From the Boulder Daily Camera (Erica Meltzer):
The leading version of the South Boulder Creek flood mitigation project would remove 893 homes in southeast Boulder from the 100-year floodplain, but it would also involve building a 23-foot berm through a recognized state natural area with endangered tall-grass prairie habitat.
That was a step too far for Boulder City Council members, and on Tuesday night, they asked the city’s flood engineers to take up discussions with the University of Colorado about using land the university owns for its future south campus instead.
The council heard an update Tuesday on the city’s flood mitigation efforts in the aftermath of 2013’s damaging floods. Boulder remains one of the cities at highest risk for flash flooding in Colorado.
The most likely flood scenarios — and the ones the city’s mitigation efforts are designed to limit — involve intense rainfall over a short period of time, not the days of sustained rain the city experienced last year.
Several council members asked what the city could do about flooding from groundwater, which caused 47 percent of the damage last year, either directly or indirectly in the form of sanitary sewer backups.
The city’s wastewater utility has developed plans to line older clay pipes to reduce groundwater infiltration during large rain events, but there’s no practical or legal way to lower the groundwater table, Jeff Arthur, director of public works for utilities, told the council…
But Arthur stressed that the flood scenarios modeled by the city’s engineers and reflected in floodplain maps are both more likely to occur than an event like the 1,000-year rain of 2013 and more likely to produce significant damage and loss of life.
The flooding in most of Boulder’s major drainageways in 2013 was the equivalent of a 10- to 25-year flood event. Only Twomile Canyon Creek experienced a greater than 100-year flood, while South Boulder Creek, Goose Creek and Fourmile Canyon Creek experienced 50- to 100-year flooding.
The city is in the planning stages of four flood mitigation efforts: Boulder Creek, Bear Canyon Creek, Gregory Canyon Creek and South Boulder Creek.
Mitigation planning efforts on Upper Goose and Twomile creeks, Skunk Creek, King’s Gulch and Bluebell Creek will start in 2016, after floodplain map revisions along those watersheds are complete.
Gregory Canyon will present significant challenges because so many homes are so close to the creek, and it will be impossible to do a mitigation plan that takes those homes out of the 100-year floodplain, short of simply removing the homes, engineers told the council. Instead, engineers are looking for ways to break up the channels to reduce the impact of flooding there.
The South Boulder Creek mitigation planning process has been underway for several years, and consultants and the city’s Water Resources Advisory Board had settled on a $46 million recommendation that would involve a major regional detention area south of U.S. 36 and smaller detention areas near Manhattan Middle School and on the Flatirons Golf Course.
Modeling of likely flood scenarios indicate a 100-year flood of South Boulder Creek would cause $215 million in damage. City Council members watched an animated simulation that showed much of southeast Boulder filling with water.
However, the large detention facility would have a significant impact on open space owned by the city that includes endangered tall grass prairie habitat, wetlands areas and a number of endangered and threatened plant and animal species.
The open space board officially objected to the plan as proposed, and the water resources board revised its position, recommending that the upstream and downstream pieces of the project be separated.
The two smaller detention areas could be built for $23 million with relatively few regulatory hurdles and would still take 294 dwelling units out of the floodplain…
Councilman George Karakehian said building the berm through open space was not politically feasible.
“I think that would be a tough one to get built in our community,” he said.
A revised version of the South Boulder Creek mitigation plan is expected to be presented to the City Council later this fall.
More Boulder Creek coverage here.