森林立地学会誌 森林立地41（1），1999，19〜24
Jpn.J.For.Environment41（1），19−24
1999
林野火災における樹木の片面燃焼に関する基礎的研究
―風洞実験による風速推定式の検討―
井上章二
愛媛大学農学部
林野火災延焼拡大の動態を解析するためには，風向・風速データが不可欠であり，火災現場から離れた気象観測所のデータを用いることが多い。しかし，火災時の熱で発生すると考えられる局地風の風向・風速を知ることは困難である。
本研究は，火災跡地の立木に残る燃焼痕（いわゆる樹木の片面燃焼痕）から火災時の現地の風向・風速を推定する方法を検討したものであり，燃焼痕に関係する因子を明確にするため風洞による燃焼実験を行った。
風向については，いずれの実験条件においても，樹木の風下側の燃焼痕が風上側に比べて高い位置まで残っており，風向は樹木の燃焼痕から推定できることが明らかとなった。
さらに，風上側の燃焼痕の高さは，風速が大きくなるほど低くなり，風下側の燃焼痕は風速とは無関係に直径が大きくなるほど高くなった。また，風下側の燃焼高と風上側の燃焼高の差（Hd）が，風速を推定する上で重要な因子であることが確かめられた。しかし，この結果を現地へ適用するためには，風洞実験と実際の林野火災とのスケールを考えると，風下側燃焼高と風上側燃焼高の比（Hr）も，重要な因子であると考えられ，次元解析の結果，次の実験式が導かれた。
風上着火：H/D＝1.86×10-6(U・D/v)1.64
風下着火：H/D＝7.24×10-5(U・D/v)1.20
ただし，H＝Hd×Hr，Dは直径，Uは風速，vは空気の動粘性係数
火入れを利用した現地実験において，実験式の適用性を検討したところ，高い精度で火災時の現地の風速が推定できることが確認された。
Shoji INOUE: A fundamental study on fire-scar of
stem in a forest fire. -Estimation of wind velocity from stem-bark char by
examination using wind tunnel-
Wind direction and wind velocity data
are necessary to analyze the behavior of forest fire and its propagation. It is
possible to obtain prevailing wind data from weather stations but not the
characteristics of fire whirlwind at the site of the forest fire unless we
measure it at the time of the fire.
This paper presents a procedure of
estimation of wind direction and wind velocity from the stem-bark char remained
at the forest fire damaged area. In this investigation, in order to clarify the
relation among the factors affecting fire-scar of stem, wind tunnel experiments
were carried out.
It was observed that the height of
stem-bark char registered at the windward face of a stem is smaller than that
of the opposite face of a stem under all the experimental conditions.
Therefore, an estimate of wind direction from the remaining stems in a burned
field can easily be done.
Further it was found that, the height
of stem-bark char of windward face of a stem decreased as the wind velocity
increased. The height of stem-bark char of a stem opposite the windward
direction increased as the diameter of the stem increased. It was derived from
these results that the magnitude of the difference (Hd) in height of stem-bark
char of windward face and opposite face of a stem was an important factor for
estimation of wind velocity. From a viewpoint of field application, stem-bark
char ratio (Hr) of both faces of a stem tends to be significant as well as the
difference in height of stem-bark char of both faces, because of the
dissimilarity of scale in wind tunnel experiment and actual phenomena in the
field. Dimensional analysis was carried out for the factors affecting the
stem-bark char using the observations mentioned above.
Based on the results of the study we
propose the following equations for estimation of wind velocity when the stem
partially burned.
H/D=1.86×10-6(U. D/v)1.64 in case of heading fire,
H/D=7.24×10-5(U. D/v) l.20 in case of backing fire,
where, H is
Hd×Hr, D is diameter of stem, U is wind
velocity and v is coefficient of kinematic viscosity.
We compared the wind velocity
calculated by experimental equtions with wind velocity observed in two
prescribed fire sites. It was observed that the wind velocity could be
expressed with a high accuracy by the above proposed equations.