P. A. D. Bird

The influence of air and scale on wave impact pressures

Both laboratory and field tests that are described provide new information on the characteristics of wave impacts. Laboratory drop tests conducted using seawater and freshwater demonstrate that maximum impact pressures and rise times are influenced by both the level of aeration and the violence of the impact. A relationship is derived which enables the reduction in impact pressure caused by aeration to be estimated. This relationship is shown to provide a better means of predicting impact pressures in laboratory seawater wave tests from freshwater tests than either the Froude or Cauchy laws. Measurements are presented which show that, due to the different properties of seawater and freshwater, aeration levels are higher in seawater breakers than in freshwater breakers, even at a 1:25 model scale. The ways in which this affects the temporal variation in pressure and the scale relationships are discussed in some detail. Aeration and pressure measurements are also presented for full-scale wave impacts on a breakwater exposed to Atlantic waves. Attention is drawn to the likely role of expelled air and data included which indicate that the equivalent of up to 55% of entrained air does not necessarily prevent the occurrence of high impact pressures with short rise times.

Breakpoint generated surf beat induced by bichromatic wave groups

Abstract This paper presents new experimental data on 2-D surf beat generation by a time-varying breakpoint induced by bichromatic wave groups. The experimental investigation covers a broad range of wave amplitudes, short wave frequencies, group frequencies and modulation rates. The data include measurements of incident and outgoing wave amplitudes, breakpoint position, shoreline run-up and the cross-shore structure of both the short and long wave motion. Surf beat generation is shown to be in good agreement with theory [Symonds, G., Huntley, D.A., Bowen, A.J., 1982. Two dimensional surf beat: long wave generation by a time-varying breakpoint. J. Geophys. Res. 87, 492–498]. In particular, surf beat generation is dependent on the normalised surf zone width, which is a measure of the phase relationship between the seaward and shoreward breakpoint forced long waves, and linearly dependent on the short wave amplitude. The cross-shore structure of the long wave motion is also consistent with theory; at maximum and minimum surf beat generation, the mean breakpoint coincides with the nodal and anti-nodal points, respectively, for a free long wave standing at the shoreline. A numerical solution, using measured data as input, additionally shows that the phase relationship between the incident bound long wave and the outgoing breakpoint forced wave is consistent with the time-varying breakpoint mechanism.

A new non-dimensional number for the analysis of wave reflection from rubble mound breakwaters

Abstract Full scale measurements of the reflection performance of a rock island breakwater were obtained using an array of 6 pressure transducers, both before and after a reduction of the seawards slope of the structure. This slope reduction (1:0.82 to 1:1.55) effectively reduced maximum reflection coefficients by 15%. Comparisons of reflection coefficients with various surf-similarity parameters including the Iribarren and Miche numbers failed to provide an accurate parameterization of wave reflection for both data sets. Multiple regression analysis indicated that the shortcomings of the available surf-similarity parameters can be attributed to an overemphasis of the effects of the incident wave height (Hi) and the structure slope (tanβ) relative to the wavelength (L). On the basis of the regression analysis a new non-dimensional reflection number R = d t L 0 2 tan β H i D 2 is postulated here, which revises the relative weightings of these parameters, and introduces other physically significant parameters, including the local water depth at the toe (dt), and the characteristic armour diameter (D). This reflection number more effectively parameterizes wave reflection for these data, and for the first time provides the basis for a new scheme for the prediction of reflection coefficients (Kr) based entirely on the analysis of full scale data, where; Kr = 0.151R0.11 or K r = 0.635 √ R (41.2 + √ R) .

A practical method for the estimation of directional wave spectra in reflective wave fields

Abstract A practical method is presented for the computation of directional spectra in presence of phase locked reflections (e.g., close to coastal structures). Existing modified directional analysis methods (e.g., The Modified Bayesian Method and the Modified Maximum Likelihood Method MMLM) require an input of the effective reflection line distance. In practice the position of the reflection line is often ambiguous leading to large errors in the directional spectrum. This paper presents a refinement of Isobe and Kondos MMLM [Isobe, Kondo, K., 1984. Methods for estimating directional wave spectrum in incident and reflected wave field. Proc. 19th Conf. on Coastal Eng., Am. Soc. of Civil Eng., New York, pp. 467–483] which includes the reflection line position as an additional free parameter. The method presented here is based on minimising an error function which characterises the difference between the predicted and observed surface elevation variance observed at a number of spatially separated gauges. The new method is tested rigorously with both synthetic time-series and field data. Results indicate that the method is robust in a variety of incident wave conditions, in the presence of added noise and with only four transducers, providing the array is in close proximity to the shore. The new method of directional analysis when applied to field data produced frequency dependent reflection estimates which compared closely to those obtained by using the 2-dimensional approach of Gaillard et al. (1980) [Gaillard, P., Gauthier, M., Holly, F., 1980. Method of analysis of random wave experiments with reflecting coastal structures. Proc. 17th Conf. on Coastal Eng., ASCE, New York, pp. 204–220].

An instrument for field measurement of wave impact pressures and seawater aeration

Two new transducers have been developed and combined into an instrument for the simultaneous measurement of wave impact pressures on steep fronted coastal structures and the level of aeration in the seawater striking the structure. Design criteria for the new pressure and aeration unit are discussed. A description is given of the new transducers together with a purpose-designed data acquisition system that serves seven of the units, a video camera to record breaker shape, and an accelerometer to record the structures response to the applied loading. Performance of the new instrumentation is illustrated with examples of measurements obtained from an exposed breakwater at Alderney in the Channel Islands. The complete system operates automatically requiring only routine change of batteries and data tapes.

Surf Beat Generation by a Time-Varying Breakpoint

Surf beat generally refers to low frequency long wave motions inside and outside the surf zone. However, although Tucker (1950) showed that surf beat could be correlated with the incoming incident short wave height, considerable uncertainty still remains about surf beat generation mechanisms and a number of possibilities have been suggested. These include edge waves, bound long wave release and reflection (Longuet-Higgins and Stewart, 1962) and direct generation by a time-varying breakpoint (Symonds et al., 1982; Schaffer, 1993). This paper considers this issue and presents new experimental results using bichromatic wave groups which show good agreement with the time-varying breakpoint model of Symonds et al. (1982). A numerical solution for the combined motion of the outgoing surf beat wave and the incident bound long wave also provides a good description of the long wave structure seaward of the breakpoint.

PREDICTION OF WAVE REFLECTION FROM ROCK STRUCTURES: AN INTEGRATION OF FIELD & LABORATORY DATA

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

SURF BEAT KINEMATICS INDUCED BY RANDOM WAVES

This paper presents new laboratory data on long wave forcing by random waves breaking on a plane beach. The (outgoing) surf beat shows a strong frequency dependence and is linearly dependent on the incident short wave amplitude. Seaward propagating long waves are also positively correlated with incident short wave groups. Skewness in the long wave surface motion is negative seaward of the breakpoint, but positive inside the surf zone. The data suggest that a time-varying breakpoint is the dominant long wave forcing mechanism for the experimental conditions considered.