Landscape-scale patterns of forest tornado damage in mountainous terrain
Cannon, J. B., J. Hepinstall-Cymerman, C. M. Godfrey, and C. J. Peterson
Landscape patterns created by natural disturbance such as windstorms can affect forest regeneration, carbon cycling, and other ecological processes.
We develop a method for remotely measuring tornado damage severity and describe landscape-scale patterns of tornado damage. We examine the extent and distribution of damage severity and gaps created by tornadoes, and examine how topographic variation can influence tornado damage severity.
Focusing on two April 2011 tornadoes that struck the Chattahoochee National Forest (CNF) in Georgia and the Great Smoky Mountains (GSM) in Tennessee, we used supervised classification of aerial photographs to map damage severity. We report the extent and distribution of damage severity from each track and characterize patterns of damage using FragStats. Using topographic overlays, we test hypotheses regarding how physiographic features such as valleys and ridges affect tornado damage severity.
Tornado damage severity estimates were significantly correlated with ground-truth measurements. The 64-km CNF track damaged 1712 ha (>25 % severity), while the 26-km GSM track damaged 1407 ha. Tornado damage severity was extremely variable and frequency of gap sizes drastically decreased with size, with many small gaps and few very large gaps, consistent with other types of wind damage. Damage severity declined as tornadoes ascended ridges and increased as they descended ridges. This effect was more consistent on shallow slopes relative to steeper slopes.
This study outlines an objective methodology for remotely characterizing tornado damage severity. The results from this study fill an important gap in ecological understanding of the spatial components of the forest tornado disturbance regime.
Estimating Enhanced Fujita Scale Levels Based on Forest Damage Severity
Christopher M. Godfrey; Chris Peterson
Enhanced Fujita (EF) scale estimates following tornadoes remain challenging in rural areas with few traditional damage indicators. In some cases, such as the 27 April 2011 tornadoes that passed through mostly inaccessible terrain in the Great Smoky Mountains National Park and the Chattahoochee National Forest in the southeastern United States, traditional ground-based tornado damage surveys are nearly impossible. This work presents a novel method to infer EF-scale categories in forests using levels of tree damage and a coupled wind and tree resistance model. High-resolution aerial imagery allows detailed analyses based on a field of nearly half a million trees labeled with their geographic location and fall direction. Ground surveys also provide details on the composition of tree species and tree diameters within each tornado track. A statistical resampling procedure randomly draws a sample of trees from this database of observed trees. The coupled wind and tree resistance model determines the percentage of trees in that sample that fall for a given wind speed. By repeating this procedure, each wind speed value corresponds with a distribution of treefall percentages in the sampled plots. Comparing these results with the observed treefall percentage in small subplots along the entire tornado track allows estimation of the most probable wind speed associated with each subplot. Maps of estimated EF-scale levels reveal the relationship between complex terrain and wind speeds and show the variability of the intensity of each tornado along both tracks. This approach may lead to methods for the straightforward estimation of EF-scale categories in remote or inaccessible locations.
Empirical approach to evaluating the tornado fragility of residential structures
Roueche, D. B., F. T. Lombardo, and D. O. Prevatt
Tornado-induced wind load modeling has advanced significantly in recent years, but comparison of the experimental or numerical models to observed tornado damage is limited. This paper describes the development of empirically derived tornado fragility functions for residential structures following the 22 May 2011 Joplin, Missouri tornado. The fragility functions were created by combining a residential structure damage assessment of the Joplin tornado using the enhanced Fujita (EF) scale and a tornado wind field model conditioned to tree-fall patterns. The damage states for the fragility functions used the degrees of damage (DOD) for one- and two-story residential structures from the EF scale. The empirical tornado fragility functions were compared to analytically derived fragility functions for straightline winds from the FEMA Hazus hurricane model to provide a first attempt at ascertaining how different or similar failure wind speeds are for residential structures under tornado-induced and straightline wind loads. Median failure wind speeds from the empirical tornado fragility functions increased monotonically with DOD, from 33.4 m/s33.4 m/s for DOD1 to 85.6 m/s85.6 m/s for DOD9. Median failure wind speeds of the empirical tornado fragility functions and Hazus hurricane straightline wind fragility functions differed by 5% or less when suburban terrain was assumed throughout the damage path, and 25% or less when open terrain was assumed. The results suggest that tornado load amplification factors are at worst no more than 55% for residential structures, and may be lower than 10%.
Damage path width and discontinuity in 50 tornado tracks through forested landscapes
Zenoble, M., and C. J. Peterson
Tornado damage-path width is a necessary component for calculation of area impacted, which allows estimation of hazards. To date, rarely has variation in damage path width or path discontinuity been a focus. In this paper, using a damage threshold of >25% canopy damage, we quantify width and discontinuity in 50 tornado paths in forested areas. Tornado-path starting and end points were overlaid on Google Earth imagery obtained ≤24 months after the tornadoes, and damage-path width (or absence of damage) was measured for severities >25% canopy loss, at fixed intervals. Width was measured only where both sides of the damage path were clearly defined by forest tree damage, thus many points were excluded from our analysis. Given our threshold level of forest canopy damage, no EF0 tornadoes showed remotely visible damage, and analyses were thus restricted to ≥EF1 tornado paths. Variation in remotely visible damage width was quantified as coefficient of variation, which ranged from 0.227 to 0.852, with a mean of 0.531 among the 50 paths. Discontinuity in remotely visible damage also varied among damage paths; up to 45% of the total number of measured points within a path lacked visible damage. Almost 40% of tornado damage paths exhibited such discontinuity along 20% or more of their path length. We suggest that the long, narrow EF-scale contours (particularly for ≥EF1) often reported after storm surveys may mask extensive width variation in severe damage and substantial portions of tornado paths with no severe damage.
Improved near-surface wind field characterization using damage patterns
Daniel Rhee, Franklin Lombardo
Tornadoes have caused significant damage and casualties in the past decades. These losses have spurred efforts toward tornado-based design, which requires an accurate estimate of the near-surface tornadic wind speeds. Due to the difficulty of obtaining in-situ measurements and various issues regarding Enhance Fujita (EF) scale, a promising method of estimating near-surface wind speed based on damage inflicted is developed. The method utilizes fall directions of trees and other objects with distinct fall patterns to describe the characteristics of the tornado and other wind storms. The observed fall patterns are used to estimate Rankine vortex parameters and reproduce near-surface wind field. The wind field then can be compared to structural damage as an independent method. The near-surface wind speeds of different tornado cases were estimated using this method, one of which (Sidney, IL) exhibited ‘crop-fall’ patterns and yet another (Naplate, IL) caused damage to trees and other infrastructures such as street signs. Based on the damage to structures and estimated wind speeds from tree-fall analysis, empirical fragility curves are also developed, which allows to interpret the vulnerability to tornadoes. The entire process of wind speed, wind load, structural resistance and ultimately how to mitigate damage then can be better understood.