Shiaw-Yih Tzang

Simulating typhoon waves by SWAN wave model in coastal waters of Taiwan

The SWAN wave model is typically designed for wave simulations in the near-shore region and thus is selected for evaluating its applicability on typhoon waves in the coastal waters around Taiwan Island. Numerical calculations on processes of wave heights and periods during the passages of four representative typhoons are compared with measured data from field wave stations on both east and west coasts. The results have shown that waves due to typhoons of paths 2, 3 and 4 can be reasonably simulated on east coastal waters. However, discrepancies increase for the simulated results on west coastal waters because the islands central mountains partly damage the cyclonic structures of the passing-over typhoons. It is also found that the included nested grid scheme in SWAN could improve the accuracy of simulations in coastal waters to facilitate further engineering practices.

Unfluidized soil responses of a silty seabed to monochromatic waves

Abstract A flume experimental study on unfluidized responses of a silty bed ( d 50 =0.05 mm) to monochromatic water waves had shown that pore pressure variations were generally poro-elastic in the bulk body and displayed two other characteristic features not found in previous laboratory sand tests. They were an immediately fluidized thin surface layer induced by wave stresses inside the seabeds boundary layer and a porous skeleton with internally suspended sediments due to channeled flow motions. The analyses verified that on soils beneath the measurement points, both features resulted in relatively small-step pore pressure build-ups, while the former played a primary role. Besides, laboratory observations confirmed that there were some near-bed sediment suspensions during wave actions resulting in a flat bed form over a silty bed compared to small-scaled ripples over a sandy bed with no clearly identified suspended sediments. These characteristic silt responses suggest that sediment transport is critically associated with the internal soil responses and some field-observed sediment suspensions near above sandy beaches can further be approached in the laboratory by utilizing fine-grained soils.

