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Findings from nearly two decades of research focused on the Columbia River littoral cell (CRLC), a set of rapidly prograding coastal barriers and strand-plains in the U.S. Pacific Northwest, are synthesized to investigate the morphodynamics associated with prograding beaches. Due to a large sediment supply from the Columbia River, the CRLC is the only extensive stretch of shoreline on the U.S. west coast to have advanced significantly seaward during the late Holocene. Since the last Cascadia Subduction Zone (CSZ) earthquake in 1700, with associated co-seismic subsidence and tsunami, much of the CRLC has prograded hundreds of meters. However, the rates of progradation, and the processes most responsible for sediment accumulation, vary depending on time scale and the morphological unit in question. Remarkably, the 20th and early 21st century shoreline change rates were more than double the late prehistoric rates that include recovery from the last major CSZ event, most likely due to an increase in sediment supply resulting from inlet jetty construction. In some locations detailed beach morphology monitoring reveals that at interannual- to decadal-scale the upper shoreface aggraded about 2 cm/yr, subtidal sandbars migrated offshore and decayed while intertidal bars migrated onshore and welded to the shoreline, the shoreline prograded about 4 m/yr, and 1 to 2 new foredune ridges were generated. A detailed meso-scale sediment budget analysis in one location within the littoral cell shows that approximately 100 m 3 /m/yr accumulated between − 12 m (seaward limit of data) and + 9 m (crest of landward-most foredune). Gradients in alongshore sediment transport, net onshore-directed cross-shore sediment transport within the surf zone, and cross-shore feeding from a shoreface out of equilibrium with forcing conditions are each partially responsible for the significant rates of sediment supplied to the beaches and dunes of the CRLC during the observational period. Direct observations of beach progradation at seasonal- to decadal-scale are put in context of measured or inferred changes over time scales of decades to centuries. [ABSTRACT FROM AUTHOR]

Abstract Coupled models of coastal hazards, ecosystems, socioeconomics, and landscape management in conjunction with alternative scenario analysis provide tools that can allow decision-makers to explore effects of policy decisions under uncertain futures. Here, we describe the development and assessment of a set of model-based alternative future scenarios examining climate and population driven landscape dynamics for a coastal region in the U.S. Pacific Northwest. These scenarios incorporated coupled spatiotemporal models of climate and coastal hazards, population and development, and policy and assessed a variety of landscape metrics for each scenario. Coastal flooding and erosion were probabilistically simulated using 99 future 95-year climate scenarios. Five policy scenarios were iteratively co-developed by researchers and stakeholders in Tillamook County, Oregon. Results suggest that both climate change and management decisions have a significant impact across the landscape, and can potentially impact geographic regions at different magnitudes and timescales. Highlights • A framework for comparing adaptation policies under a range of climate impact scenarios is presented. • Probabilistic simulation of total water levels capture coastal flooding and erosion hazards. • Policy scenarios were co-developed with local stakeholders to represent a range of management strategies. • Simulated future landscapes were compared using metrics related to development, property risk, and public good. • Both climate change and management decisions have a significant impact across the coastal landscape. [ABSTRACT FROM AUTHOR]

Producing accurate hindcasts and forecasts with coupled models is challenging due to complex parameterizations that are difficult to ground in observational data. We present a calibration workflow that utilizes a series of machine learning algorithms paired with Windsurf, a coupled beach-dune model (Aeolis, the Coastal Dune Model, and XBeach), to produce hindcasts and forecasts of morphologic change along Bogue Banks, North Carolina. Neural networks paired with genetic algorithms allow us to fine tune calibration parameters for the hindcast, and then a long short-term memory neural network, trained on the hindcast, produces a 4-year forecast. We compare our hindcasts to observations from 2016 to 2017 and find they successfully reproduce observed modes of dune and beach change except for seaward growth of the dune face. We compare our forecasts to observations from 2016 to 2020 and find that they produce reasonably accurate predictions of dune change except when there are significant instances of erosion during the forecast period. • A neural network paired with a genetic algorithm was used to calibrate a one-year hindcast with a coupled numerical model. • A Long Short-Term Memory (LSTM) neural network was trained on the hindcast to produce 4-year forecasts of dune morphology. • Windsurf accurately reproduces observed modes of dune and beach change except for seaward growth of the dune. [ABSTRACT FROM AUTHOR]

