The collection of river water quality data with high frequency and low cost is demanded for better environmental management of rivers, lakes and reservoirs. Although the feasibility of measuring nitrate, organics and suspended solids using in situ spectrometer has been demonstrated in previous researches, the calibration accuracy of phosphorous and some other dissolved nutrients is yet to be improved and the applicability to turbid water during floods has to be examined. For this purpose, we applied UV-Vis spectroscopy to rivers in the Kasumigaura watershed to estimate the nutrient concentrations. Water samples were collected from the study rivers and their water quality and spectra were analyzed in laboratory. Partial least squares (PLS) regression was used to develop models with which chemical oxygen demand, nitrate, nitrite, total nitrogen, total phosphorous and total suspended solids can be estimated from UV-Vis specta data. The difference method applied to raw spectra data was proved to eliminate the effect of spectra baseline and increase signal to noise ratio. Regression equations between concentrations of these water quality parameters and raw or differential spectra data were established and the results were assessed by the determination coefficients. Calibration result of phosphorous based on PLS regression was improved a lot by using differential spectra data. This study shows the potential to measure multiple components of water quality using both raw and differential UV-Vis spectra data based on PLS regression, and provides theoretical basis for high frequency and low cost water quality monitoring using in situ UV-Vis spectrometers.

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 Estimating spatial and temporal variations of surface waters is important for water resources management. The upcoming Surface Water and Ocean Topography (SWOT) mission will enhance our understanding on global water cycle by measuring water surface elevations at a high resolution. It will be benificial to combine SWOT observations to hydrodynamic modelling to overcome its limited observation frequency. We performed an observing system simulation experiment (OSSE) for estimating river channel bathymetry from water surface elevation (WSE) measured by SWOT. A Local Ensemble Transform Kalman Filter (LETKF) assimilation algorithm was applied to the CaMa-Flood hydrodynamic model to estimate WSE and bathymetry simultaneously via state-parameter estimation. Synthetic SWOT observations were generated from a “true” CaMa-Flood simulation based on the original bathymetry while the corrupted model used channel bathymetry parameters different from the true model. The LETKF via state-parameter estimation succeeded to estimate the true bathymetry using SWOT observations with 1.36m global average root mean square error (RMSE), which was improved by 68.0% from the corrupted bathymetry. Furthermore, the river discharge was also reasonably estimated. These results indicate the potential of the future SWOT mission to spatially and temporally estimate river bathymetry and river discharge on a global scale.

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 The aim of this contribution is to monitor unsteady behavior of streamflow, as well as, loops of streamflow-stage in a relatively small mountainous river. In order to determine river discharge, two methods were used: (i) the continuous slope area method (CSA), and (ii) the Index Velocity Rating Curve discharge data obtained by means of a novel fluvial acoustic tomography system (FATS) which developed by Hiroshima University. The findings showed that during a single hydological event, the temporal variations in water slope has two peaks, and water slope-stage has a positive hysteresis behavior. In addition, streamflow-stage hysteresis loops estimated by FATS are informative more than streamflow-stages estimated by the CSA method. Additionally, it was confirmed that stramflow-stage hysteresis loop increases with large-scale rainfall events.

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 Joint occurrence of natural hazards, that an extreme event of natural foece contemporaneous with other events, should become taken into account as our civilization has matured. Recent extreme value theory has introduced a bivariate generalized Pareto (GP) distribution for extremes. However it is hard to say that the original mathematical papers well explained how useful it is in contradistinction to the conventional treatment of censoring a bivariate extreme value distribution of componentwise block maxima. Thus there are few applications reported until now, though it is feasible and flexible to actual situations. This study examines the mathematical constitution and the connections in viewpoint of the flood problems.

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 We have some sense that frequency of high-intensity precipitation has recently increased. We confirmed that this sense by using observation data (AMeDAS data) and d4PDF(database for Policy Decision making for Future climate change) data. Furthermore, we estimated spatial average of annual precipitation of Japanese terrestorial area. It is approximately 1,900mm and it has increased during recent 60 years. Finally, we compared 100 year-precipitation calculated by using population (whole data of d4PDF) and thoese calculated by each member of d4PDF. As a conclusion, 100y-precipitation calculated by using one member potentially has large bias.

