Abstract
Around the Mihono-matsubara sand spit in Suruga Bay in Japan, the beach was eroded owing to the decrease in the fluvial sand supply from the Abe River triggered by excess riverbed mining before 1967, together with the discharge of sand into the deep sea via a steep slope near the tip of the sand spit. As a measure against beach erosion, an artificial headland (HL) composed of two detached breakwaters and the breakwaters (BWs) placed along the shoreline have been constructed along with beach nourishment, but the beach is barely maintained by these measures. In 2013, two large typhoons hit the coast, causing rapid beach changes around the structures, and these beach changes were superimposed on the long-term topographic changes that have occurred over a long time as a geomorphological process. In this study, their impact to the beaches was investigated on the basis of the field data.
Highlights
A sand spit can be formed on a shallow seabed under the premise that a sufficient volume of sand is continuously supplied from upcoast by longshore sand transport
This sand spit was formed by an abundant supply of sand, which was originally transported from the Abe River, the beach was eroded owing to the decrease in the fluvial sand supply from the Abe River triggered by excess riverbed mining before 1967, together with the discharge of sand into the deep sea via a steep slope near the tip of the sand spit
On the Shizuoka and Shimizu coasts including the Mihono-matsubara sand spit, severe beach erosion occurred owing to the excess sand mining in the Abe River before 1967
Summary
A sand spit can be formed on a shallow seabed under the premise that a sufficient volume of sand is continuously supplied from upcoast by longshore sand transport. The shoreline in this area has a semicircular shape, as shown, and along this shoreline, sand discharges into the deep sea via a steep slope, as mentioned later. The areas on the HL1HL5 south of the L-shape groin were eroded, whereas sand was deposited between HLs. The same topographic changes were observed between BW1 and BW4, implying that the wave-sheltering effect of various structures was reduced during extraordinary high waves, and deposited sand was discharged downcoast. The shoreline recession between the L-shape groin and BW1 was relatively small as little as 10 m Except for these areas, the shoreline changes were relatively small and the shoreline slightly retreated in front of the runway and behind the offshore breakwater of Shimizu Port, where the shoreline had been advanced by the successive deposition of sand. In the short term during the period of storm waves, marked beach changes with shoreline recession and accretion upcoast and downcoast of the structures, respectively, were observed, which differ from the long-term changes
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