This paper was carried out to make clear the characteristics of salinity intrusion in the absence of the barrage in Nagara estuary and to discuss the change of the mixing type due to the barrage using the parameter of tidal prism Pt. As a result, the following findings were obtained;
1) The mixing type changed by the seasonal variation of the river discharge.
2) Before construction of the estuary barrage, the mixing type varied from the partially mixed type to the well mixed type in summer and from the stratified type to the partially mixed type in winter.
3) The mixing type changed to the stratified type after construction of the estuary barrage.

View full abstract

A series of three-dimensional numerical simulation for turbulent round buoyant jets normally discharged into crossflow were performed with κ-ε two-equations turbulence model. The Reynolds numbers ranged from 7600 to 12000 and the ratios of jet to crossflow velocity was 3.8, 4 and 6. Comparisons with available experimental data were used to validate the numerical results. The jet entrainment is computed by integrating the volumetric flux over the jet cross-section and the drag force due to the pressure gradient is computed with integrating the pressure along the jet circumference which is defined by an assigned value of tracer concentration. It is shown that the entrainment rate into the jet increases faster in the region near the outfall, reaches the peak value at the short distance, and then decays to the level of the free jet in the far downstream. The drag force acting on the jet is large near the outfall, then quickly becomes neglectable beyond the bent-over point

View full abstract

A Lagrangian model is developed for predicting a turbulent buoyant jet discharged into wave-induced flow environments. Two entrainment coefficients are involved in the model: one describing the shear entrainment identical with that of traditional models for jets into stagnant ambient fluids while the other describing the forced entrainment due to wave-induced velocity assumed as constant herein. Model predictions are compared with experiments of buoyant jets into ambient water with shallow water wave conditions, and the agreement is very good. The present model is also validated for the limited case of zero ambient velocity. It is revealed that the waves may significantly increase the dilution achieved by the ocean outfalls. Taking into account of the wave effects on the jet mixing would make the outfall design easy to satisfy the environmental standard with lower costs than otherwise.

View full abstract

This paper describes the development of MEL2D-cv which combines a three-point combined compact difference scheme and the CIP-CSL2 scheme and the application of the model. A three-point combined compact difference scheme and the HS-MAC scheme were applied to solve momentum equations, and the CIP-CSL2 scheme was used for solving water temperature and salinity in MEL2D-cv. Five other models were compared with MEL2D-cv to verify the accuracy for the case when internal solitary waves occur in a two layer system. The diffusion of the interface due to numerical diffusion was almost the same in the case when the CIP-CSL2 scheme was used for solving density compared with the case when the CIP scheme is used. The predicted spatial shape of an internal solitary wave by MEL2D-cv was better than any other five models from the comparison with theoretical solutions.

View full abstract

Behavior of low salinity water caused by a freshet from Kitakami River to Oppa bay was studied by field measurements and satellite image analysis. In the field measurements, data of salinity and turbidity in the surface mixed layer were collected along many lines continuously by towing a measurement devices unit. The results show a high correlation between the salinity decrement and the turbidity increment transported by the freshet. In the satellite image analysis, Band-2 of Landsat-7 data was firstly normalized by adjusting the values at some reference points so that images of different times could be compared with one another. Then, a regression analysis was made for the normalized data vs turbidity data. Finally, three satellite images were converted to maps of salinity distribution in the bay via turbidity mapping. The series of salinity maps show the behavior of freshet clearly and reasonably.

View full abstract

Field observations were carried out to investigate the effects of suspended sediment from the Ishikari River and bottom material originating from the Ishikari River on water quality in the coastal area of Ishikari Bay during the snowmelt season in 2003. In particular, in terms of suspended sediment and bottom material, the forms of phosphorous were analyzed to determine the release of phosphorous into the seawater. Results showed that during the snowmelt season, adsorbed phosphate from the suspended sediment was released into the seawater near the surface and that the phosphate was not released into the seawater from the bed material according to the vertical distribution of dissolved phosphate in the pore water of the seabed.

