Bailing through the Boulder flood: One neighborhood's experience

by Sara E. Pratt
Friday, November 15, 2013

Despite everything I learned in my sedimentology-stratigraphy class about alluvial fan deposits, and the debris flows and floods that emplace them, I never once considered what those depositional events might sound like. Now I know.

During the week of Sept. 9 — when a low-pressure weather system stalled over the Great Basin and began pulling plumes of monsoonal tropical moisture upslope toward the Rockies — the city of Boulder, Colo., received nearly half a meter of rain.

The “biblical” rainfall, as the National Weather Service termed it, flooded rivers and streams across the Southwest. In Colorado, the rain led to mud and debris flows that wiped out roads and bridges, prompting the evacuation of thousands of residents isolated in mountain towns and contributing to the deaths of eight people across the state. More than 1,800 homes were destroyed and an estimated 16,000 were damaged — including my family’s in Boulder, which sustained only minor damage. Despite having experienced some of the flooding firsthand, it is hard to reconcile our experience in the city with the images of devastation we saw from around the rest of Boulder County and the state.

Boulder City

Boulder, a university town in the foothills of Colorado’s Front Range, is home to several earth-science organizations, including the Geological Society of America, NOAA’s Earth System Research Lab, the University of Colorado at Boulder’s (CU) Cooperative Institute for Research in Environmental Sciences,the National Snow and Ice Data Center and the National Center for Atmospheric Research (NCAR).

I moved to Boulder last year from New England, settling in a South Boulder neighborhood near the base of Table Mesa that boasts views of the iconic flatirons — the triangular outcrops of reddish-brown sandstone of the Fountain Formation uplifted during the Laramide orogeny — as well as views of the I.M. Pei-designed NCAR headquarters atop the mesa.

The lanes of the neighborhood’s main thoroughfare, Table Mesa Drive, are divided by what normally appears to be a small drainage ditch, but what is actually Bear Creek, a 15-kilometer-long tributary of Boulder Creek that emerges from Bear Canyon to the southwest.

Like any conscientious geologist-home buyers, we researched the local natural hazards, including the flood zone maps, with a wary eye on Bear Creek and the knowledge that although Boulder is in a semi-arid region that is prone to drought and wildfire, it also has the highest flash-flood risk of any city in the state. It was founded in 1858, as a gold-mining camp called Boulder City, on Boulder Creek near the mouth of Boulder Canyon. When rain falls in Boulder Creek’s 1,160-square-kilometer watershed, the denizens of Boulder usually know about it.

Perhaps this is why the city was ahead of its time on flood planning and mitigation. Gilbert White, the CU geographer known as the father of floodplain management, first studied Boulder Creek’s floodplain in 1957. White had the revolutionary notion that, rather than engineering the environment to try to prevent floods, humans should learn to live with them. “Floods are ‘acts of God,'” he wrote in his University of Chicago doctoral thesis, “but flood losses are largely acts of man.” In the 1960s, White advised the city, which began instituting flood-control measures and floodplain management techniques, including restrictions on residential development in the floodplain, a series of underpasses to channel water, and the placement of Boulder Creek’s bridges on hinges or lifts so they can be moved out of the path of floodwaters.

These measures all came into play during the flooding of September 2013.

A Thousand-Year Rain

Late in the afternoon on Monday, Sept. 9, rain and lightning sent players and parents scattering from the field at my son’s soccer practice. The rain continued with few breaks through Tuesday, but people did not seem concerned. Unlike in New England, rain is usually welcomed in semi-arid Colorado, where lawn sprinklers are ubiquitous. By Wednesday, however, the sentiment in the neighborhood had turned from relief to consternation as runoff rushed the storm drains and the playground puddles reached ankle depth. Soccer was canceled.

The first flash-flood warnings for Boulder Creek came on Wednesday night, with periodic blasts of the emergency alert sirens. Later that night, at our house, we discovered a window well filled halfway to the brim leaking into the basement as well as a leaking chimney. Rushing outside, we also found that a low area in the driveway was rapidly filling with rainwater and runoff, which threatened to breach the ground floor. We grabbed five-gallon buckets and began to bail, hauling the water up to the storm drain — a task we would repeat nearly a dozen times over the next few days and nights.

