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

Abstract 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 the dynamic pressure ( P dyn ) PDC erupted on 18 May 1980. recorded location, distribution, foliage preservation within medial distal parts devastated forest. Sub-meter resolution aerial photographs from a month after eruption allow distinction between standing that retained those were stripped. Heights estimated measured lengths shadows. number was counted defined areas along propagation paths PDCs. From tree heights, we toppling stresses . Overall, head to portions blowdown zone ranged 10 35 kPa. likely waned with distance, shown by increased in outer area. In addition, find clusters lee sides some hills. propose these survived because they primarily impacted lower pressures extant body, retention or stripping function body. estimate body less than maximum 12 ± 4