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\begin{document}

\title{Characterizing Dynamic Pressure Variations from Tree Damage Resulting
from the 18 May 1980 Pyroclastic Density Current of Mount St. Helens}



\author[1]{Nicole Guinn}%
\author[1]{James Gardner}%
\author[1]{Mark Helper}%
\affil[1]{University of Texas at Austin}%


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  \date{June 13, 2024}


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\begin{abstract}
Tree damage can provide insights into internal dynamic pressure changes
of pyroclastic density currents (PDC). On 18 May 1980, Mount St. Helens
erupted a laterally directed PDC that decimated \textasciitilde{}600km2
of forest, referred to as the blowdown zone. The head of the current
contained the peak dynamic pressure, which uprooted or broke off most
trees and stripped them of vegetation; however, some partially stripped
tree trunks were left standing. Tree damage was assessed using aerial
photography taken one month after the eruption. The flow direction of
the PDC was mapped from shadows of root balls of toppled trees and
directions of fallen trees. Along given flow paths, the density of
standing trees was measured by the number of shadows within 200m2 areas.
Towards the northwest, the average tree density increased from 0.01 to
0.58 (\selectlanguage{ngerman}± 0.19) trees/m2 with distance. Additionally, analysis identified
95 clusters of trees still standing in the blowdown zone, situated on
the lee sides of hills or plateaus. Blurry, cylindrical shadows versus
well-defined, cylindrical shadows distinguished standing trees with
foliage in clusters from those without. Five variables were used to
determine the heights of trees: ground slope and aspect, bearing and
length of shadows, and the sun angle above the horizon. Trees stripped
of foliage in patches have average heights of 16 ± 7m and occur where
the PDC reached 66 ± 24\% of its runout. Foliage patches have average
heights of 12 ± 7m and occur where the PDC reached 91 ± 9\% of its
runout. Tree heights in the patches indicate a localized height the peak
dynamic pressure must jump as it travels over hills and away from its
source. Patches with foliage imply that the peak dynamic pressure has
risen above the tops of the trees, whereas patches without foliage
suggest that the peak dynamic pressure was still low enough to damage
trees even though the current had jumped over topography. Outside of the
patches, increasing tree density suggests that dynamic pressure waned
with distance.%
\end{abstract}\selectlanguage{ngerman}%



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