DEVELOPING RAPID RESPONSE INSTRUMENTATION PACKAGES TO QUANTIFY STRUCTURE IGNITION MECHANISMS IN WILDLAND-URBAN INTERFACE (WUI) FIRES | NIST

Rapidly deployable instrumentation packages are being developed to be used during actual WUI fires to quantify structure ignition mechanisms. The packages are being designed to be placed near a given structure in the WUI and will provide video imaging of a structure at different vantage points as well as quantitative data on heat flux, wind speed, and relative humidity. Prior to attempting to use these instrumentation packages in real WUI fires, a series of proof-of-concept tests were conducted under prescribed fires. In these tests, a shed was used as a surrogate for a typical structure that would be found in the WUI. This presentation will focus on instrumentation package development and results from a recent deployment in a prescribed fire at Stafford Forge Wildlife Management Area in the State of New Jersey. INTRODUCTION Fire spread in the Wildland-Urban Interface (WUI) is an international problem with major WUI fires reported in Australia, Greece, Portugal, Spain, and the USA. In the USA, there have been two significant WUI fires within the past five years in California. The 2003 Cedar fire resulted in $2B in insured losses and destroyed more than three thousand homes. WUI fires can also result in mass evacuations. The most recent destructive WUI fire that occurred in Southern California in 2007 displaced nearly 300,000 homes and destroyed over a thousand structures. For WUI communities, fire risk is reduced by either reducing wildland fuel loading or by following a series of risk reduction practices. Unfortunately, the fuel treatment methods in practice are predicated on very limited scientific investigations. It is not all clear how effective these methods are with regard to preventing structure ignition. The risk reduction practices follow rule-based and empirically determined checklists and are not the result of a scientifically based effort. Quantitative data on how structures ignite during full scale field experiments is highly desirable. Not surprisingly, very few full scale field studies have been performed to understand structure ignition mechanisms. Cohen provided some insights into structure ignition mechanisms as part of the International Crown Fire Experiments conducted Canada. In these experiments, Cohen placed various target walls 10 m, 20 m, and 30 m from an approaching crown fire. The test walls were instrumented with water cooled heat flux gages to measure the temporal evolution of heat flux experienced at the target wall as the crown fire approached; data was obtained for seven different crown fires. While these experiments provided some useful insights, Cohen pointed out that the data was collected under a limited set of 1 Corresponding author: samuelm@nist.gov; +1-301-975-6891 (office); +1-301-975-4052 (fax) experimental conditions, such as fuel load, wind speed, and terrain. More importantly, fire spread in the WUI is not simply governed only by vegetative fuels to structural fuels but also structural fuels to structural fuels. The role of firebrands during WUI fire spread is not clearly understood as well. Therefore, the capability to collect in-situ information on the physical mechanisms related to structure ignition during actual WUI fires is highly desirable. To this end, rapidly deployable instrumentation packages are being developed to be used during actual WUI fires to quantify structure ignition mechanisms. The packages are being designed to be placed near a given structure in the WUI and will provide video imaging of a structure at different vantage points as well as quantitative data on heat flux, wind speed, and relative humidity. Prior to attempting to use these instrumentation packages in real WUI fires, a series of proof-of-concept tests are being conducted under prescribed fires. In these tests, a shed is being used as a surrogate for a typical structure that would be found in the WUI. This paper is focused on instrumentation package development and results from a recent deployment in a prescribed fire at Stafford Forge Wildlife Management Area in the State of New Jersey. EXPERIMENTAL DESCRIPTION Rapid response instrumentation packages that enable in situ, temporally resolved measurement of heat flux, wind speed and direction, relative humidity, and ambient temperature as well as full field video imaging were designed. NIST was invited to test the rapid response instrumentation packages by the New Jersey Forest Fire Service as part of their yearly prescribed burns intended to reduce the risk of fire spread by reducing wildland fuel loads in the New Jersey Pine Barrens. These prescribed burns were conducted at the Stafford Forge Wildlife Management Area, Warren Grove, NJ; this is land owned by the state of New Jersey. The New Jersey Forest Fire Service was in charge of the prescribed burns which included coordination, ignition, and suppression efforts. Figure 1 displays a map of the site where the