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OCR for page 73
The Medical Implications of Nuclear War, Institute of
Medicine. @) 1986 by the National Academy of Sciences.
National Academy Press, Washington, D.C.
A Review of the Physics of
Large Urban Fires
H. L. BRODE, PH.D., and R. D. SMALL, PH.D.
Pacific-Sierra Research Corporation, Los AngeZes, California
INTRODUCTION
A review of historical urban fires can help to illustrate the nature of
large fires and the devastation that they can cause. The observations and
descriptions of those fires provide the basis for understanding the much
larger fires that would result from a nuclear explosion. The focus of this
paper is on the major physical factors that are relevant to the characteri-
zation of such fires. Atmospheric responses in the vicinity of a large smoke
column are addressed, and the hazards expected to accompany nuclear
fires are briefly discussed.
HISTORY OF URBAN FIRES
Disastrous urban fires have occurred throughout history. In wartime,
cities have been bombarded, sacked, and burned. Fires have also resulted
from earthquake damage, hurricane winds, accidents, explosions, and
arson. Firebombing in World War II was aimed at the destruction of cities
and industries in both Europe and Japan. Despite the large number of city
fires, the available data are mostly anecdotal. Most of the empirical knowl-
edge of nuclear explosion fires has been obtained from the nuclear bursts
at Hiroshima and Nagasaki.
Table 1 lists several major urban fires, beginning with the London fire
of 1666. Although it destroyed an area of nearly 2 km2, only eight people
were killed because the fire moved slowly. The Chicago fire of 1871 killed
73
OCR for page 74
74
PHYSICAL EFFECTS AND ENVIRONMENTAL CONSEQUENCES
City
Area
Burned
Deaths (km2) Comments
London 16668 1.8 Burned 4 days; 32,000 homes lost
New York City 1835
Charleston, S.C. 1838
Pittsburgh 1845
Philadelphia 1865
Portland, Maine 1866
Chicago 187150 8.6 Burned 1 day; 98,500 homeless;
17,500 homes lost
Boston 1872
San Francisco 1906452 12.0 Earthquake-generated explosions
and fires; 30 ignitions; burned 3
days; 100,000 homeless
Halifax, Nova Scotia 19172,000
Tokyo 1923
1925
1932
Niigata, Japan 1925
Yamanaka, Japan 1931
Hakodate, Japan 19342,000 Generated fire storm
Takaoka, Japan 1938
Boston 19421,000 Explosion and fire; burned 3 days;
3,000 injured; 300 missing
Muramatsu, Japan 1946a
Texas City 1947510 Fertilizer ship explosion
Chungking, China 19491,000
Brussels 1967250 Burned 6 hours
Chelsea (London, England) 1973 400 homes lost
Anaheim, California 1982 500 apartments and 1 firehouse
destroyed
Philadelphia 1985 2 blocks of row houses gutted
aApproximate date.
more people and burned a larger area in less time. The San Francisco fire,
following the 1906 earthquake, resulted in greater casualties and left 100,000
homeless. The Halifax, Nova Scotia, explosion started many fires; the
casualty figures include those from the explosion and the fires. Many
people died in the intense Hakodate, Japan, fire storm in 1934. The
explosion of a fertilizer ship in Galveston Bay caused many fires in the
adjoining Texas City, Tex.
Even modern cities are vulnerable to urban fires. In April 1982, some
500 apartments were destroyed in a few hours as a wind-whipped fire
OCR for page 75
THE PHYSICS OF LARGE URBAN FIRES
75
swept through Anaheim, Calif. A flash bomb on a row house in Phila-
delphia in early 1985 led to the burnout of two city blocks. In September
1985, an arson fire virtually destroyed an industrial section of Passaic,
N.J.
These few examples illustrate that major urban fires can be started in
many different ways. In most of these fires, there were few casualties,
although property damage was extensive. When the fires spread from one
or a few ignition points, evacuation and movement from the threat was
possible.
In World War II, European cities suffered extensive fire damage. In
several of the German cities attacked with incendiary weapons, fire storms
developed (Bond, 19461. In particular, Dresden, Hamburg, Kassel, Heil-
bronn, Darmstadt, and Brunswick suffered intense area fires (see Table
21. When intense area fires occurred, damage and casualties were signif-
icantly higher.
