Natalie Wagenbrenner

A comparison of three approaches for simulating fine-scale surface winds in support of wildland fire management. Part I. Model formulation and comparison against measurements

For this study three types of wind models have been defined for simulating surface wind flow in support of wildland fire management: (1) a uniform wind field (typically acquired from coarse-resolution (~4km) weather service forecast models); (2) a newly developed mass-conserving model and (3) a newly developed mass and momentum-conserving model (referred to as the momentum-conserving model). The technical foundation for the two new modelling approaches is described, simulated surface wind fields are compared to field measurements, and the sensitivity of the new model types to mesh resolution and aspect ratio (second type only) is discussed. Both of the newly developed models assume neutral stability and are designed to be run by casual users on standard personal computers. Simulation times vary from a few seconds for the mass-conserving model to ~1h for the momentum-conserving model using consumer-grade computers. Applications for this technology include use in real-time fire spread prediction models to support fire management activities, mapping local wind fields to identify areas of concern for firefighter safety and exploring best-case weather scenarios to achieve prescribed fire objectives. Both models performed best on the upwind side and top of terrain features and had reduced accuracy on the lee side. The momentum-conserving model performed better than the mass-conserving model on the lee side.

A comparison of three approaches for simulating fine-scale surface winds in support of wildland fire management. Part II. An exploratory study of the effect of simulated winds on fire growth simulations

The effect of fine-resolution wind simulations on fire growth simulations is explored. The wind models are (1) a wind field consisting of constant speed and direction applied everywhere over the area of interest; (2) a tool based on the solution of the conservation of mass only (termed mass-conserving model) and (3) a tool based on a solution of conservation of mass and momentum (termed momentum-conserving model). Fire simulations use the FARSITE fire simulation system to simulate fire growth for one hypothetical fire and two actual wildfires. The momentum-conserving model produced fire perimeters that most closely matched the observed fire spread, followed by the mass-conserving model and then the uniform winds. The results suggest that momentum-conserving and mass-conserving models can reduce the sensitivity of fire growth simulations to input wind direction, which is advantageous to fire growth modellers. The mass-conserving and momentum-conserving wind models may be useful for operational use as decision support tools in wildland fire management, prescribed fire planning, smoke dispersion modelling, and firefighter and public safety.

Turbulence dependence on winds and stability in a weak‐wind canopy sublayer over complex terrain

The daytime and nighttime turbulence profiles within a weak-wind forest canopy were investigated using data collected within a temperate mixed conifer canopy in northern Idaho, USA. Turbulence measurements made at three heights on a single tower within a Douglas fir canopy were compared. Data were split between the daytime and nighttime to determine the relationships among the local temperature gradient, wind direction, wind speed, and turbulence levels. The total flow field distributions and vertical statistical profiles were determined for the overnight and daytime periods to observe how the overall flow changed with time of day. During the day, the wind probability distribution function was consistent between heights but depended on the canopy depth overnight. The skewness changed with the dominant wind direction. The kurtosis increased with depth into the canopy and from during the day to overnight. The range of wind speeds observed was higher under unstable conditions than stable conditions. Daytime turbulence had no dependence on wind direction. Overnight, the relationship between turbulence and wind speed changed with wind direction and canopy depth. The highest turbulence values were associated with downslope winds near the canopy top but the wind direction for the highest turbulence was variable within the trunk space.

Evidence for Gap Flows in the Birch Creek Valley, Idaho

AbstractA field study was conducted of flows in the Birch Creek Valley in eastern Idaho. There is a distinct topographic constriction in the Birch Creek Valley that creates two subbasins: an upper and lower valley. The data were classified into one of three groups based on synoptic influence (weak/absent, high wind speeds, and other evidence of synoptic influence). Gap flows commonly developed downwind of the constriction in association with the weak/absent group but also occurred in association with the two synoptic groups suggesting the potential for more diverse origins. In general, the frequency and strength of gap flows appeared to be linked to the development of the requisite thermal regime and minimization of any synoptically driven southerly winds that would suppress outflows. Gap flows were characterized by high wind speeds with jetlike vertical profiles along the axis of the lower valley. For all three groups the morning transition in the upper valley and western sidewall usually proceeded sligh...

