It is shown theoretically that surface waves incident on a beach from deep water can excite edge waves. In particular, a monochromatic wave train normally incident and reflected on a beach of constant gentle slope is found to transfer energy to edge waves through a weak resonant interaction resulting from an instability of the incident wave with respect to perturbation by edge waves. The analysis is based on the shallow water approximation and ignores the earths rotation and consequently applies only to relatively low mode, high frequency waves. Coupling coefficients. frequencies and longshore wave numbers of the excited waves are given. In accord with Hasselmann's (1967) rule, only edge waves with frequencies lower than the incident wave are excited by this mechanism. Viscous effects suggest that an edge wave with mode number zero and frequency one-half that of the incident wave (a subharmonic edge wave) is preferentially excited. The minimum incident wave amplitudes for which this resonance can occur are predicted by consideration of viscous effects. Higher order terms, not studied in detail, suggest that an edge wave with frequency equal that of the incident wave (a synchronous edge wave) may also be resonantly excited when the incident wave is strongly reflected.

Experiments show that a very strong subharmonic edge wave resonance occurs on nonerodable plane laboratory beaches when single frequency incident waves are surging and strongly reflected at the beach. Smaller synchronous edge waves occur when the basin geometry or viscous effects exclude the subharmonic. Neither type edge wave is visible on the beach face when the incident wave is so large that it plunges rather than surges, incident wave energy being dissipated in the surf zone rather than reflected at the beach face as assumed in the resonance theory. This observation is quantified, and leads to a classification of beach face dynamics into reflective and dissipative systems.

Large subharmonic edge waves on plane laboratory beaches are shown to rearrange sand tracers into accumulations which resemble natural beach cusps. These edge wave induced morphologies, however, interfere with the edge wave excitation process so the edge wave amplitudes decrease as the cusps grow.

Small edge waves can form longshore periodic morphologies by providing destabilizing perturbations on a berm properly located in the swash zone. In this case, the retreating incident wave surge is channelized into breeches in the berm caused by the edge waves.

Various theories for edge wave generation on dissipative beaches with complex incident wave fields are briefly reviewed, and differences and similarities with the reflective beach case are stressed.

Surfzone circulation, driven primarily by breaking waves, transports sediments, pollutants, and swimmers. Changes in patterns of wave breaking owing to variations in wave height and direction along curving coastlines, over irregular bathymetry, and near man-made structures can cause complex surfzone circulation, including rip currents.

Measurement of the spatial and temporal scales of nearshore chlorophyll-a (chl) is critical to our ability to understand or predict dense, and potentially harmful blooms of phytoplankton along the California Coast. Patchiness in chl may result from patchiness in nutrient availability, swimming or sinking, predation, or transport and mixing by breaking waves, currents, internal waves and tides. We performed a study during the Fall 2006 Huntington Beach (HB06) experiment to investigate the biological and physical factors that affect phytoplankton patchiness. Autotrophic phytoplankton require light and nutrients to survive. However, the nearshore euphotic zone (the critical depth for photosynthesis) is often nutrient-depleted due to uptake and planktonic growth. In order to sustain populations, phytoplankton rely on delivery of new nutrients from below the euphotic zone through upwellling or vertical mixing. or 2) terrestrial sources (from rain, ground water, tidal flushing or river outflow) that feed directly into the nearshore.

In Southern California, terrestrial runoff pollution often drains directly onto the beach, degrading water quality and leading to increased beach closures each year. Beaches are a significant piston in California’s economic engine and beach closures can have significant economic impact on local communities. Runoff pollution can be localized, as from a storm drain, or can be more diffuse with many smaller, distributed sources. In either case, upon entering ocean waters, the pollution is first mixed, dispersed, and advected within the surfzone (the region of breaking waves). However, behavior of polluted waters upon entering the surfzone is not understood, and advancing this understanding is crucial to improved beach water quality.

The dispersion of passive tracers, such as pollutants, in the surfzone was be examined using both drifter observations and nearshore wave/circulation models with the goal of understanding the mechanisms of dispersion and to develop formulations for predicting dispersion.

In this project, scientists describe the processes by which a subsurface nearshore algal bloom is carried to the surface, swept closer to the shore and ultimately blocked from entering the surf zone at Huntington Beach in Orange County, California.

Accurate coastal wave predictions are needed to support nearshore process modeling (transport, erosion) and local risk assessment (flooding, inundation). Small errors in direction or frequency have relatively large impacts to transport, run-up estimates. Nearshore prediction error is often dominated by offshore uncertainties in regional models. Detailed, frequency-direction, spectra are needed offshore to accurately estimate blocking and refraction in sheltered regions, e.g.\ the Southern California Bight (SBC).

