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Crosby, SC, Kumar N, O'Reilly WC, Guza RT.  2019.  Regional swell transformation by backward ray tracing and SWAN. Journal of Atmospheric and Oceanic Technology. 36:217-229.   10.1175/jtech-d-18-0123.1   AbstractWebsite

Beach erosion and wave-induced flooding models are often initialized in O(10)-m depth, seaward of the surfzone, with wave conditions estimated from regional nonlinear spectral wave models [e.g., Simulating Waves Nearshore (SWAN)]. These models are computationally expensive for high-resolution, long-term regional O(100)-km hindcasts, and they limit examination of the effect of different climate scenarios on nearshore processes. Alternatively, computationally fast models with reduced linear wave physics enable long-term hindcasts at high spatial (<100 m) resolution. Linear models, that efficiently transform complete spectral details from deep water through complex offshore bathymetry, are appropriate for low-frequency swell wave energy propagation. Here, two numerically different linear methods are compared: backward ray-tracing and stationary linear SWAN simulations. The methods yield similar transformations from deep water (seaward of offshore islands in Southern California) to the nearshore, O(10)-m depth. However, SWAN is sensitive to model spatial resolution, especially in highly sheltered regions, where with typical (1-2 km) resolution SWAN estimates of nearshore energy vary by over a factor of 2 relative to ray tracing. Alongshore radiation stress estimates from SWAN and ray tracing also differ, and in some cases the climatological means have opposite signs. Increasing the SWAN resolution to 90 m, higher than usually applied to regional models, yields the nearshore transforms most similar to ray tracing. Both accurate rays and high-resolution SWAN require significant computation time; however, ray tracing is more efficient if transforms are needed at relatively few locations (compared with every grid point), or if computer memory is limited. Though presently less user friendly than SWAN, ray tracing is not affected by numerical diffusion or limited by model domain size or spatial resolution.

Fiedler, JW, Smit PB, Brodie KL, McNinch J, Guza RT.  2019.  The offshore boundary condition in surf zone modeling. Coastal Engineering. 143:12-20.   10.1016/j.coastaleng.2018.10.014   AbstractWebsite

Numerical models predicting surfzone waves and shoreline runup in field situations are often initialized with shoreward propagating (sea-swell, and infragravity) waves at an offshore boundary in 10-30 m water depth. We develop an offshore boundary condition, based on Fourier analysis of observations with co-located current and pressure sensors, that accounts for reflection and includes nonlinear phase-coupling. The performance of additional boundary conditions derived with limited or no infragravity observations are explored with the wave resolving, nonlinear model SWASH 1D. In some cases errors in the reduced boundary conditions (applied in 11 m depth) propagate shoreward, whereas in other cases errors are localized near the offshore boundary. 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. However, the bulk properties of infragravity waves in the inner surfzone and runup are predicted to be largely independent of ig offshore boundary conditions, and dominated by ig generation and dissipation.

Young, AP, Flick RE, Gallien TW, Giddings SN, Guza RT, Harvey M, Lenain L, Ludka BC, Melville KW, O'Reilly WC.  2018.  Southern California coastal response to the 2015–2016 El Niño. Journal of Geophysical Research: Earth Surface. 123:3069-3083.   10.1029/2018JF004771   AbstractWebsite

Widespread erosion associated with energetic waves of the strong 2015–2016 El Niño on the U.S. West Coast has been reported widely. However, Southern California was often sheltered from the northerly approach direction of the offshore waves. The few large swells that reached Southern California were not synchronous with the highest tides. Although west coast-wide tidal anomalies were relatively large in 2015–2016, in Southern California, total water levels (sum of tides, anomalies, and wave superelevation) were lower than during the 1997–1998 Niño, and comparable to the 2009–2010 Niño. Airborne lidar surveys spanning 300 km of Southern California coast show the beach response varied from considerable erosion to accretion. On average, the shoreline moved landward 10 m, similar to the 2009–2010 El Niño. Some San Diego county beaches were narrower in the 1997–1998 El Niño than in 2015–2016, consistent with the higher erosion potential in 1997–1998. Beach retreat exceeded 80 m at a few locations. However, 27% of the shoreline accreted, often in pocket beaches, or near jetties. While adjacent beaches eroded, estuary mouths accreted slightly, and several estuaries remained or became closed during the study period. Only 12% of cliffs eroded (mostly at the base), and the average cliff face retreat was markedly less than historical values. Only two cliff-top areas retreated significantly. Although some areas experienced significant change, the potential for coastal erosion and damage in Southern California was reduced compared to the 1997–1998 El Niño, because of low rainfall, a northerly swell approach, and relatively limited total high-water levels.