In the more than 70 firebombed German cities, it is estimated that
500,000 to 800,000 people were killed. In intensity and magnitude, the
worst fire occurred in Dresden (February 1945) with 135,000 to 250,000
deaths. Hamburg experienced 34,000 to 100,000 deaths in the raids of
July 1943. Fire storms frequently killed more than 5 percent of the pop-
ulation at risk; less intense or isolated fires seldom killed as many as 1
percent of those at risk. Berlin was repeatedly bombed, but its defenses
prevented concentrated attacks, and the resulting fires never coalesced
into the inferno of a fire storm.
In the raids on Hamburg, the explosive and incendiary bombing was
concentrated in an old part of the city, comprised of a high density of
four- and five-story buildings. Almost all buildings in the area burned
simultaneously; the destruction was nearly complete, and for many escape
was impossible. Virtually all combustibles were burned out; only crumbled
ruins or empty masonry shells of multistory buildings remained. Figure 1
is an overhead photograph of a gutted section of Hamburg.
A similar, old section was burned in Dresden. Buildings were an average
of three to five stories high, were closely spaced, and were heavily loaded
with combustibles. The lack of an organized air defense allowed the Royal
Air Force (RAF) to concentrate its bombing, which led to many simul-
taneous fire starts. The intense fire completely and nearly simultaneously
burned out all the buildings in a broad area.
The concentrated B-29 firebombing of Japanese cities lasted about six
months from February to August of 1945. Firebombing raids were made
on 65 cities. Tokyo was the first city attacked, and that fire was perhaps
the most disastrous of all, burning nearly two-thirds of that city plus
Yokahama with great loss of life (perhaps 200,000 dead). Major fires
OCR for page 76
76
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OCR for page 77
THE PHYSICS OF LARGE URBAN FIRES
77
FIGURE 1 Photo of gutted Hamburg buildings on July 24, 1943, after a raid
by the Royal Air Force. Reprinted from Fire and the Air War. @) 1946 National
Fire Protection Association, Quincy, Mass. Reprinted win permission.
occurred at Osaka, Kobe, Kyoto, Nagoya, Nishinomiya, Kawasaki, Shi-
zuoka, and Kumagaya (the last city attacked).
The atomic bomb dropped on Hiroshima instantaneously lit many fires
throughout the city. Three to five minutes after the burst, dust and smoke
from the already burning city could be seen following the rising nuclear
fireball cloud.
Figures 2 and 3 are previously unpublished views of Hiroshima taken
in September 1945. Despite some reconstruction, an enormous amount of
rubble and devastation is evident. Only the skeletons of reinforced concrete
buildings and massive masonry structures or chimneys remained standing.
In the center of the city, fire damage was nearly complete.
The sketch of the damage areas at Hiroshima (Figure 4) shows that for
more than 1 mile in radius (about 1.6 km) around ground zero, the de-
struction was heavy; nearly all buildings were burned out. An appreciable
number of additional buildings were burned out to 2 miles (about 3 km);
even as much as 3 miles (about 5 km) away, some damage was experienced
and a few fires occurred. The yield of the Hiroshima bomb is now estimated
at about 15 kilotons (kt). The height of the burst was at an altitude of
1,860 feet (about 567 m). Modern strategic weapons have yields in the
hundreds and thousands of kilotons. Today, an attack on a city like Hi
OCR for page 78
78 PHYSICAL EFFECTS AND ENVIRONMENTAL CONSEQUENCES
FIGURE 2 View of central Hiroshima in September 1945 showing rubble and
reconstruction. (From the private collection of W. Shephard.)
FIGURE 3 View of Hiroshima, September 1945. (From the private collection
of W. Shephard.)
OCR for page 79
THE PHYSICS OF LARGE URBAN FIRES
INNER CIRCLES 1/2 Ml
OTHER CIRCLES 1 Ml
2 Ml
3 Ml
COMPLETE BURNOUT IN
SEVERE DAMAGE AREA
/
Em_
tY
it'
, _0
~ '
~5
A
Hi/, /
ta SEVERE DAMAGE
4.5 M12
C55 MODERATE DAMAGE
4.2 M12
LIGHT DAMAGE
7.5 M12
. _~
FIGURE 4 Map of damaged areas in Hiroshima.
1 ~_ ~
79
.,,, ,
~ :
_,, _ _, . . .
. .,
rosn~ma Would probably employ one or more weapons with much larger
yields, in keeping with either U.S. or Soviet targeting philosophy and
weapon availability.
Figure 5 illustrates the percentage of buildings that were burned as a
function of distance from ground zero at Hiroshima. At a range of more
than 1 nautical mile,* more than half the buildings were gutted by fire.
At that point, the peak overpressure of the nuclear blast wave was about
3 psi, and the fireball heat or thermal fluence was about 8 or 9 cal/cm2.
