Narrow Results

In this study, we projected the future beach loss in Japan’s 77 coastal zones due to sea-level rise (SLR) based on representative concentration pathway (RCP) scenarios and 21 models of the Coupled Model Intercomparison Project Phase 5 (CMIP5). The beach-loss curve for SLR in Japan was constructed, and uncertainties associated with SLR projections and sediment sizes were evaluated. Beach-loss rates in the future (2081–2100) were projected to be 62% for the ensemble mean RCP2.6 scenario, 71% for RCP4.5, 73% for RCP6.0, and 83% for RCP8.5, and the rates projected by the CMIP5 models for RCP4.5 ranged widely from 61% to 87%. The effect of the spatial distribution of SLR in each CMIP5 model on beach-loss rate in Japan is insignificant, while the effects of differences in the SLR values among RCP scenario and CMIP5 models are significant. The maximum uncertainties associated with sediment sizes (0.2–0.6mm) against the same SLR were assessed to be 38%. Despite significant uncertainties in the projected beach loss, results in the near future (2046–2065) reveal that the beach-loss rates between 18% and 79% differed by 60% in the near future and between 28% and 96% differed by 70% in the future. For the worst case scenario in the near future, the projected beach width is less than 10m in more than half of the 77 coastal zones, which would cause serious damage to coastal structures such as seawalls and revetments in beach areas. Thus, the development of effective measures to combat beach loss is critical for Japan’s coastal management.

Beach changes around Futtsu-misaki cuspate spit located at the mouth of Tokyo Bay and protruding west are investigated through the comparison of aerial photographs. It is found that shoreline has been eroded in recent years mainly due to lack of sand supply from the southern coast caused by the obstruction of continuous littoral transport by Shitazu fishery harbor. At the tip of the spit the seawall was destroyed and a concave shoreline was formed due to erosion as the shoreline became stable under the condition of two fixed control points at Dai-ichikaiho island and the tip of the seawall. Sand erode from the tip of the spit was found to be carried to the north coast of the spit by wave action.

Beyond the boundary and design criteria of Low Crested Breakwater (LCB), the success of LCB applications largely depends on the layout of the installation. The layout includes vertical and horizontal properties. Vertical layout related to the peak elevation position of the structure to the sea level, both to Mean Sea Level (MSL) and High Water level (HWL). Horizontal layout regarding LCB placements related to the optimal distance from the shoreline, the length of the structure, and the width of the gap between LCBs. These three parameters determine sediment volume accumulated behind LCB. This paper presents a summary of field experiences that is useful for the development of LCB as one of the methods of coastal protection. The objective of developing LCB structure is to make LCB concept as preferred coastal structure that is applicable to all types of materials commonly used for coastal protection structures.

As a study on beach erosion in a developing country, beach erosion in Kalibo City, located in the northern part of Panay Island in the Philippines, was investigated together with the deformation of a sand spit using satellite images. The study area is the shoreline of 7.7 km length between New Washington, south of Kalibo City, and the sand spit located southwest of the city. Field observation was carried out on January 16, 2014, two months after Typhoon Haiyan hit the area. In this area, the shoreline has severely receded owing to the decrease in sand supply from the Aklan River. Local optimization of shoreline protection using seawalls has been employed, resulting in no fundamental solution to the erosion.

The beaches at the mouth of the Cai River, which flows into Nha Trang Bay, Vietnam, have been eroded, resulting in the disappearance of sandy beaches on both sides of the river mouth. Satellite images taken between 2002 and 2015, and the effect of a storm that occurred in 2008 were analyzed. The most important cause of the beach erosion was sand excavation from the sand bar on the right bank to obtain materials for land reclamation on the left bank. Another cause was sand movement due to waves in the upstream direction along the left bank of the river. To recover the sandy beach, the construction of training jetties is expected to be effective, similarly to the case of the bay mouth jetty of Dam Nai Bay in Phan Rang, Vietnam.

The causes of beach erosion in Japan are classified into four main types: (a) obstruction of longshore sand transport, (b) beach changes associated with the formation of wave-shelter zones, (c) decreased fluvial sediment supply, and (d) offshore sand mining or dredging. Of these, examples associated with the formation of wave-shelter zones and decreased fluvial sediment supply were studied, typified by the changes in beaches at Otsu and Shimobara fishing ports, Takahama Port, and along the Enshu-nada coast caused by decreased fluvial sediment supply from the Tenryu River. Such erosion is closely related to land management and the "sector-by-sector" administrative system in Japan. The prevention of beach erosion thus requires both scientific research and improved coastal management through enhancing the public consensus.

