Federal and local flood-control officials are trying to reach a consensus on whether the flood-control facilities built in Davis County in the wake of floods and mudslides in 1983-84 are adequate.
Agreement between the two groups directly affects homeowners living in the disputed areas, who may be required to buy between $350 and $400 worth of flood insurance annually.The Federal Emergency Management Agency (FEMA) began remapping flood-prone areas of the county after the 1983-84 runoffs and slides caused an estimated $250 million damage across the state.
The county also reacted, selling $12 million in bonds to build debris retention basins and improve stream channels to carry more runoff.
FEMA this month told Centerville City officials that county efforts have not been enough and that the debris basins are inadequate. County public works officials counter that, with one exception, their projects are adequate because FEMA doesn't understand the unique geography and geology of the Wasatch Front.
And FEMA officials say that, too, may be true.
Centerville, meanwhile, appears to be caught in the middle of the controversy because FEMA's first review after the 1983-84 episodes was directed toward the six creeks that cut through the city. The ongoing study will eventually include the rest of Davis County.
Maps prepared by FEMA and submitted to Centerville for review greatly expand the potential flood damage areas at the mouths of the canyons.
Homeowners in areas designated as flood plain by the federal agency are required to purchase flood insurance if the home mortgage or refinanced mortgage is obtained through a federally insured financial institution.
The federally subsidized flood insurance costs $350 a year for $70,000 coverage, so officials estimate it could cost homeowners in designated areas an additional $350 to $400 annually, depending on the home's value.
With that in mind, Centerville officials are preparing to appeal
FEMA's findings, using information from the county public works and flood control department that built the debris basins and engineered the stream channel improvements.
Flood insurance, like fire insurance, is a good idea, public works Director Sid Smith told the county commissioners in a presentation this week. He said most homeowners will probably buy it eventually, with the cost automatically added onto their monthly house payment by the mortgage holder.
But Smith disagrees with the FEMA findings on the adequacy of the county's debris basins, with the exception of Centerville Canyon. That basin, near what Smith estimates is 16,000 cubic yards of mud and debris potentially ready to slide, is built to contain only 4,500 cubic yards.
But the county's estimate of about 16,000 cubic yards - which has been adopted and endorsed by the state emergency management agency - is far below FEMA's estimate of between 200,000 and 220,000 cubic yards of potential debris.
For comparison, Rudd Creek above Farmington slid in 1983, destroying several homes below it in the north downtown residential area of the city. FEMA estimates that between 90,000 and 100,000 cubic yards of debris broke loose in the Rudd Creek slide, scouring out the streambed and creating a fan of mud and rock 20 feet deep at the mouth of the canyon tapering down to 2 to 3 feet deep.
Smith said FEMA is used to dealing with clearwater flooding problems, like rivers in the Midwest overflowing their banks. The agency is not able to evaluate canyon runoff, with its inherent load of mud and rock.
Calculating runoff and potential debris flows from canyons is a relatively new area of research, Smith said, one that Davis County has pioneered and in which the county is leading the field.
Doug Gore, head of the flood insurance branch of FEMA, agrees. Maps prepared in 1982 were proved almost totally wrong by events of the next two years, he told the Centerville Council.
"Studying mudflows is a relatively new and inexact science," Gore said. "But FEMA is on the cutting edge of it." Gore praised work done by Davis County, saying the study - which he calls the "Davis model" - is being analyzed across the country.
The Davis model suggests that debris in canyons builds up to a depth of 11 to 12 cubic yards per lineal foot of channel before it is ready to wash out.
According to Fred May, a hazard mitigation planner with the state division of emergency management, Rudd Creek above Farmington was at that point in 1983 when a mudslide above it crashed into the creek, precipitating the debris flow.
Smith compared it to holding a bottle full of liquid upside down and then yanking out the cork, the mudslide off the canyon wall being the trigger event that knocked out the cork.
The channel was scoured down to bedrock and remains clean now, indicating the buildup of debris is very slow, according to May. Parrish Canyon above Centerville flowed in the 1930s, May said, and now contains between 3 and 4 feet of debris.
Centerville Canyon, which May called "worrisome," is a "pristine canyon," meaning it has no history of debris flows and a survey shows it contains between 11 and 12 cubic yards of debris per foot of channel - making it a prime candidate for a future debris flow.
"We're looking at that canyon with some amazement, that it didn't go during '83 or '84," Smith told the commissioners. "If it had come loose, it could have been twice the size of the Rudd Creek flow."
Smith said a triggering event doesn't have to be heavy spring runoff. Slides and mudflows in the 1920s and 1930s occurred in other months, after rainstorms sent heavy flows of runoff down hillsides stripped of vegetation by livestock grazing.
Smith said his fear now is a brush fire on the slopes above the canyon, followed by even a moderate rainstorm. The combination could send mud-laden runoff water into the canyon, setting off a debris flow.
Centerville City Manager David Hales said there are 600 residents in 185 homes valued at $15 million in the path of the canyon's projected debris flow pattern.
Smith estimated an adequate debris basin for Centerville Canyon could cost up to $250,000. The county has all but exhausted the $12 million in flood-control bond funds on other projects, Smith said, and still has some to finish.
Centerville city officials are preparing an appeal of the FEMA findings based on the Davis model of debris flow, and the County Commission agreed to take the issue under study.