FIELD STUDY ON BRACKISH WATER CONDITION IN AN ARTIFICIAL WETLAND LOCATED ON THE LOW WATER CHANNEL BANK OF THE LOWER ARAKAWA RIVER, TOKYO

Abstract

Variation of brackish water condition in an artificial tidal flat which is separated by a permeable dike with narrow openings from the low water channel of Arakawa River flowing in the Tokyo Metropolis was studied by field measurements and numerical flow simulation. Water level and salinity fluctuations due to the tide action were measure for about three months using water gauges and salinity meters installed in the low water channel and in the tidal flat respectively. In addition, water salinity contained in the surface soil of tidal flat was measured four times with one-month intervals after a flood of fresh water. A numerical flow simulation adopting the shallow water model well reproduced the salinity variation in the tidal flat. A model of salinity exchange between surface water and soil water suggested that the soil salinity in the tidal flat recovers less than one month after an impact of freshwater flood. The above results suggest that a stable brackish ecotone can be created in an artificial tidal flat using the numerical model as a design tool.