Vietnam War: Forest Fire as a Military Weapon - Paperless Archives

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sets did not even burn together on the first two fires, several seconday explosions were observed on each of the fires.
The timing of the secondaries was such that they could not have been caused by the incendiaries themselves, but could
only have been touched off by ground fires burning into unprotected munitions caches.
2. Running surface fire. The running surface fire burns both the debris on the forest floor and any shrubs and
other undergrowth that may be present beneath the main forest canopy.
Rates of spread vary from 3 to 12 feet per
minute. Flame heights range from 3-4 feet up to 15-20 feet. Grass fires in clearings are a special case of running surface
fire. Fire intensity is about the same as that of running surface fires in the forest, but rates of spread and flame heights
are about two and one-half times those of forest fires of the same intensity.
Running surface fires will burn off 75 to 90 percent of the leaves, twigs, and dead stems up to one-half inch in
diameter from the undergrowth. Horizontal visibility will be increased by one to two orders of magnitude depending on
the number of large stems left to block the view. Running surface fires in the forest do not burn the upper tree canopy,
but they are usually hot enough to kill leaves at heights 5 to 6 times the flame length. Within a few days, these dead leaves
will turn brown and begin to drop off. Vertical visibility will be improved if the average height of the forest canopy is low
enough for the topmost layer of leaves to be killed. The effect of heat kill on opening up the forest canopy for aerial
observation is practically identical to that achieved by spraying the forest with defoliant chemicals:
The leaves fall off,
given time, but the twigs and limbs remain. These residual twigs are often sufficiently dense to seriously hinder photographic
interpretation. They offer practically no hinderance to infrared scanning, laser scanning or other fixed-point,
line-of-sight remote sensing techniques.
Running surface fires are a much greater threat to unprotected stores than are low intensity ground fires. A total of
15 secondary explosions were confirmed in the 50 square kilometer area burned by running surface fire during Operation
Pink Rose II, compared to three confirmed secondaries caused by the ground fires of Pink Rose 1. For the U Minh fire.
which burned as a running surface fire during many of !he afternoons in mid-April, the report of the 96th Advisory Team
notes that secondary explosions were occurring at the rate of one every 20 minutes during these periods.
3. Forest crown fire. Intense fires within the tree crowns require simultaneous intense running surface fires.
Without adequate fire at the ground surface, a crown fire will not be propagated and sustained. Crown fires ordinarily
spread at rates of 20 to 100 feet per minute, although runs of 200 to 400 feet per minute are possible for short periods.
In flat country, crown fires will seldom persist in the absence of a moderate to strong breeze, since wind is necessary to
tilt the flames into unburned foliage at the fire front.
Since a crown fire removes not only the leaves, but also the smaller twigs and branches, vertical visibility is sufficiently
enhanced for accurate identification of ground features by aerial observers or from conventional aerial photography.
Crown fires will destroy or seriously damage all equipment and supplies not stored underground or in clearings at least
one-half tree height from the edge of the forest. Crown fires can also be expected to produce casualties, particularly to
troops who are unfamiliar with forest fire behavior. Intelligence reports following Operation Hot Tip in 1965 listed two
hundred VC casualties out of an estimated population of two thousand VC in the 21 square kilometer target area. Since
only 17 percent of the target area burned as a crown fire, this seems to substantiate the thesis that casualties will be
limited to, and closely correlated with, the areas burned by crown fires.
9/ Mutch, R. W. et al. Operation Pink Rose. USDA - Forest service. May 1967, 121 pp. SECRET.
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