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  • 1
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    Comparison of Rip Current Hazard Likelihood Forecasts with Observed Rip Current Speeds (2017)
    American Meteorological Society
    Details
    Publikationsdatum: 2017-08-01
    Beschreibung: Although rip currents are a major hazard for beachgoers, the relationship between the danger to swimmers and the physical properties of rip current circulation is not well understood. Here, the relationship between statistical model estimates of hazardous rip current likelihood and in situ velocity observations is assessed. The statistical model is part of a forecasting system that is being made operational by the National Weather Service to predict rip current hazard likelihood as a function of wave conditions and water level. The temporal variability of rip current speeds (offshore-directed currents) observed on an energetic sandy beach is correlated with the hindcasted hazard likelihood for a wide range of conditions. High likelihoods and rip current speeds occurred for low water levels, nearly shore-normal wave angles, and moderate or larger wave heights. The relationship between modeled hazard likelihood and the frequency with which rip current speeds exceeded a threshold was assessed for a range of threshold speeds. The frequency of occurrence of high (threshold exceeding) rip current speeds is consistent with the modeled probability of hazard, with a maximum Brier skill score of 0.65 for a threshold speed of 0.23 m s−1, and skill scores greater than 0.60 for threshold speeds between 0.15 and 0.30 m s−1. The results suggest that rip current speed may be an effective proxy for hazard level and that speeds greater than ~0.2 m s−1 may be hazardous to swimmers.
    Print ISSN: 0882-8156
    Digitale ISSN: 1520-0434
    Thema: Geographie , Physik
    Publiziert von American Meteorological Society
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  • 2
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    Rip currents and alongshore flows in single channels dredged in the surf zone (2017)
    American Geophysical Union
    Details
    Publikationsdatum: 2017-05-01
    Print ISSN: 2169-9275
    Digitale ISSN: 2169-9291
    Thema: Geologie und Paläontologie , Physik
    Publiziert von American Geophysical Union
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  • 3
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    Extremely Low Frequency (0.1 to 1.0 mHz) Surf Zone Currents (2019)
    American Geophysical Union
    Details
    Publikationsdatum: 2019-02-04
    Beschreibung: Low-frequency surf zone eddies disperse material between the shoreline and the continental shelf, and velocity fluctuations with frequencies as low as a few mHz have been observed previously on several beaches. Here spectral estimates of surf zone currents are extended to an order of magnitude lower frequency, resolving an extremely low frequency peak of approximately 0.5 mHz that is observed for a range of beaches and wave conditions. The magnitude of the 0.5-mHz peak increases with increasing wave energy and with spatial inhomogeneity of bathymetry or currents. The 0.5-mHz peak may indicate the frequency for which nonlinear energy transfers from higher-frequency, smaller-scale motions are balanced by dissipative processes and thus may be the low-frequency limit of the hypothesized 2-D cascade of energy from breaking waves to lower frequency motions. ©2019. American Geophysical Union. All Rights Reserved.
    Print ISSN: 0094-8276
    Digitale ISSN: 1944-8007
    Thema: Geologie und Paläontologie , Physik
    Publiziert von American Geophysical Union
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  • 4
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    Comparison of rip current hazard likelihood forecasts with observed rip current speeds (2017)
    American Meteorological Society
    Details
    Publikationsdatum: 2022-05-25
    Beschreibung: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Weather and Forecasting 32 (2017): 1659-1666, doi:10.1175/WAF-D-17-0076.1.
    Beschreibung: Although rip currents are a major hazard for beachgoers, the relationship between the danger to swimmers and the physical properties of rip current circulation is not well understood. Here, the relationship between statistical model estimates of hazardous rip current likelihood and in situ velocity observations is assessed. The statistical model is part of a forecasting system that is being made operational by the National Weather Service to predict rip current hazard likelihood as a function of wave conditions and water level. The temporal variability of rip current speeds (offshore-directed currents) observed on an energetic sandy beach is correlated with the hindcasted hazard likelihood for a wide range of conditions. High likelihoods and rip current speeds occurred for low water levels, nearly shore-normal wave angles, and moderate or larger wave heights. The relationship between modeled hazard likelihood and the frequency with which rip current speeds exceeded a threshold was assessed for a range of threshold speeds. The frequency of occurrence of high (threshold exceeding) rip current speeds is consistent with the modeled probability of hazard, with a maximum Brier skill score of 0.65 for a threshold speed of 0.23 m s−1, and skill scores greater than 0.60 for threshold speeds between 0.15 and 0.30 m s−1. The results suggest that rip current speed may be an effective proxy for hazard level and that speeds greater than ~0.2 m s−1 may be hazardous to swimmers.