Numerical computations are made for the nearshore currents and beach morphology around a system of multiple submerged groins. They function in decreasing the speed of longshore currents and controlling beach erosion. Nearshore currents are computed with the PEGBIS model by Goda (2004) for directional random wave transformation, and beach morphology is estimated with the suspended sediment transport model by Katayama and Goda (2000). The computation indicates the efficacy of a submerged groin system for beach erosion control. Discussions are made on several aspects of the submerged groin system to be clarified through further investigations.

The nearshore hydrodynamic and morphological model CSHORE has been under development for the past several years, utilizing a multitude of laboratory data sets. Although the model has a physically-based foundation, all practical morphological models, including CSHORE require empirical parameters. The model is compared with a large set of field data and two representative examples are provided in detail. The storm response data sets, one from the east coast of the US and one from the west coast, are similar in bulk wave statistic but differ in that the East Coast case has a significant storm surge. The model performs reasonably well without site-specific calibration, but some improvement is realized by increasing the effect for the dissipation due to breaking waves on the West Coast.

Beach changes of five pocket beaches located on the west shore of Miura Peninsula were investigated through the comparison of aerial photographs and bathymetric survey data. The clockwise rotation of the shoreline was observed in every case, resulting in shoreline recession in the north part of the beach and accretion in the south part. It was found that a port breakwater was extended at the south end of the pocket beach to form a wave calm zone under the conditions of the oblique wave incidence from the southwest. A small river also flows into the sea at the south end of the pocket beach because of the wave calmness by the natural headland, but sand supplied from the river became difficult to move out of the wave-shelter zone of port breakwater, accelerating sand deposition near the port immediately offshore of the river mouth. The dredged sand was disposed offshore, resulting in the decrease in sand volume of the beach. These are the common anthropogenic factors related to the beach erosion of the pocket beaches. To soundly maintain these pocket beaches, sand backpass is necessary along with the construction of some auxiliary facilities to prevent longshore sand transport from returning to the port.

On the basis of bathymetric survey data, aerial photographs and sampling data of seabed materials, sand budget analysis considering the grain size is carried out, taking the Shonan coast as an example. A diagram showing sand budget is proposed, in which longshore sand transport, rate of windblown sand, and grain sizes not only near the shoreline zone, but also in the offshore zone are shown. The loss and input of sand across the shoreline due to windblown sand and dredging in the ports are also summarized. This diagram can be effectively used for the coastal management of the coast.

On the Akiya coast a breakwater of Kuruwa fishing port was extended, inducing longshore sand transport from outside to inside the wave-shelter zone. Sand accumulated inside the wave-shelter zone, whereas beach erosion occurred on the nearby coast. A tracer test using sand of d₅₀=0.5, 1.5 and 15 mm showed that coarse material was deposited near the shoreline and slowly moved alongshore, whereas fine material was carried offshore. The contour-line-change model considering changes in grain size was applied to the coast to predict the effect of beach nourishment using material with different grain sizes. Gravel nourishment was confirmed to be effective for recovering a stable beach without damage to the offshore fishing ground. Beach nourishment using 7,600 m³ of gravel with d₅₀=15 mm was carried out in 2007 on the Akiya coast. The nourishment gravel was deposited near the shoreline forming a steep slope of 1/5, indicating that the nourished beach is very stable.

On September 6, 2007, the Seisho Bypass extending along the Seisho coast was severely damaged over a length of 1.1 km owing to storm waves associated with Typhoon 0709, resulting in roads being closed to traffic for urgent repairs. During the typhoon, severe beach erosion occurred owing to both strong offshore sand transport and westward longshore sand transport by storm waves, and the foreshore was rapidly narrowed. However, the foreshore was recovered except the damaged area of the highway as a result of the effect of normal waves. These beach changes were predicted using the contour-line-change model incorporating Fukuhama et al.'s concept. The predicted results were in good agreement with the measured beach changes.

Long-term migrations of sandbars on Chirihama Beach, Japan, have been investigated using a set of field surveys collected over 9 years. The typical profile configurations are characterized by the presence of multiple bars. The height of the outer bar exceeds 4 m, when the bar evolves most significantly. The movements of the bars are significant. For the time variation, it is shown that the net offshore periodic migrations of bars have been repeated. The length of one cycle is approximately 8 years, and the interval between each cycle is approximately 3 to 4 years.