Abstract
Constructing management strategy of newly-formed salt marsh due to the Great East Japan Earthquake and following tsunami attack (March 2011) is one of major concerns for urgent recovery in the Tohoku coastal region. It is important to assess an initial situation, formation and transition process of salt marsh, and its change process since the earthquake. We examined past and present environment of salt marsh using aerial photographs from 1947 to 2015, and estimated an elevational change in the marsh bed from 2012 to 2015 by topographical survey and using officially published data.
Newly-formed salt marsh had been a part of natural wetland, which was reclaimed and used as a residential and agricultural area before the earthquake. A main factor of salt marsh formation was a great subsidence, that allowed inflow of sea water from the Moune Bay. It has been estimated that an elevation of the salt marsh bed rised about 180 mm from 2012 to 2015, and the crustal uplift contributed 72 % of the total rise. If the rate of rise is stable in the future, the salt marsh will disappear in 2034. Temporal variation of water level was strongly affected by sea tide. Salinity was relatively stable and near to sea water even during a small rainfall event, while it was strongly diluted by overflowing river water caused by the heavy rain. These results indicate that the newly-formed salt marsh lacks the stability of topography and water quality.
