Using tree damage to characterize the evolution of dynamic pressure within the 18 May 1980 pyroclastic density current of Mount St. Helens

Abstract

On 18 May 1980, Mount St. Helens erupted a laterally directed pyroclastic density current (PDC) that decimated ~600km2 of forest, referred to as the blowdown zone. The head of the current contained the peak dynamic pressure, which uprooted or broke off many trees and stripped them of vegetation. The four main objectives in analyzing post-eruption aerial photos were to locate standing trees, calculate tree heights, calculate tree density, and calculate tree height from downed trees as a function of distance from the volcano. Clusters of 10 or more closely spaced trees were left standing throughout the blowdown zone. Clusters were categorized into stripped and foliage clusters based on their shadow shape. Stripped clusters are impacted by higher dynamic pressures than foliage clusters because trees were stripped of vegetation. Because stripped clusters are located closer to the volcano, the PDC is relatively denser, therefore has higher dynamic pressure. The increasing number of stripped, isolated trees with distances indicates that dynamic pressure was declining. Values of dynamic pressure can be estimated using tree height and radius, the maximum stress at the height of tree failure, and the drag coefficient of the PDC. Two different datasets of dynamic pressure were derived, that constrain the minimum and maximum dynamic pressures of the PDC. The dynamic pressures estimated from toppled trees tended to yield lower values than those estimated from standing trees. Standing tree clusters were affected by dynamic pressures that were on average 12 kPa lower than the dynamic pressures that impacted isolated, standing trees. The dynamic pressure of isolated, standing trees decreased from 35 kPa in the last 30% of the distance traveled by the PDC to around 15 kPa near the edge of the blowdown zone. Tree damage can provide insights into internal dynamic pressure changes of PDCs.