ABSTRACT

The river mouth delta of this study is located on the center of the eastern coast of Ariake Bay, Kumamoto, Japan. Two earthquakes scaled of 7 on the Japanese scale struck Kumamoto Prefecture on April 14 and 16, 2016, and then about the 0.4 m ground subsidence was occurred. A large amount of sediment from the Shirakawa River was discharged into the river delta due to a following-flood. The clinoform along the water-route approximately propagated 150 m in the offshore direction and the bed level increased by about 1.5m. The estimated sediment discharge was 1.1×105m3/yr./km2 during 2014 to 2016, corresponds four times during 1978 to 1997.

INTRODUCTION

Two earthquakes scaled of 7 on the Japanese seismic intensity scale happened in Kumamoto Prefecture on April 14 and 16, 2016. Then, a large-scale slope collapses occurred in the vicinity of Mt.Aso. The unstable region on the slopes extended up to 5.9 km2. Besides, about 0.4 m ground subsidence happened around the mouth of the Shirakawa River. Subsequently, a large amount of sediment from the river was discharged into the delta because of a heavy rain induced flood on June 21, 2016. That has caused a great damage to fishing. Also Mt. Aso erupted on October 8, 2016 and about 5.x106 tons of volcanic ashes have released to the air. Some of the ashes have deposited around the crater with a thickness of about 1.5 m.

Extreme weather events; such as the storm surges, tropical cyclones, and heavy rainfalls, have been estimated to increase in both frequency and magnitude due to climate changes. River-mouth deltas are very sensitive to changes of time-varying driving forces. Therefore, numbers of vulnerably of sites are rising as the result of the potential of sediment supplied by the Kumamoto earthquakes and the volcanic eruption. The quantitative changes of mudflat profiles of rive-mouth delta are important for coastal protection and management. Comparing field data with previous data, the characteristics of the mudflat profiles after the Kumamoto earthquakes could be clearer.

PREVIOUS RESEARCHES ON STUDY SITE

The field site of this study is located on the center of the eastern coast of Ariake Bay in Japan, which is a closed inner bay at the mouth of Shirakawa River as shown in Fig.1. The length, width, and depth of the bay are 97 km, 20 km and 20 m, respectively. The total area of the bay is about 1700 km2.

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