The surveyed area was composed mainly of industrial or commercial
buildings, with some residential structures intermingled among them.
The bomb that was dropped on Nagasaki also caused intense fires,
though not as widespread as those at Hiroshima because the bomb was
exploded over an industrial area, much of which was not highly built up.
. .
*One nautical mile is about 6,076 feet, 1,851 meters, or 1.15 miles.
OCR for page 80
80
100
4J
c' 80
a'
Q
-
3 60
an
m
cn
it
9
o
PHYSICAL EFFECTS AND ENVIRONMENTAL CONSEQUENCES
PEAK OVERPRESSURE (psi)/THE RMAL (cal/cm2)
oo
~d. Co
Cal
co ~
~oo
Cal
-- ~I ~1 1 1
_~
_~
-
Co ~
~Cal ~
40
20
_y,>~
\,
\~
0 0.5 1.0 1.5
GROUND RANGE (nmi)
FIGURE 5 Hiroshima fire destruction (nmi = nautical miles).
A number of enclaves of residential and commercial development, how-
ever, were scenes of intense fires.
From the limited past experience, it is clear that nuclear-caused urban
fires can do extensive damage over very wide areas. Although such fires
would be influenced by many factors, intense and widespread fires appear
inevitable in the event of a nuclear attack on urban areas. Our understand-
ing, based on historical evidence and a study of nuclear explosions, is
summarized in Table 3.
CHARACTERISTICS OF LARGE-SCALE URBAN FIRES
Several obvious facets of large-scale urban fires are unusual and deserve
characterization. Some of these unusual features are attributable to the
uncommonly large size of such fires. An intense large-area fire could
exhaust most of the urban-area fuel in a matter of a few hours; some stores
of flammables such as underground oil storage tanks or large stocks of
rubber goods could, however, continue to smolder and burn for days. And
fire in the rubble of collapsed buildings may burn more slowly.
OCR for page 81
THE PHYSICS OF LARGE URBAN FIRES
TABLE 3 Summary of Large-Scale Urban Fire Experience
Nonnuclear
Most urban war damage is due to fire.
Much fire damage occurs due to accidents and natural disasters.
Hiroshima
The central area was nearly completely burned out.
Firebreaks were not effective.
Some fires spread beyond the initial ignition area.
Possibly, many blast-induced ignitions took place (overturned hibachis or charcoal
braziers).
Nagasaki
Target layout led to smaller, separate fires.
Some areas developed small fire storms.
Weather and topography influenced damage.
Nuclear tests
No large-area fire experiments were conducted.
Numerous ignition thresholds were measured.
Thermal phenomena were studied extensively.
81
Intense large-area fires have been known to create unusual drafts, with
winds approaching hurricane velocities. Air temperatures in and near these
huge fires may exceed the temperature of spontaneous ignition for most
burnables. The rising column of hot air, smoke, ash, and combustion
gases from a large urban fire can be expected to rise more rapidly, cool
more slowly, and otherwise behave differently from that of a single house
~-
re.
Table 4 lists some of the parameters and factors needed to characterize
large-scale fires. The burning area may be described in terms of the height
of the flames, the rate of burning or rate of heat released, the nature of
combustion gases, average temperatures, and the amount of buoyancy
created by the fires. Those factors, interactively combined with the city
layout, can yield a large-scale urban fire model.
The column above the burning region can be described in terms of its
rate of rise, the altitude that it achieves, the periodic or transient toroidal
motions, the radiation and chemistry of the gases and particulates that are
carried aloft, and their interaction with winds and temperature changes in
the atmosphere.
City fires after a nuclear burst are different from fires that spread from
a point or along a front: large areas would burn simultaneously. The
diameter of such fires can be comparable in scale to the height of the
atmosphere. Area fires that are many kilometers in diameter can generate
high winds and raise a column of smoke and water vapor that can reach
OCR for page 82
82
PHYSICAL EFFECTS AND ENVIRONMENTAL CONSEQUENCES
TABLE 4 Large-Scale Fire Features for Which Models Are Needed
Fire size (area)
Duration
Surface winds
Air temperatures
Flame heights
Fuel densities
Combustion rates
Heat release
Gas/smoke/ash
Plume heights
Plume dynamics
Vorticities
Atmospher~c/meteorologic influences
Blast damage influences on fire propagation
Multiple burst effects
the tropopause.* This f~re-generated plume can pump smoke and ash into
upper levels of the atmosphere, forming large, spreading clouds. Con-
ceivably, firebrands could be dropped at great distances from the fire
itself, leading to additional fires.