Wind Erosion of Soils Burned by Wildfire

Wind erosion and aeolian transport processes are largely unstudied in the post-wildfire environment, but recent studies have shown that wind erosion can play a major role in burned landscapes. A wind erosion monitoring system was installed immediately following a wildfire in southeastern Idaho, USA to measure wind erosion from the burned area (Figure 1). This paper describes initial findings related to horizontal sediment flux and PM10 (particulate matter with a diameter of less than 10 µm) emissions from the burned area. Several wind erosion events were monitored in the months following the fire. The largest wind erosion event to-date occurred in early September 2010 and produced 570 kg m-1 of horizontal sediment transport within the first meter above the soil surface and generated a large dust plume that was visible in satellite imagery. The peak PM10 concentration measured on-site at a height of 2 m in the downwind portion of the burned area was 690 mg m-3. Measured horizontal sediment fluxes and PM10 vertical fluxes are on the upper end of values reported in the wind erosion literature. These initial results indicate that wildfire can convert a relatively wind-resistant landscape into one that produces as much soil and dust as some of the most wind erodible landscapes known, including the US southern high plains and the Loess Plateau in China.

Modeling Post-Fire Ash and Dust Emissions in Complex Terrain

Aeolian erosion has been understudied compared to its fluvial counterpart in post-wildfire environments; however, aeolian and fluvial mechanisms operate in concert to influence these disturbed landscapes, often with synergistic effects which may lead to increased total erosion in these environments. Blowing dust and ash from burned areas can impact visibility, air quality, water quality, soil productivity, and nutrient transport. Current wind erosion models are not capable of predicting dust emissions in these disturbed areas because (1) little is known about the erosion mechanisms that govern emissions from burned soil and ash and (2) wind models have not been adapted to predict local terrain effects on winds in the mountainous regions where wildfires often occur. In this paper we present a modeling approach for predicting ash and dust emissions in post-fire environments with complex terrain as well as calibration and validation methods for model evaluation. We propose linkage of an existing computational fluid dynamics (CFD) code with an existing dust algorithm to generate gridded PM10 vertical fluxes from burned landscapes. This paper outlines the development of the linked model and describes the ongoing CFD validation effort, which is a critical first step in model development.

An Evaluation of NDFD Weather Forecasts for Wildland Fire Behavior Prediction

AbstractWildland fire managers in the United States currently utilize the gridded forecasts from the National Digital Forecast Database (NDFD) to make fire behavior predictions across complex lands...

Evaluating the Effectiveness of Burned Area Emergency Rehabilitation (BAER) Road Treatments

Wild fire severity has increased over the last decade, requiring Burned Area Emergency Response (BAER) teams to assess larger burn areas for potential risks and causing associated rehabilitation costs to escalate. Risk to forest roadways is one of the assessments made by BAER teams. There is a large suite of BAER road treatments that are typically employed, however, little is known about the effectiveness of these treatments in the years following a fire. There is a need for information regarding what types of BAER road treatments are being employed and how treatment effectiveness varies with respect to site-specific parameters such as weather, soil, and road properties. The purpose of this study was to evaluate the effectiveness of implemented road treatments and to establish a monitoring protocol for future evaluations. This study reports on-going data collected from three post-fire locations, one that burned in eastern Washington in 2006, one that burned in central Idaho in 2007, and one that burned in northern California in 2008. Measurement of the effectiveness of drain dips, drain dips with armor, ditch maintenance, replacement or upgrading or new culverts, culvert inlet armoring, culvert basin excavation, hydromulch, WoodStraw™, agricultural straw, PAM (polyacrylamide) alone, agricultural straw with PAM, and placement of surface aggregate was begun. Treatments designed to directly protect the road were successful. Hydromulch reduced sediment from cut slopes at a cost of

An evaluation of three wood shred blends for post-fire erosion control using indoor simulated rain events on small plots

4.16 per kg of sediment saved per km per year (

Downscaling surface wind predictions from numerical weather prediction models in complex terrain with WindNinja

6,080 per ton of sediment saved per mile per year). After three years cut slopes treated with hydromulch average nearly 60% ground cover compared to untreated cut slopes with an average of 40% ground cover. There was no difference in depth of wheel ruts between aggregate surfaced road sections and those on native surfaces. The study is on-going.