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L. H. Holthuijsen

Abstract

Directional energy distributions of wind-generated waves were observed with a relatively high directional resolution in fairly homogeneous and stationary wind fields in fetch-limited conditions using stereophotography of the sea surface. In a situation that is traditionally considered as the ideal fetch-limited wave-generation situation, the shapes of the observed distributions were found to agree well with the cos2s(θ/2) model proposed by Longuet-Higgins et al. (1963). In non-ideal situations in which the wind slanted across the upwind coastline or in which the upwind coastline was irregular, the shapes of the directional distributions were strongly influenced by the geometry of the upwind coastline. This suggests that the process of wave generation is directionally decoupled.

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L. H. Holthuijsen and T. H. C. Herbers

Abstract

Conventional observations of waves carried out with a buoy in open sea conditions were supplemented with simultaneous visual observations of whitecaps to identify breaking events in the buoy records. A statistical wave-by-wave analysis of these records indicates that such seemingly obvious parameters as wave steepness or wave asymmetry cannot be used to separate breakers from nonbreakers and the breaking occurs at wave steepness values much less than the theoretically expected steepness of a limiting wave. The observed fraction of breaking waves varied from about 0.10 to about 0.16, depending on wind speed. Two-thirds of the breaking waves were breaking in one-third of the wave groups for which a H rms-threshold definition was used.

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G. Ph van Vledder and L. H. Holthuijsen

Abstract

The directional response of ocean waves in turning wind situations has been studied with detailed wind and wave observations in open sea and with numerical simulations of the physical processes involved. The observations were acquired with pitch-and-roll buoys in the central and southern North Sea. They are selected and corrected to represent locally generated homogeneous wave fields in deep water. The response time scales thus obtained agree well with one published dataset. The disagreement with other published datasets is shown to be due to differences in analysis techniques, at least partially. The numerical simulations are carried out for homogeneous situations in which a constant wind suddenly shifts direction or rotates. These simulations show that the atmospheric input to the waves tends to rapidly turn the mean wave direction to the new wind direction. This, however, is opposed by whitecapping dissipation and nonlinear wave-wave interactions. The effect of whitecapping on the turning rate of the waves is of the same order of magnitude as that of the atmospheric input (but of opposite sign), but that of the nonlinear interactions is one order of magnitude smaller. Both observed and simulated time scales depend on the stage of development of the wave field, but the simulated time scales are considerably larger than the observed time scales.

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L. H. Holthuijsen, A. J. Kuik, and E. Mosselman

Abstract

From the premise that the net growth of wave energy induced by wind is centered around the wind direction, a relaxation model for the response of the main wave direction to changes in the wind direction for young sea states is derived. The time scale of this relaxation model is found to be equal to the time scale of the wave energy growth. A quantitative version of the model, based on universal growth rates of the waves under the local wind is found to be consistent with observations obtained in this study and with a published dataset.

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A. J. Kuik, G. Ph van Vledder, and L. H. Holthuijsen

Abstract

A simple, computationally efficient method is proposed as a standard procedure for the routine analysis of pitch-and-roll buoy wave data. The method yields four directional model-free parameters per frequency: the mean direction, the directional width, the skewness, and the kurtosis of the directional energy distribution. For most applications these parameters provide sufficient directional information. The estimation procedure and error characteristics of the parameter estimates are discussed and illustrated with computer simulated data. An optional interpretation of the combination of skewness and kurtosis as an indicator of uni-modality of the directional energy distribution is suggested and illustrated with field observations.

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J. C. Dietrich, J. J. Westerink, A. B. Kennedy, J. M. Smith, R. E. Jensen, M. Zijlema, L. H. Holthuijsen, C. Dawson, R. A. Luettich Jr., M. D. Powell, V. J. Cardone, A. T. Cox, G. W. Stone, H. Pourtaheri, M. E. Hope, S. Tanaka, L. G. Westerink, H. J. Westerink, and Z. Cobell

Abstract

Hurricane Gustav (2008) made landfall in southern Louisiana on 1 September 2008 with its eye never closer than 75 km to New Orleans, but its waves and storm surge threatened to flood the city. Easterly tropical-storm-strength winds impacted the region east of the Mississippi River for 12–15 h, allowing for early surge to develop up to 3.5 m there and enter the river and the city’s navigation canals. During landfall, winds shifted from easterly to southerly, resulting in late surge development and propagation over more than 70 km of marshes on the river’s west bank, over more than 40 km of Caernarvon marsh on the east bank, and into Lake Pontchartrain to the north. Wind waves with estimated significant heights of 15 m developed in the deep Gulf of Mexico but were reduced in size once they reached the continental shelf. The barrier islands further dissipated the waves, and locally generated seas existed behind these effective breaking zones.

The hardening and innovative deployment of gauges since Hurricane Katrina (2005) resulted in a wealth of measured data for Gustav. A total of 39 wind wave time histories, 362 water level time histories, and 82 high water marks were available to describe the event. Computational models—including a structured-mesh deepwater wave model (WAM) and a nearshore steady-state wave (STWAVE) model, as well as an unstructured-mesh “simulating waves nearshore” (SWAN) wave model and an advanced circulation (ADCIRC) model—resolve the region with unprecedented levels of detail, with an unstructured mesh spacing of 100–200 m in the wave-breaking zones and 20–50 m in the small-scale channels. Data-assimilated winds were applied using NOAA’s Hurricane Research Division Wind Analysis System (H*Wind) and Interactive Objective Kinematic Analysis (IOKA) procedures. Wave and surge computations from these models are validated comprehensively at the measurement locations ranging from the deep Gulf of Mexico and along the coast to the rivers and floodplains of southern Louisiana and are described and quantified within the context of the evolution of the storm.

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