Ludka, BC, Guza RT, O'Reilly WC.  2018.  Nourishment evolution and impacts at four southern California beaches: A sand volume analysis. Coastal Engineering. 136:96-105.   10.1016/j.coastaleng.2018.02.003   AbstractWebsite

Four southern California beaches were nourished with offshore sand placed as subaerial pads several meters thick, m wide, and spanning between 500 and 1500 m alongshore. Three nourishments constructed with coarser than native sand, placed in 2012 at Imperial, Cardiff and Solana Beaches, elevated subaerial sand volumes for several years even when exposed to the energetic winter waves of the 2015-16 El Nifio, followed by a stormy 2016-17 winter. As these relatively resilient pads were overwashed, landward tilted subaerial profiles (accretionary crowns) formed at the eroding front face of the originally flat-topped pads and pooling occurred in the backbeach. At Imperial Beach, nourishment sand helped prevent waves from directly impacting riprap fronting houses, while groundwater flooding behind the pad was observed at a location where the pad was elevated similar to 1.6 m above the street. As the nourishments retreated, alongshore oriented spits grew downdrift from the eroding face. The alongshore displacement of the subaerial center of mass of the 2012 nourishments is positively correlated with the seasonally varying S-xy (the alongshore radiation stress component). After four years, the net southward drift of the Imperial Beach nourishment contributed to the winter 2016 closure of the Tijuana River mouth and the associated hyper -polluted and anoxic estuary conditions. Nourishment impacts on sand levels on rocky reefs were not unambiguously detectable in the background of natural variability. Over several years, gains or losses in the total sand volume (integrated from the back beach to 8 m depth, over the few km alongshore survey spans) are sometimes comparable to nourishment volumes, suggesting relatively large interannual sediment fluxes across the control volume boundaries. The clearest trend in total volume is at Torrey Pines; during 16 years since the 2001 nourishment, about 300,000 m(3) of sand has been lost. If the trend continues, the thinning veneer of sand will be removed more often from the subaerial winter beach, exposing rocks and cobbles.

Fiedler, JW, Smit PB, Brodie KL, McNinch J, Guza RT.  2018.  Numerical modeling of wave runup on steep and mildly sloping natural beaches. Coastal Engineering. 131:106-113.   10.1016/j.coastaleng.2017.09.004   AbstractWebsite

Runup on ocean beaches includes steady wave setup and oscillating swash, often decomposed into wind generated sea-swell (SS), and lower frequency infragravity (IG) waves. We show that the numerically fast, open-source numerical model 1D SWASH predicts accurately the bulk properties of runup observed on two natural beaches (one steep and one shallow sloped) for a range of incident wave conditions. The runup tongue shape was measured with a scanning lidar, and the waterline location was defined in both the observations and model with a 10 cm depth threshold. Runup is reasonably accurately predicted with energetic (e.g. 5 m significant height) incident waves, even though the assumption of 1D bound waves significantly overpredicts infragravity energy at the offshore boundary in 10 in depth. The model-data comparisons are limited by statistical chatter, often larger in runup than offshore because runup energy is concentrated in the relatively narrow infragravity IG band with low effective degrees of freedom.

Cuomo, G, Guza RT.  2017.  Infragravity Seiches in a Small Harbor. Journal of Waterway Port Coastal and Ocean Engineering. 143   10.1061/(asce)ww.1943-5460.0000392   AbstractWebsite

A method is developed to estimate harbor seiche at Marina di Carrara, Italy, from the properties of wind-generated incident waves outside the harbor. A linear model of the spatial structure of amplified seiche modes is combined with empirical estimates of the response of each mode to variable incident wave forcing. These empirical coefficients parameterize the complex nonlinear transfer of energy from wind waves to lower frequency seiche. As at other small harbors (<1 km2 surface area) on ocean coasts, and consistent with previous analyses at Carrara, the observed seiche is relatively energetic at several periods between about 1 and 15 min that are highly amplified theoretically, and the spatial structure of modeled and observed seiches agree as well. The longest seiche (≈15 min) mode is almost spatially uniform within the harbor and dominates with low-energy, short-period incident wind waves (measured 1 km offshore of the harbor). Increased wave energy and longer periods excite shorter period (1–3 min) seiche modes with more complex spatial structure, including small areas of high amplification, which have led to operational issues. The energy in each of the six most energetic seiche modes is related in this paper empirically to offshore incident wind wave height and peak period, allowing detailed predictions of harbor seiche from routine wind wave forecasts. The approach appears applicable to relatively small, shallow harbors with reflective quay walls, in which the exterior harbor mouth is exposed, and the interior sheltered from energetic wind-generated waves.

