Wen-Son Chiang

Measurements of wave modulation and breaking

To investigate the evolution of wave modulation and wave breaking, a series of elaborate experiments were conducted in a super wave flume (300 m × 5.0 m × 5.2 m) at Tainan Hydraulics Laboratory. Two wave trains of initial regular waves and initial imposed sidebands were generated by a wave maker with active wave absorption using an electro-hydraulic server feedback control system. The wave profiles were recorded by 66 high-resolution capacitance-type wave gauges positioned along the wave flume, and connected to a specially designed data acquisition system to collect a large number of parallel inputs in the long flume. Experimental data were obtained on wave modulation and related wave breaking. The long time evolution of the nonlinear wave train and the corresponding wave spectra are demonstrated for both breaking and non-breaking cases. Specifically, the periodic modulation and demodulation of wave train is observed at post-breaking stage.

Steepness effect on modulation instability of the nonlinear wave train

The long-term evolution of a nonlinear wave train in deep water with varied initial wave steepness between 0.1 and 0.3 was experimentally investigated in a super wave flume (300m long, 5m wide, and 5.2m deep). The initial wave train was the combination of one carrier wave and a pair of imposed sideband components which is the most unstable mode, referred to as sideband instability theory. Sixty-six wave gauges were installed downstream along the wave flume to simultaneously measure the evolution of a wave train. Increasing modulation of the wave train was observed due to sideband instability until a critical value which either wave breaking is initiated or maximum modulation is reached. The near recurrence of the initial state of the wave train is exhibited for the nonbreaking case. An effective frequency downshift of the wave spectrum accompanied with wave breaking is observed as the initial wave steepness is larger than 0.11. At postbreaking, the wave train reveals periodic modulation and demodulation, ...

FLOW STRUCTURES IN SWASH ZONE

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.

Large transient waves generated through modulational instability in deep water

The long-term evolution of nonlinear wave train in deep water with varied initial wave steepness is investigated experimentally in a super wave flume (300 m long, 5 m wide, 5.2 m deep). The initial wave train is the combination of one carrier wave and a pair of imposed sideband components. Increasing modulation of wave train is observed due to sideband instability until a critical value which either initiates wave breaking or reaches the maximum modulation. The observed maximum local wave steepness increases rapidly with the increase of the initial wave steepness, and levels off at initial wave steepness roughly equal to 0.15 despites that the data exhibits a little scattering. The normalized crest elevation at peak modulation increases rapidly with initial wave steepness and approached a maximum value almost equal to 3.5 which corresponds to initial wave steepness around kcac = 0.15. The results reveal that the large transient wave such as freak wave could be generated during the propagation of nonlinear wave trains in deep water through sideband instability.

Laboratory and modeling study on modulated wave properties

Development of Benjamin−Feir instability is investigated under laboratory conditions and by analytical modeling. Nonlinear properties of the wave train with discrete spectrum are also investigated. The mechanically generated waves are composed of several discrete waves, while the newly generated harmonics are still combined into discrete spectra with the same frequency step. The technique proposed in this study allows us to study accurately the nonlinear variations in main properties of each harmonic with fixed frequency, such as amplitude, phase speed, and wavenumber along the wave tank together with velocities of wave packet crests, especially for the large transient waves. The phase speeds of short waves increase near large transient waves, and the velocities of longer waves are close to the values determined by the linear theory of waves. The relative long wave accompanied by short waves can dramatically change the local kurtosis and skewness of the wave field. They may play an important role for the generation of large transient wave and provide an opportunity for triggering of the freak waves.

Measurements of high concentration sediment plume in the estuary with strong tidal currents

With the facts of high erodibility (3-6mm/yr) resulting from rapid plate convergence (80 mm/yr) accompanied by frequent large earthquakes and the intensified typhoon precipitation intensity and floods, the occurrence frequency of hyperpycnal events of Taiwanese rivers can reach to more than 1 event/yr, which is O(2) magnitude larger than the worldwide average. Taiwan is an ideal place for hyperpycnal flow study for the better chance to obtain the observational evidence. In present study, the typhoon Kalmaegi (2008) flood driven high sediment concentration fluvial discharge are focused, data obtained from field surveys at the estuary of Jhuoshui River, the second largest river in Taiwan, are investigated. The observational evidence of strong tidal current supported gravity flow was identified.