By the numbers
Permits issued through Dec. 31 for flood recovery, restoration and repair work
Unincorporated Boulder County: 194
Sources: Cities of Boulder and Longmont, and Boulder County
The frightening images of houses wrenched off their foundations and collapsing into rivers are part of our history now, but some experts caution that another wave of less dramatic damage attributable to the September flood could be on the near horizon.
"I believe that we have a sort of second round of flood damage coming," said Andrew Kelsey, principal of Ascent Group in Boulder and past president of the Structural Engineers Association of Colorado.
"That's what I've been telling clients. I fully expect there to be additional issues once they begin to dry out."
Some experts say the concern is overblown — even an "urban myth" — but Kelsey is not alone in voicing worry.
"The groundwater issue has mitigated for the vast majority," said Balaji Rajagopalan, a professor in the University of Colorado's Department of Civil, Environmental and Architectural Engineering.
"But it is very possible, or likely, there will be pockets where the groundwater could remain elevated for a period of time and as a result could exacerbate the structural issues, locally."
Because of the dramatically differing terrain in Boulder and Broomfield counties, the groundwater table is not the same in every area — and, therefore, does not rise or fall uniformly. In areas with proximity to streambeds, or at the base of hillsides, it is likely to still be higher than others.
And the interplay of the groundwater with varying types of soil would also result in differing dynamics beneath, and surrounding, a home's foundation.
"If the soil is sandy, it tends to respond quickly to changes in soil moisture," said Ross Corotis, another CU engineering professor.
"If it is mostly clay, then it takes a longer time for the water to come in and eventually go out," he said. "Someone might think the house doesn't have an issue and then two months later might find that the doors are not closing right, as the water is squeezed out of the clay. That can easily take months."
He emphasized that the soil beneath and surrounding a home's foundation can be transformed or influenced by a variety of environmental factors — not the least of which would be water. Parts of Boulder saw more than 17 inches of rainfall from Sept. 9 to Sept. 16, while creeks and tributaries running out of the foothills in many places overflowed their banks and plotted erratic new courses across the landscape.
In all, Boulder saw a record-breaking year in 2013 with 34.15 inches of moisture recorded. In addition to the record-shattering rain of September, April saw record snowfall of 47.6 inches.
"There might be soft spots in the soil, under the surface, that were changed by the high water table as the water washes through it," Corotis said. "It could weaken some soil relative to other parts, so you might see differential settlement due to the changed characteristics" of the soil.
'We're going to see changes'
The city of Boulder's planning and development services center has seen the number of permits issued for flood-related repairs recently drop substantially, according to public works spokesman Nick Grossman, from 124 in November to just 37 in December. The numbers in other jurisdictions also dropped way off in December.
A total of 641 flood recovery restoration and repair permits were issued by Boulder between the time of the flood and Dec. 31. Longmont issued 372, and for unincorporated Boulder County, the figure in that same period was 194.
"However," Grossman said, "there are still homeowners and some property owners who are obtaining city of Boulder permits to perform flood restoration and repair work."
He said while the flood is long since over, its ripple effects are not.
"One of the issues we are encountering is the elevated water table, and that's basically the depth below ground where the soil is saturated," Grossman said. "The groundwaters are higher than where they usually are, and they are expected to remain higher for a while.
"In terms of how that affects individual structures, some property owners may continue to experience water seeping into their basements and yards, and some buildings that have sump pumps may notice their sump pump is running more than usual."
Grossman said private property owners should work with experienced professionals to help prevent further damage or seepage, and work with their insurance companies "to see if they are covered for the damages that have occurred or are taking place."
Kathleen Tierney, director of the CU Natural Hazards Center, said such was the force of the September flood that it dramatically affected not only the landscape, but what she called the "risk-scape."
The transformation of the land, the deposits of debris in culverts and streambeds, the deposits of sediment in sewer systems and elsewhere, Tierney said, can have a cumulative effect going forward — including, potentially, continuing impacts on homeowners' properties.
"We're going to see changes. We haven't had a substantial precipitation event, or really much precipitation at all, since the floods," Tierney said. "And when we do, when we have snowmelt in the future and when we have heavy rain in the future, we're going to have some surprises, I would think."
Challenging an 'urban myth'
But there is not unanimity on this point. Tom Soell, president and senior structural principal at JVA Consulting Engineers in Boulder, believes the vast majority of damage done — and repairs that are needed — have long been in plain sight.
"I've heard a lot of folks speculating that there would be a lot of settling once the ground dries out, but I don't think the engineering community feels a lot of concern over the issue," Soell said.
He disputed whether the flood's lingering effects would be any more significant than what might occur as a result of the saturation typically associated with periodic heavy spring snowstorms.
In fact, Soell said, "Where we really saw an issue that was not predictable was back around the drought of 2000 or so, where soil dried to far greater depths than we'd ever seen. The amount of drying that went on was to such a depth that we started seeing some of these old brick buildings from the 1880s settling or collapsing a bit.
"Suddenly, there was a new pattern of distress that we saw, and it was because of the depth of drying."
Despite the heavy ground saturation of September, Soell said, "We do not expect to see a lot more (structural fallout) going on. That is not what we are hearing from soil engineers and the geo-technical community. I don't think there is a lot of concern with that.
"There's this urban myth that's growing, that all these things are happening."
Soell recently inspected an elevator shaft at a Boulder condominium complex where concerns had been raised that a bent rail in the shaft might be attributable to flood-damage issues. Instead, he determined that it was a result of an "interesting" installation of the elevator 10 years ago.
The debate in some circles about late-emerging flood damage has been extensive enough that there are also those who have switched positions on the question.
Geologist Ed Glassgow, owner of the Scott, Cox Associates civil engineering firm in Boulder, had initially been pessimistic about the dangers of damage continuing to surface on a delayed basis. His fears, he said, had stemmed from earlier concerns that September's havoc might be compounded by additional significant moisture.
But now, Glassgow said, "We have had such a dry fall and winter, I really think we are still in a scenario where everything is going to depend on how wet the late winter and early spring turn out to be. I'm more optimistic that we are not going to have more problems than I was two months ago."