Clarification of the properties of fluctuation of bottom sediment based on long-term monitoring around the mouth of the Tama River in Tokyo Bay

With the focus on the characteristics of temporal and spatial fluctuations of the bottom-sediment properties and their vertical distributions around the mouth of the Tama River in Tokyo Bay, the properties of surface sediments, such as grain-size distributions and water contents, had been monitored by taking bottom-sediment core samples from several sampling stations around the river mouth with the intervals of 3 month for a period of five years from 2006 through 2010. Based on the monitored data, the impact of flood events which occurred during the survey period on bottom sediment properties and the recovery processes after the flood event were evaluated. This study clarified that seasonal fluctuations of surface bottom sediment qualities clearly appear on river mouth shoals and tidal wetlands and the impact of the river flood was relatively slight and the damages were restored over relatively short period of time after the flood event. On the other hand, the flood event remarkably caused rapid decline of the water content of bottom sediments at offshore of the river mouth, and it took about three years for their bottom sediments to return to its normal as before the flood event. This paper also discusses the recovery process of the sediment properties at the offshore region and shows that the long-term evolution of the sediment properties was caused by the constant settling of new deposition of suspended particles from the water column and accumulation rate was estimated at 2.5 cm per year.