Widespread Evidence of Terminated Marine Transgressive Sand Supply and Failing Longshore Sand Transport to Eroding Coastal Eolian Sand Ramps during the Latest Holocene Time in Oregon and California (Pacific Coast, USA)
Journal of Coastal Research
beach erosion, cliff top dunes, future sea-level rise, Perched dunes
Holocene perched dunes and wave-truncated sand ramps were used to project paleoramp geometries and to estimate sand ramp erosion during the late Holocene in the U.S. Pacific Coast study area (1500 km in length). Both recently accreted sand ramp slopes (n = 24) and wave-truncated sand ramps (n = 59) in 11 sand ramp complexes (0.3–11.4 km in alongshore length) were profiled from areas of previously abundant dune sand supply. Modern sand ramp slopes (shore normal) ranged from 2 to 26°, with slope steepening resulting from upslope eolian deflation (10–20°) and downslope mass wasting by landslide (> 20°). Paleoramp surface slopes at 10, 15, and 20° were projected seaward from truncated sand ramp tops (11–97 m in elevation) to intersect with projected basal ramp horizons (4-m elevation). Differences between the 10° slope paleoramps and the corresponding modern sea cliffs yield eroded ramp cross-sectional areas (78–16,770 m2). The eroded cross-sectional areas were multiplied by corresponding alongshore lengths to estimate truncated ramp complex erosion (1.29 × 105–6.41 × 107 m3, mean 12.62 × 106 m3). Modern beach sand volumes in shorelines fronting the 11 truncated ramp complexes are estimated to range from 1.05 × 105 to 1.47 × 106 m3. The average modern beach sand volume of 4.25 × 105 m3 represents only 3.4% of the average eroded ramp complex volume, as estimated from differencing the 10° projected slopes and the modern sea cliff profiles. Near-surface 14C ages of buried archaeological materials in the 11 truncated ramp complexes (932 ± 672 cal BP 1σ, n = 20) demonstrate terminal ramp accretion and associated ramp truncation by beach retreat during the latest Holocene time. The widespread sand ramp erosion, following 3.0 m of sea-level rise during the last 3.0 ka, serves as a warning for the U.S. Pacific Coast and other high–wave energy cliff-backed shorelines for potential beach response to near-future sea-level rise (1–3 m) predicted to occur from ongoing global warming.
© Coastal Education and Research Foundation, Inc. 2019. Peterson, C.D.; Kingen, K.E.P.; Erlandson, J.M.; Kaijankoski, P.; Meyer, J., and Ryan, C., 2019.
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Peterson, C.D.; Kingen, K.E.P.; Erlandson, J.M.; Kaijankoski, P.; Meyer, J., and Ryan, C., 2019. Widespread evidence of terminated marine transgressive sand supply and failing longshore sand transport to eroding coastal eolian sand ramps during the latest Holocene time in Oregon and California (Pacific Coast, USA). Journal of Coastal Research, 35(6), 1145–1163.