Many parameters should be considered in predicting fire damage from
such large-scale urban fires. Table 5 lists some of these factors. Fires can
result from either blast disruption or thermal (fireball radiation) ignitions.
For the latter, the atmospheric transmission of visible and infrared light
is important, as are the reflection and scattering of light from snow cover
or cloud decks. For blast-induced ignitions, the frequency of open flames,
electrical discharges, or sparks from electrostatic discharge or metal fric-
tion from motions induced by the blast wave can be correlated with the
density, type, usage, and content of hilil~lina~ anti their cilrrniln~linec ~nr1
^. .
nre suppression measures.
~ _~_~>V _.A^ ~^ V ~^ ~ ~ ~^ ~
In some cases, the blast can blow out an incipient thermal ignition, but
it can also fan and spread an established fire. The blast can expose fuels
by breaking up structures, thus leading to the possibility of additional
ignitions by subsequent nuclear bursts.
Multiple bursts on or near the same urban area can exacerbate the fire
damage. A second burst can more readily light fires in the debris of a
preceding burst. It can also scatter burning debris from the first burst and
thus contribute to the spread.
High-altitude bursts could burn cities yet cause relatively little direct
blast damage. A city protected by a low-altitude antiballistic missile de-
fense system could thus be damaged even though no missiles or bombs
actually reach it.
A surface burst radiates about half as effectively as an air burst. In
addition, its fireball lies lower on the horizon, and at large ranges, burnable
*The tropopause is the altitude at which the air ambient temperature begins increasing
with altitude; it is viewed as the dividing line between the lower atmosphere and the
stratosphere.
OCR for page 83
THE PHYSICS OF LARGE URBAN FIRES
83
material is more likely to be shielded from it. Yet even surface bursts are
capable of causing large fires by thermal and blast-disruption ignitions.
A surface burst also leaves a crater, throws ejecta, and causes intense
local radiation fallout.
Rain and snow have helped suppress natural fires, but intense urban-
area fires may not be subject to weather effects most of the combustibles
in a city are inside the buildings and thus are dry. The disastrous Dresden
fire occurred in February with snow on the ground and clouds overhead.
Civil defense preparations could make considerable difference at fire
peripheries, but most passive measures are of limited value. Window
coverings will be blown away. Firebreaks are otherwise of little value if
fires start on both sides of them or large numbers of wind-borne burning
firebrands are earned across them. Even with electrical and gas utilities
turned off, spark ignition from static electricity discharges or metal scrap-
ing on stone or other metal during blast disruption can light leaking volatile
fuels or other flammable materials. The removal of all gasoline or diesel
vehicles could help, but short of tearing down a city, it is hard to greatly
reduce a city's propensity for burning. Active firefighting during or after
a nuclear attack seems quite impractical due to the overwhelming number
of ignitions, blast-caused debris, and the continued hazards to firefighters.
The bulk of the heat that emanates from a nuclear fireball comes out
mostly in a major pulse whose power is illustrated in Figure 6 (left-hand
scale). The integral of that pulse, or the total accumulated amount of
radiated heat, is indicated by the upper curve with the nght-hand scale.
For a 1-megaton (Mt) explosion, the peak occurs at about 1 second, and
the pulse lasts some 7 or 8 seconds. It is a brief, but intense, release of
heat. Roughly one-third of the total weapon yield shines away in this
thermal pulse.
TABLE 5 Variables in Fire Damage Prediction
Weapon yield
Burst height
Thermally induced fires
Visibility/transmittance
Ignition thresholds
Fire propagation probabilities
Clouds/snow cover reflectance
Multiple bursts
Blast-fire interactions
Blast-induced fires
Building construction
Building contents/usage
Building density
Firespread
Firebreaks
Topography
Weather
Countermeasures/civil defense
Preparation/evacuation
Fire fighting
Repair/recovery
OCR for page 85
THE PHYSICS OF LARGE URBAN FIRES
85
Relatively simple formulas can be used to estimate the amount of heat
per unit area, or the number of calories per square centimeter, that will
be felt at various distances. This in turn can be used to predict where fires
will develop. Equation 6 indicates that the fluence decreases as the inverse
square of the distance from the burst.
Q~ 1.07Wk~T/R2, (cal/cm2) `6'
where R is the slant range from the burst (in miles), and T is the ~ans-
missivity or attenuation due to passage through the intervening air.
