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Horton B.P.,University of Pennsylvania | Horton B.P.,Nanyang Technological University | Sawai Y.,Geological Survey of Japan | Hawkes A.D.,Woods Hole Oceanographic Institution | Witter R.C.,Coastal Field Office
Marine Micropaleontology | Year: 2011

At Pichilemu, in the northern third of the rupture area of the moment magnitude scale (Mw) 8.8 2010 Chile earthquake, deposits of the tsunami accompanying the earthquake consist of a lower layer of medium to fine sand (mean grain size of 200μm) containing rock clasts, overlain by a thin, silty, very fine sand (mean grain size of 125μm) layer. Based on a sedimentological model, most (90%) of the deposit is finer than 401-408μm suggesting tsunami flow velocities were between 7m/s and 13.5m/s. Ostracods were common in the upper layer along with a small number of broken benthic foraminifera and a single planktonic foraminifera. Diatoms were abundant throughout. Species assemblages represent a mixture of diatoms from differing environments, life forms and substrate preferences. We attribute the mixed assemblages to turbulence within the water column during tsunami inundation, with erosion of beaches and salt marshes followed by redeposition of sand and mud inland. Breakage of fragile diatom valves in the lower layer may also support transport by turbulent flow. A higher abundance of diatom species with mud substrate preferences in the upper layer implies a decrease in flow velocity from lower to upper layers. © 2011 Elsevier B.V.

Hapke C.J.,U.S. Geological Survey | Adams P.N.,University of Florida | Allan J.,Coastal Field Office | Ashton A.,Woods Hole Oceanographic Institution | And 4 more authors.
Geological Society Memoir | Year: 2014

The coastline of the USA is vast and comprises a variety of landform types including barrier islands, mainland beaches, soft bluffed coastlines and hard rocky coasts. The majority of the bluffed and rocky coasts are found in the northeastern part of the country (New England) and along the Pacific coast. Rocky and bluffed landform types are commonly interspersed along the coastline and occur as a result of relative lowering of sea level from tectonic or isostatic forcing, which can occur on timescales ranging from instantaneous to millenia. Recent research on sea cliffs in the contiguous USA has focused on a broad range of topics from documenting erosion rates to identifying processes and controls on morphology to prediction modelling. This chapter provides a detailed synthesis of recent and seminal research on rocky coast geomorphology along open-ocean coasts of the continental United States (USA). © The Geological Society of London 2014.

Witter R.C.,U.S. Geological Survey | Jaffe B.,U.S. Geological Survey | Zhang Y.,Oregon Health And Science University | Priest G.,Coastal Field Office
Natural Hazards | Year: 2012

Coastal communities in the western United States face risks of inundation by distant tsunamis that propagate across the Pacific Ocean as well as local tsunamis produced by great (M w > 8) earthquakes on the Cascadia subduction zone. In 1964, the M w 9.2 Alaska earthquake launched a Pacific-wide tsunami that flooded Cannon Beach, a small community (population 1640) in northwestern Oregon, causing over $230,000 in damages. However, since the giant 2004 Indian Ocean tsunami, the 2010 Chile tsunami and the recent 2011 Tohoku-Oki tsunami, renewed concern over potential impacts of a Cascadia tsunami on the western US has motivated closer examination of the local hazard. This study applies a simple sediment transport model to reconstruct the flow speed of the most recent Cascadia tsunami that flooded the region in 1700 using the thickness and grain size of sand layers deposited by the waves. Sedimentary properties of sand from the 1700 tsunami deposit provide model inputs. The sediment transport model calculates tsunami flow speed from the shear velocity required to suspend the quantity and grain size distribution of the observed sand layers. The model assumes a steady, spatially uniform tsunami flow and that sand settles out of suspension forming a deposit when the flow velocity decreases to zero. Using flow depths constrained by numerical tsunami simulations for Cannon Beach, the sediment transport model calculated flow speeds of 6.5-7.6 m/s for sites within 0.6 km of the beach and higher flow speeds (~8.8 m/s) for sites 0.8-1.2 km inland. Flow speed calculated for sites within 0.6 km of the beach compare well with maximum velocities estimated for the largest tsunami simulation. The higher flow speeds calculated for the two sites furthest landward contrast with much lower maximum velocities (<3.8 m/s) predicted by numerical simulations. Grain size distributions of sand layers from the most distal sites are inconsistent with deposition from sediment falling out of suspension. We infer that rapid deceleration in tsunami flow and convergences in sediment transport formed unusually thick deposits. Consequently, higher flow speeds calculated by the sediment model probably overestimate the actual wave speed at sites furthest inland. © 2011 Springer Science+Business Media B.V.

