Erosion properties tested on pile burn footprints
October 28, 2008
A team of scientists meticulously monitored water flowing down a dusty rill Monday as they conducted experiments in the scorched remains of a recently burned pile of brush near Lake Tahoe.
As part of the first-ever in-depth experiments to determine how prescribed forest burning affects soil erosion in the Tahoe Basin, the team from Integrated Environmental Restoration Services and Em Consulting tested charred craters left by last week’s pile burns near Chinquapin Condominiums in Tahoe City.
Though the test spot is no bigger than the rain shadow left by a car, the impact of their data will effect how decisions are made throughout the Tahoe Basin.
Having already monitored baseline conditions before Calfire’s prescribed burn project, Em Consulting Hydrologist Drea Traeumer and Integrated Environmental scientists teamed up to run rain and rill simulators directly on the footprint of the burned piles.
While the effects of fuels reduction programs on soil properties cause tension around Lake Tahoe, Integrated Environmental rain simulators hope to shed light on the potential for erosion problems caused by water flow.
“We are happy to cooperate with the project,” said North Tahoe Fire Protection District Forest Fuels Program Manager Stewart McMorrow, who helped oversee the prescribed pile burns last week. “It’s important to know what the true effects of pile burning are.”
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Discussing a slow environmental process like erosion often causes conflict because it is not easily seen. Hoping to provide “facts, not opinions,” the Integrated Environmental project is a step towards educated management level environmental decisions.
“There’s a lot of dialogue from people who think they know what’s going to happen,” said Kevin Drake Monitoring Manager for Integrated Environmental. “We’re coming up with a body of data to have dialogue with concrete information.”
Data taken from the post-burn tests is only the beginning of a complicated process.
After testing is complete, Traeumer will incorporate the information into the Watershed Erosion Prediction Project, a multidimensional model used to predict erosion properties on a large scale.
“Models are complex ‘if then’ statements,” said Drake. “They’re never going to be totally accurate but they help create educated management level scenarios for planning.”
“We are trying to calibrate the WEPP models for the Tahoe Basin with as much local data as possible,” said Traeumer.
Integrated Environmental uses two simulators for the project. The first spurts water out of 900 hypodermic needles to simulate rainfall. A steel perimeter hammered flush with the ground will then help the team gauge how much water gets in the soil, how much runs off, the kind of sediment in the runoff and the size of the particles.
The second simulator mimics surface water flows called rills ” gullies where water concentrates to form a flow channel. The rill simulator is a basic flow gauge attached to hose that will shoot water down a pre-dug rill into a catch basin. Rill velocity, depth and flow will help determine the area’s rill erodibility, a key component of the Erosion Prediction Project.
While typical rain events are not heavy enough to produce substantial visual evidence of erosion, water flows for the rain simulator will be set at 4.7 inches per hour, exceeding the 100-year, one-hour rain event in Lake Tahoe of four inches an hour.
Though the project will produce useful data, more testing is required for a bank of data on the erosion properties of pile burn sites.
“Burns are like fingerprints, every one is different” said Rachel Arst, Integrated Environmental Senior Restoration Planner.
While similar tests are taking place near Tahoe Mountain Road in South Lake Tahoe, the types of soil and piles burned vary between the locations. While Chinquapin’s burned piles consisted mostly of thick brush and bushes, the South Lake Tahoe piles were mostly large logs.
As Integrated Environmental continues erosion model projects they hope to test in more extreme conditions and highly protected areas.
“We go into a project like this with an open mind,” said Arst when asked if there was anything in particular they hoped to find. “Every research project leads to more data and more questions.”