WAVE FIELD BEHIND THE PERMEABLE DETACHED BREAKWATER

The Labrador Sea Extreme Waves Experiment (LEWEX), is an international basic research programme concerned with full-scale measurements, analysis, modelling and simulation in test basins of 3-dimensional seas. The research is carried out in order to assess the significance of 3-dimensional sea states in engineering applications. The first phase of the programme full scale wave measurements in the North Atlantic Ocean was performed at a site and time that had a high probability of encountering severe sea states. The present publication shows examples of measured bi-modal directional sea spectra obtained with the WAVESCAN buoy and directional sea spectra measured with an airborne Synthetic Aperture Radar (SAR). Directional spectra of gravity waves are obtained with the SAR both in open waters and below an ice cover. Further work is needed in order to verify SAR-measurements with in-situ observations. In-situ measured directional spectra are also compared with hindcast spectra from the 3G-WAM model. Hindcast significant wave heights were found to be lower than the in-situ measurements.The littoral drift model developed at DHI and ISVA, see Deigaard et al. (1986b) has been extended to include the effects of the irregularity of the waves, of a coastal current and a wind acting on the surf zone. Further, a mathematical model to simulate the near-shore current pattern along a barred coast with rip channels has been developed. The influence on the littoral drift of the irregularity of waves, wind, coastal current, and rip channels is discussed. It is concluded that irregularity of waves and presence of rip channels must be considered while coastal current and wind action are of minor importance.At present, the Prodhoe Bay oil field in Alaska contributes a substantial amount of the domestic oil production of the United States. Oil is also expected to be present on the continental shelf of Alaska, and it is estimated that approximatedly 28 percent of the total U. S. reserve is located beneath the shallow ice covered seas of the Alaskan continental shelf. To expolre and to exploit these oil rich resources, engineers are confronted with hostile oceanographic conditions such as high tides, waves, strong currents and sea ice. The same area is also rich in fishery resources. Being one of the most productive fishing grounds in the nothern Pacific, the potential ecological impact due to an oil spill is of a major concern. This paper describes the methologies used for the development of a modeling system for the oil risk analysis. The system is designed with generality in mind so it can be used for other coastal areas. The development of three dimensional models used in the modeling system described here have been published in the earlier International Coastal Engineering Conferences (Liu and Leendertse, 1982, 1984, 1986) and a report published recently by RAND (Liu and Leendertse, 1987). In the oil-spill risk analysis, these three dimensional hydrodynamic models are coupled to a two-dimensional stochastic weather model and an oil weathering model.The two projects (LUBIATOWO 79 and LUBIATOWO 86) were aimed at study pore pressure behaviour in natural sand bed in the coastal zone of the Baltic sea under the action of storm waves. During both projects, the wave induced pore pressures at the various levels in the sea bed were measured. The collected data were used to verify the applicability of the various theoretical approaches. In the conclusion, the range of the application of the particular analytical method is given.The rapid recession of the shingle bank of Hurst Beach (up to 3.5m/yr) makes it an excellent natural laboratory for the study of the factors which influence the stability of shingle beaches. Studies have included: the significance of long period, high energy, swell waves the classification and quantification of overwash processes run-up and seepage characteristics the effect of settlement of the underlying strata and the implications for practices in shingle nourishment. The studies have revealed the distinctive character of shingle beaches as compared with the more fully researched sand beaches. More detailed research on shingle beaches is justified particularly in relation to (i) the run-up characteristics including its interaction with swash cusps and (ii) the influence of the subsidiary sand fraction on the beach characteristics.Environmental assessment, engineering studies and designs were completed for a new 26.5 m3/s seawater intake system in the Persian Gulf. The original intake facility consisted of a curved, 60m breakwater with one end attached to the shoreline, a settling basin immediately adjacent to the shoreline and dredged to a maximum depth of approximately 5m, and a pumphouse structure located on shore such that the seaward wall formed one side of the settling basin. The facility located on an island in the Gulf, which served multiple seawater uses, had experienced both structural and operational problems, the latter consisting principally of excessive ingestion of sediment and seaweed. These factors plus the requirement for additional demands for seawater beyond plant capacity caused the owner to initiate a study of alternative intake systems, produce a design for the most effective solution and construct the new intake system.A two-dimensional wave prediction model suitable for use on personal computers is described. The model requires the twodimensional time-dependent wind field as input. Output consists of wave height, wave period, and wave direction estimates at all grid points on a computational grid representing an enclosed or semi-closed basin. Model predictions compare favorably with observations from a wave research tower in Lake Erie. A formula is provided to estimate how long a model simulation would take on a personal computer given the surface area of the computational domain, the grid size, and the computer clock speed.Cullera Bay is a neritic ecosystem placed on the Spanish Mediterranean Littoral largely influenced by the Jucar River, that brings about lower salinities than surrounding waters, and broad variations of its values. An extensive research, with 9 samplings throughout the year, was carried out, measuring both physical and chemical parameters, and the planktonic communities. The trophic status of the ecosystem, the spatial and temporal variations of the nutrients and the planktonic communities were studied, evaluating the influence of the river loads and the littoral dynamics. Some essential basis to allow a suitable emplacement of waste waters disposals along the Valencian littoral are set up in order to minimize the gradual eutrophication of this coast.In the last two years a whole of studies was realized in order to determine precise solutions to the regeneration of Villajoyosas beach, in the Spanish mediterranean coast. Investigations were carried out to the surrounding coastal areas based in field investigations and laboratory analyses of the beaches materials.

Theoretical Analysis and Sph Simulation for the Wave Energy Captured by a Bottom-Hinged OWSC

This paper examines one type of wave energy converter, the Oscillating Wave Surge Converter (OWSC) with its bottom hinged on the sea bed. The simplest form of OWSC is a flapper connected with a power take-off (PTO). Theoretical analysis based on a 2D linear potential theory elucidates the mechanisms of the wave energy capture by a bottom-hinged OWSC in terms of impedances associated with the wave field, the flapper body, and the PTO. Criteria of impedance canceling and matching to maximize the energy capture factor can be deduced. Smoothed Particle Hydrodynamics (SPH) is established for simulating the same problem, and the trends of SPH results match that of the theoretical prediction well. Therefore, SPH can be regarded as a reliable numerical tool for designing and optimizing OWSCs.