The ~ansmissivity, and thus fluence, decreases exponentially because
of absorption by moisture or pollutants in Me air (equation 7~. Some
increases can occur, however, by forward scattering of light. A linear
term corrects for this scaKenng enhancement. The transmissivity can be
approximated by
T ~ (1 + l.9R/V) expel-2.9RIV),
(7)
where V is the visibility in miles (12-mile [about 19 km] visibility is
considered a clear day).
As indicated by equation 5, the levels of thermal fluence, or the amount
of heat that it takes to light various susceptible fuels, is a matter of a few
calories per square centimeter. Table 6 lists thresholds for a few materials.
From 3 to 10 cal/cm2 should prove sufficient to light likely fuels at yields
from 20 to 100 kt. At the larger yields, it takes somewhat more total
energy in calories per square centimeter to ignite susceptible matenals.
Factors that can influence the development and spread of fires in urban
areas are listed in Table 7. Although they represent a number of complex
factors, recent studies (Brode and Small, 1984) have attempted to model
TABLE 6 Approximate Threshold Radiant Exposure Needed for Ignition
Threshold Radiant Exposure (cal/cm2)
Fuel 35-kt Yield 1,400-kt Yield
Dry leaves 4 6
Dry grass 5 8
Newspaper (text) 6 8
Cardboard carton 16 20
Rayon (black) 9 14
Canvas 12 18
Cotton shirt 14 21
Heavy cotton drapes 15 18
Black rubber 10 20
SOURCE: Glasstone and Dolan (1977).
OCR for page 86
86
PHYSICAL EFFECTS AND ENVIRONMENTAL CONSEQUENCES
TABLE 7 Factors in Target Susceptibility to Fire
Construction (related fire susceptibilities)
Contents (fuel load and ignition sources)
Adjacent structure susceptibility, proximity
Proximity of vehicles
Window area
Weather conditions (cloud cover, snow cover)
Terrain (uphill spread/shadowing)
Disruption sources (open fires, electrical transformers, etc.)
Fuel volatility and dispersion
Multiple burst/exposure factors
Exposing susceptible fuel to second burst
Blowing firebrands
An- .
Tire suppression
their influences. These studies reveal that, in general, fires tend to burn
out the entire center of the area around ground zero and create at least
some probability of damage out to many miles.
A number of these variables (see also Table 5) were assigned ranges
or uncertainties and were combined statistically. The probabilities for fire
damage as a function of distance from ground zero for a generic city are
plotted in Figure 7 for 50-kt and 1-Mt explosions. For a 1-Mt explosion,
the mean distance to the point at which 50 percent probability of damage
would occur is about 7 miles (about 11 km), but the range could be greater
or, under certain circumstances, much smaller. The two-sigma values
bound 95 percent of the expected variations, i.e., the damage would be
expected to fall outside of these extreme curves only 5 percent of the time.
Only 1 time in 40 might one expect an urban area to be 50 percent burned
out at less than 2 miles (about 3 kin) or to be 50 percent destroyed beyond
6 miles (about 10 km) from a 50-kt airburst.
The results shown in Figures 6 and 7 are for generic cities. The range
of possible fire sizes could be narrowed by choosing specific cities and
weather conditions. Nevertheless, there are many variables that influence
the prediction of fire size, and thus there may remain considerable un-
certainty in damage or casualty prediction. While it may be prudent to
assume and plan for the worst case, it should be noted that smaller values
may be equally probable.
MODELING LARGE-FIRE ENVIRONMENTS
Despite the rather large number of disastrous area fires, there exist little
technical data. Observations by survivors are, in most cases, sketchy and
seldom provide sufficient information to construct and verify theoretical
OCR for page 87
THE PHYSICS OF LARGE URBAN FIRES
8C
6C
4a
an
20
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- 8C
11
CO
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a
._
._
2
CL
87
~ 100
c
Q
-
E
._
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4 -
. _
| ~ ~ ~M an
Ground range, R (ml)
6C
4a
2C
-, _
Fire damage range summation curves: all parameters, W = 50 KT.
~ +2
Ground range, R (ml)
Fire damage range summation curves: all parameters, W = 1 MT
FIGURE 7 Fire damage range summation curves.
models. Nevertheless, when whole areas have burned simultaneously,
unusual and extreme conditions have resulted. Survivors of the fire storms
at Hamburg, Dresden, Hiroshima, and other cities recall similar experi-
ences. Extreme temperatures and high-velocity fire winds were reported
in each of these fires.
