Numerical simulation of liquid surface vibration in a rectangular tank using a refined particle method is introduced in this study. The original MPS method has not solved the problem to decrease energy dissipation. The authors simulate the forced vibration and obtain a good result comparing the simulation result with the result of an experiment and a finite element method. In the simulation of free vibration, the water level agrees well to a nonlinear analytical solution. The study suggests that the precise method of the gradient operator is useful to reduce the energy dissipation of the original MPS.

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A new finite volume method has been presented to solve the quasi 3-D flow model on unstructured grid. In addition to the volume integrated average for each mesh cell, the point values defined on cell boundary are also treated as the model variables. The volume integrated average is updated by a finite volume formulation, while the point value is computed by a point-wise Riemann solver. The cell-wise local interpolation reconstruction is built based on both the moments, which results in a scheme of almost third order accuracy. We do a numerical test of channel flow in continuous bends and validate our numerical model by comparison with the observed data and the shallow water model. It is clear that the horizontal velocity profile both at the free-surface and at the bottom and the momentum transport for axis direction are reasonably predicted with the effect of secondary currents in the curved section of the channel.

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A computational method to solve shallow water equations has been investigated with an SMAC method which is usually employed in the simulation for incompressible fluids. In particular, this numerical method is implemented in the unstructured collocated grid system with the distributed memory system to increase the parallel efficiency. The developed computational method was applied to the 1D dam-break problem and the free-surface flows in a meandering open channel. As a result of the 1D dam-break simulations, it was confirmed that this method improve the numerical stability. While, in the case of the meandering open channel, it was confirmed that the predicted water depth and depth-averaged velocity distributions are qualitatively in good agreement with the experimental results and that the reasonable parallel efficiencies are attained by parallel computations.

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Diffusion model with Data Assimilation of observed SF6 concentrations using Green’s Functions was applied to concentrations hindcasts estimating the movement of water mass released SF6. The SF6 concentrations were hindcasted with the currents and the released concentration flux estimated by the assimilation of the observed SF6 concentrations into the model in order to track the dye patch. Sets of hindcast experiments were carried out to evaluate the dye patch trajectories, increasing the observed data for the data assimilation. These experiments suggested that the data assimilative model could provide a good approximation of the trajectory in two days after the data assimilation. The time series of max point of the concentrations and its covariance matrix calculated by this model provided the effective information to estimate the movement.

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A quasi-three dimensional numerical model for flood flows was developed. The governing equation with the effect of secondary flow on the momentum equation was used in the model. The velocity profile of secondary flows proposed by Engelund(1974) was used. The model is based on finite volume method using HLL (Harten, Lax and van Leer(1983)) numerical flux, which is one of a Riemann solver. The model is verified against two experimental data of flows in curved channel and in river confluence. It shows that the model can reproduce the complex behavior of the flows with reasonable accuracy.

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This study presents some considerations on the possibility of practical utilization of MPS method for simulating snow avalanches from engineering points of view. The MPS method is a powerful tool to reproduce flow phenomenon with large scale surface deformation. In order to apply this method to snow avalanches, we introduce a constitutive equation of Bingham and Dilatant fluid. Erosion-deposition and effects of drag force due to air are also incorporated into the model. The modified model was applied to some cases and evaluated the model performances through comparison with experimental results and observed data.

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An estimation method of flood discharge hydrograph is developed that couples a 2-D unsteady flow model considering the effect of vegetation toppling and a 2-D model for bed elevation change due to bed load transport. The method is applied to a flush flood from the Haizuka dam in the Jouge River where vegetation became submerged with deformation in large areas. Measurements of water level hydrographs at several points, the discharge with ADCP at a weir and bed elevation change at a point were carried out during the flood. It is shown that the estimated discharge hydrograph gives reasonable agreement with the measured result and that the reliable data of bed elevation is crucial to accurate estimation of discharge hydrograph in a steep slope river.

