The tenth Five-Year Project of China Earthquake Administration installed about 40 YRY-4 type high-resolution borehole strainmeters over the mainland China in order to enhance its capability of earthquake-forecasting attempt. The strainmeters are installed in rocks at a depth around 40m and resolve strain changes to the order of 10-11. Measurements are sampled every minute. There are four gauges horizontally placed, 45-degree apart, in an YRY-4 strainmeter, whose measurements are denoted as Si (i=1, 2, 3, 4) in our study. The similarity of the two curves of S1 +S3 and S2 +S4 of Guza recordings gives sufficient credit to the data. Among the sites, Guza is located the nearest (about 140km), from the epicentral area of the Wenchuan earthquake. Even months before the earthquake, it had already been noticed at Guza that the initially smooth curves had become badly interfered from time to time by minor steps or unsymmetrical pulses with periods of minutes~hours. The interferences were dominantly compressional and mostly on the order of 10-9. They were not corresponding to weather changes. Other sites are at least 300km away and did not observe such anomalies. A comparison of the interferences with the long-term and coseismic changes shows a good consistence in the sense of strain variety among them, which suggests a tectonic cause of the abnormal signals. High-passed data show obvious relevance of the abnormal signals to the Wenchuan quake in time. They became stronger as the great event approached, reached the highest at the main shock and diminishing while aftershocks have been dying away. We put forth an Overrun Rate Analysis (ORA) to give out a quantitative description of the interferences. The high-passed interferences can be depicted as positive or negative big values overrunning the normal level. Overrun Rate of Numbers, denoted as Ron, is defined as the total number of overrun points in one day, and Overrun Rate of Strength Ros the daily sum of amplitudes of
Keen, A. S.; Holman, R. A.
Parametric depth-induced-breaking dissipation models have shown great skill at predicting time averaged wave heights across the surf zone. First proposed by Battjes & Janssen (1978), these models balance the incoming wave energy flux with a roller dissipation term. This roller dissipation term is estimated by calculating the dissipation for one characteristic broken wave and then multiplying this quantity by the fraction of broken waves. To describe the fraction of broken waves, a typical assumption asserts that wave heights are nearly Rayleigh distributed [Thornton & Guza (1983)] allowing a sea state to be described by only a few parameters. While many experiments have validated the cross shore wave height profiles, few field experiments have been performed to analyze the probability distribution of breaking wave heights over a barred beach profile. The goal of the present research is to determine the distribution of broken and unbroken wave heights across a natural barred beach profile. Field data collected during the Surf Zone Optics experiment (a Multi-disciplinary University Research Initiative) in Duck, North Carolina, consisted of an array of in-situ pressure sensors and optical remote sensing cameras. Sea surface elevation time series from the in-situ pressure sensors are used here to resolve wave height distributions at multiple locations across the surf zone. Breaking wave height distributions are resolved based upon a combination of the pressure sensor and optically based breaker detection algorithm. Since breaking is easily able to be tracked by video imaging, breaking waves are flagged in the sea surface elevation series and binned into a broken wave height distribution. Results of this analysis are compared with model predictions based upon the Battjes & Janssen (1978), Thornton & Guza (1983) and Janssen & Battjes (2007) models to assess the validity of each wave height distribution model.
Haines, John W.; Sallenger, Asbury H.
Mean cross-shore currents observed across a barred surf zone are compared to model predictions. The model is based on a simplified momentum balance with a turbulent boundary layer at the bed. Turbulent exchange is parameterized by an eddy viscosity formulation, with the eddy viscosity Aυ independent of time and the vertical coordinate. Mean currents result from gradients due to wave breaking and shoaling, and the presence of a mean setup of the free surface. Descriptions of the wave field are provided by the wave transformation model of Thornton and Guza . The wave transformation model adequately reproduces the observed wave heights across the surf zone. The mean current model successfully reproduces the observed cross-shore flows. Both observations and predictions show predominantly offshore flow with onshore flow restricted to a relatively thin surface layer. Successful application of the mean flow model requires an eddy viscosity which varies horizontally across the surf zone. Attempts are made to parameterize this variation with some success. The data does not discriminate between alternative parameterizations proposed. The overall variability in eddy viscosity suggested by the model fitting should be resolvable by field measurements of the turbulent stresses. Consistent shortcomings of the parameterizations, and the overall modeling effort, suggest avenues for further development and data collection.
van der Westhuysen, André J.
