Wildland-Urban fires have plagued the United States for centuries. One urban-wildland fire in the recent past includes the Hayman Fire in Colorado. The devastation caused by this fire was massive; the Hayman Fire burned 137,000 acres and destroyed over 600 structures. As a consequence, fires in the urban wildland interface can have a devastating effect on human life, property loss, and local economies. Embers or fire brands are produced as trees and other objects burn in urban-wildland fires. These embers are carried up in the atmosphere and may be transported by winds over long distances (1/2 to 1 mile or more depending on the winds). Hot embers ultimately come to rest and may ignite surfaces far removed from the fire, resulting in fire spread.
This process is commonly referred to as spotting or ember showers. Understanding how these hot embers can ignite surrounding surfaces is an important consideration in mitigating fire spread in communities. Unfortunately, not much information is available regarding the ignition properties of surfaces in contact with burning embers. The lack of specific knowledge on the ability of embers to ignite remote objects limits the utility of detailed computation models that could be used to predict fire spread by fire brands.
We know that pine needles in the gutters of homes are susceptible to ignition by ember showers. In addition, embers may be blown in the attics of homes and ignite materials stored there. As far as we know, no study has tested these suppositions under laboratory scale conditions until recently. The International Business Insurance Institute has recently constructed a test facility in upper South Carolina to test the effects of many different building materials by ember showers. They constructed a building with 100 plus fans that blow embers at a test house constructed on the floor below. The goal of this study is to understand how blow or lofted embers created by urban-wildland fires ignite the impacted surface.
The apparatus allowed for the ignition and deposition of single and multiple embers onto a target surface. The ability to deposit multiple embers onto a target surface is important, as most homes and other structures are bombarded by ember showers in wild-land urban interface fires. The moisture content of the surfaces used was varied and the test surfaces considered were pine needle beds, and shredded paper beds. Shredded paper beds were used as a substitute for typical cellulosic fuels that would be found in attic spaces. Pine needle beds were intended to simulate gutters filled with pine needles.
The building was used here as a wind tunnel to investigate the influence of an air flow on the ignitability of surfaces. In the ember transport process, the embers are formed, ignited, and land upon surfaces. In order to be a threat to the environment, the embers must land on surfaces and still be burning. It was apparent from the single ember ignition studies that it was possible to ignite shredded paper beds, like would be found in an attic, from single glowing ember impact. Based upon these findings, the amount of embers is clearly an important parameter which must be considered although it can only take one in the right place with the right fuel at the right time to cause a home fire and embers are very good at finding these places.
This result suggests that it may not require a large flux of embers to ignite a home, provided that the embers are able to penetrate into attic spaces or in crawl spaces. The results obtained for the pine needle beds suggest that a shower of glowing embers is required to ignite structures when glowing embers impinge upon materials found outside structures. For single flaming embers, it was possible to ignite pine needle beds and shredded paper beds. Based upon these findings, the amount and concentration of embers, the size of the embers as well as the degree of the air flow were important parameters to determine the ignition risk of a surface.
Remember that just because your house is in an organized, manicured community doesn’t mean that you are safe from an ember shower from above. Given the right weather conditions and a hot rolling fire, embers can be carried from ½ mile to 1 mile ahead of the fire front and deposited in your gutters loaded with pine straw or on your roof among the leaf accumulation there. Your house could ignite from above!
I saw a video of C-130 air tankers dropping water on fires in the city limits of Austin Texas a few years ago. Houses were igniting in the city far from the wildfires in the surrounding wildlands by embers carried aloft and dropped on the unsuspecting communities. Firefighters would be battling fires on one street and a house 2 blocks away would suddenly ignite by the embers dropped from above.
We are all at risk from wildfires.
Be Safe, be Aware and Be Firewise!
For more information on Ember showers, contact Frank Riley, Executive Director, Chestatee/Chattahoochee RC&D Council at firstname.lastname@example.org.