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 The general purpose of hydrological statistics is to decide probable hydrological variables corresponding to plan scale by using the most fitted probability distiribution for extreme hydrological data. Conventional hydrological statistics focus mainly on the estimation of the probability distribution that fits extreme hydrological data. However, observation data of unprecedented heavy rain deviate greately from the adopted probability distribution. Therefore, the method for evaluating such heavy rainfall has not been established in hydrological statistics. In this research, we aim to show a theoretical framework to evaluate unprecedented heavy rainfall by introducing confidence interval of probability distribution models.

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 A severe storm hit Northern Kyushu on July 5, 2017, and caused very intense precipitation around Asakura City, Fukuoka. Then the intense precipitation caused hillside collapses and floods there. The purpose of this study is to investigate characteristics of the intense precipitation by means of statistical and physically-based approaches. First, frequency analysis of precipitation at Asakura was conducted by means of the generalized extreme value distribution (GEV). Next, the severe storm was reconstructed by means of a regional atmospheric model. The results of the frequency analysis show that the maximum 1hr, 3hr, 6hr, and 12hr precipitation at Asakura on July 5, 2017 exceeded the value when the non-exceedance probability equals to one. On the other hand, the reconstruction results indicate that the intense precipitation was caused by the topography of Northern Kyushu and by the stationary front coming down from the Japan sea. In contrast to the frequency analysis, the physical mechanisms causing the intense precipitation may not be rare because such a behavior of a stationary front is not rare.

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 The precipitation and its return period of a heavy rainfall was analyzed spatially using radar AMeDAS composite precipitation. Relationship between depth-area (DA) was calculated about heavy rainfall events during maximum annual flood events. Spatial distribution of return period for precipitation was created using GEV and PWM. Comparison of rainfall distribution between 100 and 10 years return periods found area of frequent heavy rain in the Yoneshiro basin. In addition, relationship between return period of rainfall and area (RA) about the heavy rainfall when a flood occurred, was estimated. We selected 2007 and 2013 rainfall events according to past flood events. As the result, the area of 50 years return period in 2013 is approximately 14 times larger than that in 2007. And, the area of 100 years return period in 2013 is 50 times larger than that in 2007

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 A finite-volume hydrodynamic simulator based on centroidal Voronoi tessellation (CVT) is proposed to realize smoothly varying local mesh refinement. The fundamental properties and generation techniques of CVT, weighted CVT and boundary-fitted CVT are explained with a suitable scalar advection scheme. Hybrid parallelization of the shared and distributed memory models has been implemented to achieve efficient long-term high resolution predictions. The simulator has been applied to the brackish lake Abashiri and the salinity fluctuations in the halocline were compared with the measured data. The measured salinity fluctuations due to the internal Kelvin wave are well simulated both in phase and magnitude.

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 We developed a conceptual model for bottom layer volume in order to evaluate bottom layer volume exchange due to river flood with a large peak discharge. Three-dimensional environmental fluid dynamic model was applied to calibrate the conceptual model with a good agreement. It was found from 3D hydrodynamic model analysis that bottom layer volume is exchanged greater when the maximum displacement of density interface is larger than the critical amplitude. It was revealed that occurrence of total amount of rainfall more than 50 mm was three times in the periods from 2012 to 2016 as much as from 1990 to 2011, which caused more exchange of bottom layer water.

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 Reservoir sedimentation is one of the most important problems for sustainable management of hydropower dams. Among several countermeasures, sediment sluicing is characterized in the sediment routing options which can be environmental friendly. We have already discussed that the properties of two hydropower dams which would define effects of sediment sluicing were analyzed such as relations of trap efficiency of reservoirs and the ratio of flood water depths above river bed and spillway crest elevation. In this study, as a result of analyzing 17 hydropower dams by previous method using trap efficiency, it was shown that sediment sluicing can be installed in many dams. For the case study, feasibility of sediment bypass tunnel and gated outlet were studied in the dam reservoir which will be unsuitable for sluicing with existing spillway, and we found the possibility that gated outlet will be effective.

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 In this study, two intensive field suveys at a reservoir coverd with blue-green algae were conducted in the summer of 2016. Chl-α data showed a clear diurnal vertical miglation of cyanobacteria. Even when the thermoclines were formed in different depths, the similar vertical miglation pattern was observed. The surface Chl-α fluctuated greatly in the range between 8 and 60 μg/L in a day. The vertical miglation reached the depth of 5 m from the surface. It was suggested that blue-green algal was mostly growing in the inflow area of the dam lake because of the low concentration of nutrients (N, P) in the rage from the surface to depth of 2 m. In this situation, Microcystis ichthoblabe, which is resistant in low phosphorus concentration environment, could form a bloom state.

