[Music]
in the irpg you'll find information on
how to calculate the size of a safety
zone look at the last paragraph it
states that these calculations are based
on radiant heat only when in need of a
safety zone the more likely scenario may
be that wind and slope are directing the
much hotter convective heat towards you
and a larger area should have been
estimated Brett Butler of the fire lab
in Missoula is doing research which can
help you with that calculation
while this research is still ongoing
there is enough important information
out there now to help you think about
fire behavior in general and how it
affects safety zones I'd like to think
of convective and radiant heating in in
terms of a campfire if we're standing
around a campfire and it's right in
front of us here we're feeling radiant
heating if we hold our hand over the top
of the fire then we're experienced
convective heating but if you hold your
hand there you're gonna get burned
relatively quickly so when you have two
convective heating you need much larger
distances to be safe from burn injury so
the current safety zone guidelines are
based on assumption of radiant heating
only flat terrain and no wind how many
fires do we work on and are we at risk
on where those are the conditions right
the critical factor is that when you
have wind or slope involved in some
cases the energy from the fire can be
propagated much further distances away
from the flame front then flat terrain
with no wind so that's really what we're
trying to understand for every unit
volume of vegetation that burns so if
you know a bunch of needles are
compacted into something like a football
you're going to need six thousand
footballs equivalent footballs of air to
burn that just for the reaction process
that means there has to be a lot of air
coming in
so we've got a flame burning on a slope
air has to come in from all sides as the
slope gets steeper and steeper those
molecules of air that are being drawn in
or blocked by the slope so what happens
is you have fewer molecules coming in on
the uphill side and you get a low
pressure region and that just pushes the
flame the higher pressure on the
downhill side pushes the flame down
against the slope and then the flame
attaches to the slope you have hot gases
that are propagated up the slope and
wind is going to do the same thing so it
could be flat terrain you have a fire
wind blowing on it which pushes the
flame over what happens in either case
is by pushing the flames over so they're
close to the surface you enhance the
convective heating of fuels ahead of the
fire front that increases the rate of
spread increases the depth of the flame
and increases the amount of energy
that's being released when it comes to
safety zones the result is much larger
distances to be safe mid-90s I worked
with Jack Cohen at the lab and we you
know we asked the question what
constitutes an effective safety zone
there was very little information so we
did a relatively simple analysis and
came up ultimately with a guideline
that's currently implemented in the irpg
which is four times the flame height
equals the distance that you need to be
away from the fire there were some
problems I see with that equation one is
that you have to estimate flame height
either based on observations of past
fire behavior or just your understanding
of what the fire may do in the future
that and there's lots of uncertainty in
doing that it depends on experience
levels and observations and how we
perceive fire where I wanted to go in
the future was to get away from half the
estimate flame height and generally you
can make a general rule of thumb is that
flame height equals two times the
vegetation height
plus or minus right so so if we take
that to times vegetation height and put
in our original guideline four times two
times the veg height then equals our
distance for our safety zone four times
two is eight so you end up with eight
times the vegetation height would be the
distance under the current guideline but
that doesn't account for wind and slope
so we need eight times the vegetation
height times some slope wind factor
we'll just call it the Delta that equals
the distance that you need to be to
prevent burn injury you know of course
there's all kinds of assumptions built
in the eight times the vegetation height
but the real challenge is coming up with
that slope wind factor and that's the
work that we've been doing in the past I
know almost ten years now it based on
the measurements we have collected and
the computer simulations we've done
indicates this slope wind factor is
somewhere between one and ten I think
it's probably between actually between
one and five to nail this down we need
to do some more work we need some more
measurements we need some more computer
simulations but we're just trying to get
some better ideas some numbers some
quantitative information to kind of hang
our hat on and use as a target so that's
on your gonna be ongoing probably for
several more years and we're working on
some tools some other ways to present
the information maybe a table where all
the calculations are done maybe a phone
app we expect to have a beta version of
a phone app that's a safety zone
calculator available this year this fire
season but that the information I be
thinking about when I'm on a fire this
summer is is the metal that I'm walking
by big enough giving the wind and slope
that we expect later in the day
you