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

Abstract EN

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.