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 The Shinhori River flowing in the central part of Nagoya City has problems of water pollution and bad smell. We conducted field observations of water quality and investigated measures for water quality improvement. From the filed observation of water quality, it is found that salt water is always staying in a bottom layer and generates a density stratification. As a result, an existence of poor oxygen in the bottom layer was significant due to the lack of vertical mixing. Since a river-bed aggradation was observed in the downstream area, we investigated the influence of river-bed configuration on the tidal flow. A vertical 2D numerical model can reproduce the salinity and DO behaviors in tidal conditions. By using this model, we tested the effects of the change of river-bed shape and water installation. It is suggested that bed excavation at the downstream end is not effective and the clear water installation from the bottom layer is most effective for the dissolution of salinity stratification and the improvement of DO.

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 The water quality in the intertidal lower reach of River Fukuro is affected by the salt wedge. The treated sewage is discharged in the middle of the river, which might afftect the river water quality. To reveal the effect of the discharge, we carried the intensive field surveys and performed a 3-D ecological simulation for the river. We found that treated sewage flowed not only to downstream but also to upstream by the entrainment of the salt wedge. Moreover, the branch river of River Fukuro was also affected by the salt wedge.

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 A new analysis method using an optimal interpolation on environmental sea temperature, which can take account of spatial non-uniformity of environmental sea temperature, is proposed with the aim of applying to environmental impact assessment (EIA) or environmental monitoring of warm water discharged from power plants. As the result of applying the proposed method to the environmental survey data with a thermal power plant where warm water is discharged as surface buoyant jet, the optimal correlation length, the important parameter of optimal interpolation, is estimated about one and a half times of maximum distance between observational points. In addition, the enclosed area of 2°C higher dispersion area of warm water according to the preliminarily prepared information is evaluated to be adequate for the criteria of warm water impact area for selecting the observational points to analyze the environmental sea temperature. Finally, the estimated dispersion area of warm water in the surface layer based on the proposed method is certified to be consistent with the numerical simulation result for dispersion prediction of warm water.

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 The swimming cost is one of the most important quantities for comprehension and assessment of fish migration. We propose a generalized exact formula for the swimming cost per unite time during upstream migration of isolated fish. The formula assumes a quadratic relationship between the flow speed and the swimming speed, which turns out to reasonably handle the qualitatively different experimental observation results of the fish swimming behavior available in the literature. Starting from the quadratic relationship, with the help of an ordinary differential equation derived from an optimality principle, the exact formula of the swimming cost is derived with the help of an elementary calculus technique. The derived swimming cost is an increasing and convex function of the swimming speed, which agrees well with the reported results for a wide range of fish species. Applicability and limitation of the present formula are discussed with an emphasis on biological meaning of its strict positivity and the range of swimming speed. The obtained results of this paper give new insights into modeling and analysis of upstream fish migration.

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 In this study, authors carried out some experiments those using immature fish; Tribolodon hakonensis which had low swimming capability in a rocky ramp fishway with transverse slope.
 As the results, in Case2 b, run-up ratio R_r decreased 10 ~ 30 % in comparison with Case2 a where had high swimming capability of the fish. Therefore, invention to form a variety of flows regardless of small and large of the flow quantity is necessary. In Case3, a variety of flows was formed by securing of water depth regardless of small and large of the flow quantity. Moreover, run-up ratio R_r became more than 60(%) even if swimming capability of the fish was low. In addition, fish run up at the point which was 4BL ~ 7BL(cm/s) in these experiments.

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 Studies on biology and environment of eels Anguilla japonica have been advanced in recent years. However, little is known about fish ladder for eels and migrating characteristics of eels. Especially, special fishways are necessary for demersal fish like young eels. This study was made to evaluate effects on migration rate of eels under the condition that diameter of protrusions and discharge per unit width in fish ladder are changed. It was found that migration rate increases with an increase of discharge per unit width in each of diameter of protrusions. Besides, sinuosity of eels are increasing, and averaged migrating speed is decreasing with an increase of discharge per unit width and diameter of protrsions.

