Northern Arizona may have seen a ‘nonsoon’ last year, but as Coconino County and City of Flagstaff officials look at the risk of post-fire flooding off of the Museum Fire burn scar, they aren’t counting on the weather to stay dry again.
This month, county and city staff say they will begin efforts to prepare areas of Flagstaff and other sections within the county for the risk of flooding off of the burn scar.
Since the Museum Fire burned nearly 2,000 acres of the Dry Lake Hills north of Flagstaff in 2019, concrete Jersey barriers and thousands of sandbags have been stacked along streets and around homes throughout the neighborhoods of Shadow Mountain, Grandview and Sunnyside.
That fire burned 52% of the watershed that feeds into the Spruce Avenue Wash, and more than half of that area was burned severely or moderately. That wash feeds directly into the city, passing through the community of Mount Elden Estates before going into city limits.
Flooding in the area could impact as many as 400 homes and 50 businesses.
So far, the last two monsoons have brought little moisture, and any flooding has been minimal.
But that hasn’t put city and county staff at ease, and at a recent joint meeting between the Flagstaff City Council and the Coconino County Board of Supervisors, staff told the elected officials that the risk of flooding is still very real.
Joe Loverich with JE Fuller Hydrology said the Museum Fire burn scar has recovered very little in the intervening two years.
With little rain, large amounts of ash-baked dirt still cover much of the area and there are few plants left to hold that dirt in place when the rains do come, Loverich said.
In addition to eliminating ground cover and plants that can help hold soil in place and soak up rain, severe fires are also known to burn hot enough to turn sections of the soil to a glassy substance that increases the amount of runoff generated.
Loverich said their modeling shows that an hourlong storm that brings one or two inches of rain to the now-burned watershed could create a significant flood event. He said they modeled that kind of storm because, while it would be a significant event, it is not altogether uncommon. Between 2011 and 2020, there were four storms that fit those specifications near the current burn scar.
Based on the model, water is largely contained to only the spruce wash and the neighboring streets as it moves through Grandview, but as water enters Sunnyside, it appears to spread out significantly and impact much of the area with as much as a foot of water.
Loverich said based on how little rain the last two monsoons have brought, they believe the model is accurate.
Last year, a small rainstorm occurred over the watershed, creating small flows off the burn scar.
“It turned out to be a fairly small storm, but what it did is it showed that the watershed is very responsive,” Loverich said.
Public Works Director and Deputy County Manager Lucinda Andreani said given the possibility of flooding, the sandbags that have been up around homes for two years will have to remain in place. She said county and city staff will be starting on efforts to shore up and repair those sandbag walls ahead of the monsoon season.
Andreani said those efforts will begin in mid-May.
The thousands of sandbags used by the city and county in the aftermath of the fire are supposed to have a lifespan of about four years. But Andreani said they have seen between 30% and 40% of the bags have degraded and will need to be replaced over the next several months.
“As we know, sandbags degrade over time, particularly with the level of UV sunlight exposure that we've had over the last two years,” Andreani said.
As was done last year before the monsoon season, Andreani said city and county crews will place pallets of bags throughout the Sunnyside neighborhood. From there, property owners and residents will be able to take the bags and use them to rebuild the walls near their homes.
The city is also working out a plan to dispose of the bags that have deteriorated, and the related cinders and sand that are no longer useful, Andreani said.
The Arizona Conservation Core will help build and repair sandbag walls for older or disabled residents who may be unable to do that work on their own.
City spokesperson Jessica Drum said the efforts will also include widespread outreach on the part of both the city and county. One thing they will be emphasizing to residents is the importance of the sandbag walls being comprehensive.
Removal of any one section can compromise the effectiveness of everyone’s work and impact adjacent properties, Drum said. And that could even leave a resident liable to pay for the damage.
Adrian Skabelund can be reached by phone at (928) 556-2261, by email at email@example.com or on Twitter at @AdrianSkabelund.