By Penelope Purdy
Denver Post Columnist
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.
Once the Corps saw the studies, the law required it to determine if its dams could withstand the PMP storm, said William Miller, head of the Corps Cherry Creek project. Most, including the concrete Chatfield Dam, would survive. But the Cherry Creek Dam, which is an earthen structure, would fail.
After several public meetings in the Denver area, the Corps narrowed its preventive plans to five alternatives, each affecting some part of the public.
But all would leave the existing water level in Cherry Creek Reservoir alone – because even if the entire reservoir were drained, a PMP storm would rap idly fill the basin behind the dam, causing the structure to collapse. Thus, no matter what else the Corps does, operation of Cherry Creek State Park, which Colorado leases from the federal government, would be unchanged.
The alternatives are:
– Raising Cherry Creek Dam by 15 feet. In a PMP storm, some Aurora and Greenwood Village neighborhoods would get about 6 feet of water in their homes. If the dam is kept at its present height and a PMP event occurs, though, these same homes would still get about 2 feet of flooding.
– Building a dam in Castlewood Canyon State Park, to replace a dam that collapsed in a 1933 flood. The new “dry dam” would not hold any water except during a PMP storm. However, the Colorado State Parks Department worries about a concrete structure bisecting the popular park near Parker. Castlewood Canyon also is home to the Preble’s jumping mouse, already a candidate for the endangered species list.
– Raising Cherry Creek Dam 9 feet and building a new 300-400 foot spillway. However, the corps is studying whether the new spillway would still cause flooding downstream.
– Building a dry dam at West Cherry Creek and raising the existing Cherry Creek Dam by 7.5 feet.
– Constructing a dry dam at Scott Road and raising the existing Cherry Creek Dam 12 feet.
– Layering concrete across the face of the existing Cherry Creek Dam and raising the dam at both edges by 11 feet. The new structure would funnel water over the concrete reinforcement, thereby preventing the dam’s collapse.
This option avoids environmental and political problems caused by the 15-foot dam rise and dry dam options. But the Corps isn’t sure yet if the modification would cause flooding downstream.
The Corps plans to issue a draft environmental impact statement, recommending one alternative, for public comment this winter. The project could be finished within a decade.
The Cherry Creek Dam debate is a classic case of risk analysis colliding with politics. The costs of doing nothing – extreme in this case – must be weighed against the price of doing something, however controversial.
But the debate could devolve into a feud between the high-income homeowners, who live near Cherry Creek Reservoir and oppose any enlargement of the existing dam, and lower-income residents along the South Platte, whose lives and worldly possessions could be lost in a catastrophic flood.
Meantime, local and federal officials, whose input and votes are needed to fund the project, must try to grasp the complex science involved. Last week, weather service expert Johnson visited the Cherry Creek Dam for a first-hand look at the problem. His conclusion:
“Having driven across that dam, and looked at all that housing and development that’s downstream, I find it completely believable that failure of that dam would kill 10,000 people.”
Penelope Purdy is a member of The Denver Post editorial board.