NUMERICAL MODELING OF WAVE-INDUCED ROTATIONS OF A BOTTOM-HINGED FLAPPER WITH A SPH MODEL

In this study, a Smoothed Particle Hydrodynamics model for simulating wave-induced rotations of a bottom-hinged flapper was established in a 2-D numerical wave flume. The simulated rotating angles illustrated that the flapper could swing back and forth following harmonic wave loadings. The simulations were also seen to be in good agreement with experimental data, confirming the applicability of the present numerical model. The simulated hydrodynamic behaviors at different phases showed that the flapper moved downstream under the wave crest and upstream under the wave trough following the elliptical form of water particle trajectory. The energy conversions of a flapper during an average wave cycle showed that larger rotating angle ranges could result in higher energy conversions. However, smaller rotating angle ranges could result in higher captured efficiency.

APPLICATION OF THE EEMD METHOD TO INVESTIGATE PORE PRESSURE BUILD-UPS IN A WAVE-FLUIDIZED SANDBED

Yung-Lung Chen, Department of Harbor and River Engineering, National Taiwan Ocean University, Keelung, Taiwan, R.O.C., d92520007@mail.ntou.edu.tw Shiaw-Yih Tzang, Department of Harbor and River Engineering, National Taiwan Ocean University, Keelung, Taiwan, R.O.C., sytzang@mail.ntou.edu.tw Shan-Hwei Ou, Graduate Institute of Environmental Management, Tajen University, Pingtung, Taiwan, R.O.C., oush@mail.tajen.edu.tw

Numerical Assessment on Wave Energy Resources in Coastal Waters of Northeastern Taiwan

To assess wave power resources at a marine energy test site in Keelung coastal waters, the SWAN (Simulating WAve Nearshore) model [1; 2] is applied to obtain wave conditions for assessing the wave energy resources. The ocean surface wind velocity by CCMP (Cross-Calibrated Mutli-Platform) is first adopted in SWAN model simulation. Comparisons with field measurements of AWCP (Acoustic Water Column Profiler) station in Port of Keelung and of ADCP (Acoustic Doppler Current Profiler) station offshore NTOU (National Taiwan Ocean University) during periods form Jul 1st to Dec 31st of 2010, show that simulated significant wave heights agree well with measured values except in periods of typhoons and strong northeastern monsoons. However, the simulated peak periods are generally underestimated than the measurements. The same results can also be seen in simulated wave energies. The resulting simulated wave energies agree with measurements better at NTOU test site than at Keelung station.Copyright

HHT Analysis on Pore Pressure Dissipation of Wave-Induced Fluidization in a Sandy Bed

Wave-induced pore pressure variations during the stage of increasing excess pore pressure consist of the mechanism of generation of fluidization. Moreover, in post-fluidization stage, pore pressure variations not only reveal the dissipation mechanism of fluidization but also the wave-fluidized bed interactions. Past results from a series of lab flume tests have further illustrated that pore pressure variations in a fluidized response are nonlinear and nonsataionary. Hence, the HHT method was further applied to analyze the pore pressure measurements in this study. The results demonstrate that after the dissipation of excess pore pressures the amplitudes of fundamental and higher-frequency components begin to decay. Meanwhile, the amplified amplitudes of fundamental and higher-frequency components during fluidization response would decrease with decreasing thickness of fluidized soil-layer in consecutive tests.Copyright

Experimental Study on Regular Wave-Induced Scour and Settlement of a Submerged Obstacle Upon a Fluidized Sandbed

Bed scouring and structure settlement due to interactions of regular waves and a submerged rectangular obstacle in the shallow water regions upon a fluidized fine sandy bed are experimentally investigated. Both impervious and pervious structures are installed on the test soil bed and synchronous measurements of water surface, bed surface, suspended sediment concentration and pore pressures are carried out at locations upstream and downstream. The measurements illustrate that interactions between waves and structures have relatively mild affects on the bed scouring in an unfluidized response. The interactions become more intense in the fluidized responses with significant wave decay over and increasing suspended sediments near above the fluidized beds oscillating with similar period to the loading waves. Meanwhile, the submerged structure starts to settle into the fluidized bed and significant scouring occurs. The interactions are typically greater in a RF test than in a NRF test. More than 90% of the total settlement occurs in a single event of the RF response so that the impervious and pervious structures become half buried and totally buried after the event. The settlements further affect the interactions of following loading waves and the fluidized beds. As a result, scouring on both sides of the half buried impervious obstacle is greater than those of the totally buried pervious structure.Copyright