森林立地学会誌 森林立地41(1)，1999，25〜30
Jpn.J.For.Environment41（1），25−30
1999
林野火災延焼地域内の風向・風速の動向
―火入れを利用した現地実験―
井上章二
愛媛大学農学部
前報において，筆者は火災跡地に残された樹木の燃焼痕から火災時の風速を推定するための実験式を提案し，現地実験においてこれらの実験式の適用性を検討した。本研究では山口県秋吉台と福岡県平尾台における火入れを利用した現地実験において，前に提案した実験式を用いて詳細な燃焼動態の解析を行った。
試験地内に実験杭として皮付きスギ丸太を立て，火入れ終了後，火災時の風速を推定するために，片面燃焼高を測定した。主風の風向・風速を試験地の周辺，数カ所で測定し，さらに，火災動態図作成のために燃焼時の様子をビデオ撮影した。
秋吉台試験地の燃焼動態は複雑であり，片面燃焼痕から推定された風向は，一定の方向を示さなかった。しかしながら，主風あるいは実験式から推定された局地風の情報，および，記録されたビデオテープから分析された燃焼の状態を総合することによって燃焼動態を6つのブロックに分けることができた。風速とこれら6ブロックの燃焼速度との関係について検討した結果，燃焼方向と風向が一致している場合は，風速の増大につれて燃焼速度は増加し，燃焼方向に対して逆風の場合は一定の燃焼速度であった。
平尾台試験地では，斜面燃上がりの燃焼動態であり，片面燃焼痕から推定された風向は一定であり，主風向とは逆で斜面傾斜方向と一致していた。実験式によって推定された風速と斜面の最大傾斜角との関係を実験杭個々について検討した結果，両者に一定の傾向は認められず，実験杭位置における局所条件の影響ではないかと考えられた。しかしながら，実験杭個々の値の代りに傾斜階で区分し，その範囲の平均風速との関係を調べると，最大傾斜角が増大するにつれて，風速が増加する傾向を示した。
Shoji INOUE: Behavior of wind direction and wind
velocity in the site of forest fire −Field
experiments using opportunities of prescribed fire−
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.