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A basin-wide stream network model was developed for stream temperature prediction in a river basin. The model used Horton’s geomorphologic laws for channel networks and river basins with stream ordering systems in order to connect channel segments from sources to the river mouth. Within the each segment, a theoretical solution derived from a thermal energy equation was used to predict longitudinal variation of stream temperatures. The model also took into account effects of solar radiation reduction due to both riparian vegetation and topography, thermal advection from the sources and lateral land-use. Comparison of the model prediction with observation in the Ibo River Basin of Japan showed very good agreement for the thermal structure throughout the river basin for almost all seasons, excluding the autumnal month in which the thermal budget on the stream water body was changed from positive to negative.

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Temporal and spatial variation of water temperature has predominant effects on physical transport processes and biological and chemical transformations of substances in water area. Consequently, adequate understanding of water temperature changes is important facet of water quality management. In this study, field observations were carried out at the downstream reach of Honmyo River located at northern part of Isahaya Regulation pond. Furthermore numerical model for simulating water temperature changes was proposed and applied to this water area. Observed results show that thermal stratification is diurnally developed during dry weather condition. Redevelopment of the stratification after its dissipation due to the inflow of flood is subjected to temporal variation of wind speed. Although there is a room to tune the model parameters, proposed model simulates the observed results with acceptable error. Computed results show that thermal stratification is developed in all computed region. During the redevelopment of stratification, it takes a few days to raise water temperature in deeper layers.

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Habitats of aquatic creatures (fishes Oncorhynchus masou masou, Plecoglossus altivelis altivel and Cyprinus carpio, fireflies Luciola cruciata and Luciola lateralis, and frogs Anura sp) in the Natori River basin located at the middle of Miyagi prefecture were evaluated dynamically using the water temperature as one of the environmental indices. HSI (Habitat Suitability Index) and WUA (Weighted Useable Area) of aquatic creatures were quantitatively calculated from numerical map information and hydrological simulation with a heat budget model. As results, general HSI of fireflies increased but of frogs decreased by adding the factor water temperature. Migration of Plecoglossus altivelis altivel could be represented by the variation of WUA.

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In this study, a new assessment of river environment using carbon and nitrogen stable isotope analysis is proposed. The stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) in the sediments between river mouth and upstream end are measured at seven rivers in Okinawa Prefecture, Japan. The result indicates that δ¹⁵N is affected by nutrient load from catchment area and δ¹³C is determined by the mixing ratio of terrestrial organic matter. The prediction equations of δ¹³ C and δ¹⁵N in the riverbed sediments are proposed and applied to the observation results. The predicted value have a good agreement with the observed value, indicating that δ¹³C and δ¹⁵N reflect the characteristics of catchment area, and are useful to evaluate the river environment.

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Since the traditional river basin assessment has not been employed from the ecological viewpoint, the sound river basin management was not completed. In this paper, introducing the concepts of food chain, benthic organisms, and fishes for ecological system, the river basin simulation model based on physical dynamics of discharge and toxic-chemical is proposed. The sustainability of aquatic organisms and the accumulation impacts of toxic-chemicals in fish bodies are considered through CASM and PBPK. Finally, the Kamo River in Kyoto, Japan, is applied for verification.

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The environment of riverine estuaries is characterized by many factors including tidal oscillation, river flow and morphology, and the combination of these factors makes each estuary unique in its environment. However, there are similarities among some estuaries. We have tried to classify Japan’s 109 riverine estuaries based on physical environmental factors like tidal oscillation, wave energy, and high and low river flow. 109 riverine estuaries were divided into 5 classifications. Fish species monitoring data have also been analyzed to categorize riverine estuaries and the results were compared to 5 classifications divided by physical factors. Since our physical and biological classification both seemed to be related to the salinity of the estuary, these two classifications showed fair conformity.

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Nitrate concentration of groundwater was estimated from land use, amount of total nitrogen fertilizer, amount of absorbed nitrogen by plant and actual nitrate concentration of each catchment. The estimated nitrate concentration was in agreement with the actual concentration in the studied area. However, water balance is necessary for estimation under the different weather condition. The concentration estimated from amount of total nitrogen fertilizer minus absorbed nitrogen by plant divided by precipitation minus evapotranspiration was in agreement with the actual concentration and then nitrogen fertilizer was thought to be dissolved on the surface and solution was thought to infiltrate into soil.