    Beschreibung: Funding was provided by the National Science Foundation (1232910, 1332705, and 1536365), and by National Security Science and Engineering and Vannevar Bush Faculty Fellowships funded by the assistant secretary of Defense for Research and Engineering.
    Beschreibung: 2018-02-28
    Schlagwort(e): Coastlines ; Coastal flows ; Waves, oceanic ; Forecast verification/skill ; Probability forecasts/models/distribution ; Statistical forecasting
    Repository-Name: Woods Hole Open Access Server
    Materialart: Article
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  • 5
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    On the dynamics of the Zanzibar Channel (2015)
    John Wiley & Sons
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    Publikationsdatum: 2022-05-25
    Beschreibung: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 6091–6113, doi:10.1002/2015JC010879.
    Beschreibung: The Zanzibar Channel lies between the mainland of Tanzania and Zanzibar Island in the tropical western Indian Ocean, is about 100 km long, 40 km wide, and 40 m deep, and is essential to local socioeconomic activities. This paper presents a model of the seasonal and tidal dynamics of the Zanzibar Channel based on the Regional Ocean Modeling System (ROMS) and a comparison of the model and observations. The seasonal dynamics of the channel is forced by remote processes and the local wind. Remote forcing creates the East African Coastal Current, a portion of which flows through the channel northward with a seasonally varying magnitude. The local wind enhances this seasonality in the surface Ekman layer, resulting in a stronger northward flow during the southwest monsoon season and a weak northward or occasionally southward flow during the northeast monsoon season. The tidal flows converge and diverge in the center of the channel and reduce the transport in the channel. The remotely forced, wind-forced, and tidal dynamics contain 5%, 3%, and 92% of the total kinetic energy, respectively. Despite their low kinetic energy, the remotely forced and wind-forced flows are most relevant in advecting channel water to the open ocean, which occurs in 19 days at the peak of the southwest monsoon season. The channel is well mixed, except during brief periods in the two rainy seasons, and temporarily cools between December and February. The dispersion of passive tracers is presented as an example of potential model applications.
    Beschreibung: National Science Foundation Grant Numbers: OISE-0827059 , OCE-0550658 , OCE-0851493 , OCE-0927472
    Beschreibung: 2016-03-12
    Schlagwort(e): Zanzibar Channel ; Ocean Modeling ; East Africa Coastal Current ; Monsoon ; Channel dynamics
    Repository-Name: Woods Hole Open Access Server
    Materialart: Article
    Format: application/pdf
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  • 6
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    Rip currents and alongshore flows in single channels dredged in the surf zone (2017)
    John Wiley & Sons
    Details
    Publikationsdatum: 2022-05-25
    Beschreibung: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 3799–3816, doi:10.1002/2016JC012222.
    Beschreibung: To investigate the dynamics of flows near nonuniform bathymetry, single channels (on average 30 m wide and 1.5 m deep) were dredged across the surf zone at five different times, and the subsequent evolution of currents and morphology was observed for a range of wave and tidal conditions. In addition, circulation was simulated with the numerical modeling system COAWST, initialized with the observed incident waves and channel bathymetry, and with an extended set of wave conditions and channel geometries. The simulated flows are consistent with alongshore flows and rip-current circulation patterns observed in the surf zone. Near the offshore-directed flows that develop in the channel, the dominant terms in modeled momentum balances are wave-breaking accelerations, pressure gradients, advection, and the vortex force. The balances vary spatially, and are sensitive to wave conditions and the channel geometry. The observed and modeled maximum offshore-directed flow speeds are correlated with a parameter based on the alongshore gradient in breaking-wave-driven-setup across the nonuniform bathymetry (a function of wave height and angle, water depths in the channel and on the sandbar, and a breaking threshold) and the breaking-wave-driven alongshore flow speed. The offshore-directed flow speed increases with dissipation on the bar and reaches a maximum (when the surf zone is saturated) set by the vertical scale of the bathymetric variability.