View full abstract

In this paper, we report the results of a study of the variation of shear stress and the bottom drag coefficient C_D with sea state and currents at a shallow site in San Francisco Bay. Via field experiments, we find that themodel of Styles & Glenn, though formulated to predict C_D and shear stress under ocean swell on the continental shelf, accurately predicts enhanced drag and mean stress under wind waves in an estuary, albeit only very close tothe bed. Higher up in the water column, the steady wind-driven boundary layer at the free surface overlaps with the steady bottom boundary layer, and this overlap needs to be accounted for to accurately model shear stress.
Applying the enhanced C_D of Styles & Glenn to the estuarine circulation model TRIM-3D of Gross et al and Inagaki et al, we find that enhanced drag under wind waves rctards the flushing of contaminants and sediment in South San Francisco Bay, and it enhances channel-shoal asymmetry.

View full abstract

The aims of this paper are to estimate the residence time of sea water in Tokyo Bay from field observation results and to apply the multi-box vertical one dimensional model (MEL1D-MB) to reproduce the salinity distribution. The field observation was carried out at 27 points inside of Tokyo Bay one or two times per month from July 2002 to June 2003. The residence time of sea water was obtained by using the salinity from the field observations and the estimated fresh water which flows into Tokyo Bay. The residence time is found to be 45-60 days in winter and 15-35 days in summer, respectively. The application of the MEL1D-MB was carried out and the reproduced salinity distribution comparatively agreed with the observational results. The residence time obtained from the MEL1D-MB also agreed with the residence time from the field observations.

View full abstract

The concept of ‘environmental standard’ has been applied to coastal environmental impact assessment, EIA. The standard values have been determined for each water quality and for each divided coastal area. The achievement of improving water qualities depends on whether the values estimated by EIA are larger than the standard values or not. The uncertainly and complexity of the natural system, however, make it difficult for traditional approaches to come up with simple answers. To facilitate this integrative approach the basic concept of fuzzy set theory is introduced. In addition, the integrated evaluation function is also introduced. In the present study, the hypothetical Akashi Strait breakwater project is evaluated to test the usefulness of EIA used as a case study to examine the usefulness of this methodology: various models and data set are assembled and analyzed. Water qualities of COD, total nitrogen and total phosphorus are used to make EIA in Osaka Bay.

View full abstract

The characteristics of wave-induced residual currents and residual hydrodynamic force caused by plural submerged asymmetrical roughness in oscillatory flows were experimentally investigated. Firstly, the mechanism of residual currents induced by wave were discussed on the basis of hydrodynamic force on an asymmetrical roughness in an oscillatory flow tank and velocity around plural asymmetrical roughness in a vertical 2-dimensional wave tank. Secondly, some parameters affecting a wave-induced residual current were discussed. It is found that residual currents are produced by inertia force and drag one due to an asymmetry of roughness in an oscillatory flow. Furthermore, the directions of residual hydrodynamic force and a residual current are reversed in some condition of longitudinal interval of asymmetrical roughness.

View full abstract

When the tsunami ascend the river, it may cause the various disasters along the river, thus many river have equipped the sluice gate for prevention of the tsunami propagating in the river. If the sluice gate can not close completely, the tsunami can transmit upper side of the gate. In this paper, laboratory experiments were conducted to investigate the characteristics of the transmitting tsunami on the sluice gate. The experimental results are showed that the wave height of transmitting tsunami is governed by the opening height of the gate, is not mostly influenced by discharge. In case of opening height is low, the strong eddies generate on the part of out flow from the gate. And the eddies cause to decrease the wave height of transmitting tsunami. Furthermore, a numerical model considering the eddy loss was developed, the governing equations applied nonlinear dispersive wave theory. The computation results were compared with experiments results and the results agree with experimental results, the validity of the model was proved.

View full abstract

This paper deals with the estimation of wave climate conditions at the entrance of Ise Bay which have not been extensively investigated. First, based on the statistical analyses of wind and wave data measured over several years at 3 buoy stations, climatic conditions of winds and waves at the entrance area are discussed. Next, a long term wave hindcast system on a nested grid with high space resolution of 1 km is applied location by location under 20-year ECMWF and 51-year NCEP/NCAR surface wind data conditions. Findings are that the system reproduces well the wave climate condition evaluated from the measurement data at each of the 3 stations and that any wave height climate parameter does not indicate an increasing or decreasing trends with statistical significance over the recent 41-year period.