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We investigated the runoff properties of SS, TN, and TP in the Mimitori River and the Takizawa River streaming in agricultural areas in Iwate prefecture, Japan. Field observations provide that the following findings: 1) the concentrations of SS, TN, and TP increase in paddling and transplanting period in paddy fields, 2) TN and TP concentrations strongly correlate with SS, 3) the loads of SS and nutrients are much higher during rainfall time. To examine the possibility to control SS sorbing nutrients, we carried out experimental studies on the charging and flocculation of the suspended particles as well as the resuspension of flocculated sediments. The results clarified that the flocculation behaviors qualitatively follow the DLVO theory and the resuspension behaviors are similar to those of natural sediments. Finally, we suggest that increasing calcium ions over 2mM is effective to settle down the particles.

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As a new measure for reduction of non-point pollution, we apply storm-water reservoirs in which basket mats are wholly and partially installed to improve flow patterns and increase residence time of particulate matter of non-point sources. To clarify the reduction effect of non-point sources using the improved storm-water reservoirs, we conducted field measurements on the bottom sediments in reservoirs located within the watershed of Lake Inba-numa. The measured results reveal that the improvement of the storm-water reservoirs may increase appreciably the trap effect of non-point pollution especially in the basket mats located near the inlet. These facts indicate that the partial improvements neat the inlets may be effective for the reduction of non-point sources in the whole watershed of Lake Inba-numa.

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In the Ashigaike Regulating Reservoir, not the runoff from the watershed but the conveyed water from the outside of watershed is stored for the improvement of the water quality and the strengthening of the irrigation water supply ability. In this reserch, the method not only to decrease TN load but also to preserve the effect of dilution by water conveyance was considered by simulation using distributed water quality tank model. As a results, it was clarified that the following methods are suitable; 1)Water conveyance would not be executed until the storing rate of the reservoir decreases an appropriate value. 2)The runoff water from the watershed would be stored. 3)Reuse irrigation would be executed. 4)The water which equals the volume saved by reuse irrigation is conveyed and released during non-irrigation period.

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The Shiretoko is a pennusla where a unique interaction occurs between the terrestrial and oceanic systems, which are linked by nutrient exchange processes. Nutrient rich floating sea ice is transported to the coast from the Okhotsk Sea and salmon and trout move upstream and feed inland animals, such as bears and birds. Conversely, nutrients flow into the ocean through rivers. It has been reported that this unique nutrient circulation is being disrupted by climate change and it is thus important to understand what mechanisms drive nutrient circulation in Shiretoko and maintain a unique ecological system. In this study, stable isotope analysis is used in combination to determine the contribution of land-derived (LDN) and marine-derived nutrients (MDN) in the Rausu River basin. Stable isotopes ratios, δ¹³C and δ¹⁵N, show a higher contribution of MDN before the snow-melt floods, and that MDN are distributed largely in the downstream region of the basin.

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Understanding current environmental condition is one of the most important issues for achieving effective policy for not only environmental protection but also disaster prevention. Element distribution in the water environment at Kathmandu area was investigated in this study in order to understand current environmental conditions in the areas. The characteristic elements observed in this area were considered to be Cu and Sr, the concentrations of which seem to be relatively higher than those in the other Asian areas. Water contamination problem in that area was also serious, and urgent and effective countermeasures are required. These were caused mainly by organic substances, therefore infrastructures for environmental protection such as sewage treatment plant and waste treatment facility should be prepared and operated sufficiently. Besides, water quality monitoring system also should be developed for environmental protection and for future application of water quality information to disaster countermeasures.

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In the previous studies, high turbidity is revealed to occur in Tokoro River and to affect social economical and environmental activity in Okhotsk area. Also, it is demonstrated that turbidity from fields is one of the most influential factors on high turbidity in Tokoro River. However, it is not clarified how high turbidity is provided from fields and forests areas. Therefore, this study attempted to understand the mechanisms of the occurrence of high turbidity and the role of fields and forests areas on the occurrence of high turbidity. In 2 hectors field and forest areas, SS concentration due to the impact of rainfall is investigated in 2009, which reveals that SS concentration is associated with rainfall intensity by using exponential function. Also, it is found that SS concentration in field tends to increase with rainfall intensity more rapidly than forest areas. The proposed equation for evaluating SS concentration is confirmed to have high applicability by using distributed hydrological model.

