Abstract
Shoreline changes along the coasts between the Tenryu River mouth and Omaezaki Point facing the Pacific Ocean were investigated using aerial photographs. Beach has been eroded on the coast located near the east end of the coastline far from the river mouth. Although beach erosion on the nearby coasts to the Tenryu River mouth was triggered by the decrease in fluvial sand supply and the obstruction of longshore sand transport by a port breakwater, beach erosion on a coast far from the river mouth was due to the effect of ground subsidence associated with crustal movement, and partly due to the effect of windblown sand.
Highlights
The coastline west and east of the Tenryu River has been formed as a fluvial fan of the TenryuRiver (Fig. 1)
Severe beach erosion has been occurring on the Omaezaki coast located 40 km east of the Tenryu River mouth and near the east end of an arc-shaped shoreline, even though the eastern shoreline closer to the river mouth has been stable
It was found that ground subsidence due to crustal movement was a key factor causing the beach changes in this area, along with the predominance of eastward longshore sand transport
Summary
The elevation of the sand dunes increases inland east of the Kikugawa River, as shown, with a location where the elevation reaches over 20 m high in front of the hills These sand dunes were assumed to be formed by the deposition of windblown sand transported inland from the shoreline, and deposited on the coastal lowland with the shoreline advance since the Jomon Transgression of Sea approximately 6000 years ago. The rate of ground subsidence increases from the Tenryu River mouth to Omaezaki Point, at a maximum of 6-8 mm/yr near Omaezaki Point, and the rate of ground subsidence is as large as 4 mm/yr along the coastline in Suruga Bay. Figure 14 shows the vertical change in ground elevation at the tide gauge station with reference to the mean sea level on the basis of the observation data of tide level over the past 50 years, excluding the variation in the tide level originating from oceanographic and meteorological disturbances (Geospatial Information Authority of Japan, 2013). The contributions in change in sand volume owing to ground subsidence and windblown sand were removed for the correction of the distribution of longshore sand transport
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