One of the typical channel damages in gravel bed rivers classified as segment-1 is the occurrence of continuous bank erosion due to the development of large meandering channels and the elucidation of its mechanism is important for river management. In this study, we focused on the development of meandering channel caused by bar formation with effects of bank erosion and vegtation by using movable bed experiments and numerical simulation. In particular, it was shown that the meandering channel is developed due to bed aggradation caused by sediment supply through bank erosion. Results of the simulation show bank erosion and invasion of excessive vegetation in bars are factors which strengthen such a large meandering channel.

 The measures for promoting sediment deposition along river bank by using spur dikes were studied experimentally, focusing on the effects of head shape of submerged impermeable spur dike and permeability of permeable spur dike on sediment deposition under a condition that suspended sediment was dominant. The relation between flow structures made by spur dike group and sediment deposition mechanism was considered. As to the sediment deposition rate, it was revealed that an upstream-side head work on submerged spur dike was effective and 50% permeability on permeable spur dike was adequate. Furthermore, configuration of sediment deposition was well explained by flow structures around spur dikes.

 Formative hydraulic conditions of static alternate bars with sediment transport was discussed using flume experiment. Normally, alternate bars in a straight channel move down during floods. On the other hand, bars in a curved channel does not move. In this study, by changing the flow direction using spur dikes, we attempted to create a quasi-flow in the curved channel and static alternate bars were formed in a straight flume. As a result, in the case of using the spur dikes of the length about 10% of the channel width, alternate bars did not propagate. However, there was no sediment transport on bars. On the other hand, under the hydraulic conditions where no sand bar is generated without spur dikes, static alternate bars were formed using spur dikes and sediment was transport on the entire bed in the flume.

 In this paper, we describe the experimental results for the prevention method of compound channelization using the hydrobarrier spur dikes and the results of applying the spur dikes to the field. Hydro-barrier sluice is a transparent sluice with several channels in the body of the sluice. This spur dikes have the function of locally accelerating the flow velocity through the sluices to prevent sedimentation in the downstream region. The effect was observed in Azuma River, Aichi Prefecture. It was found that the single sluice was more effective in preventing sediment accumulation using flume experiments. In addition, when three spur dikes were installed in succession along the flume bank, the effect of the three units working together was observed, and it was found that the effect was the highest at a flow rate at which the water level was higher than the crest of the spure dike.

 In the Satsunai River in Hokkaido, reopening of the inlet of closed side channels had been used as a measure to reactivate the river morphology change, thus to increase vegetation diversity and to reduce a risk of embankment failure. To evaluate the performance of the reopened side channel in activating morphological change and reducing the risk of embankment scour, we modeled the morphological change of river reach between 31.0 km and 28.8 km from the downstream confluence during the 2019 flushing flood by using the hydro-morphological model, iRIC Nays2DH. By comparing the simulated and field-measured results, we found that numerical simulation can capture the trend of river morphological change around the bifurcation introduced by the reopening, but the erosion in the reopened side channel was overestimated compared to field-measured data. The morphology of the side channel was quite stable to the flushing flood based on the field measurement after both 2019 and 2020 flushing floods. The insignificant morphological response of the reopened side channel to the flushing flood is inefficient in reactivating the river bed. By creating an artificial mound to narrow the inlet of another side channel near the bifurcation, more discharge has entered the bifurcation and increased the efficiency of the reopened side channel according to the simulation.

 It is reported that levee breaching and river channel processes with bank erosion recently occur during floods. For disaster mitigation, it is of great importance to understand the mechanism and predict the flow fields accurately by computational model. In this study, hydraulic experiment of river channel process with bank erosion is carried out and numerical simulation is conducted, by coupling the 3D flow model which solves simultaneously overtopping and seepage flows and the equilibrium sediment transport model. It is shown that the development process of meandering channel can be reproduced reasonably, while the averaged meandering amplitude is slightly underestimated.

 In this study, a new method for estimating the flow discharge, roughness coefficient, riverbed level, and sediment discharge based on the temporal variations of the water surface profile was constructed. The method is based on the 1D shallow water and continuity equation for the sediment discharge and is used to estimate the flow discharge, roughness coefficient, bed level, and sediment discharge with roughness and riverbed convergence calculations. Since small-scale riverbed configuration changes the resistance of a river during floods, the method is verified against experimental data on sand waves such as dune I, dune II, transition I, and flatbed. This shows that the present estimation method can reproduce the flow discharge, roughness coefficient, average bed level, and sediment discharge although there are still some problems in reproducing local bed levels and roughness coefficients.