View full abstract

Water quality in a vertical section along Takesaki-Ohmuta line of Ariake Sea was observed under diverse vertical density distributions in the summer season of 2003. A high turbidity layer is formed near the bottom during the ebb tide period. The density stratification suppresses the mixing between the upper layer and the lower one. Both the generation of the high turbidity layer and the density stratification induce the rapid consumption of dissolved oxygen over the whole of the lower layer. Fluxes of Chl.a and SS take large values in the western area of Ariake Sea even under non-stratification field. When the stratification due to the freshwater is formed, primary production in the west area of Ariake sea become larger then that in the east area, and the difference of the between the fluxes west and east areas may become more apparent.

View full abstract

The Ariake Project II was operated in July 2003 to grasp the whole features of water movement and water quality change in the Ariake Sea. This study reports the results obtained from a part of the project. Especially, the features at an innermost part of the Ariake Sea are described in detail. Observation along vertical measure lines at fixed points and a long-term observation at observation tower are combined to consider the mechanism of water movement and water quality fluctuation. As a result, the project can be effective to study the summer feature of the Ariake Sea with meteorological conditions, such as rain, wind, and other watershed conditions.

View full abstract

In order to reveal the formation processes of high turbidity layer in the stratification period of Ariake Sea, field observations were made at a fixed position in the western area for 13 hours. A strong twolayered stratification had been formed during the observation period. The high turbidity layer which is generated by the strong tidal currents at the flood and ebb tides is in about 5 m height from the sea bottom. At the low tide period, the turbidity layer with relatively high concentration is formed over the whole depth of the lower layer. On the other hand, the concentration becomes lower at the high tide period, though the turbidity layer spreads over the lower layer. The shear layer thickness of tidal currents coincides with the high turbidity layer thickness. When the current speed near sea bottom becomes more than 25 cm/s, the turbidity increases rapidly.

View full abstract

The concentrations of chlorophyll-a and the tidal current velocities have been measured in the head of Hakata Bay over the one tidal period under the phase that the times of the high tide and the meridian passage are about 12 o'clock. It is hyper-eutrophic sea area enclosed by the man-made island and faces to Wajiro intertidal sand-flat. The concentration of Chl.a on the tidal flat increases in the onshore direction during the high tide because of the production of phytoplanktons and the suspension of benthic microalgal, and decreases in the same direction during the ebb tide because of their settlement. The spatial distributions of the Chl. a-concentration in the sea area have been compared in the flood tide and ebb tide periods. The primary production increases uniformly the Chl.a-concentration over the whole sea area, but the difference at the edge of the tidal flat is very small. The obtained results suggest that a large amount of phytoplanktons settle down on the tidal flat or taken by benthos.

View full abstract

Removal properties of inorganic nitrogen compounds in the water using a tidal bottom-mud, or muddy sediment, were investigated on the basis of experimental data. The nitrogen removal rate of muddy sediment was the highest of three type sediments: sandy, mud-sandy and muddy sediments. The nitrogen removal rate of sediment was largely dependent upon the denitrification activity near the RPD (Redox Potential Discontinuity) layer of sediment. However, the nitrogen removal rate decreased rapidly as the RPD layer approached to the anaerobic condition owing to the death of macro-benthos in the sediment. The relation between nitrogen removal rate, substrate (NH₄⁺) concentration and temperature was almost expressed by an equation (2) which was obtained from the rate of ferment reaction.

View full abstract

A field survey has been performed both in the estuary of the Banjo river and in the Saeki bay in order to investigate the effect of water quality on the clam habitat in tidal flat. The result of water quality measurements shows that dissolved oxygen depletes due to fine sediment in the bottom of the Banjo river. Dissolved oxygen profiles near the sediment/water interface on the tidal flat are simulated based on the experimental results of the oxygen consumption rate by the sediment. The dissolved oxygen concentration profile varies considerably near the sediment/water interface and rapidly becomes zero just below the interface regardless of the shear velocity, because all DO that reach the sediment surface are utilized instantly. There is not enough oxygen both at the sediment surface and inside the sediment. A tidal flat that has biologically active sediment accumulating is therefore an inappropriate place for the clam habitat.

View full abstract

Nearshore circulation induced by wave breaking under the low and mean tide conditions around the tidal flat was investigated by the calculation of waves and nearshore currents, taking the tidal flat in Moriye Bay in Oita Prefecture as the example. The development of longshore currents toward thebay bottom was confirmed by accretion and erosion condition on the beach in the short-term and the formation of sand spit in the very shallow water surrounding the tidal flat in the long-term. It is concluded that construction of artificial structures such as port breakwaters remarkably obstructed continuous water circulation in the bay, resulting in the retardation of diffusion of fluvial sediment on the tidal flat.

