Quantifying Firebrand Exposures for WUI Ignition Potential

Summary

The Wildland-Urban Interface (WUI) is defined as the location where structures and communities meet or intermingle with undeveloped wildland. In the U.S., over 46 million homes in over 70,000 communities have been built in the wildland-urban interface and are potentially at risk from WUI fire. Within the last 100 years in the U.S., six of the top 10 most damaging single fire events involving structures were WUI fires. Since 2002, an average of over 3,000 structures per year have been lost to WUI fires in the U.S. Additionally, annual costs from these fires can amount to over $4.5 B for suppression, alone, and the total cost of the WUI fire problem has been estimated as $14 B [1]. Despite the increasing frequency and losses from WUI fires, compared to fires within structures, there has been relatively little research on WUI fire spread. This is especially the case for fires ignited from firebrands (or embers), which has been shown from post-fire damage assessments to be a major source of structural ignitions in WUI fires. Without research, building codes and standards do not provide adequate protection to structures within the WUI. More specifically, codes and standards do not adequately address the range of exposures during a WUI fire – especially firebrand (ember) exposures. This project focuses on the research and development of a device to quantify the threat of firebrand (ember) exposure from WUI fires on structures and structural materials. This device (referred to as an “emberometer) will enable accurate quantification of critical characteristics of firebrand (ember) exposure, such as flux (m-2s-1 or g/m2s) and energy (e.g., temperature), based upon their importance for solid fuel ignition and technical feasibility. The results of this research will allow for improved fire-resistance of building components, structures and communities – via improvements to building codes and standards, as well as the WUI Hazard Scale [2] – which will increase the resilience of WUI communities in the U.S. and around the world.

Description

Objective - To quantify the threat of firebrand (ember) exposure from WUI fires on structures and structural materials.

What is the new technical idea? Despite the increasing frequency and losses from WUI fires, compared to fires within structures, there has been relatively little research on WUI fire spread. This is especially the case for fires ignited from firebrands (or embers), which is a main cause of structural loss within the WUI. For example, NIST research into WUI fires that occurred in California (2007) [4], Texas (2011) [5], and Colorado (2012) [6] have demonstrated that firebrands (embers) can account for 50% of the ignitions in WUI fires. Research on WUI fires has been difficult, in part, due to the fact that the subject area falls between traditional studies of building fires and forest fires, which in the past has been the responsibility of different branches of the government. However, without research, building codes and standards (namely the WUI-related codes and standards) do not provide adequate protection to structures within the WUI. More specifically, codes and standards do not adequately address the range of exposures during a WUI fire – especially firebrand (ember) exposures.

The major objective for this project is to develop the measurement science to quantify firebrand (ember) exposure during WUI fires. This project will design and develop a device, i.e., referred to as an “emberometer”, to enable accurate quantification of critical characteristics of firebrand (ember) exposure, such as flux (m-2s-1 or g/m2s) and energy (e.g., temperature), based upon their importance for solid fuel ignition and technical feasibility.  Since firebrand exposure consists of firebrands in two states – i.e., dynamic (flying) and static (in a pile) -- two modules will be developed as part of the “emberometer”. The first module will be developed to measure the dynamic or in-flight characteristics of firebrands, e.g., size, flux, and temperature, which will be referred to as “Module 1”. The second module, labeled as “Module 2”, will be developed to measure the static or ground signature characteristics of a firebrand or firebrand pile; i.e., thermal signature and mass. The “emberometer” device, including its two modules, will be developed to measure firebrand (ember) exposure in laboratory settings, controlled field experiments, and actual WUI events.  

Advances in measurement science are needed to effectively characterize and identify the conditions and mechanisms that result in a high risk of structure ignition across a range of WUI community types and conditions. Characterizing firebrand (ember) exposure will aid in further developing the WUI Hazard Scale [2] as well as provide the technical foundation for improvements to WUI-related codes, standards, and standard test methods.

What is the research plan? 

In FY16, a literature review was completed on firebrands (embers) in WUI fires. First, an endnote library was developed housing literature on various firebrand-related topics, including generation, transport, and solid fuel ignition by firebrand assault.  From this review, work was performed and completed to identify the various characteristics influential in solid fuel ignition (e.g., flux, size, mass, intensity) and, to the extent possible, the range of values associated with each of the characteristics. An understanding of the influential firebrand characteristics and associated range of values for these characteristics was vital in identifying the requirements for the measurement technology/approaches selected for each module of the emberometer.

In FY17, a design was developed for Module 1 of the “emberometer” for laboratory settings. Based on work performed in FY16, the in-flight firebrand characteristics identified as integral to understanding firebrand exposure (and eventual ignition) were size, flux (including number and mass) and temperature. Various techniques to measure these characteristics were considered and NIST researchers concluded that imaging techniques, namely particle tracking velocimetry, provided both technical feasibility and required accuracy. A design for Module 1 of the emberometer was developed and documented in a report. Additionally, in FY17, work was completed regarding the development of Module 2 – namely the identification and documentation of the techniques available to measure static-based characteristics of firebrands (e.g., temperature, mass and heat flux).

In FY18, work will be performed to fabricate Module 1 of the “emberometer” design developed for laboratory settings. Once fabrication is complete, Module 1 will be packed up and shipped to Dr. Manzello in Tokyo, Japan to test it at the Building Research Institute (BRI) (using the large-scale firebrand generator, also known as the Dragon) and the National Research Institute of Fire and Disaster (NRIFD) (using the small-scale firebrand generator, also known as the Baby Dragon). Additionally, a test matrix will be completed for the tests in Japan. Dr. Manzello will conduct tests at BRI and NRIFD to ensure that Module 1 is accurately measuring firebrand number flux, which is a “known” quantity from both the large-scale and small-scale firebrand generators. Work will be ongoing to test measurements of firebrand size and temperature as well. At the end of FY18 (Q4), the design for Module 2 of the “emberometer” for laboratory use will be completed. Also in Q4, NIST researchers will modify the design for Module 1 so that it can be used outside of the laboratory, in controlled field experiments (e.g., prescribed burns) and actual WUI fire events.

In FY19, work will be performed to fabricate and test Module 1 of the “emberometer” design developed for outdoor settings – e.g., controlled field experiments and actual WUI fire events. Additionally, Module 2 will be fabricated for use and testing in the laboratory setting. As fabrication of Module 1 and 2 is ongoing, NIST will identify collaboration/field teams that can employ the emberometer to collect exposure information on firebrands (embers) during field studies, and initial data collection to characterize firebrand (ember) exposure can begin.