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We measured in-situ pick-up rates of sediment and particulate components of nitrogen (P-N) and phosphorus (P-P) in an urban river and a lake with a new device we developed. Field sites for this study were Lake Teganuma and its influent river, Oohori River. The measured results show that the temporal variations of pick-up rates of P-N, P-P and sediment in Oohori River were appreciably found and mainly caused by hydrologic conditions and sedimentation environments. Pick-up rates of P-N, P-P and sediment in the river and lake increase with flow velocity. Note that the pick-up rates of P-N and P-P are appropriately modeled with power functions of bottom shear stress similar to that of sediment.

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For comprehensive management of sediments of inner bays, it is important to understand the suspended-sediment transports from the watershed through rivers flowing into inner bays. However the sediment transport have not been understood especially under flood-flow conditions due to few continuous monitoring data for suspended sediment transports. For this purpose, we have installed turbidity sensors in the influent rivers into Tokyo Bay to continuously monitor SS trasnports. The observed results indicate that the coefficient b in sediment rating curve (L=aQb) are closely related to land use of the watershed. Total SS transports flowing into Tokyo Bay were annually 0.6 – 2.5*106 ton. The SS trasnports in Edogawa, Arakawa and Tamagawa Rivers are 66 % of total SS transports.

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A numerical simulation model was presented to describe the material cycling along a river, in which the longitudinal cross-section of the river was divided into 4 segments (segment M, 1, 2 and 3, respectively), and the external supply, decomposition and transport of coarse particulate organic matter (CPOM) were taken into account to represent the longitudinal material and energy transports in the river ecosystem. The numerical simulation was performed under the ordinary discharge condition for 4 years. The results of the simulation show that the large amount of CPOM accumulates in the river bed at the upstream area of the river, which acts as an important source of PO₄-P and NO₃-N in downstream area of the river.

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During flood periods, rivers transport large amounts of water and nutrient loads, and have significant impacts on water quality and ecosystems in enclosed waters. In this study, observations were carried out in the Yodo River, the three upstream rivers (the R. Katsura, the R. Uji, and the R. Kizu), and the Yamato River in ordinary and flood conditions. As a result, the relationships of SS with PN, PP, and POC were formulated, and we could estimate the loading amounts of PN, PP, and POC, using these correlations and the turbidity measured by automatic monitoring system. In a similar way, from the monitoring electric conductivity data, the loading amounts of DIN and PO₄-P were estimated with a high degree of accuracy in the Katsura River, where the water quality was influenced strongly by point source pollution.

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Boundary between land and water usually forms ecotone in natural condition. Riparian vegetation pattern in lateral direction is one of the examples of such ecotone. Lateral riparian vegetation pattern is strongly dependent on the elevation relative to water level. Water level fluctuation in rivers is one of the most important factors controlling the ecotone along river banks. We have surveyed vegetation cover, morphology and water level fluctuation driven by tide in six riverine estuaries and analyzed the relation between vegetation pattern and ground level to understand the influence of tidal oscillation on the land cover of the banks along riverine estuaries. Common reed was dominant in all estuaries. Although specific height of where common reed distributes was different among estuaries, distribution of reed can be expressed generally by mean inundation depth within tidal zone. Mean inundation depth seems to control vegetation within tidal zone in riverine estuaries.

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This paper describes the effect of sea-level rise (SLR) on the salt wedge intrusion in terms of ecological system in Kushiro wetland. Kushiro wetland was registered by Ramsar Treaty and the largest wetland in Japan. A previous study demonstrates that the salt wedge intrusion may not affect ecological system of Kushiro wetland, such as loss of freshwater plants along Kushiro River. However, it is revealed that SLR may occur in the end of the 21st century, which enhances the increase in the distance of the salt wedge intrusion along Kushiro River and the loss of endangered species of Kushiro wetland along Kushiro River. This study thus aims to investigate the influence of the salt wedge intrusion on freshwater plants along Kushiro River, and to clarify the salt wedge intrusion when SLR occurs due to climate change. We attempted to investigate the influence of SLR on endangered species along Kyu-Kushiro River in which sea water is likely to intrude up to about 8 km from the river mouth. As results, it is suggested from field observations that salinity may decrease freshwater plants along Kushiro River, and it clarifies the possibility that the salt wedge intrudes Kushiro River due to SLR by using 3D hydrodynamic model, Fantom3D.

