Coastal flooding and beach retreat from coseismic subsidence in the central Cascadia margin, USA

Abstract

Prehistoric great earthquakes (Mw 8-9) in the central Cascadia margin have produced coastal subsidence (0-2 m) that has resulted in chronic lowland flooding and catastrophic beach erosion. Geologic records of wetland burial in 13 bays from Washington and Oregon are used with simple calculations to estimate likely coastal flooding and beach retreat following a great Cascadia earthquake. Plant macrofossils and peat-to-mud ratios recorded in core logs discriminate between forest, marsh, colonizing marsh, and mud-flat tidal settings. Transitions between these settings, i.e., across abrupt burial contacts, demonstrate either 0+ or -0.5, 1+ or -0.5, or 2+ or -0.5 m of paleosubsidence. Paleosubsidence from the last Cascadia event (AD 1700) decreases from 2+ or -0.5 m in the eastern reaches of south-west Washington bays to 0+ or -0.5 m in the western reaches of central Oregon bays. First-order estimates of post-subsidence flooding hazards are based on the predicted regional subsidence added to current 10- and 100-year flood elevations. At least 525 km of bay shorelines are threatened by chronic flooding following coseismic subsidence. Catastrophic beach retreat is estimated from Bruun's Rule to range from 50 to 300 m depending on assumed depth of closure, measured beach-berm heights, and predicted coseismic subsidence along the margin. Shorelines that are susceptible to catastrophic beach retreat from coseismic subsidence total at least 250 km in longshore distance.

Late Holocene Geomorphology of the Columbia River Estuary, Oregon and Washington, USA

“…Great earthquakes in the study region (Atwater, 1987;Peterson et al, 2000) have produced episodic events of abrupt submergence (0.5-2.0 m sea level rise) leading to abruptly buried wetland marshes in the Columbia River estuary and adjacent estuaries. Radiocarbon dating has been used to correlate the buried marsh horizons in the lower Columbia River estuary (Jurney, 2001) to regional coseismic susbsidence events (Peterson et al, 2012b).…”

Changes in productivity associated with four introduced species: ecosystem transformation of a ‘pristine’ estuary

“…About 30% of the 3500 ha of tidal marsh between mean high and extreme high water has been lost to diking for agriculture or expanding towns (Borde et al 2003), but little other bathymetric modification has occurred since 1977 when maintenance dredging of the Willapa River channel ended (Hedgpeth & Obrebski 1981). Sediment loads to the bay have likely varied over the past century of logging within the watershed (Hedgpeth & Obrebski 1981, Kehoe 1982, Komar et al 2004, and as a result of damming the Columbia River, the second largest river of the continental United States, which exits just south and often influences conditions at the bay's mouth (Simenstad et al 1992, Peterson et al 2000. The impacts of diking and sediment loading peaked by the mid-20th century and have since been constant or declined.…”

Historical evolution of the Columbia River littoral cell

“…These episodic events cause coseismic coastal subsidence of 0.5 to 2.5 m (Atwater, 1996) and shoreline retreat on the order of a few hundred meters (Doyle, 1996;Peterson et al, 2000). Scarp formations in the subsurface of the CRLC barriers and strand plains (detected with Ground Penetrating Radar) provide evidence of this coastal subsidence Jol et al, 1996;Peterson et al, 2010-this issue-b).…”