Using the Alignment of Forces Concept in Rx Burns
Three major forces heavily influence variations in speed and intensity of wildland fires: wind, slope and the preheating of fuels. These potent forces can work in cooperation or against each other.
Forces that complement each other (producing a cumulative effect) are said to be “in alignment.” When forces are not aligned, and are below their maximum potential to aid the spread or intensity of the fire, the forces are then “out of alignment.”
At the HEAD of the fire, the forces are most aligned with the direction of the spread of the fire. The HEEL of the fire is the point where the forces are in direct opposition to the spread of the fire. The FLANKS of a fire can be identified as the point(s) where the fire spread is working at 90 degrees of the force alignment.
The alignment of forces can alter the spread and intensity of the fire
By observing specific parts of a fire and understanding the alignments of the forces, it is possible to develop a foundation for predicting fire behavior change. Through reading the forces and their alignments on the topography, you learn to predict the resulting fire intensity and the potential change as force alignment evolves.
Consider a fire that is free burning and is spreading equally in all directions from the point of ignition. If no outside force is applied to give the fire a major direction, the burn will be round. Apply wind force to the situation and the fire will take a direction and shape caused by the wind.
Another potent force that affects the speed and intensity of the fire is that of slope. A slope in the terrain will “pull” the fire up slope, and retard the spread down slope. If the wind is applying its force in the same direction as the slope, these forces can be said to be “in alignment” with a fire potential that exceeds the potential of either single force without the influence of the other.
Preheated fuels are the third important factor or force to consider. The flammability of preheated fuels influences the speed and intensity of the fire. The hotter the fuel from solar preheating, the greater its influence to affect the fire in the direction of the hot side of the fuel. The time and aspect play an important part in this event but is not the entire story of the alignment of preheated fuel in the path of the fire. In medium brush and timber fuel one side of the plant is hot and the other is cool. The direction the fire enters the plant in alignment with the heated side is important to the amount of fuel consumed and how intense the fire becomes. There are preheat variations caused by aspect and time differences in the path of the fire and by the heated side of the plant and its relationship to the direction the fire burns into it. There are two alignments of preheat to consider.
To determine the comparative change in the rate of spread and intensity of a fire, compare the alignment of forces currently influencing the fire with the alignment of forces in the fires path. The more nearly aligned are wind, slope and preheated fuels, the greater is the potential spread, intensity and consumption of the fire in the direction of this alignment of forces.
These observations and the language used to describe the potential fire behaviors are the mechanisms to predict and communicate fire intensity changes. Knowledgeable predictions can be easily expressed in a manner that includes the basis for the prediction.
“The steep slope ahead of the fire will add considerably to the slope force, and the spread will increase after the fire establishes itself on that slope.”
“The forces of slope and wind are in better alignment for the fire at the base of that hill. The fire behavior intensity and spread will increase when the fire gets to that point on the topography.”
“When the fire burns over the top of the ridge, the slope will be out of alignment and the fuels are colder, so the fire will slow at that point.”
“The wind change that is forecast will change the alignment of forces on the South exposure. When that happens, the fire will be in better alignment.”
Observations of alignment of forces acting upon the fire are the baseline that can be used to predict how the fire will react on the topography around the fire. Learn to envision the reason for the HEAD, HEEL and FLANKS of a fire in this way. It’s easy for experienced fire officers to visualize the differences in the alignment of forces on various places on the topography. Identifying places where there is greater or less alignment of forces can greatly aid in accurately predicting changes in fire behavior.
Armed with the knowledge of how these forces affect a fire’s behavior, it is possible to manage fire behavior and to create the intensity desired. Conduct a test burn to distinguish the range of fire behavior variation that differing alignments create while within the prescriptive window. Identify the alignment combinations that create the desired intensity. Guided by this knowledge, place the fire on the ground when and where the alignment of forces are known to produce the desired intensity. Avoid torching fuel when and where the area is outside the fire intensity RX window. Set fires at specific alignments for specific results.
About Wind and Smoke
Wind direction is significant because wind is a force that will push fire and its resulting smoke, right along with it. Flames bending down wind and the smoke billowing over the terrain are both relevant since they can have an effect on a fire’s intensity. Smoke blocks solar radiation and will lower the flammability of hot fuels. It stands to reason then, that maximum fire intensities can be maintained by positioning lines of fire so that the smoke does not shade the fuels in front of the flames.
Fire can be ignited so that the intensities can be at maximum or retarded by the variations in the alignment of wind and smoke.
Use of strip head firing, (firing strips ahead of the flames) so that the fire runs in alignment with the wind and up slope) will produce maximum flame length.
To lessen fire intensity, turn the firing down slope. This produces a strip of fire that is not in full alignment with the forces of slope and wind, thus reducing intensities.
Shading Fuels with Smoke
In order to suppress intensity, string fire so that smoke shades the fuels in front of the flames. This condition lowers the flammability of the fuels that are next to be consumed and reduces the consumption of the live fuel. The fire intensity is lessened due to lower solar preheating. With significant smoke cover, actual fuel temperatures can be lowered 30 degrees or more. However, fuel shaded by smoke for only short periods is not affected sufficiently to reduce the temperature of the fuel or decrease its flammability. Fuels that have been heated are dried and reducing the temperature will not increase its moisture content.
When fuels preceding a fire are shaded, the full ranges of intensities that can be produced are narrowed because it is not possible to attain the hottest intensity. But this situation may sometimes be desirable to create. The principle thing to remember is that smoke is a “side effect” of fire; use it to your advantage when needed. Some controlled burns were ignited and burned well until the effect of the smoke shaded the fuel, lowered its flammability, and snuffed out the fire.