Witter R.C.,Coastal Field Office | Witter R.C.,U.S. Geological Survey | Zhang Y.,Oregon Health And Science University | Wang K.,Geological Survey of Canada | And 3 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2012

We test hypothetical tsunami scenarios against a 4,600-year record of sandy deposits in a southern Oregon coastal lake that offer minimum inundation limits for prehistoric Cascadia tsunamis. Tsunami simulations constrain coseismic slip estimates for the southern Cascadia megathrust and contrast with slip deficits implied by earthquake recurrence intervals from turbidite paleoseismology. We model the tsunamigenic seafloor deformation using a three-dimensional elastic dislocation model and test three Cascadia earthquake rupture scenarios: slip partitioned to a splay fault; slip distributed symmetrically on the megathrust; and slip skewed seaward. Numerical tsunami simulations use the hydrodynamic finite element model, SELFE, that solves nonlinear shallow-water wave equations on unstructured grids. Our simulations of the 1700 Cascadia tsunami require >12-13m of peak slip on the southern Cascadia megathrust offshore southern Oregon. The simulations account for tidal and shoreline variability and must crest the ∼6-m-high lake outlet to satisfy geological evidence of inundation. Accumulating this slip deficit requires 360-400years at the plate convergence rate, exceeding the 330-year span of two earthquake cycles preceding 1700. Predecessors of the 1700 earthquake likely involved >8-9m of coseismic slip accrued over >260 years. Simple slip budgets constrained by tsunami simulations allow an average of 5.2m of slip per event for 11 additional earthquakes inferred from the southern Cascadia turbidite record. By comparison, slip deficits inferred from time intervals separating earthquake-triggered turbidites are poor predictors of coseismic slip because they meet geological constraints for only 4 out of 12 (∼33%) Cascadia tsunamis. © 2012. American Geophysical Union. All Rights Reserved.

Olsen M.J.,Oregon State University | Allan J.C.,Coastal Field Office | Priest G.R.,Coastal Field Office
Geotechnical Special Publication | Year: 2012

Large movements (generally -10 cm/year) of the highly-active Johnson Creek Landslide, located on the coast north of Newport, Oregon, are problematic for maintenance of U.S. Highway 101, where roadway deformation is visible at the north and south extents of the landslide. Whereas landslide movement is continually driven by high-intensity precipitation events that lift the slide-block, shifting it seaward, previous research has shown that toe erosion may also destabilize the slide regardless of rainfall. Site monitoring through several ground-based LiDAR scans of the bluff face since 2004 enable assessment of the spatial and temporal variability of erosion. Comparisons of each 3D scan survey enable quantification of erosion rates and surface deformation, which can be used to analyze the pattern and propagation of displacements that have taken place over the past seven years. A preliminary change analysis methodology to distinguish landslide movement from erosion is presented. © 2012 American Society of Civil Engineers.

