Models show Pipeline Fire ‘significantly’ increased flood risk in Flagstaff area | Local

The 26,000-acre pipeline fire burned several watersheds in the San Francisco Peaks, increasing the threat of post-fire flooding for several neighborhoods downriver in the Flagstaff area. Recently released flood models incorporate data from ground burn severity analyzes to predict flood patterns under severe storm conditions.

Models show that thousands of homes are likely to be affected.

With a ‘wetter than usual’ monsoon season predicted, residents are urged to take preventative measures to keep their homes safe as city and county officials race to implement mitigation flooding in affected areas.

The monsoons arrived about two weeks earlier than expected, said Brian Klimowski, lead meteorologist at the National Weather Service outpost in Flagstaff.

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The U.S. Commerce Department’s rainfall forecast suggests a 33-40% chance of “above normal” seasonal rainfall between July and September. With above normal rainfall, an increased threat of flooding below the burn scars is expected.

Watersheds affected by pipeline fires drain into two distinct “zones”. The first—now called “Pipeline West”—encompasses the Chimney and Schultz Creek watersheds which drain into the Rio De Flag and eventually drain into downtown Flagstaff. The second, “Pipeline East”, encompasses the watersheds of Government Tank, Paintbrush-Siesta, Campbell, Copeland, Peaceful, Brandis, Lenox-Wupatki Trails – which drain into the neighborhoods of Timberline – Wupatki Trails, Fernwood and Doney Park.

West Pipeline

“The concerning aspect” of Pipeline West’s watersheds is their severity, said Joe Loverich, project manager for JE Fuller, the engineering firm that conducted the recent flood modeling.

“There are significant areas in Schultz Creek that were moderate or high [burn severity]Loverich said at a Flagstaff City Council meeting on Tuesday night. More severely burned soils easily repel water, correlating the severity of the soil burn and the risk of flooding.

Based on modeling by JE Fuller for Pipeline West, in the event that 2 inches of rain falls on the affected watersheds in 45 minutes, the water is expected to flow through Schultz Creek without impact until it meets highway 180 culverts near Flagstaff Fire Department Station No. 5. There it is expected to back up at culverts and flow onto Highway 180 in the Cheshire neighborhood near Stevanna Way, where it could flow to depths of 3 three inches.

From there, Loverich said the water would be inclined to take the “easiest route” to the Rio De Flag and through the streets of Cheshire at depths of around an inch, “covering the whole street”. .

The water is expected to cross Cheshire on its way to the Rio De Flag, backing out at Meade Lane and continuing south past Beal Road.

Unfortunately, there simply isn’t “enough infrastructure” to effectively transport increased flows to the Rio de Flag, Loverich said. He also pointed out that the uncertainty in the flood pattern increases as they proceed south into the Rio de Flag, as the Rio de Flag also absorbs water from several other sources.

“The Rio de Flag is a regional waterway,” Loverich said. “We’re not modeling rainfall across the entire 50 square mile watershed.”

The other caveat to modeling is that water can change its course.

“When that water comes out of the watershed, it’s not clear water,” Loverich said. “Debris, sediment, rocks going downhill can quickly alter the course of these streams. We try to predict the worst case scenario where this water would go.

For reasons of uncertainty, the models did not address flooding further into the Rio de Flag, in the downtown and south side areas. In these areas, common sense and historical knowledge will be key to preventative measures, said Samuel Beckett, director of public works for Flagstaff.

“People who have been here know where they get water,” Beckett said. “If you were out during Sunday’s storm and received water in the downtown area, you should probably consider sandbags at this point.”

Preventive measures in the Cheshire neighborhood should focus on the area where culverts on Highway 180 are expected to “become overwhelmed quickly,” Beckett said.

City public works officials are working to implement emergency mitigation measures in the area, including putting up concrete barriers at Stevanna Way, but supply issues could slow the process .

“Barriers, while ideal, are difficult to overcome,” Beckett said.