Crosby, SC, Cornuelle BD, O'Reilly WC, Guza RT.  2017.  Assimilating Global Wave Model Predictions and Deep-Water Wave Observations in Nearshore Swell Predictions. Journal of Atmospheric and Oceanic Technology. 34:1823-1836.   10.1175/jtech-d-17-0003.1   AbstractWebsite

Nearshore wave predictions with high resolution in space and time are needed for boating safety, to assess flood risk, and to support nearshore processes research. This study presents methods for improving regional nearshore predictions of swell-band wave energy (0.04-0.09 Hz) by assimilating local buoy observations into a linear wave propagation model with a priori guidance from global WAVEWATCH III (WW3) model predictions. Linear wave propagation, including depth-induced refraction and shoaling, and travel time lags, is modeled with self-adjoint backward ray tracing techniques. The Bayesian assimilation yields smooth, high-resolution offshore wave directional spectra that are consistent with WW3, and with offshore and local buoy observations. Case studies in the Southern California Bight (SCB) confirm that the nearshore predictions at independent (nonassimilated) buoy sites are improved by assimilation compared with predictions driven with WW3 or with a single offshore buoy. These assimilation techniques, valid in regions and frequency bands where wave energy propagation is mostly linear, use significantly less computational resources than nonlinear models and variational methods, and could be a useful component of a larger regional assimilation program. Where buoy locations have historically been selected to meet local needs, these methods can aid in the design of regional buoy arrays by quantifying the regional skill improvement for a given buoy observation and identifying both high-value and redundant observations. Assimilation techniques also identify likely forward model error in the Santa Barbara Channel, where permanent observations or model corrections are needed.

O'Reilly, WC, Olfe CB, Thomas J, Seymour RJ, Guza RT.  2016.  The California coastal wave monitoring and prediction system. Coastal Engineering. 116:118-132.   10.1016/j.coastaleng.2016.06.005   AbstractWebsite

A decade-long effort to estimate nearshore (20 m depth) wave conditions based on offshore buoy observations along the California coast is described. Offshore, deep water directional wave buoys are used to initialize a non stationary, linear, spectral refraction wave model. Model hindcasts of spectral parameters commonly used in nearshore process studies and engineering design are validated against nearshore buoy observations seaward of the surfzone. The buoy-driven wave model shows significant skill at most validation sites, but prediction errors for individual swell or sea events can be large. Model skill is high in north San Diego County, and low in the Santa Barbara Channel and along the southern Monterey Bay coast. Overall, the buoy-driven model hindcasts have relatively low bias and therefore are best suited for quantifying mean (e.g. monthly or annual) nearshore wave climate conditions rather than extreme or individual wave events. Model error correlation with the incident offshore wave energy, and between neighboring validation sites, may be useful in identifying sources of regional modeling errors. (C) 2016 The Authors.

Feddersen, F, Olabarrieta M, Guza RT, Winters D, Raubenheimer B, Elgar S.  2016.  Observations and modeling of a tidal inlet dye tracer plume. Journal of Geophysical Research-Oceans. 121:7819-7844.   10.1002/2016jc011922   AbstractWebsite

A 9 km long tracer plume was created by continuously releasing Rhodamine WT dye for 2.2 h during ebb tide within the southern edge of the main tidal channel at New River Inlet, NC on 7 May 2012, with highly obliquely incident waves and alongshore winds. Over 6 h from release, COAWST (coupled ROMS and SWAN, including wave, wind, and tidal forcing) modeled dye compares well with (aerial hyperspectral and in situ) observed dye concentration. Dye first was transported rapidly seaward along the main channel and partially advected across the ebb-tidal shoal until reaching the offshore edge of the shoal. Dye did not eject offshore in an ebb-tidal jet because the obliquely incident breaking waves retarded the inlet-mouth ebb-tidal flow and forced currents along the ebb shoal. The dye plume largely was confined to <4 m depth. Dye was then transported downcoast in the narrow (few 100 m wide) surfzone of the beach bordering the inlet at 0.3 ms-1 driven by wave breaking. Over 6 h, the dye plume is not significantly affected by buoyancy. Observed dye mass balances close indicating all released dye is accounted for. Modeled and observed dye behaviors are qualitatively similar. The model simulates well the evolution of the dye center of mass, lateral spreading, surface area, and maximum concentration, as well as regional (inlet and ocean) dye mass balances. This indicates that the model represents well the dynamics of the ebb-tidal dye plume. Details of the dye transport pathways across the ebb shoal are modeled poorly perhaps owing to low-resolution and smoothed model bathymetry. Wave forcing effects have a large impact on the dye transport.