Attaining Fuel Consumption Objectives
By experimenting with the introduction of fire upon topography, it is possible to attain the desired flame lengths and appropriate consumption required by the prescription objectives. This is the purpose of a test burn.
As an example, a fire’s highest intensity was 25-foot flame lengths created while in full alignment. Observations of the fuel consumption determined 95% of the live fuels and 100% of the dead fuels were burned. Positioning fire to burn at 90 degrees of alignment with slope and wind produced 10- foot flame lengths, consumed 80% of the live fuels and 100% of the dead fuel.
The prescription called for 90% of the live fuel to be consumed. Therefore it is reasonable that with the same conditions as the test fire, that flame lengths between 10 and 25 feet can be managed to. To accomplish the consumption objectives, the firing supervisor needs to manage the variable to produce 15-foot flame lengths.
The prescription called for 90% of the live fuel to be consumed. Therefore it is reasonable that under the same conditions the test fire burned, that flame lengths between 25 feet and 10 feet are desired.
To meet the burn objectives, the firing supervisor needs to manage the intensities for 15-foot flame lengths.
Keep it simple: ask the firing team to light fire in a manner to produce 15 foot flames.
The CPS method uses photographs and topography maps that depict timing and sequence of firing. The planning team delineates the planned perimeter for the prescribed burn on a topography map. The GIS mapping unit is an important aid to planning.
The normal wind for the season is depicted upon the map by drawing wind arrows. The intolerable wind velocity and direction are identified and noted.
Planning Needed in the Future.
With the assistance of a local meteorologist Campbell plans to identify and describe weather patterns that are that are desirable for individual projects. Since there are synoptic weather patterns that are especially beneficial, and others that should be avoided, it is appropriate that each burn project be reviewed for best – and worst – case weather patterns. This knowledge and some planning alongside local forecasters will help avoid undesirable weather systems and changes while burning. We have found that fuel temperature variation due to solar preheating is muted in mid winter months often producing only 15 to 20 degrees elevation from shaded fuels.
The interchange with weather specialists also helps when planning burn projects. Certain months are renown for producing specific weather conditions, like the winds of March or September’s Santa Ana winds.
Firing the Perimeter
Often, it is wise to widen control lines by burning before igniting the interior. This is a separate operation and should be considered separate from the interior firing even if the two operations are done on the same day. Time the sequence of fire so that the perimeter lines are burned first. Be sure to consider the alignment of forces on exposures before firing the line.
The scheduled time for firing the perimeter should be set so that there is a differential of fuel flammability or alignment of forces between the fuels to be burned and the exposures to be protected. This method has been used to successfully on burns even without the use of fire lines to contain the fire. Fuels that are low on the flammability curve create a favorable situation for the holding crews. Any fire that becomes established beyond the lines will occur in COLD fuels.
Timing Ignitions by the Position on the Fuel Flammability Curve
Do not ignore timing, it is critical when planning a burn. Be aware of how time will affect the fire behavior. Prescribed fire plans and wildfire tactical plans should have a “timing window.” When delays widen the window, it becomes out of synchronization with the desirable flammability relationship, and things go wrong. Both perimeter and interior firings should have a timetable established.
Too Low on the Flammability Curve
If a fire is planned when the fuel is too far down the flammability curve, the entire operation can bog down. At some point on the curve, firing will fail to carry, and no amount of torching will overcome the depressed flammability condition. When a fire tells you it won’t burn, believe it! In such an operations you’re firing beyond the time that should have been the cut off time. If the fuel flammability curve is not adequate to sustain the burn, fuel and effort are wasted and the areas must be re-fired when the fuel is higher on the flammability curve. Thus it is important to choose wisely the fuel’s position on the flammability curve when timing the firing. Timing is a critical element.
Too High on the Flammability Curve.
When exposures are threatened by spotfires, the burning operation should cease before the exposure fuels are too high on the flammability curve. When a given exposure is increasing in flammability– or coming up the curve– slop over and spot fires easily develop into escape situations. This is a timing error.
If a spot fire occurs in exposure fuels when the fuel flammability is increasing, getting worse, should you continue the firing operation? No, “the tactic has run out of time.”
As exposure fuels become more flammable, the fire behavior will get worse. More spots will occur and intensities will increase. If the fire behavior is near the threshold of control and the firing operation is continued, the fire will escape.
Timing for Intensity Management
Timing is very important to consider when managing fire intensities. Selection of the appropriate alignment of forces and the start and stop points on the fuel flammability curve, are keys to intensity management. Most burns are time sensitive. Get behind the time line and you loose the ability to manage the fire intensity to the optimal extent.
Firing plans should include the timing as well as the sequence of firing. The timing of the operation can be described in narrative form and on maps or photos of the area. All involved in the burn project should know the timing plan.
As an example, planning the timing for a burn that has two primary aspects, east and west the timing would be as follows.
At 8:00 a.m. the teams start firing the perimeter of the east aspect of the project. This ignites the east aspect at its highest flammability. Firing east aspects on the cooling side of the curve will result in the fire intensity being depressed. Afternoon firing of east aspects is liable to cause the fire to fizzle and no amount of fuel or fanning will overcome the lack of sufficient preheat.
For the west aspect, the burning is scheduled for afternoon. This aspect is the driest and has been receiving solar radiation all day long. The timing window can be set for off the peak hours to dampen intensities if it is desirable to do so. Thusly, the timing and sequence plan is formulated.
An afternoon hot slope map is prepared and the wind force displayed over the topography. An oblique photo taken in the afternoon will have the hot slopes sunlit and cool slopes shaded doing the work exactly. Adding the wind forecast on top gives a realistic display of the alignment of forces that the plan can be built around. Upon the map, start points and time intervals are placed. The division assignment sheet refers to the points on the map when describing where the crews start work and finish.
Copyright © 2010, 2017 by Doug Campbell.