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Abstract:It is inevitable to find in studies that shoreline position extracted from unknown capture time aerial photograph or satellite image was not corrected to tidal effect. In this study, an approach is introduced that can estimate the capture time of image from the solar azimuth angle and the length of shadow of a vertical object on the horizontal surface on the earth. The capture time of tze aerial photograph estimated from solar azimuth angle has a much smaller deviation than the one estimated from the length of the shadow. This error is acceptable for tidal correction purposes. The approach was also utilized to estimate the capture time of a set of satellite images on Sendai coast. Therefore, the tidal correction was implemented for shoreline positions extracted from those images.

Morphology of Cua Dai River mouth and adjacent sandy beaches in Hoi An City, Vietnam has been being eroded severely in recent years. Based on analysis of Landsat images, the long-term evolution, decadal scale, of morphology at this area since 1975 to 2015 is discussed. Morphology of river mouth in the period from 1975 to 1991 is highly similar to one in the period from 2003 to 2015, long sandspit on the left side and cuspate shoreline (cuspate foreland) on the right side, whereas the formation of double sandspit and welding between them after that was observed in the period from 1995 to 2002. From the characteristics of morphological change, there are three types of river mouth formed during the 40-year long period are classified. In addition, results of image analysis also indicate that the right bank of river shifted southward about 250 m. This is corresponding to the period with elongation of sandspit on the left after welding from offshore sandbar. The tip of cuspate shoreline on the right side was also observed to move to the south during the period that right river bank shifting southward. Besides the reduction of sediment supply to the river mouth, the shifting to south of main river channel diverting more sediment deposit on the right side of the river mouth, resulting in the erosion of the Cua Dai Beach on the left side became more serious. Countermeasure, which diverts more sediment to the left side, is highly required. Detailed effectiveness and magnitude of this structure can be evaluated based on numerical simulation.

The 2011 tsunami formed concave shoreline around the Nanakita River mouth and Akaiko breaching areas, Sendai Coast, Miyagi Prefecture, Japan. The recovery process of morphology in these areas is much depended on the longshore sediment which is transported from adjacent sandy coasts. Coastal structures on both sides of the concave portion are considered as rigid boundaries. Analytical solutions of one-line model, which describe the evolution of shoreline around the concave portion in cases without and with rigid boundaries, have been discussed. When the dimensionless of concave width to the length of bounded area is getting smaller, the dimensionless recovery time obtained from solution for the case with rigid boundaries is getting larger and asymptotic the one for case without rigid boundaries. An analytical solution for estimating the proportion of backfilling of sediment deposition in the concave portion for case with rigid boundaries is introduced. It is asymptotic to the one without rigid boundaries when the dimensionless of concave width to the length of bounded area is getting smaller. The comparison between theoretical results and measured data of cases of the Nanakita River mouth and the Akaiko breaching areas are presented.

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