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Numerous records on river works and topographic surveys conducted in the Chikugogawa river were collected and digitized, and the annual sand quantity removed from the river channel and the degradation volume during the last 50 years were analyzed. The river bed degradation was remarkable during the 1960s; however, the influences of river works were not understood because records on gravel quarrying for that period do not exist. The 1D numerical model for river bed evolution was applied to the channel along a 64 km length, and the simulation reproduced the surveyed topography. The model determined the volume of gravel quarrying during the 1960s. The simulation showed that the sediment load decreases every year and gravel quarrying has a significant impact on this load reduction.

We have conducted field observations for turbid water behavior during flood in the Kawaji dam reservoir in the summer and autumn seasons of 2002. The detailed data sets of water-temperature, turbidity, suspended solid, flow velocity, and grain-size distribution were obtained in the reservoir and its inflow river during flood. The observed data in the curved part of the reservoir indicated that we should take account of three-dimensional flow for more accurate estimation of the turbid water behavior. Furthermore, we also conducted numerical simulation of the behavior in the reservoir using 2D model. The simulated results indicated that the grain-size distribution of inflow water can largely change during flood in the reservoir.

In the Tone River Estuary, turbidity maximum (TM) is formed in the reach of 5km below the estuary barrage. The TM region is covered by fine sediment having the mean diameter of 10μm. The sediment repeats resuspension and deposition following the variation of flow field. Time variation of SS distribution in vertical showed almost the same tendency at many points in the TM region. This fact means that the measurement and analysis in one-dimension in vertical can be applied to investigate the sediment behavior in the region. Sediment pick-up rate obtained from the one-dimensional analysis on the budget of suspended sediment was found to have a high linear correlation with the shear velocity obtained by the log-low fitting for measured velocity profile. The critical shear velocity for resuspension was shown to be a small value of 1.8×10-3m/s, which was the same order as the value of ‘non-cohesive’ sediment. It would be caused by the surface bed sediment having a high water content due to the repeating motion of resuspension and deposition.

Field measurements and laboratory tests were conducted in order to investigate resuspension of the bottom sediment and its oxygen consumption (OC) in the Tone River estuary.Vertical profiles of salinity and turbidity were measured along the deepest line of the channel. And turbidity and flow velocity near the river bed of the salt wedge front were also measured continuously during about one year. The measurements showed that turbidity concentration near the bed increased when adverse flow was a maximum during a flood tide, and that a turbidity maximum occurred rapidly in the front part of the saline wedge during a flood tide.The laboratory tests of OC were conducted by using the bottom sediments which was mixed with the river water and suspended by stirrer. As a result, it was estimated that SS of 70 me was nearly equal to the sediment for OC.

Sagami River is one of major rivers in Japan and river water has been utilized over the last few decades, so the flow rate passing river mouth had been decreasing considerably. Therefore, there is a great concern that the water quality becomes worse and bad effect on the ecosystem is caused.In this study, numerical simulation was done according to the two-dimensional k-e turbulent model with the inflow being applied in the cross-sectional direction. In general, results agreed well with the measurement results, namely, for transitions between weak and moderate mixed type were in good agreement. Next, the numerical experiment was done by varying the upstream flow rate. As a result, it was shown that the salt water tended to go further in the downstream when the flow rate was increased, and the exchange of the water was promoted.

In this study, field observation was carried out in the Old Kitakami River. As a result, most of the mixture form was of weakness mixing, and it was shown that the saline wedge was almost stopped at the riffle at the 6km site. The result of the bottom sediment survey, proved that the property of the bottom sediment is different in the upstream and downstream of the riffle. Next, numerical simulation was done according to the two-dimensional k-e turbulent model. Simulation result of the saline wedge stopping at the riffle was reproduced in good agreement with the observation result Finally, by changing the flow rates, it was shown in the simulation that features of the saline wedge differed in the upstream and downstream of the riffle. These results suggest that the saline wedge is frequently stopped by the riffle in ordinary water discharge, and then, mound is formed by suspended solid settling on the top of the saline wedge.

A ground water flow model is developed with a special attention to a boundary between the unsaturated region and saturated region, and the combination of the each formula of the 1-dimensional motion, The vertical flow in the unsaturated region and the horizontal flow in the saturated region. The proposed model is apllied to the analysis of the seepage flow into the river embankment and it is verified by the result of the experiment on a large scale model.Finally, the horizontal seepage flow in unsaturated region is estimated based on the distribution of the suction simulated by the proposal model. The horizontal seepage flux in the unsaturated region is small enough to omit against that in the saturated region.

Field measurement and numerical simulation were carried out in order to investigate development and movement of the low oxygen water mass in the Tone River below the estuary barrage. The measurement data shows that low oxygen water frequently generated in the front part of salt wedge, and then developed downstream in the salt wedge gradually.Next, in order to simulate the characteristic behavior of the low oxygen water, the standard k-e turbulence model is adopted in the numerical model of salt intrusion. Furthermore in addition to the k-e model, DO-transport equation and BOD-transport equation are introduced for the purpose of simulating the characteristic behavior.Comparing the calculation results with the measurement data, the characteristic behavior are reappeared well.

Field observations were carried out in order to investigate the generation and the movement of anaerobic water in the Tone River below the Tone estuary barrage. The obtained data shows that anaerobic water frequently appeared in the front part of salt wedge, and part of the anaerobic water sometimes flows down along the interface between the salt and the fresh water. The sediment oxgen demand (SOD) coefficient estimated from the variation of DO concentration contained in the salt wedge agreed fairly well with the SOD coefficient obtained by a laboratory experiment.

The spatiotemporal distribution of seawater intrusion in the Chikugogawa estuary was analyzed, and its relationship with the temporal variation of phytoplankton and engraulid fish, Coilia nasus, was discussed. The Chikugogawa estuary is vertically well mixed for most of the year, and a salt wedge is observed only when the tidal range decreases to 2 m or less. We found that the ratio of chlorophyll-a to pheophytin-a during the semilunar cycle varied according to the change in the mixing conditions in the estuary. Further, the number of sampled estuarine fish was related to the salinity and tidal range within the estuary; the fish was caught when the salinity was low and the tidal range was large. It is necessary to regulate the fresh water discharge in the estuary in order to maintain a low-salinity region that is suitable for the migration of fish and their spawning areas to the downstream of the river mouth barrage.