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This study aims to clarify the causes of the expansion in alder forest in Kushiro Mire by analyzing the relationships among ground height, groundwater level and vegetation. In recent years, Kushiro Mire has been drying as a result of basin development, including river improvement, and the main vegetation has been rapidly shifting from reed communities to alder forest.Observation data analysis revealed that the area of alder forest is increasing at locations where the groundwater is -0.4 m or more underground. Groundwater simulation by analysis of observation data and unsteady two-dimensional model analysis indicates that groundwater levels are low at rivers and around former water channels. Because the groundwater level changes are large there, the area of alder swamp forest is expected to continue to expand.

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River landscape with sandbar is composed of several elements such as vegetation, bare area, side-pool and so on. In recent years, it has been reported that sandbar (vegetation in particular) has water purification function, but the quantification of purification is still unknown. The purpose of this study is to examine how the nitrogen is trapped and retained by vegetation of sandbar through numerical calculation of subsurface flow based on field observation. The numerical simulation was performed under the discharge and the topographical conditions of the Yahagi river. The main results of this study are that the vegetation area of the sandbar has higher potential of denitrification than bare area. Also, it was clearly showed that the distribution of vegetation affected nitrogen dynamics. Thus, it has become possible to propose the effective distribution of vegetation for the maximization of the water purification function by using the numerical simulation.

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Forestation of floodplain or sandbars follows the succession of vegetation. Thus, it enhances soil nutrients in the process, which then further accelerates the forestation. However, the nutrient process has not been elucidated yet. The nutrient budget by the colonization of riparian trees were estimated in the field observation. The allometric relations of tree biomass, separately with leaf biomass, and the knee-height diameter were obtained. At the same time, nitrogen and phosphorus concentrations of each tree organ were analyzed. Then, the yearly enhancement of stocked nitrogen in trunks and branches was obtained from the allometric relations, and the material allocation to leaves, which is essentially exchangeable, were estimated. The allocation to leaves was nearly same as the yearly stocked amount, and its proportion in the yearly uptaken amount of nitrogen was higher with older trees rather than younger ones. The exchangeable amount was more or less that of herbaceous plants. Thus, although soil nitrogen gradually increases with the succession to larger biomass species while they are herbaceous, it does not change much once it is forested.

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A dynamic model of the growth of riparian trees, based on the energy budget of the plant and widely used empirical allometiric relations, is developed, as to be capable for multi-purpose in river management. The model was applied to Salix gilgiana and Robinia pseudoacacia, common riparian species in Japanese rivers. The model successfully reproduced the morphological characteristics of these species observed on the sand bar of Arakawa river. The observation also indicated that Salix gilgiana colonized only less than 3m high from the normal water surface, while Robinia pseudoacacia did at higher than 3m, and the colonization of both species was peaked with tree biomass of approximately 100ind/ha. The mode provides the maximum biomass, thus, was approximately 20,000kg/ha.

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Field measurement and numerical simulation are carried out for an artificial flush flood in 2010 in the downstream reach of Haizuka dam located at the Jouge River to quantify the effect of flush flood on the exfoliation of attached algae. Measured items are water level, chlorophyll a (Chl-a), SS and attached algae at several locations. The relation between Chl-a attached to riverbed stones and velocity at the same location before the flood and the relation between detached rate of the algae and accumulated bed shear stress during the flood are obtained based on the field observation and 2-D numerical simulation. Based on these relations the total amount of supplied algae to water over the measurement reach during the flood is estimated to around 15.0kg in which the total amount of exfoliated algae is estimated to around 9.4kg.

