# How do I calculate resources by using FireBridge?

### Active fires, Rx burns or planning stages, use FireBridge to determine what you should have on hand! A supplement to the Podcast Session 1.

FireBridge will allow you to take a look at existing areas of your geography and conduct “PRE” fire suppression planning calculations so that you will know what you need to suppress a fire should something start in your chosen area of concern. As well as be used during large active wildland fires.

There is only a single problem with Richard Rothermels 1970’s publications on Fire Behavior Nomograms for Fire Modeling. That is his publications say nothing about what is needed to fight these Fires!

In Rothermel’s Research Paper from 1972, “Research Paper INT-115”, states in the Preface, “*… a method for making quantitative evaluations of both rate of spread and fire intensity in fuels that qualify for the assumptions made on the model. Fuel and weather parameters measurable in the field are features as inputs to the model*”

It goes on to say two paragraphs below that “*This mathematical model has been developed for predicting rate of spread and intensity in a continuous stratum of fuel that is contiguous to the ground. The initial growth of a forest fire occurs in the surface fuels (fuels that are supported within 6 feet or less of the ground).*”

So here the entire focus is initially on only being able to determine Rate of Spread and Intensity of a fire. There is Nothing that speaks of what to do once those elements are determined. This will be the normal course of action for nearly every model to this day. Rothermel’s 48 page report below.

FireBridge is based off of what Rothermel developed and I am simply using his numbers for the other side of the equation to determine resource size, type and number.

In the Structure Fire Service, they do this routinely as part of their Risk Analysis studies. In a 1994 Book titled “The Fire Department Water Supply Handbook”, by William F. Eckman, he has a chapter on the subject, in fact, Chapter 3. “*The Comprehensive Planning for fire protection begins with determining the water supply requirements of the area to be protected”*. He goes on to write that, *“One way to determine the needs (the water that is), is to divide these fire department response areas into what he calls Fire Management Areas, FMA’s ”.*

Curious, in the Federal Wildland Service we have Fire Management Officers who oversee Districts that are the Fire Management Areas already right? Once the boundaries are determined then you determine the water needs. After all, it is water that puts out fire, by cooling it below the ignition temperature.

The largest difference is that the book speaks of determining needs from the largest buildings and target hazards and such are planned and determined by a case-by-case study. In any event, every district in the USA has at least one area that is that “Bad Area” if a fire should ever get started. So begin with that.

In the Structure world, they have a handy formula for the structure fire departments to use to determine the NFF or Needed Fire Flow as it's called. And that is the amount of water at a minimum in Gallons Per Minute. It is often used by a formula from the National Fire Academy.

Calculating the Required Fire Flow

However, in the wildland arena, it is a whole different ballgame! Fortunately, with the creation of the Fire Behavior Nomograms from Richard Rothermell from the 1970s, which, are still used to this day and are the operational foundation, we can quickly estimate fire rate of spread and intensity easily. There is, however, still one glaring fact staring at you. The Fact that all suppression is a Btu relationship between that which is being generated by the fire and that which you can absorb, and if the two are NOT matched, then your fire goes out on its terms and not yours!

In the Structure fire world, the math to determine the needed fire flow for a particular building is based upon the above link but for brevity, it is included here: NFA Needed Fire Flow in GPM = L x W / 3 for 100% flow. So the length of the building x width divided by 3 would tell you what you need in GPM for 100% involvement for 1 story. So if we had a 60foot by 32foot building that was 2 stories and say 25% involved the amount of water needed would look like this then. 60 x 32 = 1,920 square feet, x 2 stories, = 3,840 square feet. 3,840 / 3 = 1280, times the % of involvement which we said was 25% is now 320. So 320 GPM is required at a minimum. If we had two floors fully involved then it would be 60L x 32W / 3 x 2floors = 1280gpm. for 100% involvement.

The determining of what a department needs to determine resources for say water delivery is then based upon a formula that looks like this:

V/OHC x CCN X EF = TWS (Total Water Supply).

V = Total Volume of the Building

OHC = Occupancy Hazard Classification

CCN = Construction Classification Number

EF = Exposure Factor

I am not going to go into anything further on this for the structure world however we can easily determine resources based on the Btu output of a fire and that is the number one thing to remember, Fires are all Btu and if you are conducting suppression efforts and are ignoring the Btu generation, then good luck to you.

**The Fire Behavior Nomogram**

The Fire Nomogram that you can find on the NWCG website Here, was largely the work of Richard Rothermel and can still be used with great success and is not difficult to learn how. Seen below is an image of a Nomogram from a fuel model of Chaparral.

Here is Fuel Model 4, Chaparral, showing the high wind side. I want you to look at this carefully. And then I want you to look at the instructions on how to use these found on the wildfireengineer.com website and there is a link to the NWCG from there on obtaining the instructions etc. If not then using this link to Faith Ann Heinsch (Faith wrote the 7 steps instructions) & CJ Johnson, USDA Forest Service who conduct this training and is on the Wildfire Learning Portal under Behave Plus will serve you well. This should be part of every Engine Operator and Captains job to learn this and this is consistent with the OPM’s PDs for what engine folks are supposed to know so what gives?!

The Fire Nomogram is an estimation of what your fire is doing for your given mid-flame wind speeds, slope, topography, and fuel type. The benefit to this is that it will spit out an estimate of the Fire Rate of Spread in chains per hour (which my document says to convert to Feet per Second!), and then it will tell you what the fire is producing in BTU per foot per second. So did anyone see the problem right off the bat here in being able to use these things?

