Today was the first experiment of its kind. Experimental burns for fire-weather interactions and fire behavior are usually performed during prescribed burns on flat terrain. Prescribed burns are a useful fire management tool to maintain low fuel (or less vegetation). If a wildland fire develops, the fire would have less fuel to consume and thus would not be as intense. Prescribed burns are rarely performed on hillsides, canyons, and valleys because complex terrain creates difficult fire management. Why you ask? Well fire spreads much more rapidly on slopes.
Upslope flow develops in the daytime due to daytime heating (the sun shines on the ground, warms the ground and the air above the ground, and warm air rises), which results in warm air that rises up and out of a canyon or valley. This upslope breeze will push a fire front further up the hill. Also, the fuels along that slope get preheated because the flames are closer to the fuel on the uphill side. The grass and shrubs are at an angle to the flames, and thus closer to the flames. They get preheated by radiant heating and ignite quicker than fuels next to flames on a flat surface. At the same time, convective heating pulls cooler air in from behind the fire front as heated air rises further upslope. Thus, the draft strengthens and increases the rate of spread. In summary, fires generally spread and grow faster upslope than on flat terrain. This is why fire management is problematic in complex terrain. Since it is so difficult to manage and contain, it is difficult to study. The SJSU Fire Lab was lucky enough to set up such an experiment with the help of CALFIRE. Many of the hillsides and valleys at the Fort Hunter Ligget Army Base are under prescribed burning this week. And they have sufficient firefighters to take care of any problems, in case one of the burns gets out of control.
Picture of hillside to be burned. Towers equipped with various meteorological instruments.
Upwind of the burn site sits a 105 ft tower to measure ambient conditions.
A piece of grass I broke in half to show how dry the fuels were. This dry grass stood about 4 feet high.
Picture just after ignition with drip torches along fire road.
Picture of flames once the fire front reached taller and thicker fuels. Must have been 20 foot flames!
We saw multiple vertical columns of smoke and flames. This rarely occurs on flat prescribed burns, but with this complex terrain, the fire front did not stay in a straight line and circular air patterns developed.