We have developed analytical models that explain many of the phe-
nomena observed in the World War II city fires and predict what might
occur for a large-yield nuclear attack on an urban area. Basically, the
models consider the simultaneous ignition of fires over a large area and
OCR for page 88
88
PHYSICAL EFFECTS AND ENVIRONMENTAL CONSEQUENCES
the resulting distribution of buoyancy (Small et al., 1984, 1985). The
buoyancy initiates a chain of interrelated effects. Pressure forces are cre-
ated, and as a consequence, a broad upward motion supported by a high-
velocity inward flow (the fire winds) is produced (Smith et al., 1975; Cox
and Chitty, 1980; Zukoski et al., 19811. This simple view neglects many
important transient features of large fires. Nevertheless, analysis of such
flows explains many of the observed phenomena. Higher velocities could
occur, however, if a swirling column develops (Carrier et al., 1982) from,
for example, topography, ambient wind shears, or fire-generated entropy
gradients (Weihs and Small, in preparation). Such flows rarely seem to
occur. In general, motions resulting from the fire-generated strong buoy-
ancy account for the high-velocity fire winds.
From our analyses, it appears that large-area city fires in World War
II, as well as those that might result from nuclear weapon explosions over
urban areas, are different from small laboratory-scale fires or isolated
building fires. In many of the large World War II fires, all combustibles
were consumed. This is not always the case for an ordinary building fire,
a fire involving several buildings, or spreading fires that burn along a
front. Furthermore, fires with radii approaching 5 to 10 km will have
convection columns or plumes that are almost as wide as they are high.
In fact, for low inversion heights in the atmosphere or strong ambient
winds, the plume may have greater width than vertical dimension.
An analysis of large-area fires should include at least three special
features. First, plume motions stem directly from fire dynamics, and
therefore, the fire source must be modeled in some detail. Second, since
the plume is likely to be fairly broad relative to its height, edge entrainment
of ambient air is not likely to be a major factor influencing the plume
equilibrium in the atmosphere. Third, the plumes above large-area fires
are more seriously influenced by atmospheric gradients, inversion heights,
and upper atmosphere crosswinds.
Our approach has been to develop a detailed analytical model of the
fire region and to calculate (in numerical experiments) the atmospheric
responses to widespread fires or, in modeling terms, large heat additions
in a finite surface volume.
The fire or source-region analysis (Larson and Small, 1982a,b; Small
et al., 1984, 1985) relates the heat addition to the production of buoyancy
and to the induction (and turning upward) of the fire winds. The analysis
is valid only in the vicinity of a large fire. Even though transient features
are neglected, this analytical view provides some insight into the principal
persistent features of large fires. In addition, the steady-state analysis
provides some insight and guides the formulation of time-dependent cal-
culations. Some sample results are shown in Figures 8 through 10. Tem
OCR for page 89
THE PHYSICS OF LARGE URBAN FIRES
y
J
4.0
3.0
8
x
-2.0
._
-
1.0
-
Temperature {°K x 293)
-
O- ~J
o
1.l Jo r
, , ,
0.25 0.50 0.75
Radius {km x 10)
FIGURE 8 Source-region temperature contours: 10-km radius fire.
89
perature ratios for the fire region are given in Figure 10. Those ratios
represent the average of both the burning structure and street air temper-
atures. The street air temperatures are, of course, lower than the fire
temperatures; nevertheless, the predicted mean values indicate an ex-
tremely hostile thermal environment for survivors of the blast. Figures 11
and 12 show that large fires can indeed generate hurricane-force winds.
Velocities on the ground approach 90 miles per hour (40 mist for the
largest fires. These derived velocities are averaged over space and time.
Actual velocities in streets or channels may be larger. In general, the fire
wind velocities are greater at the fire edge than in the center of the burning
area. Survivors attempting to escape the burning zone would meet pro-
gressively higher wind speeds.
We have also simulated the time-dependent dynamics of such large-
scale fires and the resulting atmospheric responses. A two-dimensional
implicit hydrodynamics program was used for numerical calculations that
modeled the dynamics of very large fires (Small et al., 1984, 1985~. Such
calculations employ finite difference methods to approximate the differ-
ential equations of motion. The model accounted for radiation, the buoy-
ancy generated by the heating by the fire, and the subsequent rising of
the plume in the atmosphere. The results show high velocities near the
OCR for page 90
9o
PHYSICAL EFFECTS AND E~IRONME~ CONSEQUENCES
max
E
-
4 -
o60
-
~40
o
._
20
E
._O
~O
-
/
' ' '~
20R
10
Fire radius {km)
FIGURE 9 Fire wind dependence on radius.
ground surface, a rapid decay of buoyancy above the flames, significant
periodic vortex motions around the rising column, and occasional pene-
trations of the tropopause by the plume. Significantly, the bulk of the
column and cloud (containing the smoke and ash) remains below the
tropopause, i.e., it does not penetrate into the stratosphere where it might
remain for long periods. Clearly, the structure of the atmosphere plays a
major role in limiting the plume rise.
