During this season’s winter storms, heavy rain flooded Chico’s streams and creeks.
Part of the reason flooding was so severe this season is due to the Park Fire. It burned through 75% of the area’s watershed. This vastly increased the amount of stormwater flowing into Chico’s creeks, because burned soil and vegetation is unable to absorb much rain water.
Multiple storms cause heavy water flows
Experts use the measurement cubic feet per second (CFS) to measure moving water.
Under typical circumstances, water flows in Chico’s creeks would normally be somewhere between 3,000 and 5,000 CFS after a storm. But due to the Park Fire, water flows were more than twice that amount this winter.
During the atmospheric river earlier this month, the National Weather Service accurately forecast a post-fire flow between 9,500 and 11,000 CFS.
Chico officials said local infrastructure handled the increased flow of water with some wiggle room left.
“The whole system can handle flows up to 16,000 CFS,” said David Kehn, a senior civil engineer at Chico’s Public Works Department. “This event, we experienced 10,500 CFS. So it was well within the capacity of the system.”
Experts use a system of equations to get a better idea of how much runoff could come from a burn scar. The amount of land burned and the severity help form the estimation.
Learning from the burn scar
Kehn said the Park Fire’s burn scar is unique because the fire burned 75% of Chico’s watershed. This caused increased flows compared to other fires in the past, Kehn said.
“With the Camp Fire, I think they said about 10% of the Butte Creek Watershed burned,” Kehn said. “So they were able to see increased runoff but it wasn't quite as straightforward as ours – where they're able to say ‘OK, it's going to be roughly 1.7 to 1.8 times bigger than you'd expect.’”
Kehn said this could serve as a way for experts across the state to understand future possibilities.
“Any observations that we can make, it just helps us as a state and us as a society improve and be able to prepare for these type[s] of events.”
He said the Park Fire burn scar in particular is important from a modeling perspective for the state.
“Because almost all the watershed burned. So it's a one-to-one response as opposed to other times if only half the watershed burned or a portion of it,” Kehn said. “It’s really hard to say what are the impacts from the burn scar if only a fraction of the watershed burned.”
Risks still loom
Aside from increased stormwater, burn scars also pose other risks like debris flowing into local streams, which could dislodge topsoil and upend logs. This would look less like ashy water and more like a muddy, viscous flow.
“Thank goodness we didn't experience that,” Kehn said. “But of course, all the ash that is on the top of the soil got washed away.”
That ash could still cause problems.
Post-Park Fire data found by Butte County’s Watershed Emergency Response Team (WERT) says high amounts of ash can affect aquatic wildlife. Ash from burned residential structures can carry downstream and pose hazards to water quality.
“The resulting ash and debris can contain concentrated and toxic amounts of polycyclic aromatic hydrocarbons and heavy metals such as antimony, arsenic, cadmium, copper, lead, and zinc,” the report finds.
The WERT report says it could be a few years until growth in the burn scar begins improving.