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After dam construction, the flow and sediment regime change downstream from the dam. For example, the armor coat progresses because of sediment trapping by the dam. To solve these problems, ’sediment replenishment’ has been executed recently. Nevertheless, it is difficult to maintain effects of sediment replenishment over the long term because the restored sediments are washed away easily in steep rivers, especially in slippery bedrock sections. Therefore, the authors have proposed techniques that promote gravel sedimentation by improving riverbed forms. The effects of proposed techniques such as the river widening and multistep masonry weirs were investigated using a two-dimensional numerical model. Results showed that the proposed techniques are useful to reduce tractive force in steep rivers. They can create a desirable bed state for river ecosystems with various gravel sizes.

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Fresh water mussels (order Unionoida) have one of the important biological roles for river ecosystems. However, abundance and habitat range of Unionoida are drastically decreasing, because of environmental change based on many artificial river works, such as over riverbed excavation and loss of floodplain habitat. There are three Unionoida species in the Matsuura river, and one lentic species (Anodouta lauta) has been dominantly decreased over the past 50 years. As they have low mobility, Unionoida are strongly affected by physical environmental change of river. In this study, we focused attention on the hydraulic characteristics of Unionoida in flowing water condition, and conducted some hydraulic experiment to three Unionoida species. The result indicated that lentic species (Anodonta lauta) was easily affected by flowing water than lotic species (Unio douglasiae nipponensis, Lanceolaria grayana). And the result suggested that hydraulic characteristic of Unionoida was one of the reason that caused a decrement of lentic Unionoida species in the Matsuura river.

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In order to grasp the effect of the gentle slope revetment works to exert on the biodiversity at a tidal river, we surveyed at the tidal area in the Gokase River in Miyazaki Prefecture about the distributions of physical environments (ground levels, grain diameter and ignition loss of bed material) and the benthic communities at both of the soil-covered gentle slope revetment (A), and the vertical dry stone pitching revetment (b). As the results, the physical environments were similar in a transverse direction at the B section but the A section were successively varied. The kinds, and rare species of benthos, especially insects were rich at the A section rather than the B section. The results suggested that the gentle slope revetment works had improved biodiversity.

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Tree vegetation overgrowth in river courses has become a serious engineering issue for riparian management in Japan. In order to properly manage the river courses for both flood protection and ecological conservation, it would be necessary to accurately predict the tree vegetation dynamics for long-term duration. In this study, we developed a stochastic model for predicting the vegetation dynamics with emphasis on the interaction with flood events. In the model, the flood discharge was stochastically simulated using a filtered poison process, one of the popularly used approaches in hydrological time-series generation. The model included effects of vegetation growth, loss by the flood impact, and infant tree invasion. The results of Monte Carlo simulation quantitatively indicated that a suitable equilibrium state of tree vegetation density for both flood protection and ecological conservation was achieved by a certain flood level, and it was degraded as the flood impact became smaller.

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Drag coefficients of a real tree trunk and a branch and sheltering effects of upper trunk or branch on downstream one were investigated in detail. Willow and Robinia pseudoacacia were selected for the real trees. Drag coefficient starts to decrease from relatively low Reynolds number and is smaller than that of a smooth circular cylinder in whole Reynolds number range investigated. Leaves increases the drag coefficient around 40 to 100 % with increasing the ratio of leaves in comparison to that with only branch cases. Sheltering effects becomes larger with the existence of leaves. The drag coefficient of downstream branch approaches around 95% of that of a single branch with increasing Reynolds number notwithstanding the existence of leaves. The moment by drag force of a whole tree was calculated for the strong wind event that overtuned trees in the Arakawa river. The calculated value was compared with the experimental threshold values, and was validated within a reasonable limit.

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Threshold shear stress for removing grasses by flood is important but is not well understood quantitatively for clump-type vegetation, Eragrostis curvula. New model for calculating the velocities in the vegetated layer and the surface layer was proposed and the model was validated with flume experiments. The bed shear stress in vegetation was calculated by the momentum balance in the vegetated layer, and was applied for evaluating the wash-out conditions of the grass. The threshold value for removing the clump-type vegetation was evaluated as a rate of shear stress of d90, grain diameter at which 90% volume passed through the sieve, and the critical shear stress of d90, WOI. The critical WOI for removing the grass is 1.9-2.1, and 1.8-1.9 for one layer analysis, and two layer analysis, respectively. The critical shear stress inside the vegetated region does not decrease much because there is bare land between each clump-type grass, and the flow is accelerated in the region by the momentum exchange.