Area residents are urged to use sandbags, especially near areas that could allow flood waters under the house, such as doors, windows and basements. Self-filling sandbag stations have been installed at Thorpe Park and the “Y” of Schultz Pass Road and Mount Elden Lookout Road to serve area residents.

“Protect your homes,” Beckett said. “Make sure they are safe enough to shelter in place.”

In the event of flooding, residents are advised not to move, but to stay put.

“If you live in Cheshire and are unaffected by water, and we see water on Hwy 180, please stay home,” Beckett said. “Don’t add to the traffic burden on the road.”

East Pipeline

The severity of the burns in the Pipeline East watersheds has “significantly” increased the risk of flooding, said Lucinda Andreani, deputy county executive. She said predictions based on recent flood modeling are “a bit worse” than preliminary modeling suggested.

In a 2-inch, 45-minute rain event, water will descend from the Pipeline East watersheds and pass through the Timberline and Wupatki Trails neighborhoods before reaching Highway 89.

Since the neighborhoods immediately downstream of Pipeline East had varying levels of flood mitigation infrastructure in place after the Schultz flood, some locations are expected to be hit harder than others. The Lenox-Wupatki trails watershed is likely to be “extremely reactive” to precipitation, Andreani said.

This was seen on Sunday, when a protective sediment basin below the watershed filled with “4 inches of sediment” and water “submerged” the channels in the Wupatki Trails neighborhood.

According to Civiltec Engineering Principal Chris Duza, the channels at Wupatki Trails were designed to handle approximately 150 cubic feet of water per second (cfs).

“They saw 800 to 1,000 cfs on Sunday,” Duza said.

According to a public presentation given by the county on Wednesday, rain events of last Sunday’s magnitude are “not unlikely” and are expected to occur multiple times throughout the season.

Other risk areas are those where no mitigation measures exist, such as the Copeland and Peaceful washes. Flooding is also likely below the Government Reservoir watershed, which empties near Switchback Trail and Heppel Drive. This watershed has experienced high burn severity and has no pre-existing attenuation, making it doubly vulnerable.

Once the water passes through the Timberline neighborhoods and across the east side of Highway 89, it should flow heavily into the residential sections near Girls Ranch Road, Leisure Lane, Last Chance Road, Ponderosa Lane, Campbell Avenue and Treadway Trail before splitting. , hitting the Twin Berms east of Campbell and crossing Landfill Road.

Then the waters should converge on Doney Park, entering near Vista De Oro and Dianna Place to move south and eventually “pond” between Carmel Road and Yancey Lane.

Up to 1,500 properties are potentially impacted by these flood patterns, Andreani said.

Coconino County has already mobilized resources to add emergency mitigation measures in heavily impacted areas. This includes concrete barriers installed in “high-speed” areas at Wupatki Trails, Heppel, Paintbrush and a property on Doney Park. They also cleared canals throughout the area, repaired the end trenches of the Campbell and Wupatki trails that were impacted by the tunnel and pipeline fires, and placed concrete barriers between Landfill Road and Doney Park.

In the works is an expansion of the Twin Berms between Highway 89 and Landfill Road, as well as a longer-term expansion of food mitigation systems in the area. Long-term goals will come into play later, according to Andreani. For now, the focus remains on the emergency response needed for this monsoon season.

Affected residents are strongly encouraged to purchase flood insurance and build sandbag walls. Conservation Corps partners have produced 100,000 sandbags for residential use, and on Tuesday 60 inmates at Winslow Correctional Complex will be tasked with producing more sandbags for use in the area. According to the county’s presentation, more than 600,000 sandbags are still needed.

Other Resources

The City of Flagstaff and Coconino County have compiled pipeline fire and flood resources to help residents prepare for increased flood risk.

Pipeline West flood assessment maps, insurance information and rain gauges are available at www.flagstaff.az.gov/4767/Pipeline-Fire-West-Flood-Area.

Pipeline East flood assessment maps, sandbag instructions, insurance information, hotlines, emergency notification sign-ups, address-specific mitigation “exhibits” designed to give homeowners a sandbag plan, and more can be found at coconino.az.gov/2926/SchultzPipeline – Flood Zone.

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