Local Wave Conditions Research Articles

Coastline sand waves on a low-energy beach at “El Puntal” spit, Spain

Coastline sand waves have been observed at “El Puntal” spit, located on the north coast of Spain. The spit has been monitored by an Argus video system since 2003 and the formation and destruction of sand waves has been observed. Coastline data from the video images are analyzed by means of principal components analysis, obtaining a mean sand wave length of 125–150 m and a maximum amplitude of ≈ 15 m. It is also observed that sand waves reach their maximum amplitude at about 15 days. No propagation of these sand waves is noticed during the approximately two-month-long events analyzed. Sand wave formation and evolution are examined in relation with the prevailing local wave conditions during that period. Incident waves at the west end of the spit approach from the east–northeast, with a very high angle with respect to the shoreline. Field observations suggest that sand waves may result from an instability in alongshore sediment transport caused by moderate-energy waves with a high-angle incidence.

An improved geometrical optics model for bistatic GPS scattering from the ocean surface

This paper is concerned with the properties of bistatic microwave scattering from a randomly rough surface, and specifically its application to the study of global positioning system (GPS) satellite signals reflected from the ocean. We present a discussion of some recent refinements of Kirchhoff-type models based on second-order iterations of the surface-current integral equation, and the relationship between these models and their high-frequency (geometric optics) limit. In particular, we show that use of these refinements can extend the domain of applicability of the standard geometrical optics (GO) model. It is found that GO can be nearly as accurate as a Kirchhoff-based model provided that the wavenumber cutoff, at which the surface wave spectrum must be filtered in computing the required slope moments, depends on the roughness of the ocean surface (i.e., wind speed) as well as the incident angle and frequency of the radiation. We use a GO model refined in this way to analyze GPS surface reflection data collected from an aircraft equipped with two down-looking antennas for receiving both left- and right-hand circularly polarized reflections. Concurrent measurements of the local wind and wave conditions were collected from a nearby research vessel. Measured waveforms and mean Doppler widths at both polarizations are compared with predictions from our refined GO model, and discussion is given concerning the sensitivity of the reflected radiation to various geophysical parameters and the utility of GPS reflections for remote sensing applications.

Risk analysis for capsizing of small vessels

Vessel accidents during stormy weather conditions over Kuwait's coastal waters are one of the major concerns for insurance and re-insurance companies, since such accidents can result in not only material losses but also losses of lives. In order to determine insurance rates for different types of vessels and their operators, it is desirable to estimate the risk or probability of occurrence of an accident and the resulting expected damage or losses associated with a given vessel type under the local wave conditions first. Therefore, there is a national need for such a study to provide this type of basic information on the risks or probabilities of vessel accidents caused by high waves within the Kuwait's territorial waters. The main objective of this study is thus to provide such information for the local and international agencies and companies concerned with sailing operations and maintenance, navigational safety, and particularly insurance of small recreational and commercial vessels. The study was carried out with the financial support of M/s Warba Insurance Company and the Kuwait Foundation for the Advancement of Sciences, Kuwait.

Spatial Variability of Wave Data from Todos Santos Bay, Baja California, Mexico

Abstract With tourism industry becoming the most important economic activity along the northwest cost of Baja California, new and expensive coastal developments are being built, increasing the relevance of gathering local wave data, even when this span a short period of time. Moreover, the value of this wave data improves if they are used to show that deep-water wave characteristics obtained from a remote location can be extrapolated to fairly represent local wave conditions. In this work wave observations during winter 2001–2002 in four sites within Todos Santos Bay, Baja California, Mexico, are used to document wave characteristics and their spatial variability within the Bay. The possibility of extrapolating into the Bay deep-water wave data from a remote location is also investigated. Results show a marked wave height spatial variability within the Bay, where in some places wave heights can be as much as 2.5 times higher than those measured at other sites. Wave height spatial variability is induced ma...

Constructions of surfing space at Durban, South Africa

Surfers at Durban, South Africa, reveal a tendency to cluster in a number of different spaces off the bathing beaches. While this activity appears to function in a social environment that is at the same time companionable, competitive or exclusive, the manner in which the usually robust interaction with the material environment contributes to the construction of surfing space is not visibly evident to the outsider. A survey reveals that surfers construct surfing space out of images of a normative wave environment and practices and processes that are both sensory and social. Images of the perfect wave that describe the normative wave environment sought by surfers are acquired from surfing magazines and other media sources. Knowledge of wave shapes, winds and currents is provided by sensory-derived experience gained in the waves. Individual and group attitudes and behaviour in surfing spaces are socially constructed around issues of identity and exclusion. The path of surfing space construction is shown to link images of the perfect wave with sensory-derived knowledge of local wave conditions and socially constructed attitudes on the use of these spaces.