prescribed fire experiments were conducted. To visualize the ignition of the structure as the crown fire approached, a shed with a dimension of (1.8 m × 2.4 m × 2.0 m) was used as a surrogate for a typical structure that would be found in the WUI. The shed was constructed of OSB with an asphalt tile roof and vinyl siding; figure 2 displays an image of the shed prior to the fire. The fire was ignited using a helicopter equipped with a heli-torch. A crown fire developed and approached the shed. A picture of the crown fire that developed is shown in figure 3. Figure 1 Satellite map where the prescribed fires occurred in the New Pine Barrens; the prescribed burning area was 0.1 km. Figure 2 Image of the shed exposed to the prescribed fire. Figure 4 displays a schematic of the field deployable rapid response instrumentation packages as well as the setup configuration used in these tests. As shown in the figure, the instrumentation packages consisted of a main station that was 19.5 m away from the shed and two remote stations that were placed adjacent to the shed. The main station included a laptop with custom software for data logging, two wireless cameras, a wireless radio modem, and a wireless router. Each remote station that faced the East and North, respectively, was equipped with single board computer (SBC) for data acquisition, a wireless radio modem, a total heat flux gage, an anemometer, directional flame thermometer (DFT), and a thermistor-humidity sensor. All physical signals (in volts) measured from each device in the remote station were collected through the SBC located in a thermally insulated enclosure and simultaneously transmitted to the laptop at 9600 bytes per second (bps) through a pair of wireless radio modems. Figure 3 Image of the prescribed fire. 0 DLQ 6WDWLRQ 5 HP RWH 6WDWLRQ 5 HP RWH 6WDWLRQ ' HWDLOHG YLHZ RI D UHP RWH VWDWLRQ Figure 4 Schematic of the field deployable rapid response instrumentation packages and configuration. In situ, time-resolved images of the fire are the most important features of the instrumentation packages. Six expendable wireless internet protocol (IP) cameras were installed at different view angles around the shed as well as the main station and were used to image the spreading fire front. The images were captured at 3 frames per second (fps) and simultaneously transmitted to a laptop inside the main station through a wireless router. Transmitted images were then saved in MPEG (Moving Picture Experts Group) format. The total heat flux gages (Schmidt-Boelter type; 5/8” diameter sensor) and DFT’s were used to measure the total incident heat flux from the fire front. Each was installed at the same height (from the ground; 1.4 m) and view angle. The total heat flux gages were water-cooled during the test and calibrated using a black body source before the test. Ambient temperature and relative humidity were measured using a thermistor-humidity sensor. Local wind velocity and direction were measured using a cup and vane anemometer only at the remote stations. It is important to point out the unique features of instrumentation developed as part of this effort. This included sending all data signal to a hardened location (NIST WUI Black Box or Main Station) wirelessly in order to allow the use of relatively inexpensive cameras that do not need to be hardened to survive the fire; this greatly reduced cost and distinguished our instrumentation packages from others used in wildfire experiments. It was also desired to quantify heat flux without the use of water cooled heat flux sensors. Thus, the DFTs were used and as part of this proof of concept exercise and water cooled total heat flux sensors were used for a direct comparison of the heat flux obtained from the DFT’s. DFT’s do not require water cooling and this is highly desirable since the ultimate goal of this effort is to deploy this instrumentation during actual WUI fires. Accordingly, deploying water cooled heat flux sensors is not desired. RESULTS AND DISCUSSION In prior wildfire studies, the spread of the fire front was monitored or visualized through thermocouple measurements and a series of thermally insulated CCD cameras and infrared imaging devices. However, high quality temporally resolved images of the spreading fire front spread were not available in those studies. Figure 5 displays in-situ images of the fire approaching the structure with respect to time. The data loggers were started some four hours before the fire was ignited. The instrumentation was setup within 20 minutes but due to weather conditions, the fires were not ignited until more than four hours after setup. Distinct fires were first observed at 4h 5m 28s and propagated toward the shed along the wind. As shown in the figure, ignition on the shed was not observed before the fire front passed by the structure. In the view of camera #5, only shrinkages of vinyl side on the back of the shed were observed at 4h 6m 20s before the passage of fire front. Upon the arrival of fire front the shed was partially engulfed