Young, AP, Guza RT, O'Reilly WC, Burvingt O, Flick RE.  2016.  Observations of coastal cliff base waves, sand levels, and cliff top shaking. Earth Surface Processes and Landforms. 41:1564-1573.   10.1002/esp.3928   AbstractWebsite

Concurrent observations of waves at the base of a southern California coastal cliff and seismic cliff motion were used to explore wave-cliff interaction and test proxies for wave forcing on coastal cliffs. Time series of waves and sand levels at the cliff base were extracted from pressure sensor observations programmatically and used to compute various wave impact metrics (e.g. significant cliff base wave height). Wave-cliff interaction was controlled by tide, incident waves, and beach sand levels, and varied from low tides with no wave-cliff impacts, to high tides with continuous wave-cliff interaction. Observed cliff base wave heights differed from standard Normal and Rayleigh distributions. Cliff base wave spectra levels were elevated at sea swell and infragravity frequencies. Coastal cliff top response to wave impacts was characterized using microseismic shaking in a frequency band (20-45Hz) sensitive to wave breaking and cliff impacts. Response in the 20-45Hz band was well correlated with wave-cliff impact metrics including cliff base significant wave height and hourly maximum water depth at the cliff base (r(2) = 0.75). With site-specific calibration relating wave impacts and shaking, and acceptable anthropogenic (traffic) noise levels, cliff top seismic observations are a viable proxy for cliff base wave conditions. The methods presented here are applicable to other coastal settings and can provide coastal managers with real time coastal conditions. Copyright (C) 2016 John Wiley & Sons, Ltd.

Crosby, SC, O'Reilly WC, Guza RT.  2016.  Modeling long-period swell in Southern California: Practical boundary conditions from buoy observations and global wave model predictions. Journal of Atmospheric and Oceanic Technology. 33:1673-1690.   10.1175/jtech-d-16-0038.1   AbstractWebsite

Accurate, unbiased, high-resolution (in space and time) nearshore wave predictions are needed to drive models of beach erosion; coastal flooding; and alongshore transport of sediment, biota, and pollutants. On sheltered shorelines, wave predictions are sensitive to the directions of onshore propagating waves, and nearshore model prediction error is often dominated by directional uncertainty offshore. Here, regional wave model skill in highly sheltered Southern California is compared for different offshore boundary conditions created from offshore buoy observations and global wave model hindcasts [NOAA WaveWatch III (WW3)]. Spectral ray-tracing methods are used to transform incident offshore swell (0.04-0.09 Hz) energy at high directional resolution (1 degrees). Model skill is assessed for predictions (wave height, direction, directional spread, and alongshore radiation stress) at 16 nearshore buoy sites between 2000 and 2009. Buoy-derived boundary conditions using various estimators (maximum entropy, maximum smoothness) have similar skill and all outperform WW3-derived boundary conditions. A new method for estimating offshore boundary conditions, CMB-ADJ, combines buoy observations with WW3 predictions. Although CMB-ADJ skill is comparable to buoy-only methods, it may be more robust in varying regions and wave climatologies, and will benefit from future improvements in global wave model (GWM) predictions. A case study at Oceanside Harbor shows strong sensitivity of alongshore sediment transport estimates to the boundary condition method. However, patterns in alongshore gradients of transport (e.g., the location of model accretion and erosion zones) are similar across methods. Weak, tidally modulated coastal reflection is evident in both shallow and deep buoy observations, and significantly increases the observed directional spread.

Doria, A, Guza RT, O'Reilly WC, Yates ML.  2016.  Observations and modeling of San Diego beaches during El Nino. Continental Shelf Research. 124:153-164.   10.1016/j.csr.2016.05.008   AbstractWebsite

Subaerial sand levels were observed at five southern California beaches for 16 years, including notable El Ninos in 1997-98 and 2009-10. An existing, empirical shoreline equilibrium model, driven with wave conditions estimated using a regional buoy network, simulates well the seasonal changes in subaerial beach width (e.g. the cross-shore location of the MSL contour) during non-El Nino years, similar to previous results with a 5-year time series lacking an El Nino winter. The existing model correctly identifies the 1997-98 El Nino winter conditions as more erosive than 2009-10, but overestimates shoreline erosion during both El Ninos. The good skill of the existing equilibrium model in typical conditions does not necessarily extrapolate to extreme erosion on these beaches where a few meters thick sand layer often overlies more resistant layers. The modest over-prediction of the 2009-10 El Nino is reduced by gradually decreasing the model mobility of highly eroded shorelines (simulating cobbles, kelp wrack, shell hash, or other stabilizing layers). Over prediction during the more severe 1997-98 El Nino is corrected by stopping model erosion when resilient surfaces (identified with aerial imagery) are reached. The trained model provides a computationally simple (e.g. nonlinear first order differential equation) representation of the observed relationship between incident waves and shoreline change. (C) 2016 Elsevier Ltd. All rights reserved.