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 It is important to keep suitable area by boulders to make rest area for fish. In this study, swimming behavior of Opsariichthys platypus in open channel were compared with change of boulders interval in streamwise direction over open channel bed. Swimming behaviors of Opsariichthys platypus were obtained with the aid of two sets of digital video cameras. It was found that swimming of Opsariichthys platypus was stagnant remarkably in cases where the boulders interval in downstream direction was narrow. It is because that there are more low velocity spaces in this case compared with cases where the boulders interval in downstream direction is wide.

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 Predictions of swimming behaviors of fish make it possible to design river structures reasonably. However, investigations regarding swimming behaviors of fish were conducted by means of conspecific fish that have same body length. Therefore, swimming behaviors of fish can be gauged only under restricted circumstances. It is necessary to consider effects of existences of conspecific fish that have different body length on behavioral characteristics because they live in revers. In this study, the swimming position and the fish direction are analysed in static water under the condition that the number of small Opsariichthys platypus is evened out and the number of large Opsariichthys platypus is changed. As a result, swimming behaviors as fish school are susceptible to lage fish and it is suggest that this trend become more conspicuous.

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 When fish gathering device is set in the entrance of fishway, this may affect the migration rate. It is assumed that eels (Anguilla japonica) gather by water spraying. However, little is known about effects of fish gathering by water spraying. This study was made to analysis effect of gathering eels under the condition that range of water spraying and discharge spraying are changed. It was found that water sprying has an effect on gathering fish as compared with non-spraying in this experiments. Besides, effects of gathering eels is increasing with increasing range of water spraying and discharge.

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 Free surface flow problems occur in disaster simulations such as tsunami flow in urban area. In this king of situation, a non-hydrostatic free surface numerical model is required to perform a tsunami inundation simulation. It is, however, difficult to carry out three-dimensional large-scale tsunami simulation because of the computing of the pressure Poisson equation in the incompressible Navier-Stokes equation.
 In current study, we have developed a fully explicit three-dimensional free surface model by lattice Boltzmann method with the Piecewise Linear Interface Reconstruction - Volume of Fluid (PLIC-VOF) approach. Through the classical dam-breaking problem, we demonstrated that our present model has a convergence of model accuracy according to the spacing grid sizes. Besides, Our model can calculate the interface shapes seamlessly and settle the oscillations even at the high Reynolds number flow.

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 After the Great East Japan tsunami, tsunami countermeasures using existing coastal forests were considered in Hokkaido. Effectiveness of tsunami energy reduction by the multiple defense using vegetation, moat and embankment from the sea side is tested using surge-type flow by quicky opening gate. The inflow volume to inland area side and tsunami delay were measured. Fluid force index, and moment index by fluid force were estimated by the measured data. The effectiveness of multiple defense structure by forest, moat, and embankment for tsunami energy reduction was clearly shown. The moat had an effect to dely tsunami arrival time, however, the reduction in tsunami inflow volume was not so large. The reduction effect has a possibility to be changed depending on the shape and size of the moat. When the tsunami height exceeds 1.23 times the embankment height, the tsunami reduction effect about fluid force index decrement is decreased because the tsunami accelerates when it overtops from embenkment.

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 Several tsunami waves are estimated to hit Okayama city when Nankai Trough earthquake occurs. Most of coastal area is occupied by a below-sea-level and severe tsunami inundation is expected there. Okayama city has a network of irrigation channels in the coastal area and many residential quarters are surrounded by channels. And the bed elevation of residential area is lower than water level of the channel in some parts. Thus the irrigation channels might enlarge the tsunami disaster in Okayama city. In this rsearch, the effect of irrigation channels on the tsunami inundation process was numerically investigated with or without considering the channels. As a result, it is clarified that channels hasten the tsunami proagation and inundation along them, and eventually the channel network significantly accelerates the tsunami inundation. Furthermore, from the viewpoin of calculation time and memory saving, we prposed a method to treat the networks as one-dimensional channels and showed the validity of the proposed method.

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 After the Great East Japan tsunami, multiple defense system is planned at the shore in Hokkaido Prefecture. However, tsunami usually runs up in a river and has a possibility to flood even when the tsunami does not overflow from shore line. Therefore, experiment was conducted for investigating the effects of ‘vegetation and moat’ along the river on the tsunami run-up around a river, especially paying attention to the effect of the multiple system on the opposite side of the river by refrection.
 As a result, when the vegetation is dense and the angle of ‘vegetation and moat’ and river is 30° and 60°, the water depth on the opposite side becomes higher than in the case without the multiple defense. For 60°-angle case, the energy of the standing wave generated in front of the vegetation is reduced when the wave crosses the river channel, and the flow velocity on the opposite side becomes smaller than in the case without ‘vegetation and moat’. However, when the angle is 30°, the energy reduction does not occur when the standing wave crosses the river channel, and the flow velocity is similar as in the case without ‘vegetation and moat’.

