Special Issue:
Sediment Transport and Morphological Changes at the Mouth Delta of Shirakawa River
2019 Volume 33 Issue 1 Pages 4-11
Details
2019 Volume 33 Issue 1 Pages 4-11
Intertidal mudflats are important for land conservation, flooding risk alleviation and biological environment. Mudflat profiles have been measuring monthly along some cross-shore lines exceeding 1,000 m in the vicinity of a river mouth since December 2000. For the mud profile evolution, the annual accretion rate was 4.2 cm/year and 1.6 cm/year on the right and left lines, the seasonal variation in the range -5 cm and 5 cm, and the maximum episodic variation 12.1 cm and 22.0 cm. The wind waves could play many contributions for deposition and erosion of sediment. Sediment budgets based on the monthly bed level and net sediment flux monitoring were estimated to examine the relative contributions of tides and river discharge to sediment transport. The estimated sediment budgets are interpreted using the water mass balance equation and the horizontal tidal current pattern. The intertidal flat accreted during normal discharge conditions are primarily attributable to the alongshore sediment flux toward the river mouth. However, the flat was eroded when the large offshore suspended sediment transport occurred on the flat during the large river discharge. The net alongshore tidal current causes alongshore sediment fluxes toward the river mouth on this intertidal flat adjacent to the river mouth. In observation periods, there were two times of a significant flood occurred on July 12, 2012 with the discharge water rates of 2,300 m3/s and on June 21, 2016 with 1,600 m3/s. Also 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 subside was occurred at the present field site. A large amount of sediment from Shirakawa River was discharged into the delta due to the flood. The clinoform along the water route approximately propagated 150 m in the offshore direction and the bed level increased by 1.5 m. The estimated sediment discharge was 1.1x105 m3/yr./km2 during 2014 to 2016, corresponds four times during 1978 to 1997.
