The results of a couple of trips north last week into the flood zone in the canyons below the watersheds burned by the Las Conchas fire (sub/ad req):
In Cochiti Canyon, a New Mexico Environment Department team estimated the Aug. 21 flow that caused the first round of damage at Dixon’s at 12,000 cubic feet per second of water — more than 10 times the entire current flow of the Rio Grande.
After that flood, Ralph Ford-Schmid and his Environment Department colleagues installed a new temporary flow meter in Cochiti Canyon upstream from the orchard. They put it six feet up in a tree on an embankment that was another 10 feet above the canyon bottom. “We thought it was safe that high off the ground,” said Ford-Schmid.
It was not. It was battered by Monday’s flood pulse, and Ford-Schmid said a calculation of the height of the water the second day suggested 16,000 to 19,000 cubic feet per second.
On a normal forested landscape, Meyer said, only 2 percent of the water that falls as rain flows off and downstream. Following a major fire, as much as 75 percent of the water can run off the landscape.
Once the water gains momentum, it can begin cutting little rivulets into the hillsides, “almost like the whole thing’s cat-scratched,” Meyer said.
As the running water picks up fine soil particles, it becomes more dense, which allows it to carve deeper and carry ever larger rocks and boulders, feeding on itself, creating a thick heavy slurry of flowing debris and gaining momentum as it goes. Jones’ photos of the second-day Cochiti flood show a wall of thick gray slurry plowing through the Dixon apple groves.