Offshore Wave Characteristics Research Articles

Offshore Wave Characteristics Observed by GPS Buoys on the Tohoku to Shikoku Coast, Japan

This study has estimated the monthly-mean significant wave height and period, most frequent wave direction, wave power, and wave and wind calmness ratio at the GPS buoys and their nearby coastal wave gauges on the Tohoku to Shikoku Coast, Japan. There is a similar seasonal variation in the monthly wave statistics among the seven GPS buoys on Tohoku Coast. The spatial variation among these GPS buoys is smaller than the yearly variation. The wave height at the coastal wave gauge is smaller than that at the GPS buoy owing to the surrounding bathymetry. The wave power is generally larger and the calmness ratios are lower at the GPS buoy sites than at the coastal wave gauge sites. The GPS buoys observed high waves near the typhoon center and the wave generation and propagation owing to a low-pressure system.

Extreme wave event along the Guyana coastline in October 2005

The objective of the research is to analyze in detail the causes and consequences of the unusual event at the coast of Guyana (South America) during October 16–19, 2005. Several sea defense structures were damaged and flooding of low-lying areas occurred. A data analysis of offshore wave and water level characteristics shows an abrupt change in wave direction from east to almost north on October 16, 2005 and a sudden increase in the offshore peak period up to extreme values. The offshore significant wave height was also relatively high, and these wave characteristics coincided with springtide conditions. The long-wave periods and the sharp transition in wave direction indicate that this event is associated with swell waves generated by a depression far away. An analysis of hurricanes and depressions reveals that a severe depression in the Northern Atlantic Ocean during October 11–15 was the origin of this swell event. Numerical computations with SWAN have been carried out to investigate the propagation of the offshore wave characteristics towards the shoreline. The SWAN model includes wave damping due to the presence of soft mud deposits. A calibration of the parameters has been carried out using joint offshore and onshore wave data from November 2006. The numerical simulations of the event in October 2005 clearly demonstrate that the mud banks damp the wave heights, but have almost no effect on the peak period. The resulting waves at the steep sea walls can be classified as surging waves causing severe runup and overtopping. The obtained insights are translated into practical recommendations for the Guyana Sea and River Defence Division in Guyana to build a sustainable management and maintenance of the sea defenses in the future.

Seasonal accretion and erosion patterns of a microtidal sandy beach

Monthly elevation surveys of a 1.5 km stretch of coast at Noordwijk, the Netherlands, were made (1) to quantify seasonal variability in the cross-shore position of high-, mean- and low-tide contours, beach width and beach volume, and (2) to examine the dependence of the variability on the offshore wave characteristics. Additionally, daily cross-shore positions of contours obtained from video images were used to examine daily beach response to wave characteristics between successive surveys. The more intense and frequent storm events in winter than in summer resulted in a seasonal change from a wide beach with a small volume at the end of winter to a narrow, large-volume beach in summer. The seasonal pattern in beach width resulted from the cross-shore sediment exchange between the supratidal and lower-intertidal part of the beach. In contrast to the coupling between beach behaviour and wave characteristics on a seasonal scale, beach variability between successive surveys was unrelated to day- to week-averages of the preceding offshore wave conditions partly due to the precise timing of a storm relative to the two survey moments. Moreover, daily variability determined from the video images showed that differing antecedent morphology to storms and alongshore variations in beach morphology contributed to spatially and temporally variable beach response.

Recurrent neural network modeling of nearshore sandbar behavior

The temporal evolution of nearshore sandbars (alongshore ridges of sand fringing coasts in water depths less than 10 m and of paramount importance for coastal safety) is commonly predicted using process-based models. These models are autoregressive and require offshore wave characteristics as input, properties that find their neural network equivalent in the NARX (Nonlinear AutoRegressive model with eXogenous input) architecture. Earlier literature results suggest that the evolution of sandbars depends nonlinearly on the wave forcing and that the sandbar position at a specific moment contains 'memory', that is, time-series of sandbar positions show dependencies spanning several days. Using observations of an outer sandbar collected daily for over seven years at the double-barred Surfers Paradise, Gold Coast, Australia several data-driven models are compared. Nonlinear and linear models as well as recurrent and nonrecurrent parameter estimation methods are applied to investigate the claims about nonlinear and long-term dependencies. We find a small performance increase for long-term predictions (>40 days) with nonlinear models, indicating that nonlinear effects expose themselves for larger prediction horizons, and no significant difference between nonrecurrent and recurrent methods meaning that the effects of dependencies spanning several days are of no importance.

Nearshore waves and currents over crescentic bars

The influence of crescentic sand bars on time and depth-averaged wave-induced currents is investigated. Crescent patterns are one of the most common features observed in the nearshore zone, but their morphological behavior and their influence on wave-induced currents and the intertidal zone morphology are still not well-understood. The two-dimensional coastal area model MORPHODYN is coupled with the random-wave model SWAN to compute the hydrodynamics over the French aquitan coast nearshore bars. Simulations of waves and currents are done for different tide and swell conditions on an idealized crescentic system bathymetry to investigate the influence of wave refraction over the crescentic bar on the wave focalisation in the intertidal domain and the flow patterns induced. The sensitivity of our model to the tide level and the offshore wave characteristics is revealed, as the influence of the crescentic bars on the formation and the shape of the intertidal bar system.

The dependency of inshore long waves on the characteristics of offshore short waves

This paper pursues the feasibility of establishing workable, site specific relationships between concurrent inshore long wave characteristics and offshore short wave characteristics. Such relationships would have immediate relevance to the design of wave climate date collection systems for proposed ship mooring facilities. A strong dependency was shown to exist between inshore long wave height and the height and frequency of the offshore short waves, but no similar dependency was found for long wave frequency. The possibility of a dependency between inshore long-wave frequency and the group frequency of the offshore short waves, was also pursued. This proved inconclusive because wave group frequencies in a quasi stationary sea state were shown to be an extremely variable quantity such that the wave records to which the authors had access were too short to provide stable statistical descriptions of the characteristic group frequencies. This leads to recommendations regarding the required minimum lengths of wave records when seeking data on wave group frequencies.

COMPARISON OF MODEL AND OBSERVED NEARSHORE CIRCULATION

Results from a two-dimensional numerical model for nearshore circulation induced by waves and wind are compared with observations made during two storms at a beach on Lake Michigan. Model-input data include bathymetry, offshore wave characteristics, wind histories, and local water-level changes. The predicted locations of the breaker zone are in rough accord with those observed during the storms. Data for comparison with model results consist of wave and current observations across the surf zone, especially those acquired by using a towed, instrumented sled. The comparisons show that the model often predicts peak currents near the breaker zone quite well, but underestimates the decay of wave height and the strength of longshore currents across the surf zone. Wave breaking on the bar-trough beach structure prevalent in this study apparently is not well represented by the model. An improved breaking criterion, treatment of breaking waves as traveling bores, and inclusion of horizontal mixing of momentum might add to better simulation of surf-zone currents.