View full abstract

Distribution and wetland utilization of shorebirds were observed at wetland located on the North coast of Tokyo bay in winter 2003. Dunlin and Kentish plover were dominant species appeared during the observation. Number of these shorebirds was three to four times larger during ebb tide period than those during flood tide period. Ebb tide period supplies adequate feeding area for shorebirds, and patchy distribution of shorebirds has strong relation with micro-topography of wetland. This phenomenon is attributed to feeding behavior and strategy of shorebirds. Ebb area and micro-topography are indispensable factors for constructing wetland as a bird sanctuary.

View full abstract

Genetic algorithm, GA, is one of the well-known methods to solve global optimization problems and its use is becoming widespread in many branches of engineering. In this study, this optimization method, GA, was made use in order to calibrate the numerical simulation model, and its applicability to this kind of problem was discussed. The results obtained from calibrating the water quality simulation model show that the error defined by calculated and observed results gradually decreases as the proceed of alternation of generation, and GA succeeded to find the parameter set which makes the error much smaller than by other optimization method. Although some trials executed in this study thought the necessities of several implementations for the practical use of GA, it might be concluded that this method has some extent of applicability to the automatic calibration of numerical simulation model.

View full abstract

This paper describe about technique to retrieve chlorophyll a concentration by using field spectroradiometer data with fluorometer data and the Sea-viewing Wide Field-of-view Sensor (Sea WiFS) data. Spectroradiometer data were coupled with fluorometer data to find out the best suited bands ratio to monitor the chlorophyll a concentration for inland water. Remote sensing reflectance measurements were used to evaluate the performance of several default ocean color chlorophyll algorithms for Sea WiFS data. Using the default chlorophyll algorithms used in TeraScan System (OC2-V2) and SeaDAS (OC4-V4) as the basis of comparison; this study examined the validity of these empirical chlorophyll algorithms with in situ data. Remote sensing reflectance data from spectroradiometer and in situ chlorophyll a data from fluorometer were collected for Lake Abashiri, and SeaWiFS data for Lake Saroma were used. It is shown that the chlorophyll a concentration from fluorometer and reflectance from spectroradiometer lies in exploiting the signal provided by the chlorophyll a red absorption peak near 670nm. Two-band ratio based on a ratio of reflectance 670 and 700nm provided a good correlation for a linear model, compared with blue-green two band ratio.

View full abstract

By the field observation on the water quality in Kawahara Lake, it was found that the phytoplankton biomass, of which dominant species was Gonyostomum semen (G.semen), grew and aggregated densely in the middle layer in summer season. The diel migration of aggregated layer of G.semen was observed in the field. In the daytime G.semen aggregated at the depth of about 5m and in the nighttime about 6.5m. Then, the laboratory experiment by using the microcosm was conducted to make clear characteristics on diel vertical migration of G.semen. As the results of the microcosm experiment, it was found that G.semen moved downward for the nutrient salts at nighttime, and G.semen stopped the downward migration when reached the anaerobic layer in strong reduction. Also, the upward migration of G.semen was influenced by pH and the photon flux density. In addition, there is a correlation in pH and the photon flux density in the top of G.semen's bloom layer. It was found that the optimum photon flux density for G.semen decreased with the rise of pH.

View full abstract

A field observation was curried out in an irrigation pond with a floating-leaved plant (Nymphoides indica) community to make clear the effects of aquatic plants on the ecosystem and water quality. In the observation, the appearance of the plant community and the values of various water quality items were monitored from fall to winter in 2002. Although a lot of nutrients (N, P) discharged from the plant bodies into the pond water due to withering, their concentrations quickly decreased after the temporary increases in the plant community and then phytoplankton cannot greatly multiply. Concerning phosphorus, in particular, this fact resulted from that the recycled phosphorus rapidly fell down as detritus and also being adsorbed by suspended solids after dissolution. In conclusion, it was shown that the recycle of nutrients due to withering of plant bodies scarcely gives negative influence on the ecosystem and water quality in an irrigation pond.

