Dynamic pressure evolution within the 18 May 1980 Mount St. Helens pyroclastic density current: evidence from tree damage

The effects of pyroclastic density currents (PDCs) can be devastating, so understanding their internal dynamics and evolution is important for hazard assessment. We use damaged trees located around Mount St. Helens (USA) as proxy for the dynamic pressure (P-dyn) of the PDC erupted on 18 May 1980. We recorded the location, distribution, and foliage preservation of damaged trees within the medial and distal parts of the devastated forest. Sub-meter resolution aerial photographs from a month after the eruption allow distinction between standing trees that retained foliage from those that were stripped. Heights of standing trees were estimated from the measured lengths of their shadows. The number of standing trees was counted within defined areas along the propagation paths of PDCs. From the measured tree heights, we estimated tree toppling stresses from P-dyn. Overall, P-dyn of the PDC head within the medial to distal portions of the blowdown zone ranged from 10 to 35 kPa. P-dyn likely waned with distance, as shown by the increased number of standing trees in the outer parts of the devastated area. In addition, we find clusters of standing trees on the lee sides of some hills. We propose that these clusters survived because they were primarily impacted by lower dynamic pressures extant within the PDC body, with foliage retention or stripping as a function of the P-dyn evolution in the PDC body. We estimate that P-dyn of the body was less than the estimated maximum P-dyn of the PDC head by 12 +/- 4 kPa.

Automated summary

Understanding the internal dynamics and evolution of pyroclastic density currents (PDCs) is crucial for hazard assessment due to their devastating effects. By studying damaged trees near Mount St. Helens, researchers estimated the dynamic pressure (P-dyn) of PDCs erupted on 18 May 1980. They found that P-dyn decreased with distance from the center of the devastated area, and clusters of standing trees on the lee sides of hills survived due to lower dynamic pressures within the PDC body.