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Beach evolution is one of the key issues in coastal engineering. High computational efficiency model of beach evolution is limited by strong wave nonlinearity, flow acceleration, and sediment phase lag. The XBeach model is improved to cover the three limitations in the study. First, the near-bed velocity of nonlinear wave is reconstructed by using nonlinear parameters. Second, flow acceleration and sediment phase lag are taken into account for the near-bed sediment conditions to calculate the equilibrium concentration of suspended sediment and the transport rate of bed load. The response of sediment is not necessarily instantaneous to near-bed velocity in practical problems, but separated into a phase (time) shift between sediment concentration and velocity and a phase residual denoting sediment amount in moving at flow reversal. The improved XBeach model performs better results than the original XBeach model. Numerical experiments are carried out using the improved XBeach model to investigate the effects of wave height and sediment size. The numerical experiments show that the volume and range of erosion/deposition increase with the increase of wave height and decrease of sediment size, and deposition center moves to offshore. The perspective of wave energy and phase lag can explain the erosion/deposition variation. [ABSTRACT FROM AUTHOR]

This paper presents the results of numerical study on the wave-induced currents and morphology changes due to the complicated interaction between waves and a sandy beach with shore-parallel breakwaters set on. The study was based on the SWASH wave model coupled with a sediment transport model developed in this research. The simulated results of the coupling model were compared with the published laboratory experimental data. The good agreements were obtained for both wave-induced currents and formation of salient and tombolo behind breakwaters. The models were also applied for studying the evolution of wave-induced currents associated with topography changes of a sandy beach. The wave-induced currents and sediment transport processes have a strong relationship and their interaction could reach a quasi-equilibrium state in which the changes of current pattern and bathymetry become very slow. Numerical results of shoreline response mode were also compared with a published empirical formula and confirmed good agreements. [ABSTRACT FROM AUTHOR]

Field observations were conducted during relatively mild wave conditions at a dissipative barred sandy beach to investigate the cross-shore sediment dynamics from the inner surf zone to the offshore side of the outer bar. Fluorescent sand tracers of five different colors were placed on the seabed at different cross-shore locations, and sand core samples were extracted respectively 11 and 20 days after the placement of tracers. Moreover, nine cross-shore bathymetric/topographic beach surveys were conducted, and the water level, wave, and wind conditions were observed. The objective of this research was to investigate the mechanisms of the on- and offshore-directed sediment transport during the inner- and outer-bar accretion events. The analyzed results revealed that at the inner surf zone, the sediment was dominantly transported in the onshore direction. The sediment was also transported offshore-ward due to the undertow; however, this occurred until the offshore end of the trough region. Overall, the outer bar shape affected the on-offshore sediment transport rate. The sediment movements in the onshore region and offshore region of the outer bar were not related; however, the outer bar was formed with the sediments from both the inner- and outer-bar regions. [ABSTRACT FROM AUTHOR]

Beach erosion mechanisms were investigated for Kujukuri Beach, a 60-km-long sandy beach facing the Pacific Ocean, located in Chiba Prefecture, Japan. The longshore sediment transport rate was estimated using aerial photograph analysis, physical property measurements of beach sediment samples, and land subsidence estimation owing to paleo-groundwater pumping. Alongshore distribution of sediment grain size and thermoluminescence intensity revealed that the longshore sand transport from both ends would meet at x = 40 km, where x denotes the distance from the north end of the beach. Sediment loss caused by the land subsidence was estimated at 130,000 m3/year in the south, which was observed to play an essential role in the beach erosion, as compared with the relatively small amount of sediment transport in the south. An effective and comprehensive countermeasure was proposed in which the land subsidence could be mitigated through the restoration of pumped water in the coastal zone. The countermeasure was found effective in decreasing the sediment loss by 85,000 m3/year, thus changing the sediment budget of the whole Kujukuri Beach positive. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]