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Countermeasures are investigated against coastal erosion and sediment invasion around a half-buried floodway through a series of small-scale physical model experimentations. Further, countermeasures are proposed to shorten the protruded length of the floodway across the shoreline to prevent interruption of longshore sediment transport and to excavate downdrift sediments at the mouth of the floodway to minimize the downdrift erosion as well as the sedimentation inside the floodway. Results of the study show that the shortened floodway is successful in mitigating downdrift erosion, while continuous downdrift excavation is proven to be significantly effective in reducing sedimentation inside the floodway when excavation is implemented with the same order-of-magnitude of the longshore sand transport rate. Attention must be paid, however, on the effect of wave randomness and floodway discharge during stormy waves as they further increase sedimentation inside the floodway apparently owing to onshore sediment movement enhanced by asymmetric horizontal acceleration and to a large skewness in the water particle velocity at the mouth of the floodway. [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]

This study aimed to investigate hydrodynamics and morphology around river mouth. The propagation of long period waves along the river side of sand spit around the Tenryu River mouth was analyzed by means of image analysis. A series of field surveys were conducted using video cameras to observe shoreline morphology on the river side of the sand spit. Characteristics of the swash motion were extracted and used to describe the fluid motion on the river side. On the basis of image analysis, it was concluded that the shoreline undulation was developed by sediment transport affected by edge wave. This was suggested by shoreline undulation properties found in the images and was confirmed by the edge wave wavelength calculated based on frequency of the corresponding swash motion. In addition, longshore transport instability due to oblique wave with large angle appeared to change the shape of the shoreline undulation. [ABSTRACT FROM AUTHOR]

Abstract: Prototype scale physical model tests were conducted to investigate the sheetflow sediment transport of uniform sand under different skewed-asymmetric oscillatory flows with and without the presence of relatively strong currents in the opposite direction against wave propagation. Experiments show that in most cases with fine sands, the “cancelling effect” which balances the on-/off-shore net transport under pure asymmetric/skewed oscillatory flows and results a moderate net transport was developed for combined skewed-asymmetric shaped oscillations. However, under certain conditions (T >5s) with coarse sands, the onshore sediment transport was enhanced for combined skewed-asymmetric flows. Additionally, the new experimental data under collinear oscillatory flows and strong currents show that offshore net transport rates increase with decreasing velocity skewness and acceleration skewness. Sediment movement behaviors were investigated through analysis of experimental data obtained from the image analysis technique and attempts were made to estimate and formulate the sheetflow layer thickness. Accordingly, sediment transport under oscillatory sheetflow conditions was studied and successfully explained by comparing the bed shear stress and the phase lag parameter at each half cycle. Consequently, these parameters were incorporated in an improved Dibajinia and Watanabe''s type sediment transport model. The formula is calibrated against a comprehensive experimental data (331 in total). Good agreement obtained between predictions and measurements shows that the new formula is fulfilled for practical purposes. [Copyright &y& Elsevier]

The thermoluminescence (TL) properties of feldspar were used to investigate the sediment movement processes in a fluvial-coastal system. Field samples were collected at various locations along the Tenryu River and the Enshunada Coast in Japan. After a series of pretests, an appropriate TL measuring sequence was proposed for this study. Applying this approach, the natural TL intensity of feldspar grains was measured. Owing to the young age of the research area, it was assumed that the natural TL difference was ascribed to the sample depositional environment under which different possibilities of sunlight exposure exist. Surface sediment particles in the target area were classified into three groups in terms of TL intensities, i.e., river sand with large TL signals, coastal sand with medium TL signals, and dune sand with small TL signals. Stratified configuration of the Nakatajima Coastal Dune was observed from the underground-sample TL glow curves with a top, windblown, dune-sand layer; a bottom, wave-induced, coastal sand layer; and a mixing layer in between. A rather complex sediment-movement pattern in front of the Tenryu River mouth was revealed after investigation of the underwater samples. Because of the seawater influence, acting as an ultraviolet filter to sunlight, underwater samples present a larger, high-temperature TL peak than the low-temperature peak; whereas these two are almost the same for ground samples. [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]