Wildfire Model Can Help in Predicting Behavior, Assist in Pre-Fire Planning
features
DONALD G. PERRY
Battalion Chief
Santa Barbara County, Calif.
Each year this nation experiences several thousand wildland fires. In addition, we lose about five fire fighters fighting these same fires. Millions of dollars are expended in direct suppression costs.
A key to reducing fire fatalities on wildland fires is extensive fire behavior training prior to the fire season. This will also assist command team personnel with the tools necessary for appropriate attack methods and cost-effective methods of suppression.
Wildland fire behavior has four key components that fire fighters need to review: weather, fuel, topography and time. Without question, weather is the most critical fire behavior factor. Within the weather component, wind provides the most problems facing fire fighters. A driving wind not only increases the burning rate, but it also preheats the fuels ahead of the fire. Furthermore, it can create erratic, unpredictable shift in fire spread. This one factor alone is responsible for many multiple fire fatalities.
Wildland fires pushed by high winds can push a fire 350 feet per minute and give off a thermal output (Btu/sec/ft) over a period of one hour equal to burning 5000 gallons of gasoline. Fire fighters need to know how wind speeds and directions affect their local area.
Canyon windshifts
In mountainous or valley locations, fire fighters need to know when upcanyon winds change to down-canyon. These factors associated with wind should be studied over several years to build averages.
Temperature and relative humidity factors are also important. Peak temperatures and correspondingly low relative humidities reflect low fuel moisture readings in light fuels. These factors create peak burning and fire spread rates daily between 1200 and 1400 hours.
Another key component is the time when relative humidity recovery starts. This factor often is the start of slower burning rates and lessening of fire behavior conditions. In midsummer, this factor might occur as late as 1700 hours but can occur as early as 1500 hours. As relative humidity goes, so goes light fuel moisture levels. The more moisture in the light fuel, the slower the burn rate, assuming the wind pattern is stable and normal. Relative humidity trends take 10 to 100 hours to affect medium and heavy fuel types.
It is also important to monitor frontal movement through your area and watch the effect on local wind patterns. Cold fronts will cause erratic wind shifts up to 180 degrees and wind velocity increases up to 75 mph—all in a short time. The
National Oceanic and Atmospheric Administration or the weather bureau in your area can assist you in localized weather studies. Weather is still the most unpredictable element in fire behavior and you must plan your fire behavior forecasting correspondingly.
Must know fuel
The next component of fire behavior in importance is fuel. What is there to burn? I feel that before you can become a good fire behavior forecaster, you must know your adversary, and how each different fuel type burns. It is important to know burning rates of light fuels that often serve as a preheater or primer for medium and heavy fuels. Rach fuel type gives off different temperatures and reacts accordingly. This will greatly affect your attack method. For example, you might be able to coordinate a direct attack on the flanks of a grass fire, but if the grass is replaced with a heavy chaparral, the heat would make this attack method untenable.
FIRE MODELING CONCEPTS TACTICAL REQUIREMENTS
Figure 1.
This relationship can also be used in some support equipment usages. You probably can utilize air attack on grass fires with good penetration and wet line characteristics, but with a chaparral or tree fuel bed, a guaranteed wet line is not assured.
Fire fighters need to have an understanding of fuel loading and density as it relates to continuity, arrangement, and tons per acre. Light fuels (grasses, grains, crops) provide good continuity and arrangement and a loading of to ton per acre. This means a hot fire, rapid spread when cured, but a fire with little prolonged thermal output. This is important. This is why grass fires slow up and die during the evening burn period.
Medium class fuels, such as sage and
chamise, provide a different continuity. Often interfaced with grass that provides the preheating necessary to heat the oil-laden canopy foilage, these fuel types can provide moderate rates of spread but—more important—high, untenable temperatures for sustained periods. Fire control crews are often forced to use indirect methods because of the heat. This heat also preheats surrounding air and heavier fuel types, such as chaparral and trees. The heating of ambient air often leads to fire fighter fatigue, a factor often associated with fireline injuries. Both medium and heavy fuel types will burn into evening burn periods although burn rates are slower.
The age class of fuels is also important. This is especially true in heavy (chaparral) type fuels. Unlike annual grasses, which go through a 120-day cycle and die out, heavy fuel types adapt to long, hot summer months and store water in their extensive root systems. This means that they often do not reach critical live fuel moisture levels until September or October—even later in wet years. In heavy fuels, a critical fuel moisture level is 60 percent. Correspondently, a critical fuel moisture level in light fuels is 3.5 percent.
This is why most of California’s major wildland fires occur in this time frame. In contrast, during the 1975-1978 drought years several major wildland fires occurred in late June and July.
The fire fighter is most concerned with heavy fuel beds over 20 years old. The live to dead plant material (stems, twigs) ratio changes about here and the plants start to become decadent and create real problems for fire control forces. Major fuel beds should be plotted by age class to determine potential for a major fire. An up-to-date fire history map will assist in this area.
Hazardous land features
Topography features serve as barriers and channels for wind and fire spread. A fire burning on level ground will spread two times faster on a 30 percent slope and four times faster on a 55 percent slope. Erratic fire behavior can be preplanned around narrow canyons, chimneys and saddles. These factors are also associated with many fire fatalities. Safe fireline operations will occur only if personnel recognize hazardous topography features and plan for them prior to starting their assignments.
Time provides a key factor in fire control and potential fire spread and behavior. Peak daytime conditions are usually between 1000 and 1600 hours. Nighttime conditions will vary, depending on topography and wind. By using the 1 -hour spread projections, the model can compute acreage, which in turn will give command team personnel an estimate of how many men and resources will be needed to mount an aggressive attack.
As you can see, there are many areas of fire behavior that must be researched and studied prior to a fire season. Prefire planning is a key. The more you know about your area—and specifically your fuel beds—the better.
Fire model development
However, I feel you also should develop a fire model if you have extensive wildland fire problems or provide mutual aid into one of these areas. This model will assist you in pre-fire planning, fire behavior forecasting and even predicting fire spread and containment (see figure 1).
Many hard-working shift personnel and I designed a functional fire spread model for a wildland area covering 1100 square miles. This model and study is comprised of fuel beds from 600 to 30,000 acres in size. Additional factors for our model included topography, slope, fuel, and three-year weather study.
The model was started in 1975 and is just now reaching the final field format. We used the model on two actual wildland fires during the 1979 fire season with 80 percent accuracy. The error seems to be in underestimating fire spread in medium fuel types.
A good portion of the model was based on actual fire spread tests conducted in various fuel beds. Test grids were set up in key fuel beds for the monitoring of fire spread temperatures, flame heights and erratic behavior (see figures 2 and 3).
Figures 2 and 3 show results of actual fire spread tests run in a medium fuel bed. Temperature readings were obtained by using tempils, small colorcoded pills which melt at different temperatures.
Field overlay predictor
These composites have been used to develop a field overlay that shows key factors associated wiht fire behavior and predicting probable fire size.
Figure 4 shows what a final fire model sheet will look like. This sheet should be produced on clear acetate so it can be placed directly over a topography map. The key feature of this system is the ability to estimate where the fire is going to be in one, two or three hours so the command team can order up adequate resources and manpower. It can also be used in master planning to assist in projecting major fire potential.
The fire model is functional in light, medium and heavy fuels during normal weather conditions, but it does not appear to be accurate with wind velocities in excess of 30 mph. In future fire seasons, we will attempt to modify the model to handle extreme fire behavior conditions.
Figure 4.
