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In coastal areas around big cities such as Tokyo Bay, intertidal flats decreased due to man-made reclamation lands constructions. Such a loss of tidal flats make, water quality environments deteriorated. In the study, we carried out field studies to estimate nutrient fluxes between inter tidal sediment and overlying water and intertidal water and neighbor sea water. In conclusion, we found the following results. (1) Function of this tidal flat is net source of phosphorus and net sink of nitrogen and chlorophyll-a in summer. And function of net source of phosphorus in summer changes to net sink of that in winter. (2)The fluxes of phosphorus and nitrogen from the intertidal sediment are 20% and 87% of the fluxes through the changes between tidal flat and neighbor sea water.

Although massive fish death was mostly observed in tidal rivers and mainly caused by hypoxic water mass, there was little information on DO environments in large tidal rivers. To investigate DO environments in tidal rivers with and without massive fish death, we conducted field measurements on hydrodynamics and water quality environments in the Sumidagawa and Arakawa Rivers. In summer, larger intrusion of sea water was found in Sumidagawa River than Arakawa River mainly due to the difference of river discharges, and then DO concentration in the Sumidagawa River was relatively lower than in the Arakawa River. Also found that the intrusion of hypoxic seawater mass and inflows of CSO due to hydrologic events have an important role on reduction of DO concentration in the Sumidagawa River.

It has been already reported by Nguyen et al.(2008) that at the river entrances facing the Sea of Japan, frequent occurrence of water level rise caused by wave set-up can be observed in storm waves during winter. It is hence expected that saline water intrusion is generated into the tidal estuary due to wave set-up, resulting in more frequent and more severe impact to the estuarine environment. In the present study, the additional amount of the sea water discharge caused by wave set-up is estimated from measured water level variation at the Iwakigawa and Yoneshiro river mouths in the northern part of Japan. Furthermore, theoretical expression is proposed to estimate the additional amount of sea water discharge expressed in terms of the deep water wave height and an empirical coefficient proposed Nguyen et al.(2008).

The tidal variations of groundwater salinity and groundwater discharge/infiltration through the riverbed were observed in a tidal estuary. The process of fresh water intrusion and salinity recovery in the riverbed due to flood were analyzed using observed data in relation to the circulation between hyporheic water and wide area groundwater. The groundwater discharge varies with tide in the order of 10-3cm/s, but the salinity decreases at the surface of the riverbed. The fresh water intrusion to the riverbed during flood depends on groundwater infiltration caused by river water and wide area groundwater, and the fresh water intruded twice deeper than that of tidal intrusion during non-flood period.

In order to investigate transportation of organic mud in the Nakdong River Estuary, deposited and settling organic matter sampled in the Nakdong River Estuary was compared with data in the Hiroshima Bay. And the function of tidal flat was evaluated from continuous observation of ground water level, groundwater temperature, and salinity. After considering these results, it is found that property of organic mud, which is mainly supplied from Nakdong River, is similar to the property of organic mud in the constructed tidal flat in Hiroshima bay (Onomichi Itosaki constructed tidal flat). Additionally the river-mouth bar is formed by sand due to its high permeability.

In this paper, field observations were performed in order to clarify the mechanism of groundwater level rising, and variation of groundwater quality under the influence of tidal cycle. Moreover, laboratory experiments using one dimensional infiltration apparatus were conducted to investigate the behavior of salinity in a tidal flat. Observation results suggested that groundwater level rising is occurred by rising of deeper layer of groundwater when river water level is lower than ground level. From laboratory experiments, it was cleared that separation of pore water in free water and bound water is important to consider the behavior of salinity. Ratio of free water increased with increasing of void ratio and decreasing of infiltration velocity. Salinity of bound water varied with infiltration of salt or fresh water.

Changes in concentrations of organic substances of the sediment and nutrient salts in the pore water of the riverbed were examined at a tidal area of the mouth of the eutrophic Yamato River in 2009-2010. TOC and TN contents of the sediment, together with NH4-N and NO2+NO3-N concentrations in the pore water drastically changed depending on time, month and station. Production rate of NH4-N in the pore water of the Yamato River was ca.10-100 times higher than that of laboratory experiments. These results suggest the importance of tidal change and wave in evaluating dynamics of organic substances and nutrients at the tidal area of the mouth of a river.

Behaviors of organic sediment and groundwater in tidal flats in a bend of a tidal river were investigated by field experiments. The horizontal advection was reduced by the hydraulic resistance of mangroves, and sediment which has large C/N ratio repeats deposition and re-suspension near the mangroves. At the inside of the bend, since fresh fine organic mud deposited on the sandy bed 10 cm thick, the groundwater salinity in the sandy layer was hardly influenced by the tidal variation of stream water salinity. Low salinity water flows down at the inside bend and current velocity decreases at the outside bend due to mangroves and shallow water.