View full abstract

The characteristics of bottom sediments are most important that factor of water quality in epilimnion of summer. This study of aims at to make clear bottom sediments based on flow process from heavy metals and nutrient salts. Heavy metals and nutrient salts of bottom sediments were investigated based on quantity and qualitative analysis in Lake Hibara. The characteristics of geographical distribution of heavy metals and nutrient salts were shown in bottom of this lake. Transportation of heavy metals and the process of generating were shown from the relation between particle diameter and nutrition salt.

View full abstract

A model of sedimentary oxygen demand (SOD) for a sediment bed for finite length is presented. The model describes the effect of developing a diffusive boundary layer on oxygen transfer just above the sediment/water interface and on microbial oxygen uptake inside the sediment. Variations of biomass profile inside the sediment, dissolved oxygen concentration profile near the sediment/water interface, and SOD in accordance with the developingd iffusiveb oundaryl ayer are investigatedn umerically.

View full abstract

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.

View full abstract

This paper presents a development of a numerical model that simulates flood water intrusion into a stratified reservoir and fine suspended sediment behavior. The model is based on laterally averaged vertical 2-dimensional equations and κ-ε turbulence model, which are solved by using SIMPLE scheme.
A laboratory experiment was conducted on settlement of fine suspended sediment and a tentative modeling is proposed to estimate settling velocity of fine solid. The model is validated by simulating the flood event on which the field measurement was conducted. Vertical distributions of suspended sediment concentration are well reproduced and effectiveness of the proposed model is demonstrated.

View full abstract

The behavior of fine suspended load in the reservoir was analyzed by using numerical simulation model. In this study, a vertical 2-dimensional reservoir hydraulic simulation model was employed that includes the effect of buoyancy caused by water temperature and suspended sediment. This is because density current in the reservoir caused by suspension of sediment typically shows a flow pattern in longitudinal and vertical directions. Validation of the model was tried from the view point of full year variation of sediment behavior and trap efficiency of fine sediment during each flood event. The effect of the redevelopment project being carried out was also estimated by using the model.

View full abstract

Flow characteristics of plunge flows in well-mixed and stratified reservoirs are investigated experimentally. The study is directed towards the estimation of the densimetric Froude number F_p at the plunge point and the initial mixing rate γ. The internal critical slope I_c is found to be about 1/50. It is shown that the values of F_p in well-mixed reservoirs are strongly affected by the inflow densimetric Froude number F_o and the channel slope I when plunging takes place near the inlet of a reservoir, whereas Fp becomes a constant value of 0.5 in both mild and steep channels when these effects are less pronounced, and that for the range of I=1/150-1/30 is quantifed as a linear function of I. It is also found that the values of F_p in well-mixed reservoirs with a steep channel are also well approximated as 0.5 when F_o is more than about 2.0.

View full abstract

Experimental and numerical investigations were conducted in order to understand the windinduced flow and mixing in a closed stratified water body. The two-layer density model was provided in a two-dimensional wind flume. Measurements of conductivity at several vertical points were performed, and the result of the density measurements was compared with the luminance value obtained by the image processing technique. The distributions of luminance were comparable to that of conductivity. The PIV method was also used to measure the two-dimensional velocity field near the density interface. The velocity field by the PIV was compared with the numerical results by the κ-ε model with the TVD-MUSCL scheme. It was found that the major feature of the velocity field could be reproduced qualitatively by the present numerical model.

View full abstract

Accurate predictions of inflows to dam reservoirs are the most important information for operations and managements. The water levels in the reservoirs are significantly influenced by wind and seiche resulting from gate operations and hence the computed inflow volumes are highly fluctuated with relatively large errors.
The characteristics of fluctuations in water levels of reservoirs managed by Regional Hokkaido Development Bureau are practically clarified in this paper. The present study proposes a new smoothing method of fluctuated water levels in dam reservoirs, using Fourier analysis. The proposed method can easily be applied to the practical situations where the inflows are estimated with high accuracy.

View full abstract

In this study, we introduce the new sediment extractor pipe named “Multi-Hole Suction Sand Pipe” which has several suction holes arrayed at the bottom of the pipe in the same interval. The hydrodynamics in this pipe is expressed by the governing equations which are added to a pressure loss term derived from the momentum diffusion of the sediment inlet at the suction holes. It is verified that the results of the numerical analysis based on such equations agree well with those obtained by preliminary experiments for a water flow without sediments. Applying this pipe to sediments extraction, the blockade by a sediment discharge in the pipe is caused in some cases from the shortage of the suction velocity at the hole or velocity in the pipe. Considering the results of numerical analysis and sediments extraction experiments, the latter one is significant and the effective support systems are proposed for the evasion of the blockage of a sediment discharge.

