Array Of Wave Gauges Research Articles

Wave Reflection from Breakwater

A new method is presented for estimating the reflection of a random, multidirectional sea from a coastal structure. The technique is applicable to an array of wave gauges of arbitrary geometry deployed seaward of the reflector. An expansion for small oblique wave-incidence angles is used to derive an approximate relationship between measured array cross-spectra and a small number of parameters that describe the incident-wave properties and the reflectivity of the structure. Model tests with simulated array data demonstrate that the gross properties of incident and reflected waves can be accurately estimated for wave-incidence angles less than about 30°. The new method is applied to array data acquired offshore of a permeable, rubble-mound breakwater in Monterey Bay, California. The estimated reflection coefficients decrease approximately linearly with increasing frequency. Whereas the observed reflections depend only weakly on the incident-wave energy, the fraction of the incident-wave energy flux transmitted through the breakwater decreases with encreasing wave energy, suggesting that dissipation is enhanced with large-amplitude waves.

On the measurement of directional wave spectra

The directional resolving power of various directional instruments is examined. It is concluded that pitch/roll buoys have intrinsically poor directional resolving power. This finding accounts for the broader directional spreading functions which have been reported for pitch/roll buoy data as compared to spatial array data. Improved instrument performance can be achieved by increasing the number of measurement elements in the measurement system. This is most easily achieved with a spatial array of wave gauges. A rational method for the design of spatial arrays is presented. This technique allows the number and placement of gauges in an array to be determined so as to achieve the desired instrument performance.

Wave Directional Spectra Measurements by Small Arrays in Lake Ontario

Wave records collected during the fall period of 1987 from a wave gauge array at the National Water Research Institute's tower on Lake Ontario are used to obtain detailed directional information of the wave field. Two direct Fourier transform and three maximum likelihood methods are used for calculation of directional wave spectra. Comparison of these techniques with simulated data shows that the convolutive maximum likelihood method (CMLM) performs better for the wave gauge array used in this experiment. The CMLM is used to obtain the directional wave spectra in the cases of onshore and offshore wind, and offshore wind in the presence of an onshore swell in Lake Ontario.

Directional Irregular Waves Generated in a Long Tank

Behavior of ships and offshore structures in short-crested confused sea has become a very important problem for design and safty analysis. For experimental study of this problem, it is indispensable to generate in a model basin short-crested irregular waves which have a specified directional spectrum.However, only a few model basins have ever succeeded in generation of directional waves up to the present. All of them are square basins and the available region for experiments is restricted to the vicinity of wave generators, due to the influence of side wall reflections.Recently, aiming at the generation of directional irregular waves, a new snake type wave generator consisting of 16 independent units has been installed at the towing tank of Yokohama National University. This wave maker is based on a new concept to generate the specified directional spectrum waves. Reflection of waves on the side walls, which has been always recognized as a disturbance, is utilized positively to obtain directional irregular waves propagating in a towing long tank.Directional spectra were analyzed from wave height records measured by the wave gauge array at various positions in the towing tank. Successful directional spectra were obtained for arbitrary specified power spectra and directional distribution function.This paper presents the synthesis method of wave generating signals and the practical method of measurement and analysis for the directional spectrum wave. Moreover, the spectral and statistical characteristics of measured directional waves in our towing tank are mentioned.

MEASUREMENT OF REFLECTION COEFFICIENT OF SEAWALL IN OMORA BAY

Field investigations were performed in order to establish the reflecting characteristics of a steel sheet-piling type seawall and two vertical wave-dissipating type seawalls through directional wave measurements of wave reflection systems. A pre-designed line array of several capacitance-type wave gauges was used to measure the wave field just in front of the seawall, and the MMLM (Modified Maximum Likelifood Method) was applied for the calculation of the directional wave spectrum and the reflection coefficient. The estimated directional spectra demonstrated that the MMLM has high resolution power and can sufficiently separate the incident and the reflected wave energies. As a result , it was found that the reflection coefficient of the vertical steel sheet-piling seawall is about 0.9 and is independent of the incident wave conditions, whereas those of the vertical wave-dissipating type seawalls show considerable change in correlation to the incident wave period or the mean water level.

On the dispersion relation of random gravity waves. Part 2. An experiment

Random waves are generated by wind in the first half of a wind-wave flume. The latter half of the flume is kept free from wind to measure the waves unaffected by the wind and wind-generated current. The random waves in the latter area are measured with a linear array of wave gauges, and their phase velocities and coherences are determined by a usual technique of the cross-spectral analysis. The measured results are compared with the nonlinear theory of two-dimensional random waves, which has been presented in part 1 of this paper (Masuda, Kuo & Mitsuyasu 1979). Agreement between the theory and the experiment is satisfactory, and observed characteristics of the phase velocity and coherence of the spectral components can be attributed to the effects of the nonlinearity and angular dispersion of the random waves.

Directional Characteristics Of Winter Waves OffTaishi Coast Of Taiwan

A spatial array of wave gauges installed on an observation platform has been designed and arranged to measure the local features of directional wave off Taishi coast of Taiwan. A new method, named the Bayesian Parameter Estimation Method (BPEM), was then developed and adopted to determine the main direction and the directional spreading parameter of directional spectra. Also, the Mitsuyasu's empirical formula of directional energy spreading was justified to be representative of wave field. It was shown that the bigger the waves age, the more rapid decrease in spreading. Nevertheless, the major portion of the wave energy, which is around the peak frequency, may not be completely related to the local wind condition. The relation between the parameter S^ and the wave age, as suggested by Mitsuyasu's formula, may be improper in this area. The field data were shown that the wave steepness is more related to S^ than wave age does. In general, the values of S^ decrease as the values of the wave steepness increase.