Beach nourishment has complex implications for the future of sandy shores

Beach nourishment — the addition of sand to increase the width or sand volume of the beach — is a widespread coastal management technique to counteract coastal erosion. Globally, rising sea levels, storms and diminishing sand supplies threaten beaches and the recreational, ecosystem, groundwater and flood protection services they provide. Consequently, beach nourishment practices have evolved from focusing on maximizing the time sand stays on the beach to also encompassing human safety and water recreation, groundwater dynamics and ecosystem impacts. In this Perspective, we present a multidisciplinary overview of beach nourishment, discussing physical aspects of beach nourishment alongside ecological and socio-economic impacts. The future of beach nourishment practices will vary depending on local vulnerability, sand availability, financial resources, government regulations and efficiencies, and societal perceptions of environmental risk, recreational uses, ecological conservation and social justice. We recommend co-located, multidisciplinary research studies on the combined impacts of nourishments, and explorations of various designs to guide these globally diverse nourishment practices.Accepted Author ManuscriptCoastal Engineerin

Imperial Beach Nearshore Waves

Nearshore waves at Imperial Beach. To facilitate modeling of beach profile change, wave characteristics in 10m depth spaced 100m alongshore were extracted from the Scripps Institution of Oceanography’s wave Monitoring and Prediction (MOP) system for the California coastline (http://cdip.ucsd.edu/). Wave estimates along the coastline are produced using a linear spectral refraction model initialized with 2-D spectral estimates from multiple Datawell directional buoys. For swell waves (0.04–0.08 Hz) the model is initialized with deep water buoys located seaward of the Channel Islands. For sea waves (0.09-0.5 Hz) the model is initialized with buoys located inside the islands along the mainland shelf break. Each MOP point in 10m depth has a corresponding backbeach point, defining a MOP line. MOP line orientations are chosen to minimize the distance from the backbeach to the 10m contour. Hindcast time series of wave height (H_s), peak (T_p) and average (T_a) wave period, peak (D_p) and mean (D_m) wave direction, and radiation stress estimates (onshore S_{xx} and alongshore S_{xy}) are provided at each transect line relative to the MOP estimated shore normal (orientation also provided). Additionally, time series of wave energy, E, and low-order moment directional Fourier coefficients (a_1, b_1, a_2, b_2, in true compass “from” coordinates), as a function of wave frequency, are provided at the seaward end of each MOP line on the 10m depth contour. The 10m depth wave model output mirrors the information provided by directional wave buoys or a pressure-velocity meter (PUV), and can be treated in the same way as the spectral data from these instruments when defining boundary conditions for sediment transport models. Occasionally, wave model output is degraded due to buoy malfunctions and is flagged using the "waveFlagPrimary" variable. (Good model output has waveFlagPrimary = 1.) WHEN USING THE MODEL OUTPUT IT IS IMPERATIVE THAT THE WAVEFLAGPRIMARY IS CONSIDERED IN CONJUNCTION WITH THE WAVE ESTIMATES. The nearshore wave hindcasts were validated using shallow water wave buoys (20m depth). The hourly buoy-driven wave hindcasts show significant skill at most validation sites, but prediction errors for individual swell or sea events can be large. Model skill is fair at Imperial Beach owing to a combination of swell energy sensitivity to shadowing by the offshore islands and poorly resolved model bathymetry south of the U.S-Mexico border. Overall, the buoy-driven model hindcasts have relatively low bias such that averaging over space or time is useful for minimizing noise. Best practices for using the 100m spaced, 10m depth MOP wave hindcasts, as boundary conditions for beach change models, are not well established. It is not known if alongshore averaging or smoothing of the 100m-spaced MOP hindcasts (eg. on typical sea, swell or infragravity wavelength scales) is beneficial for beach change model stability. Space-time wave averaging questions must be explored by investigators based on their specific modeling needs and goals. Using the fixed shore normal S_{xy} estimates with 2D beach change models that predict changes in shoreline orientation is internally inconsistent, so additional second-order rotations of the S_{xy} values (or direct recalculation of S_{xy} using the a_2 and b_2 Fourier coefficients in compass coordinates) based on modeled shore normal changes, will be required