View full abstract

The field observation conducted in the summer of 2000 at Lake Koyama showed that there were some types of internal waves. In this study we simulated those data using ELCOM which is a three-dimensional hydrodynamics model developed by CWR, the University of Western Australia. The results of simulations showed that there were three types of internal waves. The first one is an internal seiche generated after the stop of a sea breeze, the second one is an internal seiche after the stop of a land breeze and the final one is an internal Kelvin wave after the stop of a sea breeze. It is also cleared that in the water body of the lake these waves interacts each other and this causes the complicated distribution of water temperature and flow structures under the condition of land and sea breeze.

View full abstract

Saline water flows into Lake Shinji from the Ohashi River, and the Ohashi River connects Lake Shinji with Lake Nakaumi. Such saline water impacts on the flow and water quality of Lake Shinji. This study focuses on the behavior and characteristics of the saline water advancing in Lake Shinji. For this purpose, it is necessary to understand the physical conditions controlling the flow of the Ohashi River. This study was based on the field observation performed under large and well organized plans. The field observation clarifies that dynamic behavior of the saline water is affected by the velocity and the direction of the wind, the difference of the water levels between Lake Shinji and Nakaumi, and distribution of atmospheric pressure.

View full abstract

Nagatsura-ura Lagoon is located at the southern end of Sanriku Coast, Japan with the area of 1.41km² and the maximum water depth of 10m. The lagoon area has been used for cultivating oyster for these several decades. In the recent years, however, the appearance of anoxic water causes severe damage to the fishery industry in this area.
In the present study, filed observations of water quality, such as water temperature, salinity, dissolved oxygen (DO) concentration, and phytoplankton were carried out every one week in Nagatsura-ura Lagoon to investigation detailed process of occurrence of anoxic water and its decline. During the course of continuous measurement for 6 months from May to November 2002, abrupt recovery of DO can be measured. Based on the analysis of phytoplankton, it is concluded that the recovery was induced by water exchange between the lagoon and the sea through a narrow channel connecting these two.

View full abstract

The area of this study, Lake Utonai, is in eastern Tomakomai, Hokkaido, Japan. In recent years, the lake has been experiencing the ecological eutrophication resulted from nutrient salts and fine sediments from the rivers, which will damage the ecosystem. The lake water is affected by sea-land wind from costal regions, and by groundwater flow, due to the lake's shallow depth. This study proposed a water quality model and a water flow model for the shallow lake, considering effects of groundwater flow and sea-land wind.

View full abstract

The concentration of nutrients such as phosphorus and nitrogen increases in July to August in Watarase reservoir. This increase is caused by the release of nutrients from bottom sediments. Physical disturbance on bottom sediments is seemed to have significant role in the nutrient release. Physical disturbance on bottom sediments is mainly driven by wind waves in shallow lakes and reservoirs. Since such events occur intermittently, it is necessary to monitor continuously to detect the events. We have conducted several monitoring including current, temperature profiles, meteorology and water quality measurement in Watarase reservoir to detect the record of physical disturbances on bottom sediments and its driving force. Due to its shallowness, the reservoir is warmed to the bottom sediments in summer. The temperature of surface layer of bottom sediments exceeded 30°C. Density current penetrated to the bottom layer during river inflow in summer since the temperature of river water is cooler than that of reservoir water. Monitored records showed that this density current seems to have great disturbance on bottom sediments. Cooler and denser river inflow to such shallow reservoir should be paid more attention in analysis of material flow between water column and bottom sediments. Based on field survey and numerical simulation results, we discussed such event and its effect on water quality of a shallow reservoir.

View full abstract

Pollutants runoff with rain water is a significant source of nutrients to lakes. Especially particulate matters play important roles to transport carbon, nitrogen and phosphorus. It is important but difficult to investigate NPS loading comprehensively from generation of pollutants to arrival at a lake. Alternatively in this study, surface soil of catchments was examined as a potential source of particulate pollutants. Soil was classified by particle size. Size distribution and carbon, nitrogen and phosphate contents were examined. Phosphate bioavailability was also examined. From point of view of bioavailability, soils from dry field has rapid and slow effects. While soils from paddy field had almost same slow effect as those of dry field, they showed little rapid effects.

