By J. Patrick O’Leary
The City of Wheat Ridge is drawing up new floodplain maps, which indicate a lower flood risk in some areas and may reduce flood insurance premiums. It’s part of a project initiated by the city late last year to revise the existing maps to reflect more complete streamflow data.
“We’ve never had a 100-year flood in Clear Creek,” said Mark Westberg, project manager and floodplain manager for the City of Wheat Ridge, referring to data used to support existing floodplain maps. “We’re either wrong or due for one.”
Flood Hazard Area Delineation (FHAD) study maps mark the boundaries of a Special Flood Hazard Area (SFHA), an area with a one percent risk of flooding each year – in layman’s terms, the 100-year floodplain. The maps identify areas where homeowners with federally backed loans are required to purchase flood hazard insurance, alert people to flood risk, and affect building permits.
One percent sounds small, but over the course of a 30-year mortgage, a Special Flood Hazard Area has a 26 percent chance of flooding, which is a nine times greater risk than fire, according to the city.
The Army Corps of Engineers used typical hydrology (rainfall and runoff) calculations in creating the current maps, circa 1979, according to Westberg.
“Since good gauge data is often not available, the hydrology calculation route is the one that is most often done.”
More than 100 years of stream gauge data is being used for the revised map. Two sets of data – from stream gauges monitored by the United States Geological Survey between 1911 and 1974, and 1975 to today – show a 31 percent reduction in flood flows. For example, Clear Creek, downstream of Kipling has a flood flow of 10,000 cubic feet per second, down from the previously used calculated flood flow of 14,550 (downstream of Lena Gulch).
“We think the floodplain will go down,” said Westberg, “but we have no idea at this point whether it is six inches or three feet. In some places where it’s very wide, a lot.”
How many residents would be affected by the change, and how?
Data from the National Flood Insurance Program (NFIP), presented at the city’s most recent (March 29) annual floodplain open house in city council’s chambers, shows that 206 single-family residences are located in local Special Flood Hazard Area. Of the 260 insurance policies issued under the NFIP, 203 are in the hazard area, and 57 are outside. The insured property is greater than $62 million, with total annual premiums of $232.041. Inside the hazard area, the annual premium is $1,328; outside, $450.
Premiums are affected by a property’s elevation above or below the Base Flood Elevation, as well as coverage and deductible. Wheat Ridge’s Class 6 status under NFIP’s Community Rating System allows a 20 percent discount on premiums; the city is being reviewed for Class 5 status, which would allow a 25 percent discount. Only nine other Colorado communities are rated Class 6 or better; only four are Class 5 or better, according to NFIP.
“We have been extremely fortunate in that over 50 percent of floodplain is preserved as open space or parks,” said Westberg. That limits the amount of damage to private property, but lives are still at risk.
“My biggest concern with the ‘campers that we have along in our open space is the risk associated [with] flooding,” said Westberg. “We do have a city wide Early Warning System that targets the open space areas to warn of things like floods. But in the midst of a noisy storm, those warnings are not always heard.”
The city has 15 sirens throughout the city, concentrated along creeks.
Although draft mapping will be completed in September, the federal government’s adoption of the Physical Map Revision was originally anticipated in January 2019, with an effective date of July 2019. But Westberg said the schedule has been delayed.
Starting with a presentation to the Urban Drainage and Flood Control District – originally set for August but now February or March – that board will officially approve or adopt the FHAD.
“The FHAD would then be sent to the Colorado Water Conservation Board (CWCB) for review and acceptance. The CWCB is the state agency that oversees floodplains. Once the CWCB accepts the FHAD, then it would be forwarded to FEMA for review and approval.”
Surprisingly, neither the recent hailstorm nor the 2013 rains resulted in major flooding.
“The storm this year was mostly very large hail without a lot of rain, so we did not have any reported flooding associated with this storm,” Westberg said. “The storm in September 2013 had more rain and small hail. With that kind of storm the small hail is often washed into low areas and causes drainage systems to clog. We had a fair amount of localized flooding with that event. However, Clear Creek did have much more flow than what would occur in a large snow melt event.”
That’s not to say Wheat Ridge is in the clear till next spring.
“We could still see flooding once the monsoons hit and always present a danger for residents and visitors who walk or play along Clear Creek and around our local ponds and lakes,” said City Public Information Officer Sara Spaulding. Never driving through standing water, checking bike trails for flooding before setting out, and keeping an eye on small children are all good safety reminders, she said.
For more information, visit www.ci.wheatridge.co.us/flood.