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 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.
Highlights
In this paper, we present the results of investigating paleotsunami inundations that crossed multiple beach ridges and flooded intervening valleys in a low-elevation beach ridge plain
Previous work has not established whether the beach plain inundation occurred by (1) direct overwash of the seawardmost cobble beach ridges, or (2) bay mouth flooding and shore-parallel flow within the interridge valleys [5]
High-velocity inundation distances exceed the width of the beach plain at 1.5 km
Summary
We present the results of investigating paleotsunami inundations that crossed multiple beach ridges and flooded intervening valleys in a low-elevation beach ridge plain. The resort town of Seaside, Oregon, with a summer population of up to 50,000, is developed on a prograded beach ridge plain (Figure 1) This low-elevation plain has experienced both historical (1964) farfield tsunami [1] and nearfield paleotsunami (4-5 Cascadia events during the last 2,500 yr) [2,3,4,5]. Previous work has not established whether the beach plain inundation occurred by (1) direct overwash of the seawardmost cobble beach ridges, or (2) bay mouth flooding and shore-parallel flow within the interridge valleys [5] Discrimination of these two mechanisms is important to documenting paleotsunami runup heights and to proposing specific evacuation strategies for coastal populations in low elevation settings of beach ridge plains
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