Foreshore Applications of X-band Radar
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Beach morphodynamics
Rip current
Typhoon
Wave height
Abstract X‐band radar observations from the 2017 Inner Shelf Dynamics Experiment (ISDE) in central California show multiple persistent and pulsatory rip currents on a relatively straight coastline with alongshore‐varying bathymetry. Although past studies have assessed the characteristics of transient rip currents on alongshore uniform beaches, the relative balance of transient versus steady rip current behavior on nonuniform beaches in realistic wave conditions remains poorly understood. Here, a phase‐resolving Boussinesq‐type wave model ( funwaveC ) is used to assess the role of alongshore‐varying bathymetry and incident conditions in controlling mean and transient surf zone vorticity and velocity fields and their effect on surf zone exchange. The model simulates wave conditions chosen from the ISDE observations and utilizes both an alongshore‐varying bathymetry estimated from the ISDE radar observations and a uniform bathymetry. Results show that the variable bathymetry significantly increases the alongshore‐ and time‐averaged kinetic energy but that this increase is primarily due to the increase in the standing component resulting from mean circulation patterns, with only small changes in the transient component. A variable bathymetry also increases the spectral energy of surf zone vorticity and time‐averaged vorticity forcing at large spatial scales (>100 m). Wave directional spreading has a large impact on the alongshore‐ and time‐averaged enstrophy and on the spectral energy of surf zone vorticity and vorticity forcing at smaller spatial scales (<100 m). In the presence of a directionally spread wavefield, an alongshore‐varying bathymetry slightly increases the total exchange velocity but has little effect on its transient component.
Surf zone
Rip current
Forcing (mathematics)
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Of order one importance to any study of nearshore processes is knowledge of the bathymetry in shallow water. This is true for studies on open coast sandy beaches where surf zone dynamics drive the system, inlet environments where bathymetric evolution can rapidly change navigation channels, and in more benign, lower-energy coastal environments that evolve slowly over 10’s to 100’s of years. Difficulties in obtaining shallow bathymetry where depth-limited wave breaking occurs, submerged hazards are present, or other harsh environments has led to the development of survey systems on highly maneuverable personal watercraft (Beach, et al., 1994; Cote, 1999; Dugan, et al., 1999; MacMahan, 2001). In this work we discuss shallow water surveying from the Coastal Bathymetry Survey System (CBASS), a Yamaha Waverunner equipped with differential GPS, single-beam 192 KHz acoustic echo-sounder, and onboard navigation system. Data obtained with the CBASS in three regions will be discussed, including an energetic surf zone located in southern California during the 2003 Nearshore Canyon Experiment (NCEX), on Lake Erie in 2002 (and compared with historical surveys dating back 150 years), and around Piscataqua River Inlet, NH, in 2007. Estimated accuracy (for sandy bottoms) in water depths ranging 1–10 m are 0.07-0.10 m in the vertical, and on the order of 0.1-1 m horizontally depending on water depth and bottom slope. The high maneuverability of the personal watercraft makes very shallow water bathymetric surveys possible with acoustic altimeters, particularly in regions where airborne remote sensing systems fail (owing to water clarity issues) or where repeated high resolution surveys are required (e.g., where an erodible bottom is rapidly evolving).
Surf zone
Watercraft
Rip current
Echo sounding
Bathymetric chart
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Sembiring, L., Van Dongeren, A., Winter, G., Van Ormondt, M., Briere, C., Roelvink, D., 2014. Nearshore bathymetry from video and the application on rip current predictions for The Dutch Coast. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 354–359, ISSN 0749-0208.This paper demonstrates the potential use of nearshore bathymetry estimated from video data in a coastal operational model for the Dutch Coast, which provides daily forecast of waves, water levels and (rip) currents. Two video techniques to obtain beach bathymetry from video data- Beach Wizard and CBathy are validated against jet ski surveyed bathymetry. Both methods show good agreement with surveyed data, and bathymetric features of the beach are adequately produced. To assess the applicability of video-derived bathymetry for the purpose of rip current predictions, a model is built for Egmond aan Zee in The Netherlands, in which rip current predictions based on video-derived bathymetry are compared with those based on the jetski survey. Results show that predicted maximum mean offshore-directed (rip) velocities are in good agreement when using surveyed bathymetry (reference model) and video-derived bathymetry. Finally, to address the rip current forecasting skill, this local model is forced by wave boundary conditions obtained from a larger-scale wave prediction model, in addition to the bathymetry from video. Results show that useful information can still be obtained regarding the rip currents' flow where maximum offshore-directed velocities obtained from the predictive model resemble adequately rip currents pattern in the reference model. This paper will discuss each component of the model system and the validation result on hindcasting and forecasting rip currents.
