Abstract
In the previous investigation, the author proposed experimental equations for estimation of fire whirlwind velocity at the time of the fire from the fire-scar of a stem remained in the forest fire damaged area and observed applicability of these equations in the field experiments. This paper presents the results of analyzing the fire behavior in detail using these equations at two prescribed fire sites. One experimental site was at Akiyoshidai in Yamaguchi prefecture and the other was at Hiraodai in Fukuoka prefecture. In these experiments, Sugi (Cryptomeria japonica) logs were used as a substitute for stems. Factors affecting the fire-scar of stems were measured after burn in order to estimate wind velocity at the time of the fire using experimental equations. The direction and the velocity of prevailing wind were measured at selected points around the experimental sites. Further, scenes at both fires were recorded on videotape for making survey maps of fire behavior. At Akiyoshidai site, investigation of fire behavior was a bit complicated owing to the moisture of vegetation and soil. Wind directions estimated by fire-scar of stems in the experimental site did not show a fixed direction. However, it was found that general fire behavior was simplified into six groups by examining the fire behavior pattern with the prevailing wind data, fire whirlwind data estimated using experimental equations and the state of combustion analyzed from the recorded videotape. Relation between the fire whirlwind velocity and the spread rate of fire among these six parts was studied. In the case of heading fire, the spread rate of fire increased as the wind velocity increased, and was constant for backing fire. In the experiment at Hiraodai site, when the fire behavior was an upslope fire, the fire whirlwind directions estimated from the stem-bark char showed a fixed direction. Fire whirlwind direction agreed approximately with slope azimuth, in spite of a prevailing opposite wind direction. Further relationship between fire whirlwind velocity estimated by experimental equations and maximum slope gradient was investigated. However, it was not possible to recognize a relationship between them using individual data of slope angles and wind velocities estimated, which could be due to the differences in on-site condition of experimental logs. But, dividing the slope into ranges and taking mean values for gradient instead of individual values, it was observed that fire whirlwind velocity increased as maximum slope gradient increased.