-
c'
~ Umax
-
o
60
40
._
o
~ 20
._
x
Ol )
/
I 1 1 1 >
60 120 OH
Burning rate scale (kcal/m2-sec)
FIGURE 10 Fire wind dependence on burning rate: 10-km radius fire.
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THE PHYSICS OF WAGE URBAN FIRES
91
An example calculation shown in Figures 11 through 15 models a fire
with a 10-km radius. Such a fire would be representative of a 1-Mt yield
explosion over a very large urban area. In the first 15 minutes, the intensity
of the burning is linearly increased and then held constant at 100 kW/m2.
This simulates the development of a superfire of many thousands of ig-
nitions. Initially (Figure 1 1), there is evidence of a very turbulent motion
with several distinct rotating cells. The motions extend several kilometers
above the fire.
At 25 minutes, the fire winds are well established and extend beyond
the fire region (Figure 121. Notable in the solutions are complex transient
motions. Some local vortex motions account for periodically high cen-
terline velocities that may loft combustion products through the tropo-
pause; other vortices influence the ambient air induction (fire winds) and
the plume rise. These vortex motions vary somewhat periodically with
time. There is, however, a unifo~ity and an overall persistence to the
Mow.
The calculated plume motions show that the atmosphere plays a major
role in the equilibrium height attained by the fire products. The lofting of
fire products is limited; the tropopause effectively caps the flow (see
Figures 13 and 14~.
30
25
20
10
Velocity {mJsec)
. . . . . .
O 40
Time. 15minutes
ill 1 i
~ ~ ~ ~ ~ ~ i i i i i i
5 10 15 20 i5 30 is 40 4 5
Radial distance (km)
FIGURE 11 Velocity vectors at 15 minutes after the start of a 10-km radius
fire.
OCR for page 92
an
2S
1C
Vel=ity (ma)
^ . . . . . .
U ~Time = 25 minuls
Ii iT in iT Is 1 it i is 1 I .1 1
Radial Dana ~m)
BOUT 12 Velocity vector ~ 25 minu~s aDer me s1= of a 10-- radius
Ha.
so~
ask
an .
~ = ~ m-=
OGURE 13 Seam lines in atmosphchc chculabon generated by a 10-km radius
0~ 40 minutes after ignition.
OCR for page 93
OFFS
30 .
\
10
6
/
0 5
P]
~ = ~ ma
10 15
20 as 30
HI ~ (a
35 40 45 so
FIGURE 14 Seam lines in atmospbedc circulation chewed by a 10-km radius
hm 1 bow ^r ignidon.
30
TIme = 75 ml-`
0 51 0 1 5 20 25 30 35 40 45 ~
Rad~I daunt (km)
FIGURE 15Seam lines Move a 10-- radius hm ~ 1 hour 15 minutes after
. . .
1gnlOon.
OCR for page 94
94
PHYSICAL EFFECTS AND ENVIRONMENTAL CONSEQUENCES
At 75 minutes (Figure 15), the bulk of the circulation is well below the
tropopause but it extends many kilometers beyond the 10-km fire radius.
Evident above the main flow is another layer containing combustion prod-
ucts that originally penetrated to higher altitudes and then fell back. Both
layers are contained in the lower atmosphere.
Both the steady-state source-region analysis and the time-dependent
numerical simulations portray physically consistent flow fields. There are,
however, a number of simplifications and assumptions contained in both
models. That there is reasonable agreement between theory and experiment
lends confidence to these models. Nevertheless, it is important to recognize
deficiencies: processes such as turbulence, radiation, and heat release
require improved modeling. The development of these models is contin-
uing.
SUMMARY
The concentrated attacks on urban centers during World War II resulted
in several city fires in which large areas burned simultaneously. Extreme
temperatures and wind fields were created by those fires. Despite a well-
organized German civil defense, firefighting, rescue operations, and emer-
gency medical aid were severely limited in many of the fires and totally
ineffective in the intense fire storms.
Even larger area fires are likely to result from nuclear weapon bursts.
A 1-Mt yield weapon can start fires over several hundred square kilo-
meters a fire area many times larger than those in Hiroshima, Hamburg,
or Dresden. In fact, a greater area may be damaged by fire than by blast.