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 This paper describes development of tsunami inundation database for applications to inundation analysis based on the post-event survey results in the 2011 off the Pacific Coast of Tohoku Earthquake. Data quality check, classification method, and adding of additional information were discussed for inundation analysis. Finally, total 680 point data were constructed and its usefulness to verification of numerical modeling is confirmed.

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 A 13-year-long wave hindcasting at each of the prescribed points around Japan and Korea on the Northern Pacific Ocean was conducted by using the first generation model such as a backward ray tracing model(BRTM) associated with high space resolution nesting grids under the MSM(Meso Scale Model)-based input winds condition provided by JMA(Japan Meteorological Agency). Also a wave hindcasting over the same year period on the entire region was made by use of the third generation model SWAN with medium-size space resolution grid under the same wind conditions. Comparison between hindcasts and measurements for significant wave heights and wave periods acquired at not only offshore stations around Japan and Korea but also coastal stations in the Japan Sea reveals the following findings; 1) for offshore waves and coastal waves, both models yield reasonable estimates respectively. 2) for offshore waves, error statisitcs suggest the evaluation ability of SWAN is slightly better than BRTM, but wave period statistics indicate a measure of underpredictability with SWAN. 3) for coastal waves, BRTM with high space resolution grids produces a more accurate estimate than SWAN with a medium-size space resolution grid.

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 In the general-purpose ocean model, the vertical spatial resolution near the seafloor decreased with increasing water depth, and there was a problem in evaluating the suspension dynamics near the seabed accurately over a wide water depth zone..Therefore, in this research, we applied a vertical distribution model to the lowest layer of the σ coordinate system to construct a boundary layer model that can appropriately evaluate the concentration of suspended matter near the bottom of the ocean regardless of water depth change. It is confirmed that this model can be applied to simple waterways and real sea areas and that this model appropriately evaluates sediment concentration and concentration flux in the vicinity of the bottom surface to be overestimated by the usual differential method.

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 At the mouth of the Maite River flowing into Nakatsu tidal flat which faces the Suonada Sea, a rivermouth jetty using gravel net was extended to block longshore sand transport from the nearby coast into the river mouth. The effect of the jetty appeared within two months after the installation of the jetty, and sand deposition inside the river mouth was mitigated. Sand was deposited imeadiately west of the jetty, and it was found that such a sand mound was used for horeshoe crab Tachypleus tridentatus as a spawning ground. The recovery of sandy beach west of the jetty was effective for the protection of sand dune as a natural levee.

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 Erosion is significant in the vicinity of Cua Lo River mouth, which is located in Central Vietnam. Despite the implementation of countermeasures in the recent years, no effect has been brought and erosion is still in progress. Therefore, in order to clarify the mechanism of erosion at the estuary, a series of Landsat images and Google Earth images were analyzed to clarify the characteristic change of the estuary topography. From the 1970s the estuary has been moved consistently to the coastal drifting direction, and its traveling distance has reached 1,700 m. In order to quantitatively investigate such characteristics, the tip coordinates and the area of the sandbar on the left of the estuary were calculated. In particular, the area of the sandbar has increased remarkably, and the increasing rate is almost constant. Using this value, the rate of sand volume transported along the left sandbar by the waves was estimated and it is in good agreement with the longshore sediment transport rate of the surrounding beach.

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 Landslides frequently occurred in the Shirakawa basin due to the Kumamoto earthquakes occurred in 2016, a large amount of sediment flowed into the river channel of Shirakawa River. They were deposited on an intertidal flats at the mouth of the Shirakawa River. That is concerned about the influence on the water environment and the fishery industry. Therefore, in this study, in order to examine the influence of the sediment on the intertidal flat at the mouth of the Shirakawa River, We conducted a field survey and estimated the state of the river sediment budget. Regarding the field survey, We conducted grain size analysis and bed elevation survey on the intertidal flat at the mouth of the Shirakawa River. As for the estimation of the state of the river sediment budget, We estimated the amount of bed load using turbidity data. The results showed that the sediment deposited was fine. As for the ground height on the intertidal flat at the mouth of the Shirakawa River, the period from May to July was rising, however the period from July to January was declining. Regarding the estimation of the state of the river sediment budget, the amount of sediment that flowed into the intertidal flat due to the flood of last year contain a large suspended load and it was about 10 times compared with the whole year of it.

