Tom's Blog

Friday, April 1, 2011

First big burn of the year

Getting prescribed burns done requires a lot of planning, appropriate weather, and a good crew. Because of the cold March, our burns have been considerably delayed, but yesterday everything came together and we got a lot done.

For burn weather, I mostly follow the NOAA web site, and especially a page called "Forecast Discussion". Beginning on Monday, a surface high pressure system with a very dry air mass was predicted to hover over the area and continue through Thursday. The weather was to be coldddd in the mornings, but to warm up a bit by afternoon. The high Thursday was predicted to be about 50 F, with low humidity and light winds out of the south. Good conditions for a burn, and the DNR gave us the go-ahead. Actually, although the wind was light, it was variable, and shifted from north to south and back again.

Because we were doing several large and complicated burns, we needed a large crew. We had 17 people, 7 from contractors, 3 of our own PVC crew, and 7 volunteers (including Kathie and me). We needed them all!

Thursday morning broke clear and cold, with light winds and a temperature of 22 F. Because of the low temperature, we could not use our shallow well, and had to borrow water from a neighbor. We needed a huge amount of water, and by the end of the day we had gone back twice for refills. Fortunately, I had anticipated the cold and Marci had brought all our water backpack to her basement, and filled them in the morning of the burn. (There is nothing worse than ice crystals in a water backpack.) We met at 9 AM to start getting things ready, and we were finally able to start lighting at 11:15 AM.

We burned the south-facing slope, which is a steep hill of about 20 acres, with little bluestem prairie remnant below and bur oak savanna above. The fuel was primarily prairie grass, but in the upper more shaded savanna, the fuel also had a lot of bur oak leaves. With the light wind, the very steep slope was more critical, and was the dominant factor.

A computer program called BehavePlus 4.0 can be used to model fire behavior. One inputs parameters such as fuel type (in our case, tall grass), fuel moisture in %, wind speed, and slope steepness. The output is either flame height or rate of fire spread. With our input parameters, flame length varied from 6 to 11 feet, depending on the windspeed and the rate of spread was pretty fast.

Obviously, with such high flame lengths, it is desirable to minimize the size of the fire. For the past several years we have been keeping the width of the fire line low by using the "strip headfire" technique. This technique uses four drip torches. After putting in a blackline at the top of the ridge, next to our mowed fire break, the other three drip torches moved in parallel to the contours, the uppermost first, then the second, and then the third at the bottom of the slope (along Pleasant Valley Road). The steep hill encourages the fire to move up, but the fire quickly reaches the black line above and goes out. The drip torch operators are in communication by two-way radio, to ensure that they move across the slope in staggered formation.

The most critical part of this procedure is the creation of the blackened area at the top of the slope. The photos below show Susan and Stephanie working on this process. (Several more people with backpack water cans remain farther behind to keep the fire line under control.) Also, when this crew reaches the far end of the slope, there is a tricky phase when a vertical black line must be created from the top to the bottom of the slope (at Pleasant Valley Road).

The first photo below shows Chris lighting the third strip from the top and the next photo shows the slope in a later stage, when it was mostly black.

The whole burn took about 90 minutes.

After a short lunch break, we moved to Black Earth Rettenmund Prairie for two more burns. In all, a fine day.

Note: if you don't have enough personnel to use the strip headfire technique, you can do the whole burn as a backburn from the top. This works, but takes quite a bit longer. (The last photo above, which was taken just before the lowest drip torch arrives, shows such a backburn in progress.)


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