Narrow Results

The relationships between medium-term shoreline fluctuations, offshore wave energy flux and longshore current velocity were investigated on the basis of data sets from 1987 to 2001 (15 years). In this analysis, the spectral density of the data was calculated, and the frequency components from the investigated period (15 years) to 1000 days were recomposed and defined as the medium-term fluctuation. A multiple linear regression analysis was performed to evaluate the associations between the medium-term fluctuations of shoreline change rate and that of the major dependent variables, which are the offshore wave energy flux and the longshore current velocity. The results suggested that a medium-term fluctuation of the shoreline change rate at the field was affected by both the medium-term fluctuations of the offshore wave energy flux and the longshore current velocity at approximately the same rate. The fluctuation range of the medium-term shoreline fluctuation was nearly the same range of the seasonal shoreline fluctuation, i.e. approximately 20 m. This indicates that for beach management, not only seasonal shoreline fluctuation but also medium-term shoreline fluctuation is needed to be considered.

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.

This study assesses the short- and long-term changing position of the shorelines along the barrier lagoon and mud section of the Nigeria coast using remote sensing techniques. Five shoreline positions, covering a 30-year period (1986–2016), were extracted from medium resolution multi-spectral Landsat satellite imageries using both manual and semi-automatic shoreline extraction methods. Approximately, 533 orthogonal transects were cast using DSAS at simple right angles along the entire coast at 250-m interval. The shoreline change analysis was calculated using the Net Shoreline Movement and the End Point Rate techniques. The results show that the shoreline is highly dynamic; with the average rate of erosion estimated to be 28.08m/year and the average rate of accretion estimated to be 20.56m/year. While the persistence of erosional tendencies was found mainly in the Okesiri-Abereke-Aiyetoro parts of the mud section of the shoreline, the accretional tendencies was found mainly in the Aboraji-Araromi (barrier lagoon) and the Ajegunle-Jinriwo-Awoye (mud section) parts of the shoreline. The high dynamism of the shoreline is mainly attributed to the increasing frequency of storm surges in the area with over 13 incidents experienced within this period. This study submits that addressing coastal erosion and flooding problems in Nigeria should be based on the system boundary model where the coastal process and dynamics are constantly monitored holistically rather than locally or regionally as it is currently being done. This will also ensure the incorporation of the extent, frequency and intensity of extreme event in the development of adaptation measures.

On Embudu Village Island in the Maldives, the seasonal movement of a pair of sand spits can be observed, which are generated by the seasonal changes in wave direction associated with the tropical monsoon. Field observation of the beach changes on this coral cay was carried out in May 1992. Satellite images of the cay taken between February 2005 and November 2013 were compared to investigate the planar changes of the cay. The BG model (a model for predicting three-dimensional beach changes based on Bagnold’s concept) was used to predict beach changes of the coral cay. The calculated beach changes were in good agreement with those measured. It was confirmed that the wave-sheltering effect of the island itself played a decisive role in the beach changes.

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.

Shoreline identification using satellite images is compared with in situ shoreline measurements in the Yucatan Peninsula to evaluate its potential for studying shoreline changes in places with a paucity of data. This study firstly tests the detection limits of shoreline identification by comparing a SPOT image with ground shoreline measurements, and secondly we show examples of overlaying satellite-derived shorelines from three different years to assess the ability of the technique to quantify real shoreline changes. The mean (-0.19 m) and the standard deviation (4 m) between the ground and satellite-derived shoreline are much smaller than the pixel size. Shoreline changes of more than 30 m were measured between images spanning several years (2004, 2006 and 2008) in areas near to coastal structures and near urban areas without coastal vegetation.

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.