View full abstract

The Barato River in northern Sapporo was formed from a cutoff meander of the Ishikari River. Since the river is in an urban area, it receives urban wastewater. As a largely enclosed water body withlittle exchange of water but with inflow load from the surrounding area, the Barato has undergone eutrophication, as evidenced by water bloom. We conducted a comprehensive study on the hydrological and material cycles of the Barato River basin. We clarified some causes of the water quality problem of the Barato River. One cause is rainfall runoff from the sewer system. Another is interaction between water and bottom deposits. The Barato River adjoins a backwater section of the Ishikari River, where flow alternates with backflow. Such a hydrological mechanism is considered to significantly affect the water and heat balances of the Barato River. In an effort to solve the eutrophication problem, we are conducting observations and simulations to identify additional causes, toward suggesting an effective water quality improvement method.

View full abstract

It is very important to realize a sound and sustainable water environment at urban areas. In order to attain a desirable water environment, sewage system has aimed to be accomplished, but its final goal cannot easily be obtained for lots of reasons. One of the reasons is, of course, due to the non-point pollutantsources, i.e. the diffusive pollution. Others are due to the careless behavior confusing the wastewater with stormwater, and also to the detachment of sediments accumulated in a sewer system.
In this paper, pollutant runoff mechanism at urban area completely facilitated the so-called sewage system has been investigated, through not only the field observation but also the predictive computation. Estimating the detachment rates of T-N by the equation k=a×R^b×L^c, temporal variation of its concentration can be predicted by a mathematical computation, fairly well. Understanding the pollutant runoff mechanism, effectivemanagement can be executed as a strategy for a desirable water environment.

View full abstract

One of non-point sources in the watershed of urban rivers is a river-bed deposit which may easily suspend as a river-flow velocity increases, referred to here as an easily suspended deposit (ESD). To evaluate ESD environments in an urban river, we conducted field measurements in Oohori River with a new device which can easily measure a pick-up rate of the ESD under various flow conditions. The observed data shows that the temporal and spatial variations of the ESD environments are caused due to hydrologic events and changes of flow rate from North-Chiba water conveyance channel. When the pick-up rate P_k is modeled as a power law dependence on bottom stress τ_b, P_k=ατ_bη, the measured results suggest that the coefficients, α and η, also vary appreciablyin time and space.

View full abstract

The water pollution in rivers and lakes is not adequately improved, though much pollutant load from point sources has been decreased. The pollutant runoff from non-point sources in wet weather condition brings relatively more impact to the water pollution than point sources in recent years. Investigation and analysis of pollutant runoff have been carried out during recent years, but amount of runoff pollutant is not adequately evaluated. Especially, the increase of pollutant load from non-point sources in an urban area strongly influences urban water environment.
In this study, a residential area is especially focused on among urban area, and investigation of storm and pollutant runoff was carried out in several rainfall events. And deposition and sedimentation on road surfaces, roof surfaces and drainage channel beds were examined at several sampling points during a few weeks. Investigation shows that the first flush of pollutant runoff consists of particle pollutant materials. Particle pollutant runoff was mainly derived from impervious area, for example roof and road surface, and dissolved pollutant runoff was derived from sedimentation in a drainage channel bed. On-site pollutant reduction measures were estimated by the storm and pollutant runoff model prepared by using of investigation data. Simulation showed that 2mm rainfall storage from roof runoff brings 30% reduction of maximum pollutant runoff rate during one rainfall event.

View full abstract

To clarify the cause of pollutant loads in Oohori river, one of the main influent rivers into Lake Teganuma, we have attempted to perform a GIS analysis on pollutant loads in Oohori river. The comparison between the calculalted and observed pollutant loads for COD, T-N and T-P indicates that the calculated results give acceptable agreements with observed results. We also compare the calcuated pollutant loads in 1991 and 2002, showing that the contribution of the domestic waste water appreciably decreases with the increase of extension rate of sewerage and then the industrial waste water and non-point source are relatively dominat. The spatial distribution of the point and non-point sources in the watershed is examined with the GIS analysis.