Ludka, BC, Gallien TW, Crosby SC, Guza RT.  2016.  Mid-El Nino erosion at nourished and unnourished Southern California beaches. Geophysical Research Letters. 43:4510-4516.   10.1002/2016gl068612   AbstractWebsite

Wave conditions in Southern California during the 2015-2016 El Nino were similar to the 2009-2010 El Nino, previously the most erosive (minimum beach widths and subaerial sand levels) in a 7 year record. As of February 2016, Torrey Pines Beach had eroded slightly below 2009-2010 levels, threatening the shoulder of a major highway. However, Cardiff, Solana, and Imperial Beaches, nourished with imported sand in 2012, were on average 1-2 m more elevated and more than 10 m wider than in 2009-2010. Monthly subaerial sand elevation observations showed that the nourished beaches remained consistently wider than unnourished beaches under similar wave conditions. In contrast to a 2001 nourishment at Torrey Pines built with native sized sand that was removed from the beach face during a single storm, these relatively coarse grained nourishments protected shorelines for several years, and during the significant wave attack of the 2015-2016 El Nino, as of February 2016.

Ludka, BC, Guza RT, O'Reilly WC, Yates ML.  2015.  Field evidence of beach profile evolution toward equilibrium. Journal of Geophysical Research-Oceans. 120:7574-7597.   10.1002/2015jc010893   AbstractWebsite

An equilibrium framework is used to describe the evolution of the cross-shore profile of five beaches (medium grain size sand) in southern California. Elevations were observed quarterly on cross-shore transects extending from the back beach to 8 m depth, for 3-10 years. Transects spaced 100 m in the alongshore direction are alongshore averaged into nineteen 700-900 m long sections. Consistent with previous observations, changes about the time average profile in many sections are captured by the first mode empirical orthogonal function (EOF). The first EOF poorly describes sections with hard substrate (less than roughly 80% sandy bottom) and also fails near the head of a submarine canyon and adjacent to an inlet. At the 12 well-described sections, the time-varying amplitude of the first EOF, the beach state A, describes the well-known seasonal sand exchange between the shoreline and offshore (roughly between 4 and 7 m depth). We show that the beach state change rate dA/dt depends on the disequilibrium between the present state A and wave conditions, consistent with the equilibrium concepts of Wright and Short (1984) and Wright et al. (1985). Empirically determined, optimal model coefficients using the framework of Yates et al. (2009a, 2011) vary between sections, but a single set of globally optimized values performs almost as well. The model implements equilibrium concepts using ad hoc assumptions and empirical parameter values. The similarity with observed profile change at five southern California beaches supports the underlying model equilibrium hypotheses, but for unknown reasons the model fails at Duck, NC.

Fiedler, JW, Brodie KL, McNinch JE, Guza RT.  2015.  Observations of runup and energy flux on a low-slope beach with high-energy, long-period ocean swell. Geophysical Research Letters. 42:9933-9941.   10.1002/2015gl066124   AbstractWebsite

The transformation of surface gravity waves from 11 m depth to runup was observed on the low-sloped (1/80) Agate Beach, Oregon, with a cross-shore transect of current meters, pressure sensors, and a scanning lidar. Offshore wave heights H-0 ranged from calm (0.5 m) to energetic (> 7 m). Runup, measured with pressure sensors and a scanning lidar, increases linearly with (H0L0)(1/2), with L-0 the deep-water wavelength of the spectral peak. Runup saturation, in which runup oscillations plateau despite further increases in (H0L0)(1/2), is not observed. Infragravity wave shoaling and nonlinear energy exchanges with short waves are included in an infragravity wave energy balance. This balance closes for high-infragravity frequencies (0.025-0.04 Hz) but not lower frequencies (0.003-0.025 Hz), possibly owing to unmodeled infragravity energy losses of wave breaking and/or bottom friction. Dissipative processes limit, but do not entirely damp, increases in runup excursions in response to increased incident wave forcing.

Hally-Rosendahl, K, Feddersen F, Clark DB, Guza RT.  2015.  Surfzone to inner-shelf exchange estimated from dye tracer balances. Journal of Geophysical Research-Oceans. 120:6289-6308.   10.1002/2015jc010844   AbstractWebsite

Surfzone and inner-shelf tracer dispersion are observed at an approximately alongshore-uniform beach. Fluorescent Rhodamine WT dye, released near the shoreline continuously for 6.5 h, is advected alongshore by breaking-wave- and wind-driven currents, and ejected offshore from the surfzone to the inner-shelf by transient rip currents. Novel aerial-based multispectral dye concentration images and in situ measurements of dye, waves, and currents provide tracer transport and dilution observations spanning about 350 m cross-shore and 3 km alongshore. Downstream dilution of near-shoreline dye follows power law decay with exponent -0.33, implying that a tenfold increase in alongshore distance reduces the concentration about 50%. Coupled surfzone and inner-shelf dye mass balances close, and in 5 h, roughly half of the surfzone-released dye is transported offshore to the inner-shelf. Observed cross-shore transports are parameterized well ( r2=0.85, best fit slope 0.7) using a bulk exchange velocity and mean surfzone to inner-shelf dye concentration difference. The best fit cross-shore exchange velocity u*=1.2x10-2ms-1 is similar to a temperature-derived exchange velocity on another day with similar wave conditions. The u* magnitude and observed inner-shelf dye length scales, time scales, and vertical structure indicate the dominance of transient rip currents in surfzone to inner-shelf cross-shore exchange during moderate waves at this alongshore-uniform beach.

