Burn Area Flooding Ecology


The changed ecology of burned areas significantly increases the risk of flooding and flash flooding.

- Burned vegetation changes the water balance on denuded hillsides

- Burned organic matter on the soil increases water repellency

- Runoff is increased, carrying debris rapidly downslope

Additionally, runoff flows and debris combine with autumn leaves to clog drains for localized ponding and flooding on and around roadways.

See also https://www.weather.gov/riw/burn_scar_flooding

Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Email: jnull@ggweather.com  
Web: http://ggweather.com
Twitter: https://twitter.com/ggweather
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3 responses
The trouble with this type of posting is that it sensationalizes a statistical issue. The hydrophobicity is dependent on the type of vegetation and the intensity of the fire. Fires are quite heterogeneous. The root systems take a few years to decompose. The likelihood of mudslides depends on the slope, the depth of the soil, the intensity of the fire, the sequence of rains and the timing of fire response species germination (rain and temperatures) and the speed at which resprouting species recover, both which hold the slopes. There are likely a few places with slide potential, but this type of posting will encourage a response to 'do something' like seeding with grass. Grass seeding inhibits recovery of vegetation and increases future fire potential (flash fuels) and likelihood of slides (poor vegetation recovery).
Please note that the post does not mention mudslides. It talks about flash flooding and debris flows.
Yes, but the bottom right figure is clearly a mudflow, and with rock and plants might be called a debris flow. The recent fires were mostly grassland, chaparral and mixed evergreen forest (that weren't houses). The diagrams reflect conifer forest, a chemically different type of place. My only concern is not arguing about this, just that management practices tend to worsen the situation rather than improve it. You may feel that you're just providing information, but it won't be received that simply.