CEDAR CITY — Iron County has experienced multiple serious floods over the last few years, with sediment and debris identified as “main issues” impacting residents and businesses. As a result, the Natural Resources Conservation Service has developed a plan it says could mitigate flood risk.
Lower Coal Creek Watershed project
“The heart of the project is flood mitigation — preparing for that 100-year storm,” said Jason Dodds, a project engineer with the service. However, despite its name, a 100-year storm won’t only occur once every hundred years.
“Another way to understand the 100-year event is a 1% chance in any given year — in 1982 and 1983, they were back-to-back,” said Iron County Engineer Richard Wilson. “It just means you have a 1% chance. It doesn’t say it’ll take 100 years for it to happen again.
“The other issue that a lot of people misunderstand is: because of statistical accumulation, the 100-year event is guaranteed to happen in 26 years, because each year if you miss the 100-year event, (you’re twice as likely) to have the 100-year event.”
The plan for the Lower Coal Creek Watershed project is to make flood control improvements in the creek from Interstate 15 to 6600 North, with the assistance of its co-sponsors, Iron County and Cedar City. Bowen, Collins & Associates is the engineering firm associated with the project. It would be funded by the service through 1954’s Watershed Protection and Flood Prevention Act, Dodds told Cedar City News.
“A couple years ago, we had a flood in Coal Creek,” he said. “This project was actually in the works prior to that, and so that just showed us exactly what was going to happen when we get high flows, and it matched a lot of our modeling very well — it wasn’t a surprise.”
The flooding in question occurred in August 2021, and although it wasn’t a 100-year flood, many residents were impacted, Dodds said.
“It showed that sediment and debris are the main issues, not necessarily water,” he explained.
Coal Creek experiences “flood flows” approximately three times every 10 years, producing an average of 33-acre-feet or 53,000 cubic yards of sediment, Dodds said while presenting the proposal at a Cedar City Council meeting last November. Both the city and county have worked proactively to reduce flood risk.
“Coal Creek’s been dealt with down past Brad’s; that channel worked beautifully,” he said. “We had lots of floods — all that water came through — No flooding up in town. … We just need to continue that on.”
Without additional flood improvements, however, Wilson said many people throughout Iron County could be required to purchase flood insurance once the 2022-2028 Iron County Floodplain Mapping Project is complete and accepted by the Federal Emergency Management Agency.
“If we do this, those properties are not in the flood zone anymore because now we’ve handled it,” he said.
Dodds said the project’s objectives include mitigating flood risk, maintaining irrigation flows and groundwater recharge, and avoiding induced flooding, Dodds said, explaining, “If we want to take the water and say it doesn’t go here, where does it go?”
The proposed work includes removing sediment from the channel and widening it, and improving irrigation efficiency, he said.
The service would use gravel pits as detention basins to capture and store sediment. One pit would take an estimated 15-35 years to fill, two would fill in about 40-100 years, and three could be used for 55-140 years, assuming the water typically contains 20-60% sediment. Dodds said the conservation service hopes to begin with two pits, using the third if necessary.
The pits are 50-100 feet deep, with footprints ranging from 20 to 40 acres, and are located on the east side of North Airport Road, northeast of Cedar City Regional Airport. Filled pits can potentially be mined for gravel, depending on the type of sediment, and then reused as detention basins, Dodds said.
“The county uses (the gravel),” he said. “The city uses it….
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