Table of Contents
Journal of Geological Research
Volume 2010, Article ID 276989, 22 pages
http://dx.doi.org/10.1155/2010/276989
Review Article

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

1Geology Department, Portland State University, 1721 SW Broadway, Portland, OR 77207, USA
2Geography Department, University of Wisconsin at Eau Claire, P.O. Box 4004, Eau Claire, WI 54702, USA
3Horning Geosciences, 808 26th Avenue, Seaside, OR 97138, USA

Received 4 February 2010; Revised 6 June 2010; Accepted 10 August 2010

Academic Editor: Steven L. Forman

Copyright © 2010 Curt D. Peterson et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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 ) 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 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.