Spydell, MS, Feddersen F, Olabarrieta M, Chen JL, Guza RT, Raubenheimer B, Elgar S.  2015.  Observed and modeled drifters at a tidal inlet. Journal of Geophysical Research-Oceans. 120:4825-4844.   10.1002/2014jc010541   AbstractWebsite

Material transport and dispersion near the mouth of a tidal inlet (New River Inlet, NC) are investigated using GPS-tracked drifters and numerical models. For ebb tide releases, velocities are largest (> 1 ms(-1)) in two approximately 30 m wide channels that bisect the 1-3 m deep ebb shoal. In the channels, drifter and subsurface current meter velocities are similar, consistent with strong vertical mixing and 2-D hydrodynamics. Drifters were preferentially entrained in the channelized jets where drifter cluster lateral spreading rates mu(in) were small (mu(in) approximate to 0.5 m(2) s (1)). At the seaward edge of the ebb shoal, jet velocities decrease linearly with distance (to <= 0.2 ms(-1), about 1 km from shore), and cluster spreading rates are larger with mu(out) approximate to 3 m(2) s(-1). Although the models COAWST and NearCom generally reproduce the observed trajectory directions, certain observed drifter properties are poorly modeled. For example, modeled mean drifter velocities are smaller than observed, and upon exiting the inlet, observed drifters turn north more than modeled drifters. The model simulations do reproduce qualitatively the spreading rates observed in the inner inlet, the flow deceleration, and the increase in mu(out) observed in the outer inlet. However, model spreading rates increase only to mu(out) < 1 m(2) s(-1). Smaller modeled than observed mu(out) may result from using unstratified models. Noncoincident (in space) observations show evidence of a buoyant plume (Delta rho = 1 kg m(-3)) in the outer inlet, likely affecting drifter lateral spreading. Generally, drifter-based model performance is good within the inlet channels where tidal currents are strongest, whereas model-data differences are significant farther offshore.

Gallien, TW, O'Reilly WC, Flick RE, Guza RT.  2015.  Geometric properties of anthropogenic flood control berms on southern California beaches. Ocean & Coastal Management. 105:35-47.   10.1016/j.ocecoaman.2014.12.014   AbstractWebsite

Coastal flood riskfrom coincident high tides' and energetic waves is concentrated around low-lying urban areas. Municipalities construct temporary sand berms (also known as sacrificial dunes) to manage potential flooding, however the relationships between berm geometry (e.g., height, width and length) and performance are not understood. Concomitant pressures of sea level rise and urbanization will increase active beach berming. Effective future coastal flood risk management will depend upon optimizing berm efficacy relative to geometry, placement, and water levels. Here, 34 individual berms at seven southern California locations are characterized using 18 LiDAR datasets spanning nearly a decade. Three berm classifications emerged based on deployment duration: event, seasonal and persistent. Event berms, deployed to manage specific storms or high water events, are triangular in cross-section, relatively low volume (similar to 4 m(3)/m) and low crest elevation (similar to 5 m NAVD88). Seasonal berms are larger, volumes vary from 6 to 28 m(3)/m, and average crest elevations are between 5.3 and 6.4 m. A persistent berm, captured in all LiDAR data for that area, is the largest (48 m(3)/m), longest (1.2 km), and highest mean crest elevation (7 m NAVD88) of all study berms. Total water levels, estimated using observed tides and a regional wave model coupled with an empirical runup formula, suggest that overtopping is rare. Currently, event berms are vulnerable to wave attack only a few hours per year. However, even with modest sea level rise (similar to 25 cm) or El Nino conditions, exposure increases significantly, and substantial nourishments may be required to maintain current flood protection levels. (C) 2014 Elsevier Ltd. All rights reserved.