In this particular image of the Nomogram (above): The Fires Energy release is shown/given to you on a Btu per foot per second rate, but the rate of spread is in chains per hour. Sorry, but this is not very useful like this. A rate of spread on a per-hour basis when the heat is given on a per-second basis is just not workable to the boots on the ground. In order to use the ROS in a more sensible manner, we need to convert that ROS to Feet per Second! then the Fire’s intensity will match what the rate of spread is indicating, and this will make it much faster at calculating needed resources and types!

Download my FireBridge explanation. This will make much more sense to you on the ground. FireBridge Calculation Procedures. Using the document that is on the bottom of that linked page, will lay out the steps necessary on how to use the Nomograms to be much much more effective and effective on the fire ground! Chains per hour are NOT workable when your Btu generation is on a per-second basis.

Furthermore, water coming out of the hose is in Gallons per minute but has a Gallon per Second equivalency that can be used directly. Simply take the GPM and divide by 60. so 60gpm = 1 gallon per second.

A Darley JMP 500 pump is right around 6.7 Gallon Per Second (GPS), to 8.33 GPS. the Darley HSE 250 would be around a 4.17GPS.

The Hale 250’s and Waterous 500gpms, or whatever brand you use, if the capacity is listed in gallons per minute, dive that by 60 to get the GPS equivalent. Why? because it will more closely match what a fire's Btu estimation is in the per-second range.

For example, if we have a fire that is 6ft Chaparral and its dead fuel moisture is 4% and live is 120%, and the midflame wind speed is 14mph on a 60% slope, producing, as an example, 224 chains per hour, ROS, an HPA (Heat per Unit Area) of 2,700 and a Btu/S/Ft of approximately 11,097. So here is what this is telling you.

First, the ROS in Chains per hour of 224, you convert that to feet per second to get an accurate rate of forward travel. 224 x 66ft = 14,784 feet forward in 1 hour. so now divide that by 3600 to get the feet per second. 14,784 / 3600 = 4.11 feet per second. So this also means that is how many feet it is taking in every second. 4.1 feet. So ask yourself this question, how many feet is your stride?

Next, take that 4.1 and multiply it by the Btu per square foot of 2700. 4.1 x 2700 = 11,097 Btu per second. so again how much Btu is that booster nozzle shoving out on a per-second basis? if you are flowing 20gpm then it is 20/60 = .33 GPS. so how much heat does that absorb? let us find out.

FireBridge says to determine the Btu capacity of the flow by using altitude and water temp first.

So if our fire’s altitude is at 5,000ft MSL and the Temp is 60F, then the Btu per pound of water is 1123. So .33 gallons of water = .33 x 8.34 lbs per gallon = 2.75lbs x 1123 = 3,088Btu. Not a match for our fire. two booster nozzles at 3,088 are only 6,176.5 Btu so still not going to work. Are you seeing and understanding what is happening here, folks?

**Working from another direction!**

Let us work it from another direction. We know that the fire is producing 11,097Btu per second so what do we need in water to stop it? Take the 11,097 Btu and divide that by the Btu per pound of water to determine the number of pounds of water first.

11,097 / 1123 = 9.88 lbs of water per second required.

Now divide that by the amount of weight per gallon of water.

9.88 / 8.34 = 1.18 Gallons. So 1.18 gallons per second is what is needed. that is now a 1.18 x 60 = 70.8 gallons per minute of flow. and 70.8 gallons per minute is now worked back to the Btu standpoint to verify.

70.8 x 8.34 = 590.47 lbs worth

590.47lbs x 1123btu = 663,097.81 Btu/min

663,097.81 Btu/min, divided by 60 seconds = 11,051.63 Btu/sec. However, if we just used 1.2GPS then 1.2 x 8.34lbs per gal x 1,123 Btu/lb = 11,241 Btu of absorption. so this means that 71 Gallons Per Minute is more than what the fire is producing and it will go out.

This is also used for fires with a Length and width dimension like the NFA formula. instead of using nozzles now, we simply use aircraft and the total number of drops is the number of aircraft you would have to have in the air simultaneously to make sequential drops. You are creating an aerial nozzle and making it rain!

If you simply use a single aircraft, on a load and return basis and the Btu generation is large enough, then by the time you returned with your next load the fire is already into new fuel and the heat re-established. So what is the point here then? On the other side, sometimes you cannot get multiple aircraft into an area due to visibility, etc. That is not the point of this post, the whole point is to let you know that you can determine what a fire needs to suppress it, or even HOLD IT. The delivery might pose another issue, however.

Looking at the dimension of a fire that is say 4.1 feet of active fire line width and a fire line length of a mile is an area of 4.11 x 5280 = 21,700.8 square feet. Now multiply this by this by the HPA of 2700 = 58,592,160 Btu every second going up.

This means that you would need the following:

58,592,160 / 1123 = 52,174.67 pounds of water.

52,174.67 / 8.34 = 6,255.96 gallons

6,255.96 / 800 = 8 SEATS

6,255.96 / 3000 = 2 LATS

6,255.69 / 180 = 35 Type 3 helicopters.

You can use FireBridge like the Structure Fire Services uses their methods of needed water supply for their situation only with our world, you look at the Btu loading first, obtaining the fuel type, then determine a series of worst-case scenarios and then you can determine resource need and capability type from that.

Next time we’ll discuss how Heavy Equipment can factor into this as well on Suppression Efforts and contrary to what folks this, making dozer lines 4, 5 and 6 blades wide is NOT the method that I would subscribe to when your dozer lines are always at 90-degree angels to your fire line and winds will always push embers across them.

Thank you for reading and any questions hit the about link to send me an email. There will be many more FireBridge posts here.

Hydro.