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This paper summarizes a field survey on physical features of and vegetation on a dry riverbed around a slit-type erosion control dam in Ozo river. Field surveys cover the range 130m to upstream and 200m to downstream of the dam and were performed in 2001 before the construction, and 2008 and 2009 after the construction. We developed a relationship between physical property of substrata and incidence of species of vegetation on the same property of substrata for each species. This incidence ratio is obtained from the observed data in 2001.Predicted mesh numbers and observed mesh numbers for all species observed in three years were shown in one figure. An estimation line between prediction and observation which is developed by Generalized Linear Mixed Model can give reliable relationship even for cases of sparse incidence of vegetation. It is shown that variation of vegetation incidence can be predicted by physical property of substrata. However, it is also shown that substrata and vegetation in studied area are still under succession stage in short period after the impact of construction.

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Nakamura pointed out that the notch should be placed on one-side compared with alternate to shorten the amount of staying time of fish in a pool. However, there is little information on the suitable orifice location for shorten the amount of staying time of fish in a pool. In this study, orifice location in a pool-and-weir fishway is changed alternate or one-side. Behaviors of ayu, Plecoglossus altivelis altivelis, were obtained with the aid of two sets of digital video cameras. It was found that when orifice location is one-side, the consecutive migration rate becomes high and the amount of staying time of fish in a pool becomes shorten, as compared with those of alternate orifice.

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It has been considerably investigated on the factors that ayu are triggered migration from ocean to river. However, there are few studies that observed migration characteristics of ayu in the river. There is no collective view of migration characteristics of ayu in the river, because contradict observation results were obtained in the fishway. One of the reasons may depend on the school population, because ayu has a tendency to follow the swimming front ayu. In this study, the behavior of fish school of ayu in running water was analyzed. It was found that staying tendency increases with an increase of school population, so that the migration rate decreases with an increase of school population. The distance between each ayu decreases with an increase of school population. Further, it was found that the fluctuation of streamwise ground speed in upstream migration type is high and also that the fluctuation of spanwise ground speed in downstream migration type is high.

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Swimming behavior of isolate or a couple of fish in running water has not been investigated. Recently, Onitsuka et al. investigated on the swimming behavior of isolated ayu in running water. In this study, the swimming behavior of a couple of ayu in running water was investigated. It was found that the swimming speed in the streamwise direction increases with an increase of the flow velocity. In contrast, the swimming speed in the spanwise direction is constant, irrespective of the flow velocity. The turning angle of swimming trajectory decreases with an increase of the flow velocity. The swimming speed for each branch decreases in compared with that of isolate ayu.

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This paper deals with the hydraulic and hydrologic properties of the habitat of fish in a river and surrounding area. It especially focuses on the habitat of the Acheilognathus Longipinnis in the lower reach of the Yodo River, because the restoration of its habitat is the most important and urgent work for the Yodo River eco-system. The estimation of flood disturbance required for the life history of the fish was examined in the Akagawa sand bar and Shirokita Embayments, where both areas were its most important habitat 40 years ago, through the return period of the magnitude of the flood discharge, the frequency of the water surface elevation, which were obtained by the statistics of the water surface elevation records, and the magnitude of the order of the velocity and the shear velocity and their distributions, which were obtained through the 2-D numerical simulation. The results will be usable for the selection of the location of the newly constructed habitat, and its design.

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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.

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Hydrologic regimes play a major role in determining aquatic ecosystems. However, most research has not focused on the effects of flow characteristics on fish species richness in freshwater at the global scale. In this paper, we evaluated flow characteristics in world rivers and showed general trends of rivers and distinctive features of Japanese rivers. The results showed that daily mean discharge and duration and frequency of floods significantly correlated with fish species richness. The present study suggests that it is important for the prediction of fish species richness in freshwater to consider not only average flow volume but also other flow characteristics such as variability and seasonality.

