The El Niño-Southern Oscillation is the dominant mode of interannual climate variability across the Pacific Ocean basin, with influence on the global climate. The two end members of the cycle, El Niño and La Niña, force anomalous oceanographic conditions and coastal response along the Pacific margin, exposing many heavily populated regions to increased coastal flooding and erosion hazards. However, a quantitative record of coastal impacts is spatially limited and temporally restricted to only the most recent events. Here we report on the oceanographic forcing and coastal response of the 2015–2016 El Niño, one of the strongest of the last 145 years. We show that winter wave energy equalled or exceeded measured historical maxima across the US West Coast, corresponding to anomalously large beach erosion across the region. Shorelines in many areas retreated beyond previously measured landward extremes, particularly along the sediment-starved California coast.

A novel large wave flume experiment was conducted on a fixed, barred beach with a sediment pit on the sandbar, allowing for the isolation of small‐scale bed response to large‐scale forcing. Concurrent measurements of instantaneous sheet layer sediment concentration profiles and near‐bed velocity profiles were obtained on a sandbar for the first time. Two sediment distributions were used with median grain diameters, d50, of 0.17 and 0.27 mm. Sheet flow occurred primarily under wave crests, where sheet thickness increased with increasing wave height. A proportionality constant, normalΛ, was used to relate maximum Shields parameter to maximum sheet thickness (normalized by d50), with bed shear stress computed using the quadratic drag law. An enhanced sheet layer thickness was apparent for the smaller sediment experiments ( normalΛ = 18.7), when directly compared to closed‐conduit oscillatory flow tunnel data ( normalΛ = 10.6). However, normalΛ varied significantly (5 <  normalΛ < 31) depending on the procedure used to estimate grain roughness, ks, and wave friction factor, fw. Three models for ks were compared (keeping the model for fw fixed): constant ks = 2.5 d50, and two expressions dependent on flow intensity, derived from steady and oscillatory sheet flow experiments. Values of ks/d50 varied by two orders of magnitude and exhibited an inverse relationship with normalΛ, where normalΛ ∼ 30 for ks/d50 of O(1) while normalΛ ∼ 5 for ks/d50 of O(100). Two expressions for fw were also tested (with the steady flow‐based model for ks), yielding a difference of 69% ( normalΛ ∼ 13 versus normalΛ ∼ 22).

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Rising seas coupled with ever increasing coastal populations present the potential for significant social and economic loss in the 21st century. Relatively short records of the full multidimensional space contributing to total water level coastal flooding events (astronomic tides, sea level anomalies, storm surges, wave run-up, etc.) result in historical observations of only a small fraction of the possible range of conditions that could produce severe flooding. The Time-varying Emulator for Short-and Long-Term analysis of coastal flood hazard potential is presented here as a methodology capable of producing new iterations of the sea-state parameters associated with the present-day Pacific Ocean climate to simulate many synthetic extreme compound events. The emulator utilizes weather typing of fundamental climate drivers (sea surface temperatures, sea level pressures, etc.) to reduce complexity and produces new daily synoptic weather chronologies with an auto-regressive logistic model accounting for conditional dependencies on the El Niño Southern Oscillation, the Madden-Julian Oscillation, seasonality, and the prior two days of weather progression. Joint probabilities of sea-state parameters unique to simulated weather patterns are used to create new time series of the hypothetical components contributing to synthetic total water levels (swells from multiple directions coupled with water levels due to wind setup, temperature anomalies, and tides). The Time-varying Emulator for Short-and Long-Term analysis of coastal flood hazard potential reveals the importance of considering the multivariate nature of extreme coastal flooding, while progressing the ability to incorporate large-scale climate variability into site specific studies assessing hazards within the context of predicted climate change in the 21st century.Plain Language Summary Predicting extreme coastal flooding is a present-day societal need and will only become more relevant as mean water levels increase due to sea level rise. However, the number of processes contributing to such events is too high for relatively short observational records to have measured all of the constructive combinations of waves, surge, wind, and sea level anomalies. We present a framework designed to create hypothetical combinations of relevant flood hazard potential processes by simulating the climate and weather patterns that drive coastal flooding. Including large-scale oceanic and atmospheric patterns as the drivers of coastal hazards reveals the climate a coastal community is most vulnerable to, which will be increasingly more important to understand as the climate changes during the 21st century.

Horizontal and vertical pressure gradients may be important physical mechanisms contributing to onshore sediment transport beneath steep, near‐breaking waves in the surf zone. A barred beach was constructed in a large‐scale laboratory wave flume with a fixed profile containing a mobile sediment layer on the crest of the sandbar. Horizontal and vertical pore pressure gradients were obtained by finite differences of measurements from an array of pressure transducers buried within the upper several centimeters of the bed. Colocated observations of erosion depth were made during asymmetric wave trials with wave heights between 0.10 and 0.98 m, consistently resulting in onshore sheet flow sediment transport. The pore pressure gradient vector within the bed exhibited temporal rotations during each wave cycle, directed predominantly upward under the trough and then rapidly rotating onshore and downward as the wavefront passed. The magnitude of the pore pressure gradient during each phase of rotation was correlated with local wave steepness and relative depth. Momentary bed failures as deep as 20 grain diameters were coincident with sharp increases in the onshore‐directed pore pressure gradients, but occurred at horizontal pressure gradients less than theoretical critical values for initiation of the motion for compact beds. An expression combining the effects of both horizontal and vertical pore pressure gradients with bed shear stress and soil stability is used to determine that failure of the bed is initiated at nonnegligible values of both forces.