Cardiff and Solana Beaches Nearshore Waves

Nearshore waves at Cardiff and Solana Beaches. To facilitate modeling of beach profile change, wave characteristics in 10m depth spaced 100m alongshore were extracted from the Scripps Institution of Oceanography’s wave Monitoring and Prediction (MOP) system for the California coastline (http://cdip.ucsd.edu/). Wave estimates along the coastline are produced using a linear spectral refraction model initialized with 2-D spectral estimates from multiple Datawell directional buoys. For swell waves (0.04–0.08 Hz) the model is initialized with deep water buoys located seaward of the Channel Islands. For sea waves (0.09-0.5 Hz) the model is initialized with buoys located inside the islands along the mainland shelf break. Each MOP point in 10m depth has a corresponding backbeach point, defining a MOP line. MOP line orientations are chosen to minimize the distance from the backbeach to the 10m contour. Hindcast time series of wave height (H_s), peak (T_p) and average (T_a) wave period, peak (D_p) and mean (D_m) wave direction, and radiation stress estimates (onshore S_{xx} and alongshore S_{xy}) are provided at each transect line relative to the MOP estimated shore normal (orientation also provided). Additionally, time series of wave energy, E, and low-order moment directional Fourier coefficients (a_1, b_1, a_2, b_2, in true compass “from” coordinates), as a function of wave frequency, are provided at the seaward end of each MOP line on the 10m depth contour. The 10m depth wave model output mirrors the information provided by directional wave buoys or a pressure-velocity meter (PUV), and can be treated in the same way as the spectral data from these instruments when defining boundary conditions for sediment transport models. Occasionally, wave model output is degraded due to buoy malfunctions and is flagged using the "waveFlagPrimary" variable. (Good model output has waveFlagPrimary = 1.) WHEN USING THE MODEL OUTPUT IT IS IMPERATIVE THAT THE WAVEFLAGPRIMARY IS CONSIDERED IN CONJUNCTION WITH THE WAVE ESTIMATES. The nearshore wave hindcasts were validated using shallow water wave buoys (20m depth). The hourly buoy-driven wave hindcasts show significant skill at most validation sites, but prediction errors for individual swell or sea events can be large. Model skill is high at the sites in north San Diego County. Overall, the buoy-driven model hindcasts have relatively low bias such that averaging over space or time is useful for minimizing noise. Best practices for using the 100m spaced, 10m depth MOP wave hindcasts, as boundary conditions for beach change models, are not well established. It is not known if alongshore averaging or smoothing of the 100m-spaced MOP hindcasts (eg. on typical sea, swell or infragravity wavelength scales) is beneficial for beach change model stability. Space-time wave averaging questions must be explored by investigators based on their specific modeling needs and goals. Using the fixed shore normal S_{xy} estimates with 2D beach change models that predict changes in shoreline orientation is internally inconsistent, so additional second-order rotations of the S_{xy} values (or direct recalculation of S_{xy} using the a_2 and b_2 Fourier coefficients in compass coordinates) based on modeled shore normal changes, will be required

An early warning system for wave-driven coastal flooding at Imperial Beach, CA

AbstractWaves overtop berms and seawalls along the shoreline of Imperial Beach (IB), CA when energetic winter swell and high tide coincide. These intermittent, few-hour long events flood low-lying areas and pose a growing inundation risk as sea levels rise. To support city flood response and management, an IB flood warning system was developed. Total water level (TWL) forecasts combine predictions of tides and sea-level anomalies with wave runup estimates based on incident wave forecasts and the nonlinear wave model SWASH. In contrast to widely used empirical runup formulas that rely on significant wave height and peak period, and use only a foreshore slope for bathymetry, the SWASH model incorporates spectral incident wave forcing and uses the cross-shore depth profile. TWL forecasts using a SWASH emulator demonstrate skill several days in advance. Observations set TWL thresholds for minor and moderate flooding. The specific wave and water level conditions that lead to flooding, and key contributors to TWL uncertainty, are identified. TWL forecast skill is reduced by errors in the incident wave forecast and the one-dimensional runup model, and lack of information of variable beach morphology (e.g., protective sand berms can erode during storms). Model errors are largest for the most extreme events. Without mitigation, projected sea-level rise will substantially increase the duration and severity of street flooding. Application of the warning system approach to other locations requires incident wave hindcasts and forecasts, numerical simulation of the runup associated with local storms and beach morphology, and model calibration with flood observations

Cardiff and Solana Beaches Sand Level Survey Information

Information on sand level surveys conducted at Cardiff and Solana Beaches. Complete lists of all survey filenames, dates, depth zones surveyed, alongshore regions surveyed, south and north-most surveyed MOP line indices with good coverage, regions influenced by nourishment, and vehicles and transects driven are included in this file

Imperial Beach Sand Level Survey Information

Information on sand level surveys conducted at Imperial Beach. Complete lists of all survey filenames, dates, depth zones surveyed, alongshore regions surveyed, south and north-most surveyed MOP line indices with good coverage, regions influenced by nourishment, and vehicles and transects driven are included in this file

README

Overview README file for the entire repositor