The improvement of the environment in the sea by the creation of tidal flats and shallow waters utilizing dredged sand

Abstract

Extensive tidal flats and shallow waters used to exist in Mikawa Bay. About 17% of them have been lost by land reclamation, etc. It is known that deterioration of water purification ability due to the loss of many tidal flats and shallow waters has caused various environmental problems such as red tide, oxygen-depleted water and so on. In order to make navigation safer, the Ministry of Land, Infrastructure and Transport (MLIT) dredged Nakayama Waterway at the mouth of Mikawa Bay between 1998 and 2004. The MLIT and Aichi Prefecture are jointly created tidal flats and shallow waters by the dredged materials in environmental rehabilitation inside Mikawa Bay. In the districts of created tidal flats and shallow waters, monitoring surveys have been conducted on environmental conditions such as water quality, sediment quality, and benthos. Here, we report on the monitoring results of tidal flat creation utilizing dredged sand in the Mito and Nishiura districts at the innermost part of Mikawa Bay. From the second year after the creation of the tidal flat, bivalves such as Ruditapes philippinarum and Mactra chinensis increased. The recovery of benthos, in particular, is likely to be due to the reduction of oxygen-depleted water as a result of the creation of the shallow site. Improvement of water purification ability is expected due to the increase of bivalve shellfish, which has high water purification ability.