In the first hours of Thursday, Sept. 12, CU closed and began evacuating students living on the lower floors of dormitories. The Boulder Valley School District also canceled and wouldn’t open again until the following Wednesday, an unprecedented closure for rain. All day Thursday, Table Mesa residents warily watched Bear Creek. By late afternoon, it began to overflow its banks at the bottom of the hill, flooding the local branch of the public library. Up the hill, the creek was still contained within its banks, but the road was closed as a precaution.

That evening, we began to hear the clatter and clunk of rocks being carried down the street. One block away, a culvert had become blocked, and rocks, trees and mud surged up onto Table Mesa Drive. Throughout the night, we listened to the clatter of alluvium being emplaced and periodically bailed the window well and driveway.

Friday morning brought sunny skies and the sound of bulldozers. Residents surged out into the streets to assess the damage and commiserate with one another. Some brought wheelbarrows and raced the bulldozers to collect outwash debris to use as landscaping stone. The cleanup had begun, but the storm wasn’t over. It would continue to rain on and off through the weekend. Although not as heavy, the rains fell onto already saturated soils, exacerbating the flooding.

Records and Resilience

By the time the storm ended on Sunday, Sept. 15, Boulder had received a record-breaking 43.5 centimeters of rain. More than 22 centimeters had fallen in one day, nearly double the previous one-day record set in 1919. The storm brought Boulder’s total year-to-date precipitation to 76.5 centimeters, topping the previous annual record of 76 centimeters set in 1995 — with three months still to go in the year.

At the flood’s peak late Thursday night and early Friday morning, U.S. Geological Survey stream gages clocked Boulder Creek at more than 140 cubic meters per second, up from a mere 1.5 cubic meters per second on Monday. The precipitation and streamflow rates qualified it as a 1,000-year rain event and a 25- to 50-year flood.

As I write this, a few days after the rain has stopped, the rumble of boulders moving down the street has been replaced by the roar of bulldozers, the thunder of rescue helicopters overhead and the constant thrum of pumps emptying sodden basements.

Damage from the flood can be seen everywhere. Sand, rocks and outwash debris line streets with torn-up landscaping and trees. Roadsides and sidewalks near the creek are mangled or missing. Nearly every house has a debris pile of wet carpet, soggy drywall, and muddy and ruined furniture and personal belongings heaped at the curb. Although there were no fatalities in the city of Boulder, one building did collapse and thousands of homes were flooded with water, mud, debris and sewage. One of the town’s two water treatment plants was swamped and had to be taken offline; however, there were no boil-water orders in the city, and clean tapwater continued to flow throughout the flood.

Overall, the city of Boulder did not experience the destruction that happened in 18 other Colorado towns, including Longmont, Jamestown and Lyons, where residents were cut off for days by rising water and washed-out roads; mudflows destroyed homes, in several cases killing people inside; and more than a thousand residents had to be evacuated by helicopter. Especially hard hit was Fourmile Canyon, where a September 2010 wildfire left slopes barren leaving the canyon vulnerable to flash flooding and mudslides.

Statewide, damages are estimated to reach $2 billion. The road to recovery will be long. Some residents will not be able to return to their homes for months, if ever. But the resilience I’ve seen displayed so far as the community rapidly mobilized to begin the cleanup has been heartening.

Flood Control

What struck me most about this flood was the degree to which the damage varied from place to place, even from house to house. Some people lost everything, including their lives. In our neighborhood, several houses were flooded with 60 centimeters of water; at the other end of the block, some had geysers of raw sewage shooting from the plumbing. In between, some suffered no damage at all.

The lesson of the Boulder flood is that planning and preparedness at the municipal, state and federal levels are crucial determinants of the impact a natural disaster will have on a population as a whole and its ability to recover. That said, individual outcomes will always depend on many chaotic variables — whether a boulder becomes wedged in a culvert or the local topography diverts, or directs, floodwaters — that will forever be beyond our complete control.

Perhaps the early settlers of Boulder City already knew this. One morning after the flood, as my son and I navigated around rocky flood debris on the walk to school, he suddenly realized that the town had been named for the large rocks that wash out of the canyons. Then he asked about the origin of the word “boulder,” which I did not know. Later, we looked it up and learned the etymology derives from the Old Swedish word “bulder,” meaning a rumbling noise. Now I know.

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