    Beschreibung: National Security Science and Engineering Faculty Fellowship; Vannevar Bush Fellowship; Office of the Assistant Secretary of Defense for Research and Engineering; NDSEG; ONR; NSF
    Beschreibung: 2017-11-08
    Schlagwort(e): Rip currents ; Alongshore flows ; Surf zone ; Nearshore
    Repository-Name: Woods Hole Open Access Server
    Materialart: Article
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  • 7
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    Extremely low frequency (0.1 to 1.0 mHz) surf zone currents. (2019)
    American Geophysical Union
    Details
    Publikationsdatum: 2022-10-20
    Beschreibung: Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 46(3), (2019):1531-1536, doi:10.1029/2018GL081106.
    Beschreibung: Low‐frequency surf zone eddies disperse material between the shoreline and the continental shelf, and velocity fluctuations with frequencies as low as a few mHz have been observed previously on several beaches. Here spectral estimates of surf zone currents are extended to an order of magnitude lower frequency, resolving an extremely low frequency peak of approximately 0.5 mHz that is observed for a range of beaches and wave conditions. The magnitude of the 0.5‐mHz peak increases with increasing wave energy and with spatial inhomogeneity of bathymetry or currents. The 0.5‐mHz peak may indicate the frequency for which nonlinear energy transfers from higher‐frequency, smaller‐scale motions are balanced by dissipative processes and thus may be the low‐frequency limit of the hypothesized 2‐D cascade of energy from breaking waves to lower frequency motions.
    Beschreibung: We thank R. Guza, T. Herbers, and T. Lippmann for their leadership roles during the SandyDuck and NCEX projects and the CCS (SIO), PVLAB (WHOI), and FRF (USACE) field teams for deploying, maintaining, and recovering sensors in harsh conditions over many years. Funding was provided by ASD(R&E), NSF, and ONR. The data can be obtained via https://chlthredds.erdc.dren.mil/thredds/catalog/frf/catalog.html and https://pv‐lab.org.
    Beschreibung: 2019-07-02
    Schlagwort(e): Surf zone currents ; Nearshore processes ; Breaking waves ; Vorticity
    Repository-Name: Woods Hole Open Access Server
    Materialart: Article
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  • 8
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    Hydrodynamic and morphodynamic responses to surfzone seafloor perturbations (2016)
    Massachusetts Institute of Technology and Woods Hole Oceanographic Institution
    Details
    Publikationsdatum: 2022-05-26
    Beschreibung: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2016
    Beschreibung: Holes and channels were excavated in the surf zone on an ocean beach near Duck, NC, and observations of the subsequent evolution of waves, currents, and the modified seafloor were used to investigate nearshore dynamics. In one set of seafloor perturbation experiments, deep holes with steeply sloping sides were excavated in the inner surfzone seafloor. Observations of the infilling holes were used to make the first field estimates of the surfzone morphological diffusivity, which describes the rate of seafloor smoothing by downslope sediment transport. To improve the temporal resolution of bathymetric estimates, a mapping method was developed to combine infrequent, spatially dense watercraft surveys with continuous, spatially sparse in situ altimeter estimates of the seafloor location. In another set of seafloor perturbation experiments, channels were dredged across the surf zone with the propellers of a landing craft. Alongshore variations in wave breaking caused by the perturbed bathymetry resulted in strong rip currents in the channels under some conditions, whereas alongshore currents bypassed the channels under other conditions. The dynamics of the circulation response for changing wave forcing, bathymetry, and tidal elevation are investigated using the observations, a numerical model, and a parameter based on wave properties and bathymetry.
    Beschreibung: This work was supported by the Assistant Secretary of Defense for Research and Engineering, the National Science Foundation, the Office of Naval Research, the WHOI Coastal Ocean Institute, the National Science Foundation Graduate Research Fellowship Program, and a National Defense Science and Engineering Graduate Fellowship.
    Schlagwort(e): Hydrodynamics
    Repository-Name: Woods Hole Open Access Server
    Materialart: Thesis
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