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 Hydropower dams have been rapidly developed in the upstream Mekong River Basin without considering the downstream related issues. This research aims at investigating possible impacts of six hydropower dams of the Lancang cascade in the main Mekong River on flow regimes, including the magnitude, timing, and inter-and-intra variations, of the Vietnamese Mekong Delta (VMD) based on time series of historical flow data at TanChau station as a protocol. The results show that the impacts of the first four dams in the Lancang cascade on flow regimes in the VMD are not obvious. However, flow regimes in the VMD have been significantly altered when all six dams in the Lancang cascade were completed. In flood season, median discharge and maximum water level decrease by 13.5% and 20.2% respectively. High and low stage durations decrease by 55% and 66% when six dams existed. Flows from November to February are significantly reduced, causing difficulty for irrigation in the VMD. In dry seasons, discharges generally increase while water levels decrease due to river bed incision and reduction in sediment supply from the upstream, causing an increase in tidal influence accompanied by saltwater.

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 In recent years, forestation of rivers has become one of major problems in river management. In this study, we predicted potential habitat of willows, one of the major factors of forestation, by using vegetation map created by the Biodiversity Center of Japan, GIS, and maximum entropy modeling (Maxent). The base map for setting managerial objective and analyzing the factor of forestation was created by using potential distribution map and actual distribution data for willows in Hokkaido, Honshu, Shikoku, and Kyushu . The results of Maxent show that the area under the curve (AUC) exceeded 0.7 in all regions. TWI was the most influential environmental condition in most regions.

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 The purpose of this study, which was done by using non-uniform flow calculations and subsurface water analyses, is to clarify the influence of a river meander restoration project on the river water level and the groundwater level. In the Kayanuma District of Shibecha, which is at the upper reaches of the Kushiro River in Kushiro Mire, a project to restore the meanders of the river channel was implemented several years ago. In this nature restoration project, a river channel that had been straightened was restored to its former natural channel with meanders.
 In this study, hydraulic estimation was done on the changes in the river water level and the ground water level before and after the restoration project. The estimation found that the river water level had risen in the meander-restored river section and at the reaches upstream of that section after the restoration project. The calculation and estimation showed that the groundwater level had risen simultaneously with the river water level in and around the meander-restored river section. The results of this study, i.e., quantitative estimations of rising river and ground water levels resulting from restoration of meander channels, demonstrated that the restoration of river channels can be effective in restoring the natural environment of a mire.

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 Large amount of nutrient salt and dissolved organic matter is transported from Kushiro River basin to coastal area. In present paper, seasonality of transport ratio in dissolved inorganic matter from upstream catchment to downstream reach was estimated by using multiple regression mothod incorporating Bayesian theory into major ion pattern in stream water. Transport ratios could be well verified and estimated in several percent of errors. Seasonality of contribution ratio was clearly observed in comparison with uncertain errors. Ion representing each catchment should be extracted in advance by using statistical test.

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 We developed a simple benthic-pelagic coupling ecosystem model to understand changes in long-term water quality. A features of this model is the division of pelagic and benthic ecosystem models, and the calculation of each ecosystem component individually. The method used an existing two dimensional vertical pelagic ecosystem model designed at a macro-scale for long-period prediction coupled to a simple one dimensional vertical benthic ecosystem model configured with Phosphorus and Oxygen bio-chemical processes. Using the coupled ecosystem model, we reproduced the water quality of Ofunato Bay from 1993 to 2015. This suggested that the deposition rate of detritus and the concentration of organic matter at the seabed were decreased due to increased seawater exchange resulting from the collapse of a breakwater at the mouth of bay during the 2011 Tohoku earthquake tsunami.

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 Environmental DNA (eDNA) analysis, which has developed remarkably in recent years, has reached the stage where relatively large evaluation of the biomass of ayu (Plecoglossus altivelis) at the water sampling point is possible, in addition to the fact that the work at the site is only water sampling and the burden is small. In this research, using this eDNA analysis, we clarified the habitat of ayu during river life in wide area including Tama River and its branch, grasp the distribution of the population density of ayu, and especially in Tama River We identified areas with high population density. In addition, it was confirmed that the high range of the specified habitat density and the range where the confirmed population was large in the census of riparian waters nearly coincided, and the estimation of the population density of ayu by eDNA is consistent with the capture survey.
 Furthermore, from the results of concentration comparison with Saba and Takatsu River which quantified eDNA of Ayu by the same method as this research, the eDNA concentration of Tamagawa obtained this time is extremely high, and the Ayu resource amount of Tama River is It was suggested that there was a lot.