Recent studies have shown that the spectral wind wave model SWAN (Simulating Waves Nearshore) underestimates wave heights and periods in situations of finite depth wave growth. In this study, this inaccuracy is addressed through a rescaling of the Battjes and Janssen (1978) bore-based model for depth-induced breaking, considering both sloping bed surf zone situations and finite depth wave growth conditions. It is found that the variation of the model error with the breaker index γBJ in this formulation differs significantly between the two types of conditions. For surf zones, clear optimal values are found for the breaker index. By contrast, under finite depth wave growth conditions, model errors asymptotically decrease with increasing values of the breaker index (weaker dissipation). Under both the surf zone and finite depth wave growth conditions, optimal calibration settings of γBJ were found to correlate with the dimensionless depth kpd (where kp is the spectral peak wave number and d is the water depth) and the local mean wave steepness. Subsequently, a new breaker index, based on the local shallow water nonlinearity, expressed in terms of the biphase of the self-interactions of the spectral peak, is proposed. Implemented in the bore-based breaker model of Thornton and Guza (1983), this breaker index accurately predicts the large difference in dissipation magnitudes found between surf zone conditions and finite depth growth situations. Hence, the proposed expression yields a significant improvement in model accuracy over the default Battjes and Janssen (1978) model for finite depth growth situations, while retaining good performance for sloping bed surf zones.
Ludka, B. C.; Guza, R. T.; McNinch, J. E.; O'Reilly, W.
variations of the wave field (measured immediately offshore) are large, but shoreline changes (usually <30cm) are smaller than in Southern California. Maximum vertical variations occur just seaward of the shoreline and the nearshore bathymetry is often barred. Plant (1999) show that bar crest position at Duck has equilibrium-like behavior. We will present the results of equilibrium shoreline and profile modeling at Duck. At both sites, we diagnose sources (e.g. grain size and incident waves) of the sometimes strong observed alongshore variations in sand level change patterns. Funding was provided by the US Army Corps of Engineers and the California Department of Boating and Waterways. REFERENCES Plant, N. G., R. A. Holman, M. H. Freilich, and W. A. Birkemeier (1999), A simple model for interannual sandbar behavior, J. Geophys. Res., 104(C7), 15,755-15,776. Yates, M. L., R. T. Guza, and W. C. O'Reilly (2009), Equilibrium shoreline response: Observations and modeling, J. Geophys. Res., 114, C09014.
Lopez, Mario; Iglesias, Gregorio
focused on the long waves at the Port of Ferrol and their implications for the operations at the port. References Candella, R.N., Rabinovich, A.B., Thomson, R.E., 2008. The 2004 Sumatra tsunami as recorded on the Atlantic coast of South America. Adv. Geosci. 14, 117-128. Cecioni, C., Bellotti, G., 2010. Modeling tsunamis generated by submerged landslides using depth integrated equations. Appl. Ocean Res. 32(3), 343-350. de Jong, M.P.C., Battjes, J.A., 2004. Low-frequency sea waves generated by atmospheric convection cells. Journal of Geophysical Research-Oceans 109(C1), C01011. López, M., Iglesias, G., 2013. Artificial Intelligence for estimating infragravity energy in a harbour. Ocean Eng. 57(0), 56-63. López, M., Iglesias, G., Kobayashi, N., 2012. Long period oscillations and tidal level in the Port of Ferrol. Appl. Ocean Res. 38(0), 126-134. Okihiro, M., Guza, R.T., Seymour, R.J., 1993. Excitation of Seiche Observed in a Small Harbor. J. Geophys. Res. 98(C10), 18201-18211. Sepic, J., Orlic, M., Vilibic, I., 2008. The Bakar Bay seiches and their relationship with atmospheric processes. Acta Adriat. 49(2).