Point Source Fires

Point Source Ignitions

A prudent burning boss will often switch from strip-heading fires to point source fires as the day progresses and continuous lines of fire become too intense. When properly executed, a grid of spot ignitions will produce a fire with an intensity much greater than that of a line-backing fire but somewhat less than that of a line-heading fire. Timing and spacing of the individual ignition spots are the keys to the successful application of this method. First a line backing fire is ignited across the downwind side of the block and allowed to back 10 to 20 feet into the block to increase the effective width of the control line. A line of spots is then ignited at some specified distance upwind of the backing fire and the process continued until the whole block has been ignited.

To minimize crown scorch, ignition-grid spacing is selected to allow the spots along a line to head into the rear of the spots along the downwind line before the flanks of the individual spots merge to form a continuous flame front. The merger of successive ignition lines thus takes place along a moving point rather than along a whole line at the same time. Merger along a moving point can be ensured by beginning with a closely spaced square grid (2 chains by 2 chains is recommended). Close spacing between lines helps the individual spots develop, but ensures that the head of one spot will burn into the rear of the downwind spot before the heading fires potential flame length and intensity are reached. Of course, the closer the spacing, the more merging points you have. You must be aware that a large number of small fires burning simultaneously can produce the same kind of explosive convective energy as a single large fire because too much heat energy is released too rapidly. This situation is discussed more fully in Aerial Ignition.

Grid or Point Source Ignition Technique

Rectangular grids with wider spacing between lines than within a line should not be used initially because such a pattern may allow the spots along a line to merge into a line of heading fire before running into the rear of the downwind spots. Once the first few lines have been ignited and fire behavior has been assessed, intensity can be regulated to some extent by changing the time between ignition points within a line, the distance between points, and the distance between lines. Thus the balancing act between spacing and timing has to be continually adjusted as fire behavior reacts to both temporal and spatial changes in fuel and weather.

Intensity is decreased by widening the interval between ignition points along a line. If fireline intensity is still too high after doubling this interval while maintaining a 2-chain distance between lines, firing should be halted. Allow the area to burn with a backing fire or plow it out. Although intensity at the head of an individual spot is increased by widening the distance between lines, the average intensity of the burn as a whole is usually somewhat lower. Check to see that convergence-zone flame lengths are within tolerable limits, and that other fire behavior parameters appear satisfactory. If everything is within prescription, you can increase both between- and within-line distances. This step will reduce ignition time, and decrease the number of ignitors used. The number of convergence areas with higher intensities will also be decreased.

Experience to date shows grids up to 4 chains by 4 chains (one ignition point every 1.6 acres) can produce excellent results. The time needed to complete the burn can be reduced by offsetting successive ignition lines by one half of the within-line spacing. The heading fires from one line will then come up between the backing fires on the next line.

Factors associated with point source fires

• Assume much of the area will be burned by heading and flanking fires and very little by backing fires.
• If conditions are ideal for traditional line-backing fires, point source fires may be too intense.
• Preferred burning conditions include low (1-2 mph) in-stand wind speeds. Wind direction can be variable. Actual fine-fuel moisture should be above 15 percent.
• When underburning, start with a square ignition grid (equal distance between spots within a line and between flight lines). Two chains by 2 chains is often used.
• Always secure the downwind base line first.
• Be careful when underburning stands with a flammable understory or a heavy rough.
• Severe crown scorch is likely if fuel is too dry.
• Under the same weather conditions, fires in Piedmont fuel types tend to spread slower and be less intense than those in Coastal Plain fuel types.
• The usual changes in weather during a typical winter day may require modification of ignition patterns throughout the day. Burn until fires verge on getting "too hot." Then either quit burning or resort to backing fires only.
• Continually modify the ignition grid to take advantage of topography and changes in understory fuels.
• Costs are low because firing is rapid and no interior control lines need to be constructed.