View full abstract

Seasonal Concentrations of BOD, NH₄⁺-N and interfacial active agent derived from domestic sewage were high in winter and low in summer. The concentrations decreased with an increase in water temperature. Generally, river water contains organic compounds and Cl^- derived from domestic sewage and Cl^- was quite stable in rivers. As the ratios of BOD/Cl^- and NH₄⁺-N/Cl^- decreased down the stream and changed seasonally, organic compounds were thought to be decomposed during flow. NO₃^--N/Cl^- which organic compounds were finally decomposed into, also decreased down the stream and were high in winter and low in summer. NO₃^--N was thought to be used for photosynthesis because pH, DO values and chlorophyll concentration of the Yamato river increased with temperature and decrease of NO₃^--N concentration.

View full abstract

This paper evaluated the effects of land use on river water quality in the Kishi and Zakuro Rivers. Both basins are mainly covered by forest and orchards, and especially about 30% of the Zakuro River watershed is orchard. In the field observation, the concentrations of NO₃^- and Mg²⁺ in the Zakuro River were observed to be higher than those in the Kishi River from autumn to spring. However, there were no differences in the concentrations of the other ions and COD between both rivers. In order to examine these results, especially the effects on the concentrations of NO₃^- and COD, nitrogen and COD loadings were calculated using pollutant load factors. The calculation result of nitrogen loading implied that the discharge of nitrogen from the orchards played the most important role in the change in the NO₃^- concentration of the Zakuro River. In the meantime, there were no differences in COD loadings between both basins and therefore the COD concentrations would be approximately equal.

View full abstract

A COD value and COD load calculation method was determined using the intensive measurement results of COD values and flow rates in the Kinokawa river. For 4 years, hourly measurements lead COD values changed with flow rate. The relation between flow rates and COD values was analyzed. Using this relation, past COD values and COD loads could easily be calculated from flow rate. The used data has no data of high flow rate condition because sampling is difficult at the condition of high flow rate. With the exclusion of high flow rate (more than 1400m³/s), the calculation results were in good agreement with actual measurements. The estimated past COD loads of the Kinokawa River were very changeable and maximum COD loads reached 8 times that of minimum values for past 29 years.

View full abstract

Nitrogen and phosphorus have been treated as important substances related to the eutrophication and the water quality such as red tide. However, silicon is one of substance to compose an organic matter (Redfield ratio; C:N:P:Si=106:16:1:16). Recently, it was reported that Si variation causes the change of the species composition of phytoplankton in estuary. This phenomenon effects on the ecosystem balance. One of the reasons that the Si supply from river to sea decreases is assumed to dam constructions. However, the mechanism of Si decrease in river is not clarified due to few observation data of Si in river basin. In this study, we observed the temporal and spatial variations of DSi (Dissolved Silicate) concentration of surface waters (stream, pond, spring, river) from 2000 to 2003 in the Kinokawa river basin. From this result, we can see that DSi of pond water varies seasonally. On the contrary, DSi of spring water and river water are almost constant all through the year. Average value of DSi of river water is lower than that of other waters. Moreover, as compared our data with the DSi concentration of river water observed in 1957 (before dam constructions), we find that DSi concentration decreased.

View full abstract

For understanding pollutant transports in an urban river, the field measurements of the hydrodynamic environments and SS during one month were carried out in the estuarine section of Oohori River and Lake Teganuma. We also conducted the intensive field survey of the sediment, nutrient and organic matter transports in Oohori River under a rainy condition. The observed results show that the differences of SS fluxes between Stns. 1 and 3, located at upstream and downstream points in the estuarine section, appear appreciably in hydrologic events and depends on the erosion process of river-bed deposits and flow rate in the river. The differences of nutrient and organic matter fluxes between Stns. 1 and 3 in the hydrologic event are also characterized with the erosion process.

View full abstract

In some lakes and estuaries, yearly maximum chlorophyll a (Chl. a) concentration was observed in winter, however, the mechanism of such blooms are poorly understood. In this study, dynamics of Chl. a of the Kiso and Nagara River estuaries was investigated, by using long-term monitoring data set of water quality during 1995-1998. The chloronity (Cl concentration) was found to be a primary component to govern the temporal variation of the Chl.a concentration in the surface layer of the two estuaries. A mathematical model was constructed to analyze the phytoplankton dynamics as a function of Cl, light intensity, nutrient (DIN), and water temperature. The model calculations satisfactorily reproduced not only year to year variations in the Kiso River estuary, but also spatial difference between the two observational sites located in the Nagara River estuary, in case that nutrient limitation was incorporated.

View full abstract