Here, nearshore prediction skill using both global wave model predictions and offshore buoy observations in the offshore boundary condition is compared for swell-band wave energy (0.04-0.09Hz). Despite inherent directional ambiguity, offshore buoy observations yield a more accurate boundary condition. Analytical and ah hoc combinations global model predictions and offshore buoy observations are tested and yield worse and marginally improved predictions respectively (as compared to buoy-driven predictions).

Coastal regions often contain sheltered nearshore wave observations. Assimilating nearshore observations in regional wave models theoretically will improve regional skill. However, nonlinear wave propagation models (e.g. SWAN, WW3) are difficult to invert. Recent developments employ variational methods, but success in complex real-world environments has yet to be observed. At swell-bands wave energy propagation across narrow shelves (e.g. U.S. West Coast) is dominated by linear processes (refraction, shoaling) and well modeled by self-adjoint ray tracing. Here, a general assimilation framework is developed to estimate physically smooth (time, direction), accurate, offshore boundary conditions from offshore and sheltered buoy observations with global model predictions included as a model prior. Case studies show error reduction at validation (non-assimilated) buoy sites suggesting that assimilation of nearshore observations improves regional skill. Initial results suggest that few (1 offshore, 1-2 nearshore) buoys are needed to sufficiently resolve offshore conditions, which has implications for cost-effective buoy array design. Additionally, buoy sites with significant misfit to assimilated observations identify regions of model error suggesting missing model physics (e.g. diffraction, reflection).

Field data of waves in the surfzone and runup on ocean beaches in extreme conditions are rare; empirical prediction formulas of runup are therefore largely based on mild and moderate conditions. Here, the effects of varied offshore conditions on surfzone and runup, including wave heights greater than 7 m, are explored with two unique data sets and numerical modeling. Field studies of infragravity (IG, 0.004-0.04 Hz) waves and wave runup were conducted at Cardiff Beach, California (Winter 2012--2013) and Agate Beach, Oregon (Fall 2013). The beaches were instrumented with a cross-shore transect of pressure sensors, current meters and a scanning lidar at the shoreline measuring wave runup.

Runup saturation in the infragravity band is not observed at the low-sloping Agate beach despite considerable IG energy losses in the surfzone. That is, runup continues to increase as offshore wave height increases. The numerical model 1D SWASH accurately predicts the observed bulk runup properties on both beaches without model tuning, even though the assumption of 1D bound long waves at the offshore boundary significantly amplifies the IG energy seaward of the surfzone. Despite agreement in the bulk wave properties, the bound wave boundary condition can propagate errors in nonlinear statistics of the various sea swell and infragravity band interactions well into the domain, depending on offshore conditions. These errors are addressed via inclusion of an offshore boundary condition derived from co-located current and pressure sensor measurements, which accounts for reflection and includes nonlinear phase-coupling. Boundary conditions that can be implemented without infragravity observations (e.g. bound waves) do not accurately simulate infragravity waves across the surfzone, and could corrupt predictions of morphologic change. Infragravity wave properties near the shoreline, however, are predicted to be largely independent of IG offshore boundary conditions, and dominated by local generation and dissipation.

Sand levels were monitored at five southern California beaches for periods of 3 to 15 years, spanning a total of 18 km alongshore. Every 3 months, GPS equipped vehicles measured sand elevations on cross-shore transects from the backbeach to 8 m depth, with 100 m alongshore resolution. Subaerial observations were collected monthly above the spring low-tide line. Wave buoys and a numerical model provided hourly wave estimates in 10 m depth at each site.

These observations show that beach profile shapes (depth versus cross-shore distance) evolve consistent with the equilibrium hypothesis: under steady wave conditions, evolution is toward a unique, wave condition dependent, equilibrium beach profile. Beaches far out of equilibrium change rapidly, and as equilibrium is approached they change ever more slowly. At the sandy regions, a simple equilibrium beach state model has skill >0.5 (Chapter 2, [Ludka et al., 2015]).

Repeated nourishments over multiple decades, costing hundreds of thousands of dollars, are a primary beach management strategy worldwide, but the wave-driven redistribution of nourishment sand is poorly understood. At four survey sites, 50,000-300,000 m^3 of imported sand was placed on the subaerial beach over alongshore spans between 300-1300 m. Wave conditions in the months after placement were similar at all sites, but the subaerial nourishment pads eroded and retreated landward at different rates. A pad built with native-sized sand washed offshore in the first few storms. In contrast, nourishments with coarser than native sand remained on the beach face for several years and protected shorelines during the significant wave attack of the 2015-16 El Niño (Chapter 3, [Ludka et al., 2016]). These relatively resilient and coarse subaerial pads stretched alongshore in a pattern consistent with seasonally shifting, wave-driven alongshore currents. Natural gains and losses in the total sand volume budget, integrated spatially over each site, are sometimes larger than the nourishment contributions (Chapter 4, in prep for Coastal Engineering).