 Scour occurred by flood and so on cause lowering bearing capacity for structure and serious damage. Mechanism of sediment movement have been evaluated by tractive force conventionally, but in late years it has been thought that hydrodynamic forces cause change of stress in ground and that may destabilize surface soil particles. In this research, we carried out open channel experiment under movable bed condition to clarify influence of open channel flow on sediment movement, and considered focusing seepage flow in ground. As a result, seepage flow could be visualized clearly by coloration water and it is clarified that seepage behavior varied according to grain size and thickness of permeable layer. In addition, it is showed that seepage behavior is different according to thickness of permeable layer, and that generated difference of loss of open-channel flow as mainstream and measured tractive force by existence of ground. We think it leads to mechanism elucidate of sediment movement and scouring to understand seepage behavior well.

 The settlement process of plant seeds during a flood event has not yet been fully understood, including their spatial heterogeneity and governing hydrodynamics in the gravel bed surface.This study observed the amount and types of seeds settled onto the gravel beds of the Kiso River and Nagara River during flooding, as well as meas-ured the settling velocities and hydrodynamic characteristics near the gravel bed in an experimental flume. The result of the field survey showed that the density of the seeds settled had highly heterogene-ous in number, i.e., two order differences among the observation points. It also indicated that the seed density was well-correlated to the proportion of finer sediments, including the fine sand, silt, and clay. The result of the hydraulic experiments showed that the settling velocity of the seeds varied depending on not only their specific weight but the physical shape as well. Besides, the observation of the seed movement near the bed suggested that the seed settlements should be highly related to the turbu-lence eddy characteristics near the gravel bed surface.

 In the sediment reproduction to the river downstream of the dam, fine materials including turbidity are trapped in the gravel-bar in the river channel, and there is a problem that they cannot reach the sea. In this study, the hydraulic experiments were conducted to grasp the trapping mechanism of fine sediment in the turbid flow at the gravel-bar. As a result, the gravel-bar with a large grain size ratio and a small porosity in the gravel bar had higher fraction of fine materials. It was also found that the fraction of fine materials increased in proportion to length of the gravel-bar and the hydraulic gradient, and that the turbulent structure and clogging of the sub-surface flow affected the fraction of fine materials.

 The overgrowth of Egeria densa is a major issue in the Gonokawa River. It is necessary to identify the environmental factors negatively affecting E. densa in the river in order to develop the management strategies for eradicating the nonnative aquatic plant. In this study, we quantified the coverage of E. densa with UAV and analyzed the relationships between the coverage and environmental factors on preflood seasons using simple linear regression and generalized linear model. Our results indicated that the coverage was significantly and negatively related to the coefficient of variation of the river discharge in the preflood seasons, suggesting that the river discharge fluctuations could suppress the overgrowth of E. densa in the river.

 Aegagropila linnaei are natural monuments of Japan and in danger of extinction. However, the formation of Marimos has been unresolved. For the clarification of the creation of Aegagropila linnaei, a numerical computation is one of the most powerful techniques, such as a Distinct Element Method (DEM). We attempted to estimate two significant parameters for DEM, elastic modulus and viscosity coefficients of Aegagropila linnaei by applying laboratory experiments, which revealed that two parameters are constant regardless of size and density. Furthermore, we investigated Marimos by using Magnetic Resonance Imaging, which allows non-destructive investigation of Marimos underwater. The base of Marimos is revealed to move from the outer edge to the centre of Aegagropila linnaei when they grow.

 Oligotrophy in coastal areas is considered to be the main cause of deterioration in aquatic product productivity, this can lead to a decrease in algae growth capacity. In this study, we investigated how to create aquatic environment in which algae, especially diatoms, can grow in oligotrophic condition. The effect of GCA (Granulated Coal Ash) on diatoms growth was verified by demonstrating diatoms adhesion to the GCA layer covered in the site tidal range. In addition, lab-scale experiments were conducted to verify the diatom growth and iron elution properties with GCA. GCA not only eluted SiO₂ over a long period of time, but also stimulated the elution of nutrients (N, P, Fe, SiO₂) from sediments. It was found that the growth of diatoms is also facilitated by these changes in water environment. Overall, GCA is a material that can promote the growth of diatoms in the oligotrophic environments.

 The present study predicted the occurrence of the Japanese Littleneck Ruditapes philippinarum using presence/absence data from filed surveys and image analysis of mud flat surface employing deep learning around the river mouth of the Fushino River in the Yamaguchi Bay, Yamaguchi Prefecture, where the decline in the catch of the species is an issue. The shells were found inhabiting at 28 out of the 193 surveyed sites in the Yamaguchi Bay. Of the 448 pictures, each from present or absent sites, 69.5% were judged correctly to be present or absent in total by deep learning. Although pictures taken at sites where the species is apparently absent (i.e. on top of roads, sandy beaches, or rocks) were judged correctly, the algorithm tended to misjudge absent sites as present. The results indicate that even though the program is currently not practicable, analyzing pictures taken by UAV with deep learning could be usable to grasp the general trends in the habitat of the species.

