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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]

A beach morphology monitoring program was initiated during summer 1997 along the Columbia River littoral cell (CRLC) on the coasts of northwest Oregon and southwest Washington, USA. This field program documents the seasonal through interannual morphological variability of these high-energy dissipative beaches over a variety of .spatial scales. Following the installation of a dense network of geodetic control monuments, a nested sampling scheme consisting of cross-shore topographic beach profiles, three-dimensional topographic beach surface maps, nearshore bathymetric surveys, and sediment size distribution analyses was initiated. Beach monitoring is being conducted with state-of-the-art real-time kinematic differential global positioning system survey methods that combine both high accuracy and speed of measurement. Sampling methods resolve variability in beach morphology at alongshore length scales of approximately 10 meters to approximately 100 kilometers and cross-shore length scales of approximately 1 meter to approximately 2 kilometers. During the winter of 1997/1998, coastal change in the US Pacific Northwest was greatly influenced by one of the strongest El Nino events on record. Steeper than typical southerly wave angles resulted in alongshore sediment transport gradients and shoreline reorientation on a regional scale. The La Nina of 1998/1999, dominated by cross-shore processes associated with the largest recorded wave year in the region, resulted in net beach erosion along much of the littoral cell. The monitoring program successfully documented the morphological response to these interannual forcing anomalies as well as the subsequent beach recovery associated with three consecutive moderate wave years. These morphological observations within the CRLC can be generalized to explain overall system patterns; however, distinct differences in large-scale coastal behavior (e.g., foredune ridge morphology, sandbar morphometrics, and nearshore beach slopes) are not readily explained or understood. [ABSTRACT FROM AUTHOR]

Dynamic cobble berm revetments are a promising soft engineering technique capable of protecting sandy coastlines by armouring the sand and dissipating wave energy to protect the hinterland against wave attack. They also form composite beaches as they are essentially mimicking natural composite beach structure and behaviour. This type of coastal protections and beaches have recently been investigated, and this led to a better understanding of their overall behaviour under varying water levels and wave conditions. However, the short-term dynamics of the swash zone (where all bed changes occur) has never been studied at high-resolution, and this is needed to fully understand the underlying dynamics of such structures and relate it to observed processes at larger scale. To do so, the revetment at North Cove (WA, USA) was monitored for a nine-day period in January 2019 over a spring tidal cycle and with offshore significant wave height reaching 6 m. A 2-D lidar was used to survey a cross-shore profile of the revetment, and record all surface changes and interaction with swashes at high spatial (0.1 m) and temporal (swash-by-swash) resolution. The revetment was found to rapidly reshape under these energetic conditions, but reached a relatively stable state during the rising tide. The analysis of bed-level changes and net cross-shore mass fluxes over the revetment showed that revetment changes are mainly driven by very small events, with some rare large bed-level changes of a magnitude comparable to the median cobble diameter. The distribution of event mass fluxes nearly balanced out over the duration of a tide, meaning that positive and negative fluxes tended to be symmetrical. Furthermore, measured net fluxes magnitude were 18 times smaller than the absolute fluxes, which demonstrated the dynamic stability of the revetment as substantial movement occur on a wave-by-wave timescale but these balance out over time. The analysis of swash revealed that the revetment section where the swash reaches a maximum depth between 0.15 and 0.45 m undergoes the more extreme fluxes. Swashes deeper than 0.45 m only occurred in zones inundated more than 50% of the time, and smaller extreme fluxes were measured over the revetment section where these deep swashes were recorded. Bed level change oscillations over the revetment were observed, and the cross shore limit of these was correlated with the mean wave period at the toe of the revetment. Overall, the water depth at the toe of the revetment was identified as the key parameter to describe the energy reaching the revetment. This study enables the morphodynamics of dynamic revetment, observed in previous lab and field studies, to be explained at the swash scale, and brought new information on the sediment dynamics of composite beaches and dynamic revetments. These findings allow to suggest some generic guidance for dynamic cobble berm revetment design. Finally, the results are compared to a similar study on sandy beaches. • A novel experiment was undertaken on a dynamic cobble berm revetment/composite beach in North Cove, US. • At the swash scale, gross changes were one order of magnitude larger than net changes. • The rising tide generates more net change than the falling tide. • Locations on the revetment where swashes reach a maximum depth of 0.15–0.45 m experience the most extreme sediment fluxes. • The swash distribution showed similarities with sandy beaches and gave new perspectives for the design of dynamic revetments. [ABSTRACT FROM AUTHOR]

A submerged, low-relief nearshore berm was constructed in the Pacific Ocean near the mouth of the Columbia River, USA, using 216,000 m3 of sediment dredged from the adjacent navigation channel. The material dredged from the navigation channel was placed on the northern flank of the ebb-tidal delta in water depths between 12 and 15 m and created a distinct feature that could be tracked over time. Field measurements and numerical modeling were used to evaluate the transport pathways, time scales, and physical processes responsible for dispersal of the berm and evaluate the suitability of the location for operational placement of dredged material to enhance the sediment supply to eroding beaches onshore of the placement site. Repeated multibeam bathymetric surveys characterized the initial berm morphology and dispersion of the berm between September 22, 2020, and March 10, 2021. During this time, the volume of sediment within the berm decreased by about 40%to 127,000 m3, the maximum height decreased by almost 60%, and the center of the deposit shifted onshore over 200 m. Observations of berm morphology were compared with predictions from a three-dimensional hydrodynamic and sediment transport model application to refine poorly constrained model input parameters including sediment transport coefficients, bed schematization, and grain size. The calibrated sediment transport model was used to predict the amount, timing, and direction of transport outside of the observed survey area. Model simulations predicted that tidal currents were weak in the vicinity of the berm and wave processes including enhanced bottom stresses and asymmetric bottom orbital velocities resulted in dominant onshore movement of sediment from the berm toward the coastline. Roughly 50% of the berm volume was predicted to disperse away from the initial placement site during the 169 day hindcast. Between 9 and 17% of the initial volume of the berm was predicted to accumulate along the shoreface of a shoreline reach experiencing chronic erosion directly onshore of the placement site. Scenarios exploring alternate placement locations suggested that the berm was relatively effective in enhancing the sediment supply along the eroding coastline north of the inlet. The transferable monitoring and modeling framework developed in this study can be used to inform implementation of strategic nearshore placements and regional sediment management in complex, high-energy coastal environments elsewhere. • The morphology and dispersion of a nearshore berm was quantified with repeated mulitbeam bathymetry surveys. • A three-dimensional hydrodynamic and sediment transport model was calibrated based on observations of berm morphology. • Calibrated model hindcast simulations accurately predicted observed changes in berm morphology. • Model predictions used to predict fate of berm sediment and quantify amount of sediment supplied to to eroding coastline. [ABSTRACT FROM AUTHOR]