Hally-Rosendahl, K, Feddersen F, Guza RT.  2014.  Cross-shore tracer exchange between the surfzone and inner-shelf. Journal of Geophysical Research-Oceans. 119:4367-4388.   10.1002/2013jc009722   AbstractWebsite

Cross-shore tracer exchange between the surfzone and inner-shelf is examined using temperature and dye measurements at an approximately alongshore-uniform beach. An alongshore-oriented plume is created by releasing dye continuously for 4.5 h in a surfzone alongshore current. The plume is sampled for 13 h from the release point to 700 m downstream, between the shoreline and 250 m offshore (6 m water depth). Within the surfzone (<= 2 m depth), water is relatively warm, and dye is vertically well mixed. On the inner-shelf (3-6 m depth), alongshore currents are weak, and elevated temperature and dye co-occur in 25-50 m wide alongshore patches. Within the patches, dye is approximately depth-uniform in the warm upper 3 m where thermal stratification is weak, but decreases rapidly below 3 m with a strong thermocline. Dye and temperature vertical gradients are correlated, and dye is not observed below 18 degrees C. The observations and a model indicate that, just seaward of the surfzone, thermal stratification inhibits vertical mixing to magnitudes similar to those in the ocean interior. Similar surfzone and inner-shelf mean alongshore dye dilution rates are consistent with inner-shelf dye properties being determined by local cross-shore advection. The alongshore-patchy and warm inner-shelf dye is ejected from the surfzone by transient rip currents. Estimated Stokes drift driven cross-shore exchange is small. The transient rip current driven depth-normalized heat flux out of the surfzone has magnitude similar to those of larger-scale shelf processes. Dye recycling, from the inner-shelf back to the surfzone, is suggested by relatively long surfzone dye residence times.

Clark, DB, Lenain L, Feddersen F, Boss E, Guza RT.  2014.  Aerial imaging of fluorescent dye in the near shore. Journal of Atmospheric and Oceanic Technology. 31:1410-1421.   10.1175/jtech-d-13-00230.1   AbstractWebsite

Aerial images are used to quantify the concentration of fluorescent Rhodamine water tracing (WT) dye in turbid and optically deep water. Tracer releases near the shoreline of an ocean beach and near a tidal inlet were observed with a two-band multispectral camera and a pushbroom hyperspectral imager, respectively. The aerial observations are compared with near-surface in situ measurements. The ratio of upwelling radiance near the Rhodamine WT excitation and emission peaks varies linearly with the in situ dye concentrations for concentrations <20ppb (r(2) = 0.70 and r(2) = 0.85-0.88 at the beach and inlet, respectively). The linear relationship allows for relative tracer concentration estimates without in situ calibration. The O(1 m) image pixels resolve complex flow structures on the inner shelf that transport and mix tracer.

Spydell, MS, Feddersen F, Guza RT, MacMahan J.  2014.  Relating Lagrangian and Eulerian horizontal eddy statistics in the surfzone. Journal of Geophysical Research-Oceans. 119:1022-1037.   10.1002/2013jc009415   AbstractWebsite

Concurrent Lagrangian and Eulerian observations of rotational, low-frequency (10(-4) to 10(-2) Hz) surfzone eddies are compared. Surface drifters were tracked for a few hours on each of 11 days at two alongshore uniform beaches. A cross-shore array of near-bottom current meters extended from near the shoreline to seaward of the surfzone (typically 100 m wide in these moderate wave conditions). Lagrangian and Eulerian mean alongshore velocities V are similar, with a midsurfzone maximum. Cross-shore dependent Lagrangian (sigma(L)) and Eulerian (sigma(E)) rotational eddy velocities, estimated from low-pass filtered drifter and current meter velocities, respectively, also generally agree. Cross-shore rotational velocities have a midsurfzone maximum whereas alongshore rotational velocities are distributed more broadly. Daily estimates of the Lagrangian time scale, the time for drifter velocities to decorrelate, vary between 40 and 300 s, with alongshore time scales greater than cross-shore time scales. The ratio of Lagrangian to apparent Eulerian current meter decorrelation times T-L/T-A varies considerably, between about 0.5 and 3. Consistent with theory, some of the T-L/T-A variation is ascribable to alongshore advection and T-L/T-A is proportional to V/sigma, which ranges between about 0.6 and 2.5. Estimates of T-L/T-A vary between days with similar V/sigma suggesting that surfzone Lagrangian particle dynamics vary between days, spanning the range from "fixed-float'' to "frozen-field'' [Lumpkin et al., 2002], although conclusions are limited by the statistical sampling errors in both T-L/T-A and V/sigma.