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 After rain events, scum with malodor often appears in the brackish water reaches of urban rivers. In this study, scum generation mechanisms were observed through continuous water quality observation, capturing inflow organic sludge using sediment traps, and camera surveys in the Nomi-river, which flows through the southern part of Tokyo. The results revealed that sludge accumulated in the area around the Sanya-bridge where the riverbed is deepest and generation of anaerobic gas required for floating scum occurs quickly. Regarding the latter, most of the period from rainfall to the observation of scum shows a transition to a strong anaerobic condition by sulfate reduction action for 12 to 24 hours, and the generation of anaerobic gas required for floating scum occurs within 2 hours.

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 Numerical and mass concentrations of microplastics in sizes < 5mm were investigated based on in-situ surveys on the surface of two rivers (Edo River and Ohori River), and annual flux of floating microplastics of Edo River were estimated by using a statistically significant relationship of the concentrations with river discharge. Good correlation between MP flux and discharge (i.e., L-Q relation) was also found in Edo River. Annual flux of numerical (mass) MP concentrations on the Edo River was 1.5×10¹⁰ pieces (2.42 ton). The ratio of numerical (mass) MP flux under high flow conditions was 73.5% (84.1%) in the total flux. These results indicate that the high ratio of MP flux in flooding conditions was similar to that of macro debris flux in rivers.

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 To evaluate the effect of land cover on the dissolved iron loading to river, river water samples were collected from the rivers in Kitakami mountains in northeastern Japan in winter. The relation of dissolved iron concentration to water qualities and land cover were evaluated, and iron load unit of each land cover type were estimated. Dissolved iron was significantly correlated with DOC, specific UV absorbance (A₂₅₄) and land cover percentages of paddy field and urban area. The load unit of the dissolved iron from boroad leaf forest, paddy field, and urban area were estimated as 1.57, 139 and 439 g/km²/day respectively. Thus, the human activity in river basin are found to highly contribute to the iron loading to river.

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 To clarify changes in water pollutant loads from the land to coastal region before and after the Great East Japan Earthquake, we evaluated COD, TN, TP emissions from Miyagi and Fukushima prefectures in FY2010 and FY2012. Result indicated that COD, TN and TP emissions from sewage treatment plants in Miyagi Prefecture significantly increased 2.63, 1.24 and 1.93-fold, respectively, caused by the disaster damage. Consequently, total pollutant emissions from the whole domain in FY2012 became larger than that in FY2010 but these increments were 1–11% at most. This result suggested that the amount of water pollutant emissions in the basin at 1–2 years after the disaster almost recovered to the previous one.

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 Flood discharge capacity control is crucial for the management of vegetated rivers. Proper field investigations of vegetation species, height, and density on-site would lead to an accurate estimate of the flow conveyance. However, several reasons including hard field work limit the quantity of observation data. Consequently, under existing circumstances, we often obtain the roughly approximated values of vegetation characteristics in the field with aid of some information sources such as aerial photographs.
 In this paper, we developed a method to estimate the distributed values of vegetation density using high-resolution point cloud data obtained in a long river reach by an airborne laser bathymetry (ALB). We also checked the applicability of the estimated values by numerical simulations of the Asahi River flooding in 2011. Results showed that the distribution of vegetation density of willow tree community on a gravel bar match qualitatively with the appearance in the aerial photograph. They also showed that the simulated results using the present data are well consistent with observations for both the longitudinal water level and the distributary discharge into the Hyakken River, compared with the results using the conventional data.

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 In this study, one of SfM-MVS techniques in which a ”pole-camera” was applied was used to investigate the evolution of micro-topography on a small region of riverbank of the Shonai River. This convenient technique helps researchers to perform frequent field measurement. The result showed that the micro-highlands generated by the deposition of fine sediments on vegetations need to be settled on erosional regions in order to remain during the periods of relatively small flood events.