Komar P.D.,Oregon State University | Allan J.C.,Coastal Field Office | Ruggiero P.,Oregon State University
Journal of Coastal Research | Year: 2011

Analyses of the progressive multidecadal trends and climate-controlled annual variations in mean sea levels are presented for nine tide-gauge stations along the coast of the U.S. Pacific Northwest: Washington, Oregon, and Northern California. The trends in relative sea levels are strongly affected by the tectonics of this region, characterized by significant alongcoast variations in changing land elevations measured by benchmarks and global positioning system data. These combined data sets document the existence of both submergent and emergent stretches of shore. The Pacific Northwest sea levels are also affected by variations in the monthly mean seasonal cycles, with its extreme water levels occurring in the winter during strong El Niños. To quantify this climate control and to derive improved multidecadal sea-level trends, separate evaluations of the winter and summer-averaged measured water levels have been undertaken. The resulting pair of linear regressions for each tide gauge shows a consistent difference in the mean water levels over the years, at their highest during the winters, reflecting the total magnitude in the seasonal cycle of water levels. Of importance, the degree of scatter in the summer averages is reduced compared with the annual averages, yielding sea-level trends that generally have the highest statistical significance. In contrast, the winter records emphasize the extreme water levels associated with strong El Niños, yielding a predictive correlation with the Multivariate El Niño/Southern Oscillation Index. Both trends in relative sea levels and extremes in the winter monthly elevations produced by El Niños are important to the Pacific Northwest coastal hazard assessments, combining with the multidecade increase in wave heights measured by buoys. With these multiple processes and their climate controls, the erosion hazards are projected to significantly increase in future decades. © 2011, the Coastal Education & Research Foundation (CERF).

Ruggiero P.,Oregon State University | Komar P.D.,Oregon State University | Allan J.C.,Coastal Field Office
Coastal Engineering | Year: 2010

Deep-water wave buoy data offshore from the U.S. Pacific Northwest (Oregon and Washington) document that the annual averages of deep-water significant wave heights (SWHs) have increased at a rate of approximately 0.015. m/yr since the mid-1970s, while averages of the five highest SWHs per year have increased at the appreciably greater rate of 0.071. m/yr. Histograms of the hourly-measured SWHs more fully document this shift toward higher values over the decades, demonstrating that both the relatively low waves of the summer and the highest SWHs generated by winter storms have increased. Wave heights associated with higher percentiles in the SWH cumulative distribution function are shown to be increasing at progressively faster rates than those associated with lower percentiles. This property is demonstrated to be a direct result of the probability distributions for annual wave climates having lognormal- or Weibull-like forms in that a moderate increase in the mean SWH produces significantly greater increases in the tail of the distribution. Both the linear regressions of increasing annual averages and the evolving probability distribution of the SWH climate, demonstrating the non-stationarity of the Pacific Northwest wave climate, translate into substantial increases in extreme value projections, important in coastal engineering design and in quantifying coastal hazards. Buoy data have been analyzed to assess this response in the wave climate by employing various time-dependent extreme value models that directly compute the progressive increases in the 25- to 100-year projections. The results depend somewhat on the assumptions made in the statistical procedures, on the numbers of storm-generated SWHs included, and on the threshold value for inclusion in the analyses, but the results are consistent with the linear regressions of annual averages and the observed shifts in the histograms. © 2009 Elsevier B.V.

Priest G.R.,Coastal Field Office | Stimely L.L.,Coastal Field Office | Wood N.J.,U.S. Geological Survey | Madin I.P.,800 NE Oregon Street | Watzig R.J.,800 NE Oregon Street
Natural Hazards | Year: 2016

Previous pedestrian evacuation modeling for tsunamis has not considered variable wave arrival times or critical junctures (e.g., bridges), and did not effectively communicate multiple evacuee travel speeds. We summarize an approach that identifies evacuation corridors, recognizes variable wave arrival times, and produces a map of minimum pedestrian travel speeds to reach safety, termed a “beat-the-wave” (BTW) evacuation analysis. We demonstrate the improved approach by evaluating difficulty of pedestrian evacuation of Seaside, Oregon, for a local tsunami generated by a Cascadia subduction zone earthquake. We establish evacuation paths by calculating the least-cost distance (LCD) to safety for every grid cell in a tsunami hazard zone using geospatial, anisotropic path distance algorithms. Minimum BTW speed to safety on LCD paths is calculated for every grid cell by dividing surface distance from that cell to safety by the tsunami arrival time at safety. We evaluated three scenarios of evacuation difficulty: (1) all bridges are intact with a 5-min evacuation delay from the start of earthquake, (2) only retrofitted bridges are considered intact with a 5-min delay, and (3) only retrofitted bridges are considered intact with a 10-min delay. BTW maps also take into account critical evacuation points along complex shorelines (e.g., peninsulas, bridges over shore-parallel estuaries) where evacuees could be caught by tsunami waves. The BTW map is able to communicate multiple pedestrian travel speeds, which are typically visualized by multiple maps with current LCD-based mapping practices. Results demonstrate that evacuation of Seaside is problematic seaward of the shore-parallel waterways for those with any limitations on mobility. Tsunami vertical evacuation refuges or additional pedestrian bridges may be effective ways of reducing loss of life seaward of these waterways. © 2015, Springer Science+Business Media Dordrecht (outside the USA).

