Flash Flooding Causes Serious Damage in Northern Colorado: AIR Worldwide

Three days of intense rainfall in many areas of Colorado has resulted in severe, widespread flooding and collapsed homes, according to catastrophe modeling firm AIR Worldwide.

The greatest threat is to cities along the Front Range, including the city of Boulder and parts of the Denver metropolitan area. Essentially all water bodies in these areas including ditches, canals and streams are at capacity with many of them at flood levels. Several roads have been washed away and highways blocked due to landslides, hampering rescue attempts.

“Severe flooding began during the evening of September 11, when several areas received up to 2 inches of rainfall per hour,” said Dr. Yucheng Song, senior scientist, AIR Worldwide. “In Erie, which is located in Boulder County, the Erie Parkway was submerged under a foot of water. By the morning of September 12, a flash flood warning had been issued for the areas affected by the Waldo Canyon wildfire last year, where the lack of foliage within the burn scar has increased the risk of excess runoff.”

In Boulder County, flooded creeks include Left Hand Creek, Four Mile Creek, Coal Creek, and St. Vrain Creek. The government buildings in the city of Boulder, as well as the University of Colorado and Naropa University are closed. About 400-500 students and faculty from the University of Colorado have been evacuated from on-campus housing and several homes in the University Hill area of Boulder have reported flooding.

The Meadow Lake Dam at the Big Elk Meadows, southeast of the city of Estes Park, has been breached, causing flood warnings for the areas of Pinewood Springs and Blue Mountain. Water continues to rise today as heavy rains continue, particularly along the Interstate 25 corridor.

Portions of Boulder Canyon, Big Thompson Canyon, Lefthand Canyon, Poudre Canyon are closed. Highway 36 is washed out at the St. Vrain River. Other highway closures due to standing water or landslides are hampering rescue efforts.

According to AIR, this region often experiences heavy precipitation this time of year, but it has also been an unusually wet summer, with over 10 inches of rain inundating parts of Colorado during July and August.

Dr. Song observed, “Currently, a surge of moist air that originated in the Gulf of Mexico has combined with a low-pressure system over Utah and a cold front. The area’s mountainous topography causes the air to cool as it moves upslope, producing more precipitation. This upslope flow will continue over the next few days due to a high pressure system to the east that will keep the system virtually stationary over the next few days.”

By 12:30 PM September 12, several river gauges showed elevated water levels, with at least one indicating moderate flooding. At Boulder Creek, near Boulder, water peaked overnight at 8.88 feet and showed an average flow of 380 cubic feet/second (the average is 64 cubic feet/second), the largest ever experienced in September. Along the Big Thompson River at Loveland, the water stage has since risen above 7.5 feet and is experiencing flows exceeding 2,300 ft/s, the highest in 14 years.

According to AIR, Colorado has a long history of flood events and flood mitigation efforts that have been in place for many years. In Boulder County, these include codes and ordinances that prohibit, or limit, building in floodplain areas. In addition, channelization and detention ponds have been built in the county and several high-risk buildings located in floodplains have been removed. Despite these efforts, the area does have vulnerable structures. Approximately 70 percent of the residential construction is wood, with an estimated 40 percent having basements. The presence of a basement increases the risk for contents and building damage. Over half of the commercial buildings are steel and concrete. Unlike residential structures, these buildings often have engineering attention and are built to stricter standards, being less vulnerable but still susceptible to high water flow velocities.

AIR will continue to monitor the situation in Colorado and will provide additional information as warranted.

Source: AIR Worldwide