet per second when it shattered the Cherry Creek Shopping Center.

It would be 46 feet tall when it roared through the Sixth Avenue and Speer Boulevard intersection, trapping cars in the tunnel.

TV stations KUSA-Channel 9, KMGH Channel 7 and KDVR-Fox 31’s new offices would be in harm’s way. So would be the local Red Cross chapter and Denver Health Medical Center, home to the region’s premier emergency room.

An hour after the dam broke, the torrent would rage into Downtown’s heart, putting at risk Denver police headquarters, city hall and innumerable restaurants, retailers and apartments.

The flood would gain vigor when it merged with the South Platte River – near whose banks sit Elitch Gardens, the Pepsi Center, an electrical power station and an oil refinery.

When the torrent reached the sewage plant – which serves all of Denver and most suburbs – it would stand 48 feet high and be moving at a destructive 10 miles per hour. Experts are studying if the flood would spill human wastes into Brighton, Fort Lupton and Greeley.

The South Platte’s channel couldn’t handle the enormous volume, so flood waters would get shoved upstream, too.

At the Children’s Museum, the crest would measure 16 feet deep and clip along at 8 feet per second.

Engineers don’t yet know if the backwash would flood the Shattuck site, where the U.S. Environmental Protection Agen cy has allowed radioactive materials to be entombed. The backwash would reach as far as Englewood.

Meanwhile, since one dam spillway flows into Aurora, the flood would thunder into Toll Gate Creek. Miles away, the former Fitzsimons Army hospital, where the University of Colorado is building its new health sciences center, would be awash.

The disaster would have been preceded by several days of heavy rains, so evacuation would be nearly impossible along already flooded streets – especially because the dam break would come just two hours after water started lapping against the dam’s top.

The catastrophe would kill 10,000 people, leave 100,000 homeless, destroy $10 billion in property, inundate 39 schools, three hospitals, three police stations and one fire department.

The horrific scenario is unlikely, but not outlandish. In fact, the Cherry Creek Dam would fail if the metro area got a storm just three-quarters the size of the maximum possible deluge.

Given the consequences, the U.S. Army Corps of Engineers, which operates the dam, says even a small probability of a dam failure is intolerable. But the Corps’ worst-case storm scenario and its plans to shore up the dam have themselves stirred a flood of controversy.

So far, political pressure has come mostly from communities who want nothing done – not cities at risk if the dam collapsed.

To comprehend why the Corps is worried, it’s helpful to understand how dam safety is determined.

An older method entailed familiar measurements called the 100-year or 500year floods. Left in place long enough, a structure built to these standards eventually will face a storm bigger than the 100year or 500-year flood, and so could fail.

The newer and more rigorous method considers the worst storm that could ever strike the area around the dam. That amount of rainfall is called Probable Maximum Precipitation (PMP).

If a dam is built to withstand PMP, it should never fail in any storm, explained Jose Salas, a civil engineering professor at Colorado State University.

The PMP approach is an internationally accepted method, Salas added.

But it’s expensive to build all dams to such rigorous standards. So, the Corps uses the strict PMP standard when failure is not an option.

Since 1988, studies disturbingly have shown that the Cherry Creek Dam would break during a PMP storm, or even rainfall approaching the PMP’s severity. The failure would cause the Probable Maximum Flood, whose consequences were described earlier.

Critics of the Corps’ plans note that metro Denver has never had a storm as bad as the purported PMP.

But even if the PMP estimates were 25 percent too high, the Cherry Creek Dam would still present a hazard – because it would collapse during a storm just 75 percent of the PMP’s size.

Moreover, conditions that would trigger such a terrible event are only an amplification of a common Front Range weather pattern called an upslope. The most likely PMP scenario: Hurricanes in the Gulf of Mexico would spit big, wet air masses north into the continental United States. One or more of these rain-laden air masses would curve west, and, when they reached Colorado, get trapped against the Rockies.

New weather studies show the maximum amount of moisture such conditions could carry into the Cherry Creek drainage is much higher than any previous estimate. Scientists combined those potential moisture figures with information about past flood-producing storms in the Denver vicinity.

The result was the controversial PMP: The entire Cherry Creek drainage could get more than 24 inches of rain in three days, or about 15 inches of rain in a six-hour period. Critics challenge the estimates as outrageous for an area that receives only about 14 inches of precipitation annually.

But a 1935 storm did dump 24 inches of rain in a single day in the Cherry Creek drainage. And a 1965 event let loose 14 inches in six hours over creeks that feed the South Platte. Both caused major, deadly floods.

Those storms didn’t produce the worst-case disaster only because the intense rain fell in confined areas, not the entire drainage, the research showed.

The research included a site-specific study of Cherry Creek, said Edward Johnson, chief of the Hydrometeorological Operations Division at the National Weather Service. The division is responsible for conducting the PMP studies, on which the Corps of Engineers bases its flood estimates and dam assessments.

The research has passed three independent peer reviews, Johnson added. Continue reading

Disaster scenario for Cherry Creek Dam weighed

By John Aguilar
The Denver Post

AURORA — A failure at the Cherry Creek Dam would inundate the heart of the Denver area with a torrent of deadly water, putting more than 280,000 people, nearly 39,000 structures, and $14 billion of land and property in jeopardy.

