- Author
- Albini, F. A.
- Title
- Potential Spotting Distance From Wind-Driven Surface Fires.
- Coporate
- Intermountain Forest and Range Experiment Station, Ogden, UT
- Report
- Research Paper INT-309, April 1983, 30 p.
- Distribution
- AVAILABLE FROM National Technical Information Service (NTIS), Technology Administration, U.S. Department of Commerce, Springfield, VA 22161. Telephone: 1-800-553-6847 or 703-605-6000; Fax: 703-605-6900; Rush Service (Telephone Orders Only) 800-553-6847; Website: http://www.ntis.gov
- Keywords
- fire brands | wildland fires | forest fires | wind effects | equations | field tests
- Identifiers
- spot fires; spotting; transport of fire brand particles by thermals; strength of thermals from a line fire; spotting distance examples; power spectral density of an ergodic sequence of one-period "square wave" events
- Abstract
- This paper documents a speculative model of the process by which firebrand particles are lofted into the air through the action of buoyancy-induced airflow near the head of a wind-driven fire in surface fuels. It is postulated that the particles are lofted by strong thermals generated by the fire. The fire and the thermals it generates are idealized asbeing two-dimensional, for analytical tractability. It is further postulated that the fire generates the thermals because the intensity of the fire fluctuates with time in response to variations in windspeed or "gustiness". An increase in intensity above the average value, sustained for some period of time, is assumed to give birth to a line thermal with energy/length (or "strength") equal to the excess energy/length afforded by the intensity excursion. The author has previously published theoretical power spectral densities of intensity variations of line fires in typical wildland fuels, based on a model for the dynamic response of the fire to windspeed variations and an empirical power spectrum function for horizontal wind gustiness near the surface. A simple stochastic sequence of excursions of intensity is used as a surrogate process to approximate these power spectra and so allow explicit expression of thermal strength as a function of windspeed, mean fire intensity, and fuel type. The trajectories of particles lofted by line thermals have been described elsewhere by the author. Maximum viable firebrand height was shown to be proportional to the square root of the thermal strength, and the downwind drift distance during lofting proportional to the product of windspeed and the square root of the loft height. The equations for predicting maximum firebrand height and drift distance during lofting are summarized here in simple form for easy field use. Once the maximum viable firebrand height is known, it can be used to predict the distance downwind that the particle will travel before it returns to the ground. Equations for this calculation have been published elsewhere, and are available as pocket calculator programs. Because several elements of the model process are both speculative and not subject to direct validation, these results are to be considered tentative. Field tests of the spotting distance predictions are sought as a means of testing the utility of the model.