Barnard P.L.,U.S. Geological Survey | Allan J.,Coastal Field Office | Hansen J.E.,U.S. Geological Survey | Hansen J.E.,University of California at Santa Cruz | And 3 more authors.
Geophysical Research Letters | Year: 2011

High-resolution beach morphology data collected along much of the U.S. West Coast are synthesized to evaluate the coastal impacts of the 2009-10 El Nio. Coastal change observations were collected as part of five beach monitoring programs that span between 5 and 13 years in duration. In California, regional wave and water level data show that the environmental forcing during the 2009-10 winter was similar to the last significant El Nio of 1997-98, producing the largest seasonal shoreline retreat and/or most landward shoreline position since monitoring began. In contrast, the 2009-10 El Nio did not produce anomalously high mean winter-wave energy in the Pacific Northwest (Oregon and Washington), although the highest 5% of the winter wave-energy measurements were comparable to 1997-98 and two significant non-El Nio winters. The increase in extreme waves in the 2009-10 winter was coupled with elevated water levels and a more southerly wave approach than the long-term mean, resulting in greater shoreline retreat than during 1997-98, including anomalously high shoreline retreat immediately north of jetties, tidal inlets, and rocky headlands. The morphodynamic response observed throughout the U.S. West Coast during the 2009-10 El Nio is principally linked to the El Nio Modoki phenomena, where the warm sea surface temperature (SST) anomaly is focused in the central equatorial Pacific (as opposed to the eastern Pacific during a classic El Nio), featuring a more temporally persistent SST anomaly that results in longer periods of elevated wave energy but lower coastal water levels. © 2011 by the American Geophysical Union.

Allan J.C.,Coastal Field Office | Komar P.D.,Oregon State University | Ruggiero P.,Oregon State University | Witter R.,Coastal Field Office
Journal of Coastal Research | Year: 2012

The March 11, 2011, magnitude 9, Tohoku earthquake off the coast of Japan generated a tsunami that crossed the Pacific Ocean and impacted the shores of the U.S. West Coast. Analyses of the arrival times, wave heights, periods, and total water levels from the tsunami waves and the tides have been undertaken for 17 tide gauges along the length of the West Coast. Significant along-coast variations in wave heights were found, with the highest waves having been recorded at Crescent City (maximum height of 4.23 m) in northern California and in San Luis Obispo Bay (maximum 4.25 m) in southern California. It was concluded that similar to the Kuril Island tsunami in 2006, the particularly large wave heights that impacted the CaliforniaOregon border region, including Crescent City, California, were the result of wave refraction that focused tsunami energy into a relatively narrow band as it crossed the ocean from Japan, followed by local shelf resonance that further enhanced the wave heights and determined the dominant wave periods recorded by tide gauges. Detailed analyses of the tsunami waves at Crescent City, California, and nearby at Port Orford, Oregon, documented the differences in their heights and periods and in the profiles of the maximum measured waves. The effects to developments along the coast varied in response to the local wave conditions, moderated by the largest waves having arrived during a low tide. Undoubtedly, damage to infrastructure would have been greater had the waves arrived a few hours earlier, at the time of the Higher High tide. The resulting damage occurred almost entirely within the harbors, brought about mainly by the strong to-and-fro currents that alternately filled and then emptied the boat basins, capsizing boats and damaging docks. © the Coastal Education & Research Foundation 2012.

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