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 Tendency of vegetation overgrowth in several river channels downstream of the Kinugawa River, Japan was analyzed by using a vegetation dynamics model. In order to apply the model to the Kinugawa river channels, the model parameters were determined for the flood generation, vegetation growth rate, and infant vegetation invasion. The resultant parameters indicated that the vegetation activity of the Kinugawa river channels was relatively small. The model accuracy for the evaluation of vegetation overgrowth tendency was verified by comparison with the aerial photographs. Furthermore, a probability evaluation of vegetation state showed increment of vegetation overgrowth tendency from upstream to downstream in the Kinugawa river channels.

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 This paper investigated causes of grass invasion into river channels and developed the prediction model by using logistic regression analysis. The river channels analyzed was located in the middle section of the Kinugawa River. The logistic regression analysis dealt with data obtained from field observations in 2015 and 2016 using a UAV measurement. The results of the regression analysis showed that the grass invaded into the channels with the factors, in order of the past vegetation history, dimensionless tractive force/height from the normal water level, and shortest distance from river streams. The accuracy of the logistic reogession model became highest in the case including the past vegetation history as a explanatory variable. The prediction result of the model indicated good ability for the grass invasion probability with 75% of accuracy rate.

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 Flow with submerged vegetation is a topic of current high interest for river engineering and environment. In the present work, we study the structure of open-channel flow with submerged vegetation using linear stability analysis. The mixing-length model is employed to describe the turbulence. We consider a base flow characterized by the absence of Kelvin-Helmholtz stability, which results in a base flow which is distinct from the mean flow. This permits a more consistent study of the conditions under which Kelvin-Helmholtz inatabilities are generated. We find that the growth rate of the perturbations is maximized for moderate values of vegetation density.

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 This paper analyzed the trend of long-term change in annual mean equilibrium water temperatures and air temperatures all over Japan for 50 years, 1963-2012, and investigated their regional characteristics. For analysis of time series data of the temperatures, Mann-Kendall test was used to statistically detect the trend and Sen's slope was used to quantitatively evaluate its significance. The results showed that the temperature rising tendency in the equilibrium water temperature was common in the coastal part of the Pacific Ocean in the western part from the Kanto region. In these areas, a positive correlation was found in the rising tendency between the equilibrium water temperature and the air temperature. Furthermore, the temperature lowering trend in the equilibrium water temperature was detected more often in the northern part of Tohoku and Kyushu rigions as well as the coastal part of the Sea of Japan. In these areas, there was a negative correlation between the equilibrium water temperature and the air temperature.

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 River water temperature is one of the factors closely related with material circulation and human activities, and its numerical representation in global modeling has received increasing attention in recent years. In particular, we discovered that the relationship between water and adjacent air temperature which was pointed out in previous researches is not observed in the large rivers over low-latitude regions. In this study, we investigated the influence of the various determinants including fluvial dynamics (e.g., advection, water depth change, flood inundation) and vegetation canopy in the global river water temperature model, HEAT-LINK. It was found that considering additional factors improves the reproducibility of river water temperature variability in general, but the impact of the flooding varies for different basins. Simple experiments on vertical stratification of water temperature suggested a potential development of thermal stratification in the Amazon river.

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 An artificial flush flood have been conducting in the daownstream reach of Haji dam to improve the rivebed environment. In this area, the overgrowth of Egeria densa gives a negative impact on the river ecosystem. We investigated the change of the area covered by Egeria densa, the attached algae biomass and the benthic invertebrate biomass bofore and after the artificial flush flood.
 The result indicated that the attached algae and the cover degree of Egeria densa decreased, on the ontrary the biomass of the benthic invertebrate almost never changed. However, the detachment of Egeria densa occurred only in the center of stream with high bed shear stress duing the peak of artificial flood. This result indicates that the group of Egeria densa will expand the inhabitable area from the bank to the center of river again.

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 Relationship between the change in river bed material and the dynamics of benthic macroinvertebrates after sediment replenishment was analyzed base on monitoring data collected in the downstream of Futase Dam. As a result, grain-size refining and re-coarsening are described by a representative grain size D₂₀ because the grain size represents relatively high relation to cumulative sediment supply. It is suggested that an amount of around 10,000 m³/yr sediment supply in the downstream of the dam would not cause river bed re-coarsening. According to a macroinvertebrates analysis, net-spinning, attaching and creeping on Life type and, surface and inter-stones on Bed residence type are highly related to the cumulative sediment supply and the grain size D₂₀.

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