Coastal Engineering Journal

Experimental investigations on wave reflections, run-up and run-down and wave pressures on plane, dentated and serrated seawalls were carried out for a wide range of wave heights, wave periods, seawall slopes and in a constant water depth of 0.7 m. Based on the measurements, predictive equations are proposed. A total performance evaluation is carried out on each seawall. The improved performance of the serrated seawall compared to plane seawall is brought out. It is found that closer to deep water condition, the reflection coefficients from the serrated and the dentated vertical seawalls are only about 0.5 and 0.6 respectively compared to the fully reflecting plane vertical wall. Sloped serrated seawall is capable of dissipating the wave energy almost two times that of the sloped plane seawall. Closer to still water level, the normalized wave pressure value (ratio of maximum wave pressure to static pressure due to the water column equal to incident wave height) for 2% exceedence is 0.9, 1.1 and 1.15 for the serrated, dentated and plane seawalls. The measured relative run-up value (ratio between the maximum run up to the incident wave height) for 2% exceedence is 1.30, 1.5 and 1.6 for serrated, dentated and plane seawall respectively. Hence, the design crest level of the serrated seawall can be at a lower level than that of the plane seawall. Overall, it is found that the serrated seawall is about 20% to 40% better than the plane seawall for reducing the wave pressure, reflection, run-up and rundown. The results can be used for better hydrodynamic design of coastal structures with vertical face (like quay walls in harbors, vertical seawalls and caisson type breakwaters) and sloped structures (like seawalls, dikes, sea side sloped caissons).

The coupled motion between progressive wave and floating curtain-wall type breakwater is simulated by an incompressible Smoothed Particle Hydrodynamics (SPH) method combined with a Large Eddy Simulation (LES) model. The Naviers-Stokes equations in Lagrangian form are solved using a two-step split method. The method first integrates the velocity field in time without enforcing incompressibility. Then the resulting deviation of particle density is projected into a divergence-free space to satisfy incompressibility. The SPH method is convenient for describing free surfaces and moving boundaries of the floating body. The LES model is capable of dealing with turbulence during the wave-breakwater interactions. The translation and rotation of the floating curtain wall is calculated through an additional rigid-body-tracking subroutine. The computed wave profiles and hydrodynamic forces by the current model are in good agreement with those reported in the literature. Finally, the wave transmission and reflection characteristics are analyzed and compared with numerical results in which the breakwater is fixed.

Organized eddies generated by wave breaking have an important role on sediment transport in surf zones. In order to clarify the characteristics of intermittent sediment clouds generated by vertically-oriented eddies, the Computed Tomography (CT) technique was applied for a laboratory experiment to capture instantaneous two-dimensional distributions of suspended sediment concentration in a near-bottom horizontal plane. The Extended Bayesian Method with a smoothing filter as the prior information model was applied to obtain the optimum solution of the distribution. A specification test showed that concentration distribution was reproduced fairly well. Successive images of concentration distribution due to wave breaking were obtained and movements of sediment clouds were investigated. Concentration at the center of the measuring area was simultaneously measured for verification by a conventional optical forward-transmission-type sedimentmeter. It was found that horizontal distribution of suspended sediment concentration in a surf zone is quite inhomogeneous and the high concentration regions are formed as they indicate separated sediment clouds. Distinctive patterns of sediment clouds were observed in a relatively small number of trials. The length scale of the clouds was found to be in the order of the water depth. The primary cause of sediment cloud formation can be considered sediment pickup by intermittent eddies generated by wave breaking.

A field investigation on sediment movement and beach evolution was performed in a fluvial system composed of the Samegawa River and the Nakoso coast, which experienced significant beach erosion recently. Analyses of regional sand volume in the system revealed that the rate of decrease in nearshore sand on the beach is of the same order as the rate of accumulation of sand in dam reservoirs. Dredging of the riverbed near the river mouth also influenced nearby shoreline retreat. Long-term changes in sand movement processes were estimated from the distribution of mineral composition included in the sediment samples collected in a wide area.