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Numerous attempts have been conducted in order to recover emergent plants and submerged plants in various lakes. However, floating-leaved plants, which make water quality worse, have been rarely focused. The purpose of this paper is to evaluate the effect of mowing down the floating-leaved plants area partially in Lake Suwa, where floating-leaved plants-dominated state is observed in shallow zone. We measure water temperature, turbidity and water velocity inside and outside of the floating-leaved plants area. It is confirmed that 1) The variation range of water temperature inside of floating-leaved plants area is smaller than that of outside. 2) Floating-leaved plants lessen the vertical water circulation. 3) Outside of floating-leaved plants area is easier to be influenced by cold water than inside when cold water intrude into water grass area. 4) Turbidity inside of floating-leaved plants area is higher than outside. 5) The leaves of floating-leaved plants become the resistance of surface current.

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Lake ecosystems can be viewed as complex systems where organisms and materials interact in many ways. The features of complex systems are two. One is nonlinear interaction, and another is effect of network formation between organisms and materials. A regime shift is a result of repetition by a chain of both effects. Water level operation of Lake Biwa causes serious concern with an influence on the ecosystem. Concretely, a state shift from turbid water to clear water may occur in response to thickness of macrophytes in the Southern Lake Biwa. It would appear that phenomenon of a regime shift occurred. In this paper, we predict a regime shift for the lake. Water quality analysis model is developed under a multi-layered model to predict 2-dimensional property of water quality through a year in the lake. This study illustrates ecological impacts of the water level operation through numerical experiments.

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This paper describes the fundamental characteristics of the numerical experiments on thermal convection carried out under the condition of the northern part of Lake Biwa. Since the temporal change of vertical distribution of water temperature during the cooling period can be reproduced by means of a simple simulation model of thermal convection in the previous study, the numerical results of thermal convection simulation was firstly shown under wide range of cooling rates at water surface. We considered to clarify the basic features of these numerical data on thermal convection using a stochastic model and its mean field approximation. It was shown through the comparison of theoretical results to numerical results that the mean field approximation with some refinement is applicable to the numerical experiments to some extent.

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We have measured the concentrarion of nitrate-nitrogen (called as ’concentration’ hereinafter) in tap water as a proxy of the concentration in the Yodo river. We examined the data with some data which are obtained from public offices, and found as follows. (1) Concentration in the Yodo river depends on the discharge at the Seta weir, i.e., the concentration decreases when the discharge increases. Water in the Biwa lake whose concentration is relatively low contributes to the decreasing. (2) On the other hand, when the discharge increases, the load of nitrate-nitrogen in the Yodo river often increases. Large amount of discharge contributes to the increasing. (3) Concentration in the tap water, namely, that in the Yodo river decreases when temperature is relatively high. The possible causes are that plants and denitrifying bacteria are active when temperature is high.

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A laboratory experiment on denitrification was carried out in order to reduce nitrogen load from municipal landfill leachate. Nitrogen was efficiently removed by feeding sludge of the leachate pond into the tanks, which could activate denitrification bacteria. Although inorganic reducing agent such as iron powder was not able to make the whole water mass anoxic, denitrification took place by supplying organic matters such as methanol, hydrogen feeding agent, etc.. It is considered that small amount of anoxic water film produced on surfaces of container and carriers might contribute to denitrification, although the bulk water is kept aerobic. It is found that organic matters contained in the leachate is so insufficient that nitrification liquid circulation does not work well for denitrification.

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In designing a constructed wetland for water purification, a homogeneous vegetation bed is often adopted in order to prevent short circuit which reduces the efficiency of SS trapping. However, vegetation naturally becomes inhomogeneous under the action of water flow, causing unexpected short circuit. This paper discusses a possibility to design a channel for a "stable short circuit", which distributes SS to vegetation zones by large horizontal eddies between the channel and vegetation zones. A series of numerical experiments show that even one slightly bended channel can distribute a high ratio of SS supplied through the channel to vegetation zones with the aid of horizontal eddies. This fact suggests that hydraulic design of artificial short circuit can be an alternative strategy for design of constructed wetlands.

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