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Proceedings of the Fourth Conference on Coastal Dynamics, held in Lund, Sweden, June 11-15, 2001. This collection contains 109 papers detailing the state of the art in the physical aspects of coastal and inlet environments. It provides a valuable source of information from the latest research to the newest engineering applications. Topics include: effects of coastal structures, wave properties in shallow water, nearshore sediment transport, wave-current interaction, planform modeling, breaking waves, inlet morphodynamics, long waves, suspended sediments, nearshore wave modeling, morphodynamic modeling, inlet hydrodynamics, inlet sediment transport, rip currents, ripples and bedforms, nearshore circulation, micro-scale processes, regional studies, nearshore bars, shoreline and transport modeling, long-term morphodynamics, swash hydrodynamics, beach nourishment, swash and graded sediments, cross-shore response, monitoring, intertidal processes, and rhythmic features.
Beach morphodynamics
Swash
Coastal engineering
Rip current
Surf zone
Bedform
Longshore drift
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Bathymetry is a major factor in determining nearshore and surf zone wave transformation and currents, yet is often poorly known. This can lead to inaccuracy in numerical model predictions. Here bathymetry is estimated as an uncertain parameter in a data assimilation system, using the ensemble Kalman filter (EnKF). The system is tested by assimilating several remote sensing data products, which were collected in September 2010 as part of a field experiment at the U.S. Army Corps of Engineers Field Research Facility (FRF) in Duck, NC. The results show that by assimilating remote sensing data alone, nearshore bathymetry can be estimated with good accuracy, and nearshore forecasts (e.g., the prediction of a rip current) can be improved. This suggests an application where a nearshore forecasting model could be implemented using only remote sensing data, without the explicit need for in situ data collection.
Rip current
Surf zone
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Rip currents are one of the most dangerous coastal hazards for swimmers. In order to minimize the risk, a coastal operational-process based-model system can be utilized in order to provide forecast of nearshore waves and currents that may endanger beach goers. In this paper, an operational model for rip current prediction by utilizing nearshore bathymetry obtained from video image technique is demonstrated. For the nearshore scale model, XBeach1 is used with which tidal currents, wave induced currents (including the effect of the wave groups) can be simulated simultaneously. Up-to-date bathymetry will be obtained using video images technique, cBathy 2. The system will be tested for the Egmond aan Zee beach, located in the northern part of the Dutch coastline. This paper will test the applicability of bathymetry obtained from video technique to be used as input for the numerical modelling system by comparing simulation results using surveyed bathymetry and model results using video bathymetry. Results show that the video technique is able to produce bathymetry converging towards the ground truth observations. This bathymetry validation will be followed by an example of operational forecasting type of simulation on predicting rip currents. Rip currents flow fields simulated over measured and modeled bathymetries are compared in order to assess the performance of the proposed forecast system.
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Ground truth
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Sandy beaches exhibit significant morphological adjustments to hydrodynamic forcing at different time-scales. This study describes the morphodynamics of the subaerial and subtidal sections of an enclosed beach at seasonal and interannual scales over a 5-year period (2015-2020). Typically, the beach presents a longshore bar trough (LBT) state at the end of the winter, driven by net offshore sediment transport. The decrease in wave-energy over the spring and summer induces onshore sediment transport, and the nearshore morphology evolves into more reflective states (rhythmic bar beach, RBB; transverse bar rip, TBR; and low tide terrace, LTT). Seasonal changes in offshore wave conditions can result in large subaerial-subtidal exchanges of sediment even though the annual balance is almost nil. Morphological changes are not uniform in the alongshore direction. The southern beach end is influenced by persistent rip channels that induce a small negative sediment balance at the yearly scale. In the central and northern sections rip channels are less persistent, as a result the subtidal bar usually welds to the shoreline earlier than in the south. This research highlights the presence of large alongshore morphological variations related to cross-shore sediment exchange differences associated with persistent rip channels, on a beach with almost negligible annual sediment balance.
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Swell
Rip current
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Two profile models have been compared with field data measured at the Egmond sandy beach site (The Netherlands) within the framework of the European COAST3D project. Three hydrodynamic storm events and two morphodynamic events (storm and post-storm periods) have been selected for model comparison. The two profile models UNIBEST-TC and CROSMOR are process-based models; the UNIBEST-TC model is a deterministic model, whereas the CROSMOR model is probabilistic model based on a `wave by wave' approach. The models show reasonable results for wave height, longshore current and offshore bar migration.
Beach morphodynamics
Surf zone
Rip current
Wave model
Longshore drift
Wave height
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Temporal interactions of shape of rip currents and coastal bathymetry are studied with numerical wave simulation. Boussinesq equations for coastal waves, which involve coastal currents as a solution, are combined with equations of littoral drift and deformation of coastal bathymetry. Uniform slope and the beach cusp with regular spacing are used as typical model coastal geography, and Tayuhama beach located in Niigata city, where the in situ rip current was observed was used as objective of field scale simulation. Results show that interaction between currents and bathymetry change enhances the rip currents more than the case with constant bathymetry. The shepe of rip currents and bathymetry changes obtained by the numerical study agree with theoretical study by Hino.
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Rip current
Surf zone
Wave height
Significant wave height
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