That nightmare scenario is why federal officials are taking steps to assess the safety of the 140-foot-high earthen dam on the border of Denver and Aurora and lay out ways — such as raising the dam, building a second spillway or adding more relief wells — to make it safer.

(Helen H. Richardson, Denver Post file)

On Saturday, the Army Corps of Engineers will take its Dam Safety Modification Study for Cherry Creek Dam to the public to get feedback on how to move forward.

Another meeting was held Thursday night.

John Palensky, the Army Corps’ Cherry Creek Dam safety-study manager, said any risk associated with the 65-year-old dam is because it is perched over a densely packed urban corridor, not because of problems with its structural integrity.

In other words, he said, “the risk is skewed by the consequences.”

“There is not an operational or structural deficiency with this dam,” Palensky said. “If this dam was out in the middle of nowhere, we wouldn’t care about it.”

That said, Cherry Creek Dam has received an elevated risk rating from the Corps because of its large downstream population. The agency began screening its dams — it has about 700 across the country — after Hurricane Katrina in 2005 to determine each structure’s risk level.

(Helen H. Richardson, Denver Post file)

The Corps also runs the Bear Creek and Chatfield dams in the metro area. The Chatfield Dam was completed in 1975 after floods 10 years earlier caused millions of dollars damage to Denver and the South Platte River basin.

The Cherry Creek Dam also was built as a flood-control structure. It has never come close to overtopping since its opening in 1950, according to the Corps. Its spillway, which would divert rising water from Cherry Creek Reservoir long before the top of the dam was reached, has never been used.

The highest water level recorded in the reservoir was 5,567 feet in 1973 — just 15 feet above its normal level, said Army Corps dam-safety engineer Steve Butler.

The top of the dam is at an elevation of 5,646 feet.

The Corps has projected that it would take about twice as much precipitation as was dumped on Boulder County during the historic floods of September 2013 to cause water to spill over the top of Cherry Creek Dam. The chances of an overtopping are one in 58,800 in any given year, the agency projects.

But Butler said a storm of that size, while “very unlikely,” is not outside the realm of possibility. And the consequences of such an event would be so severe that they can’t be ignored, he said.

An overtopping of the dam would send 143,900 cubic feet of water per second crashing over Interstate 225, through Kennedy Golf Course and along 11 miles of Cherry Creek to the confluence of the South Platte River in downtown Denver — wrecking houses, sweeping cars off roads and potentially flattening entire neighborhoods.

A breach would send that much more water downstream.

And the damage wouldn’t be limited to the metro area. The inundation area would stretch all the way into Nebraska, according to Corps modeling.

Scott Field, head of the Denver Office of Emergency Management and Homeland Security, said a failure of the Cherry Creek Dam is such a distant possibility that it doesn’t keep him up at night.

But he said the world of emergency management nevertheless operates “in the world of worst-case scenarios.”

His office has been looking at ways to evacuate people as quickly as possible along Cherry Creek should it become necessary. He hasn’t taken a position on whether any particular measures or safeguards, such as raising the dam or modifying the spillway, should be implemented.

Part of the reason is that the Corps hasn’t attached costs to any measures.

“We’re still at the ‘everything is on the table’ point in this discussion,” Field said.

John Aguilar: 303-954-1695, jaguilar@denverpost.com or twitter.com/abuvthefold

Cherry creek dam safety meeting

When: 10 a.m. Saturday

Where: Cafeteria at Campus Middle School, 4785 S. Dayton St., Greenwood Village


20150123_120322_Cherry-Creek-Dam-Disaster-Scenario-Map

By the numbers

5,567

Highest water level, in feet, recorded at Cherry Creek Reservoir, in 1973 — 15 feet above its normal level

5,646

Elevation at the top of Cherry Creek Dam

 

No Vote, No Trial, No Citizen Input On Hentzell Park

By Charles C. Bonniwell, The Glendale Cherry Creek Chronicle June 2014

For over two years Denver park advocates who formed the entity Friends of Denver Parks have attempted to assert the rights of citizens of Denver as guaranteed by the Denver City Charter and the Colorado State Constitution to vote on matters critical to them and in particular on whether Mayor Michael Hancock could simply trade away 11 acres of open space land for development at Hentzell Park for a rundown office building in downtown Denver.

The city, led by Assistant City Attorney David Broadwell, has blocked all efforts for citizens to have that say. First he claimed that Denver City Charter Sec. 2.4.5., that requires “approval of a majority of registered voters” for the sale or lease of any park or any portion of any park, does not apply since it had not officially been designated a park not withstanding all appearances to the contrary, and even prior statements by the Mayor of Denver in 1979 that the property was “dedicated park land.”

When parks advocates sufficiently gathered signatures under their right of referendum and initiative as seemingly guaranteed by of the City Charter Sec. 8.3.1 and the State Constitution, Broadwell instructed the City Clerk and Recorder Debra Johnson to reject the petitions. He claimed that the swap was an administrative not a legislative action and that vitiated any right of the citizens on the matter.

Municipal law experts noted that the City Clerk and Recorder position was deliberately made by the City Charter as a separately elected officer so not to be under the control of the Mayor of Denver directly or indirectly through the City Attorney. Nonetheless, Johnson took the instructions from Broadwell and rejected the petitions.

Read complete article here