Young, AP, Guza RT, Dickson ME, O'Reilly WC, Flick RE.  2013.  Ground motions on rocky, cliffed, and sandy shorelines generated by ocean waves. Journal of Geophysical Research-Oceans. 118:6590-6602.   10.1002/2013jc008883   AbstractWebsite

We compare ground motions observed within about 100 m of the waterline on eight sites located on shorelines with different morphologies (rock slope, cliff, and sand beaches). At all sites, local ocean waves generated ground motions in the frequency band 0.01-40 Hz. Between about 0.01 and 0.1 Hz, foreshore loading and gravitational attraction from ocean swell and infragravity waves drive coherent, in-phase ground flexing motions mostly oriented cross-shore that decay inland. At higher frequencies between 0.5 and 40 Hz, breaking ocean waves and wave-rock impacts cause ground shaking. Overall, seismic spectral shapes were generally consistent across shoreline sites and usually within a few orders of magnitude despite the diverse range of settings. However, specific site response varied and was influenced by a combination of tide level, incident wave energy, site morphology, ground composition, and signal decay. Flexing and shaking increased with incident wave energy and was often tidally modulated, consistent with a local generation source. Flexing magnitudes were usually larger than shaking, and flexing displacements of several mm were observed during relatively large incident wave conditions (Hs 4-5 m). Comparison with traffic noise and earthquakes illustrate the relative significance of local ocean-generated signals in coastal seismic data. Seismic observations are not a simple proxy for wave-cliff interaction.

Doria, A, Guza RT.  2013.  Estimating changes in near-shore bathymetry with subaerial surveys. Journal of Atmospheric and Oceanic Technology. 30:2225-2232. AbstractWebsite

Surveys of the subaerial beach (e.g., landward of approximately the MSL depth contour) are widely used to evaluate temporal changes in sand levels over large alongshore reaches. Here, seasonal beach face volume changes based on full bathymetry beach profiles (to similar to 8 m in depth) are compared with estimates based on the subaerial section of the profile. The profiles span 15 years and 75 km of Southern California shoreline, where seasonal vertical fluctuations in near-shore sand levels of a few meters are common. In years with relatively low winter wave energy, most erosion occurs above the MSL contour, and subaerial surveys capture as much as 0.8 of the total (relatively small) seasonal beach face volume change. In response to more energetic winter waves, beach face erosion increases and occurs as deep as 3 m below MSL, and subaerial surveys capture as little as 0.2 of the total beach face volume change. Patchy, erosion-resistant rock and cobble layers contribute to alongshore variation of the subaerial fraction of beach face volume change.

Rippy, MA, Franks PJS, Feddersen F, Guza RT, Warrick JA.  2013.  Beach nourishment impacts on bacteriological water quality and phytoplankton bloom dynamics. Environmental Science & Technology. 47:6146-6154.   10.1021/es400572k   AbstractWebsite

A beach nourishment with approximately 1/3 fine-grained sediment (fines; particle diameter <63 mu m) by mass was performed at Southern California's Border Fields State Park (BFSP). The nourishment was found to briefly (<1 day) increase concentrations of surf-zone fecal indicator bacteria (FIB) above single-sample public health standards [104 most probable number (MPN).(100 mL)(-1)] but had no effect on phytoplankton. Contamination was constrained to the nourishment site: waters 300 m north or south of the nourishment were always below single-sample and geometric mean [<= 35 MPN.(100 mL)(-1)] standards. Nourishment fines were identified as a source of the fecal indicator Enterococcus; correlations between fines and enterococci were significant (p < 0.01), and generalized linear model analysis identified fines as the single best predictor of enterococci. Microcosm experiments and field sampling suggest that the short surf-zone residence times observed for enterococci (e-folding time 4 h) resulted from both rapid, postplacement FIB inactivation and mixing/transport by waves and alongshore currents. Nourishment fines were phosphate-rich/nitrogen-poor and were not correlated with surf-zone phytoplankton concentrations, which may have been nitrogen-limited.

Rippy, MA, Franks PJS, Feddersen F, Guza RT, Moore DF.  2013.  Factors controlling variability in nearshore fecal pollution: The effects of mortality. Marine Pollution Bulletin. 66:191-198.   10.1016/j.marpolbul.2012.09.003   AbstractWebsite

A suite of physical-biological models was used to explore the importance of mortality and fluid dynamics in controlling concentrations of fecal indicator bacteria (FIB) at Huntington Beach, CA. An advection-diffusion (AD) model provided a baseline to assess improvements in model skill with the inclusion of mortality. Six forms of mortality were modeled. All mortality models performed better than the AD model, especially at offshore sampling stations, where model skill increased from <0.18 to >0.50 (Escherichia colt) or <-0.14 to >0.30 (Enterococcus). Models including cross-shore variable mortality rates reproduced FIB decay accurately (p < 0.05) at more stations than models without. This finding is consistent with analyses that revealed cross-shore variability in Enterococcus species composition and solar dose response. No best model was identified for Entero coccus, as all models including cross-shore variable mortality performed similarly. The best model for E. colt included solar-dependent and cross-shore variable mortality. (C) 2012 Elsevier Ltd. All rights reserved.