Coastal Engineering Journal

In-situ and laboratory measurements of particle setting velocity through different techniques, such as settling tubes and video systems, suffer from human and random sampling errors and lack of accuracy. The development of a method to improve the accuracy of the settling velocity measurements has been a challenge for researchers. We propose a method using a laser diffraction particle size distribution analyzer (Shimadzu, SALD-2000J) to estimate the particle settling velocity with increased accuracy. SALD-2000J measures continuously the average intensity of light scattered and the particle size distribution of mud flocs, which were used to estimate the particle settling velocities. The SALD-2000J method produced high accurate results saving both labor and time. The predicted settling velocity presented the same order of magnitude of those reported in previous studies. The settling velocity was used in the simulation of particle size distribution variation, producing a relative error of particle size distributions (obtained from SALD-2000J and the simulation) interior to 5%. The results suggested that the proposed method can be applied to the prediction of settling velocities of mud flocs and inorganic particles both in fresh and saline water environments.

This study aims to develop a new numerical model for predictions of the time-varying shear current field under breaking and broken waves. The present model differs from the other similar existing models in that the model explicitly determines the bottom shear stress so that it satisfies both force balance and mass-conservation equations while most of the other existing models leave the bottom shear stress as one of the calibration parameters to be fitted. The present model consists of two sub-models, a wave model and a flow model. The wave model is based on modified Boussinesq equations with evolution and dissipation of surface rollers. The flow model is based on Reynolds' equations with turbulence closure models and computes vertical profiles of wave-induced shear current field. The numerical model is applied to various existing and newly performed experimental cases which cover various breaker types and bed profiles with different bed roughness. The present model showed overall good predictive skills of various surf zone hydrodynamics such as wave heights, mean water levels and undertow profiles. It should also be highlighted that the model has only a few calibration parameters and the predictive skill of the model is relatively less sensitive to these parameters.

Two two-dimensional (2D) models for flow through a circular patch of circular cylinders in a channel, the multi-body model and porous model, are established using the scale adaptive simulation (SAS) turbulent model. It is verified that the multi-body model could predict well the average velocity field, turbulent structures and vortex in all cases. The porous model could only predict well the flow pattern and the large-scale vortex motion and fails to predict the small-scale motion. For cases of high solid volume fraction or low cylinder Reynolds number, the differences in prediction between the two models are small even in the region inside the patch. For the case of low solid volume fraction, the performance of the porous model is acceptable only in the region away from the patch. If predictions are required only for large-scale motion away from the patch, the more computational efficient porous model is a good modeling tool.

The solitary wave as well as objects drifted by run-up wave interacting with elastic breakwater was investigated using an Arbitrary Lagrangian Eulerian (ALE) method. A numerical breakwater-flume coupling model was developed in conjunction with experiments to best understand the wave impact. In the experiment, the solitary wave was generated and a physical breakwater model was used. The experimental data of the wave pressure and water-on-breakwater were then used to validate the established simulation model. Comparisons indicate good agreements between simulations and experiments. After the validation, the ALE method based simulation model was applied to practical engineering analyses. Effects of the design parameters of breakwaters on wave loads were investigated. In full-scale simulation for collision of the drifting object, two types of colliding objects were used. Effects of mass and initial position of the drifting object on collision force were discussed. It was found that the wave pressure and structural stress of rear wall increases significantly when the breakwater width decreases. In addition, the drifting object brought larger collision force compared to the solitary wave, and the mass and the initial position of drifting objects had great effects on the breakwater's dynamic response.

The 2011 Great East Japan Earthquake and Tsunami caused severe damage along the affected coastal area. This study analyzed seabed subsidence and morphological changes along the Ishinomaki Coast based on bathymetry measured shortly before and after the 2011 event. In general, seabed subsidence ranged from 0.32 m to 0.91 m. In addition, massive erosion on the seabed due to the 2011 tsunami occurred locally, in front of the Naruse River mouth and at the west end of the Ishinomaki Coast. Moreover, the depth of closure generated by the tsunami in these areas was higher than in others. The sandspit in front of the river mouth was severely eroded and the riprap protection at the west end was completely destroyed. Thus, there were no buffers in these areas during the tsunami. Therefore, tsunami-induced bed stresses were higher and may have occurred in deeper areas. Other areas, which were protected by relatively unharmed concrete structures, did not suffer from severe erosions. This suggests that the coastal protection structures protected both the land and the seaward side during the tsunami.

Little is known about the strength of the land-side slopes of the Vietnamese sea dikes under overtopping attack. Some grass-covered slopes were tested by a wave overtopping simulator. The paper describes the simulator and presents the test results. The tested grass covers could withstand overtopping rates varying from 20 l/s to 100 l/s per meter of dike length in some hours. Damage usually started at bare spots, at the transition between the slope and the toe, at the transition between different materials, and around objects (e.g. big trees). These features reduce the strength of a grass cover and therefore should be avoided.