Anthropogenic climate change shown by local wave conditions in the North Sea

In the central North Sea we have observed an increase in the frequency of eastwardly propagating waves in the last 4 decades. To assess the significance of this change, wave statistics for the 20th century were reconstructed with a statistical model. With a linear multivariate technique (redundancy analysis), monthly mean air pressure fields over the North Atlantic and Western Europe were downscaled on the intramonthly frequency of directional wave propagation. When compared against this reference, the recent change appears statistically significant at the 5% level. In order to investigate the reason for this local climatic change, the reconstruction was compared with the down- scaled results of control and transient GCM scenarios (ECHAM4-OPYC3) and with the results obtained in a high-resolution time-slice experiment with increased concentrations of greenhouse gases and aerosols. Both estimates are qualitatively consistent with the changes observed in the last 4 decades. We suggest that the recent increase in eastward propagation is a local manifestation of anthropogenic global climate change.

Wind Waves and Moveable-Bed Bottom Friction

Abstract Effects of moveable-bed bottom friction for wave observations and wave modeling are investigated using a state-of-the-art bottom friction model. This model combines the hydrodynamic friction model of Madsen et al. with a moveable-bed roughness model based on Grant and Madsen. Analyzing the present model for idealized swell cases, it is shown that swell might result in wave-generated sand ripples. The large change of roughness corresponding to initial ripple formation results in a preferred wave height for swell, related to bathymetric scales as generally occur in shelf seas away from the coast. The corresponding wave-generated bottom roughness is not defined by the local wave conditions, but is related to the overall energy balance of the wave field. Sediment data thus is imperative for the interpretation of observed decay rates and friction factors for swell. For idealized depth-limited wind seas, near-bottom wave motion is expected to generate partially washed-out ripples and moderate sheet-flo...

An energetics total load sediment transport model for a plane sloping beach

Bagnold's energetics‐based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed. In both the bedload and suspended load, the transport rate vectors are found to be composed of a velocity‐induced component directed parallel to the instantaneous velocity vector and a gravity‐induced component directed down slope. The model is applied to idealized surfzone conditions, leading to estimates of the local longshore and onshore‐offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions. The model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions. The results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics.

Transport and Deposition of Suspended Sediments on Estuarine Tidal Flats: ABSTRACT

The suspended sediments in water flushing on and off an estuarine tidal flat have been examined for relative changes in concentration as a function of wind speed, local wave conditions, salinity, temperature, and tidal currents. The shallow water was sampled by plumbing the upper 20 cm of the sediment with buried plastic pipes and pumping water samples from an orthogonally spaced matrix of intake valves positioned across the tidal flat to permit observation of natural sedimentary processes. The sediment interface has been modified over parts of the matrix by introducing roughness elements in the form of small closely spaced stakes and stabilizers, such as eel grass. The data show a decrease in sediment concentration as the water floods over the tidal flat primarily as a function of dilution by more marine waters. As high slack tide is reached, sediment deposition occurs when wave heights are at a minimum. This deposition appears to continue even on the ebbing tide until water depths are extremely shallow, at which time a minor resuspension usually is noted. In contrast, if a slight surface chop develops, the falling water maintains the sediment in suspension and flushes it out on the ebbing tide. End_of_Article - Last_Page 601------------

A Subrecent Beach Ridge Complex in Southeastern Louisiana

Formation of the Moreau-Caminada chenier plain on the southeast Louisiana coast started about 3000 years ago. Autochthonous and detrital peat layers and laminae were found interlayered in the beach ridges. The very fine-grained, well-sorted beach sand contained 1.4 to 7.6 percent detrital lignite grains, eroded from pre-existing peaty marsh deposits. Some time after chenier progradation had ceased, sediments of a more recent Mississippi delta lobe surrounded the chenier plain. Later, a substantial portion of the plain was eroded and a reversal of littoral drift direction took place, possibly as the result of the growth of the Mississippi Plaquemines-Balize delta lobe. Original shore outline and drift and sediment source directions are the most important determining factors in the formation of chenier beach ridge configurations. Periods of mudflat and beach ridge formation were not related to major shifts in the positions of active Mississippi delta lobes but to fluctuations in local wave conditions and sediment supply.