 The low-water channel banks of large rivers flowing through megalopolis, which have been developed as hydrophilic spaces, are recently recognized as the space for nature-rich river works because of its potential for habitats of a variety of aquatic lives and plants. A unique micro-scale brackish biotope can be created in the tidal flat along the brackish water river reach. In this study, a numerical model was developed for water salinity of inflow into a brackish tidal flats in the Gotannno Wand in the Arakawa River, which flows through the Tokyo metropolitan area, and a numerical model for salinity exchange between surface water and bed sediment in the tidal flat. The combination of the two model successfully reproduced the soil salinity variation during the six months from the late autumn to the early summer of 2020.

 After a rain event in urban rivers receiving combined sewage outflows, organic sludge (OS) can float on water surface as scum, causing foul odor and worsening the riverscape. Addressing these problems requires first to characterize the physical behavior of OS. In this study, physical properties and characteristics of erosion and deposition of OS were examined by riverbed cross-sectional survey, physical tests of sediment and scum, and one-dimensional simulation of unsteady flow in the Shakujii River estuary.

 The scum generated in the estuary has dry density of 1,760 kg/m³, where its main source is considered to be the OS containing large amount plant materials with ignition loss of 28 % or more. In addition, it was found that scum hardly occurs after floods causing maximum tractive force of 2.0 - 2.5 N/m² or more. These floods cause largely to erode accumulated OS on the riverbed and inhibit the deposition of organic matters sourced from upstream.

 The objectives of this study are to (1) compare the results of fish environmental DNA(eDNA) metabarcoding from glass-fiber filters with different pore sizes and (2) compare these results with the results of fish collection surveys. We collected eDNA water samples and fishes in five rivers. We filtered the sampled water through the filters with three different pore sizes and analyzed the three types of filters individually for the eDNA metabarcoding. Our results showed that the filters with relatively small pore sizes (GF/F and GF/A) were effective in terms of the coverage of the collected fishes and the similarity to the collected fishes.

 The aim of this study is to evaluate the usefulness of environmental DNA (eDNA) analysis for monitoring seasonal biomass variations of aquatic organisms in the Takatsu River, western Japan. We conducted underwater observation, surbernet collection and eDNA water sampling for Plecoglossus altivelis, Tribolodon hakonensis, Cottus pollux, and Stenopsyche marmorata over two years at roughly monthy interuals to compare the results between the sampling methods. The biomass and eDNA fluxes of P. altivelis, C. pollux, and S. marmorata showed seasonal variations corresponding to their life history. Our results suggested that the eDNA analysis could trace the seasonal biomass changes of the aquatic organisms in the river.

 In this study, we examined the spatial fish distribution patterns in the river segment from the Haji Dam to the three tributary junction section (~40 km) in the Gonokawa River using environmental DNA metabarcoding. We also examined the relationships between the fish assemblage patterns and environmental factors such as hydraulic environment and invasive aquatic plant Egeria densa using the multivariate statistical analyses. Our results indicated that the fish assemblage structure spatially changed as the distance from the Haji Dam increased possibly because of the effects of the dam and weir on the fish assemblages in the river segment. The abiotic environmental factors such as river discharge fluctuation and depth were better correlated with the fish assemblage patterns than the coverage of E. densa in the downstream of the weir.

 We aimed to clarify appropriate time of a day for evaluating spawning intensity of Ayu Plecoglossus altivelis altivelis in the Takatsu River pouring to the Sea of Japan, where the tidal range is small. Water sampling for environmental DNA (eDNA) analysis was carried out three times a day (daytime, one hour after sunset, and three hours after sunset) throughout the spawning season of the species at one to two week intervals. In 2019, late September and late October were assumed to be the peaks of downstream migration based on temporal changes in the daytime eDNA concentration. Spawning was recorded only for one hour after sunset in late October, the putative peak in spawning for the natural population. Thus, surveys undertaken one hour after sunset is also desirable in the Takatsu River for accurately judging spawning conditions of the fish. Nonetheless, no difference in eDNA concentration between the two nighttime hours was observed in the remaining survey occasions, indicating that the spawning of the fish could roughly be estimated based on the results of the surveys three hours after sunset.

