Paleotsunami Inundation of a Beach Ridge Plain: Cobble Ridge Overtopping and Interridge Valley Flooding in Seaside, Oregon, USA

摘要

The Seaside beach ridge plain was inundated by six paleotsunamis during the last ～2500 years. Large runups (adjusted >10 m in height) overtopped seawardmost cobble beach ridges (7 m elevation) at ～1.3 and ～2.6 ka before present. Smaller paleotsunami (6-8 m in height) likely entered the beach plain interior (4-5 m elevation) through the paleo-Necanicum bay mouth. The AD 1700 Cascadia paleotsunami had a modest runup (6-7 m height), yet it locally inundated to 1.5 km landward distance. Bed shear stresses (100-3,300 dyne cm-2) are estimated for paleotsunami surges (0.5-2 m depths) that flowed down slopes (0.002-0.017 gradient) on the landward side of the cobble beach ridges. Critical entrainment shear stresses of 1,130-1,260 dyne cm-2 were needed to dislodge the largest clasts (26-32 cm diameter) in paleotsunami coulees that were cut (100-200 m width) into the landward side of the cobble ridges.