Abstract: Detailed wave energy resource assessments are necessary for the planning and design of wave energy converters. The waves in the U.S. Pacific Northwest have been identified as very energetic making this coast potentially suitable for wave energy harvesting. Several efforts to harvest this resource are under way in this region, however no long term, high resolution description of the resource is presently available. Here, the results of a 7 year hindcast are presented at a 30 arc-second resolution using the numerical models WAVEWATCH III and SWAN. The hindcast accuracy was quantified by comparing to measured buoy data yielding linear correlation coefficients ∼0.90 for the significant wave height. This study describes the alongshore variability of the resource over the continental shelf. The general decline of the wave power with depth is explained by considerations of wave refraction and shoaling. Further, due to wave refraction, areas off the central and southwest Oregon coast are identified that show increased wave power at 50 m of water in comparison with the 250 m value. These areas also show increased temporal variability. In addition, areas with preferentially narrower wave spectra in both frequency and direction are identified off southwest Oregon. Further, general trends in the directionality of the resource indicate a systematic switch in the wave direction with latitude. The seasonality of the resource is also assessed in terms of variability and trends relevant to the planning and deployment of wave energy converters. The continental shelf is mapped in terms of the coefficient of variability, which is greater (smaller) than unity during the summer (winter) and regardless of the season smaller in southwest Oregon. [Copyright &y& Elsevier]

Abstract: Sediment supply provides a fundamental control on the morphology of river deltas, and humans have significantly modified these supplies for centuries. Here we examine the effects of almost a century of sediment supply reduction from the damming of the Elwha River in Washington on shoreline position and beach morphology of its wave-dominated delta. The mean rate of shoreline erosion during 1939–2006 is ~0.6 m/yr, which is equivalent to ~24,000 m3/yr of sediment divergence in the littoral cell, a rate approximately equal to 25–50% of the littoral-grade sediment trapped by the dams. Semi-annual surveys between 2004 and 2007 show that most erosion occurs during the winter with lower rates of change in the summer. Shoreline change and morphology also differ spatially. Negligible shoreline change has occurred updrift (west) of the river mouth, where the beach is mixed sand to cobble, cuspate, and reflective. The beach downdrift (east) of the river mouth has had significant and persistent erosion, but this beach differs in that it has a reflective foreshore with a dissipative low-tide terrace. Downdrift beach erosion results from foreshore retreat, which broadens the low-tide terrace with time, and the rate of this kind of erosion has increased significantly from ~0.8 m/yr during 1939–1990 to ~1.4 m/yr during 1990–2006. Erosion rates for the downdrift beach derived from the 2004–2007 topographic surveys vary between 0 and 13 m/yr, with an average of 3.8 m/yr. We note that the low-tide terrace is significantly coarser (mean grain size ~100 mm) than the foreshore (mean grain size ~30 mm), a pattern contrary to the typical observation of fining low-tide terraces in the region and worldwide. Because this cobble low-tide terrace is created by foreshore erosion, has been steady over intervals of at least years, is predicted to have negligible longshore transport compared to the foreshore portion of the beach, and is inconsistent with oral history of abundant shellfish collections from the low-tide beach, we suggest that it is an armored layer of cobble clasts that are not generally competent in the physical setting of the delta. Thus, the cobble low-tide terrace is very likely a geomorphological feature caused by coastal erosion of a coastal plain and delta, which in turn is related to the impacts of the dams on the Elwha River to sediment fluxes to the coast. [Copyright &y& Elsevier]

Coastal hazards often result from the combination of different simultaneous oceanographic processes that occur at multiple spatial and temporal scales. To predict coastal flooding and erosion, it is necessary to accurately represent hydrodynamic conditions. For this reason, here we present a stochastic, climate based wave emulator that provides the hydrodynamic conditions needed for these predictions. The emulator can generate an infinitely long data series maintaining its statistical properties at different time scales, from intra-storm to inter-annual variability, and its link to large scale climate patterns. The proposed methodology relies on the use of weather types and an autoregressive logistic regression model forced with different variables to simulate daily scale chronology. Considering the dependencies of wave conditions on the different weather types, the intra-storm chronology is solved by means of shuffling and stretching historical wave sequences. To demonstrate the replicability of this emulator worldwide, we have applied the model to 3 different locations and found good agreement when compared to the historical data. Furthermore, to illustrate and explain the strengths and limitations of the emulator, we present a different application for each of the different locations. • A new stochastic climate-based wave emulator was developed as a tool for probabilistic assessments of coastal hazards. • The emulator generates infinitely long data series maintaining historical properties at different time scales. • The methodology can be reproduced worldwide and can be adapted to generate hydrodynamic conditions under climate change. [ABSTRACT FROM AUTHOR]