In August, Moab was hit by a record-setting flood, rounding out an active monsoon season. The August flood caused enough damage that the city declared a state of emergency; since then, the city’s been actively assessing its infrastructure damage.
City staff have hauled out 120 tons of debris from the trail bridge along 100 West, 180 tons from the Aggie Boulevard storm drain and retention pond, 60 tons from the bridge along 300 South, and 450 tons from a pond near the golf course, which staff call the “frog pond.” Hundreds of hours of work are still needed, according to Public Works Director Levi Jones—already the streets department has clocked 930 hours and the parks department 400 hours on flood cleanup.
During a city council meeting on Nov. 8, City Engineer Chuck Williams provided an update on possible future flood mitigation projects. The real issue, Williams pointed out, is that the City of Moab is in a major drainage corridor: As the city has grown, developments have encroached on the wetlands and swamps near the Colorado River that builders in the 1950s largely avoided. According to Federal Emergency Management floodplain maps, most of the northern half of the city—from Mill Creek to nearly where 500 W. meets Highway 191—would be flooded in a 500-year event.
Two creeks—Mill Creek and Pack Creek—pass through town on their way from the La Sal Mountains to the Colorado River. Within city limits, there are 6.2 miles of creek corridors; many of the city’s roads, utility pipes, and recreation areas (parks) pass over or through those corridors. When the creeks flood, floodwaters can wipe out major pieces of Moab’s infrastructure, which is what happened in August.
“When we have flooding, damages occur not just in the creek corridor but also outside that corridor, when the creek jumps the creek bed,” Williams said. “…Here in Moab, we live in an erosive environment.”
The city has yet to hear back about receiving funding from FEMA, the Federal Highway Administration, the Natural Resource Conservation Service, or the Insurance Trust.
“When you do flood mitigation, meaning repairs after flooding events, the engineers and hydrologists go through this process of evaluating how we might address that—it’s not one size fits all,” Williams said. Williams said there are three mitigation concepts the city could work within: taking no action at all, taking structural action, or taking nonstructural action.
The “no action” concept would mean the city returns the system to what it was just prior to the August flood. This would be the easiest concept to undertake if the city receives no funding at all, though Williams said he expected the city would receive at least some. He added that if the council decides to go with this option, the city should at least attempt to acquire land easements surrounding bridges in town to more easily stabilize the streambanks when they inevitably flood. When land surrounding bridges is owned privately, as much currently is, city staff have been able to do work on it, but only with the property owner’s permission—owners have no responsibility to provide streambank stabilization for the city.
“Even then, we don’t go in 100 feet, we only go in 10 feet,” Williams said. “And that’s honestly not enough.”
Williams also presented a few potential structural modifications. First, the city could replace the box culvert at 300 S. (which creates the bridge) with a true bridge—the culvert has been an issue during flooding events as far back as the 1970s, as it easily collects debris which then forces water up and over the road. Other modification ideas included installing a storm drain from 300 S. under the creek to 100 W., which would ease some of the constriction that Mill Creek faces in that area; maximizing conveyance under roadway bridges by returning the channel depth to its original elevation; and building and maintaining a series of upstream detention/debris basins.
One of Williams’s largest ideas was to establish, construct, and manage the creeks as a drainage system, rather than a recreational and transportation system, which would be a massive undertaking. City staff would have to choose a design frequency of protection—like protection for a 50- or 100-year flood—and construct a channel system to that frequency.
The structural projects are in no way quick fixes: Many projects would take at least a few years to complete.
Nonstructural modification ideas included implementing a real-time flood warning system—Williams said during a flood, he checks the USGS gages in town, but those aren’t meant for flood purposes, and thus max out their readings around 1100 cubic feet per second. Williams also proposed establishing and maintaining vegetation along the Mill Creek corridor and upgrading the city’s floodplain damage prevention ordinance, which would include increasing the finished floor elevation in new developments to be one foot above the 100-year base flood elevation.
The city council didn’t vote on anything during the meeting—Williams presented the ideas as things to mull over to eventually create a Corridor Drainage Master Plan.
“Our recommendation is that we do pursue this corridor drainage master planning effort,” said City Manager Carly Castle, “so we can come up with a suite of these options, to determine which is going to be best tailored to solve this problem. It’d consider our financial capability, public engagement, recreational use, drainage use, and transportation use, of that area. At this moment, I don’t think we can say, ‘this is the one little fix that will prevent it from happening again.’ It’s such a big problem that it’s going to require a lot of time to plan and a lot of effort to implement.”