High mean temperatures, hurricane-force winds, and toxic gases would
characterize the street-level environment. Analysis shows that the fire wind
velocities increase with the size and intensity of the burning city and peak
at approximately 90 miles per hour (40 mist. Local wind values in natural
channels, between buildings, or in streets may be somewhat higher; and
gusts to even higher speeds can be expected. Intense fires, and thus a
more severe fire environment, are likely in the more densely built cities.
Our calculations show that large-area fires will produce high mean air
temperatures. Flame convection and radiative heating would produce a
hostile temperature environment throughout an intensely burning city,
even in the streets. Extreme thermal conditions and noxious gas accu-
mulations would also be likely in shelters not properly designed to diffuse
or dissipate the heat load and filter the smoky and poisonous air.
The induced fire winds would be drawn into the burning city from
surrounding areas. Measurable velocities may be felt as far as 40 km from
OCR for page 95
THE PHYSICS OF LARGE URBAN FIRES
95
the fire, and significant wind speeds may be felt as far as 10 km from the
fire edge. This inflow would feed and fan the fires and replace the gases
of the rising plume or smoke column. The smoke would mostly be con-
tained in the lower atmosphere, although some may be injected to higher
altitudes. The possible long-term or climatic effects of these fires are
currently being investigated by a number of agencies and laboratories.
The smoke load injected into the atmosphere by a nuclear war is the
subject of a continuing study by us. A previously published study provided
detailed estimates of smoke from attacks on nonurban targets (military
strategic forces) (Small and Bush, 19851.
REFERENCES
Bond, Horatio, ed. 1946. Fire and the Air War. Boston: National Fire Protection Asso-
ciation.
Brode, H. L., and R. D. Small. 1984. Fire Damage and Strategic Targeting. PSR Note
567 (DNA-TR-84-272). Santa Monica, Calif.: Pacific-Sierra Research Corp.
Carrier, G. F., F. E. Fendell, and P. S. Feldman. 1982. Firestorms. TRW Report 38163-
6001-UT-00. Redondo Beach, Calif.: TRW Systems.
Cox, G., and R. Chitty. 1980. A study of the deterministic properties of unbounded fire
plumes. Combust. Flame 39:191-209.
Glasstone, S., and P. J. Dolan. 1977. The Effects of Nuclear Weapons. Washington, D.C.:
U.S. Department of Defense and U.S. Department of Energy.
Larson, D. A., and R. D. Small. 1982a. Analysis of the Large Urban Fire Environment.
II. Parametric Analysis and Model City Simulations. PSR Report 1210. Santa Monica,
Calif.: Pacific-Sierra Research Corp.
Larson, D. A., and R. D. Small. 1982b. Analysis of the Large Urban Fire Environment.
I. Theory. PSR Report 1210. Santa Monica, Calif.: Pacific-Sierra Research Corp.
Nielsen, H. J. 1970. Mass Fire Data Analysis. DASA Report No. 2018. Chicago, Ill.: IIT
Research Institute.
Small, R. D., and H. L. Brode. 1983. Thermal radiation from a nuclear weapon burst.
LLNL-CONF-8305107. Pp. 211-216 in Proceedings of the 17th Asilomar Conference
on Fire and Blast Effects of Nuclear Weapons. Monterey, Calif.: Lawrence Livermore
National Laboratory.
Small, R. D., and B. W. Bush. 1985. Smoke production from multiple nuclear explosions
in nonurban areas. Science 229:465-469.
Small, R. D., D. A. Larson, and H. L. Brode. 1984. Asymptotically large area fires. J.
Heat Trans. 106:318-324.
Small, R. D., D. Remetch, and H. L. Brode. 1985. Atmospheric motions from large fires.
American Institute of Aeronautics and Astronautics Paper 85-0458. Paper presented at
the 23rd Aerospace Sciences Meeting, Reno, Nev., January 14-17.
Smith, R. K., R. B. Morton, and L. M. Leslie. 1975. The role of dynamic pressure in
generating fire wind. J. Fluid Mech. 68:1-19.
Weihs, D., and R. D. Small. In preparation. On the possibility of large area fires swirling.
Los Angeles, Calif.: Pacific-Sierra Research Corp.
Zukoski, E. E., T. Kubota, and B. Cetegen. 1981. Entrainment in the Near Field of a Fire
Plume. Report NBS-GCR-81-346. Washington, D.C.: National Bureau of Standards.
Representative terms from entire chapter:
fire damage