 This study assessed microhabitat conditions of juvenile Lefua echigonia using field observed ecohydraulic data and a data-driven habitat model. A series of monthly field surveys were conducted in a spring-fed small urban stream in Tokyo, Japan for 2 years. All data were used for habitat suitability modelling using Random Forests (RF). It was found from the surveys that the juvenile L. echigonia appeared in April and increased by June. Variable importance from the RF computation suggested that microhabitat of juvenile L. echigonia was influenced by flow velocity and water temperature. Response curves illustrated the important instream habitat conditions such as higher water temperature above 17℃, and the presence of vegtation. High habitat potential for higher water temperature was evident as most juvenile L. echigonia were cought in early summer. The importance of lower flow velocity and higher percentage of vegetation coverage that reduces flow velocity reflected the species’ characteristics during its drifting stage. Future works should consider the habitat condition of larva and fly for an improved conservation design of L. echigonia because fish experiences high mortality during these life stages.

 Physical habitat assessment relies on the hypothesis that observed species-habitat relationships represent species’ habitat requirement. However, physical habitat conditions are spatiotemporally dynamic and field measurement such as depth-averaged velocity can over- or underestimate habitat suitability of flow conditions where a fish individual positions. This can result in fault habitat suitability assessment of a target species. To cope with this issue, swimming ability of fish can be used as an indicator of swimming cost under a given water velocity, thereby having a potential for the use in physical habitat assessment on the basis of bioenergetics. This study aims at assessing critical swimming speed (CSS) of Lefua echigonia based on a laboratory open channel experiment. In addition, we conducted a habitat restoration experiment to quantitatively evaluate the effect of large-sized gravels (815 cm in diameter) as a shelter habitat for L. echigonia in fast flowing water (U_0.6 = 30 cm/s at the center). In this experiment, seven cases of allocation of large-sized gravels on the channel bed were tested and spatial distribution of bottom velocity was measured to relate with successful swimming for 60 min. As a result, CSS of L. echigonia was found to be 17.2 ±3.22 cm/s and 4.4±1.19 BL/s (BL: body length). This was in line with the estimated CSS of a small migrating benthic fish Gymnogobius sp. In the habitat restoration experiment, the number of fish remained after 60-min experiment increased with the area of water velocity lower than 17.2 cm/s. This result supports potential applicability of CSS as an index for the assessment of habitat restoration works.

 The exposure height of stones can be used to evaluate a river bed environment. An estimation model for the exposure height was developed herein to reduce the measurement cost and estimate the wide-area exposure height more easily. To improve the model’s accuracy, the size classifications of the stones in the model were further divided, and several improved model configurations were tested. The improved models were applied to various rivers to compare their accuracies with that of the previous model. The improved models generally exhibited better accuracy than the previous model. In the tested cases, dividing the cobble classifications (64–256 mm) into several parts was quite effective in improving the model’s accuracy. In fact, two parts of the divided number were sufficient for improving the accuracy.

 A nature restoration of the Kuroiwa Moor has been conducted for compensation of environmental impacts on recession area of a small hydropower plant in the Ochiai River in northeast part of Okayama Prefecture. In order to evaluate the effects on the hydropower efficiency, we estimated the flow duration curves under different conditions of vegetation patterns in the watershed. One of the causes of wetland degradation in the Kuroiwa Moor is decreases of the wetland water level induced by forestation and increase of evapotranspiration in the watershed. Then we focused on the historical vegetation change and built a tank model with incorporating evapotranspiration and canopy interception effects. Using the model we evaluated Hydropower Development Potential which has an influence on hydropower generation. As a result, Hydropower Development Potential under the vegetation distribution of 1948 is the highest among those of 1948, 1976 and 2017. The results of this study can be applied as a model of small hydropower development in quasi-national parks.

 This study surveyed on the concentration variation of total and labile fraction of metals in a river during rain events. Moreover, we examined the applicability of DGT-Passive-Sampling for understanding the occurrences of the labile fraction in rainy river without grab sampling. As a result, total metal concentrations of Al, Fe, Mn, Cu, Li, Ti, and V tended to increase with the increase of SS concentration. On the other hand, there was no clear trend in the variation of the labile fraction concentration with the SS concentration. Furthermore, the estimated concentration of labile Mn and Cu acquired by DGT-Passive-Sampling were at the same order of magnitude with the results of grab sampling.

 To consider a countermeasure for mitigation and preventation of plastic release into rivers, we attempted to identify plastic product by analyzing microplastics (MP) collected from river water and river bank of three rivers: Ara River, Tama River and Hikichi River. Consistently, MP concentrations were the highest in the midstream of the three rivers. And also, the size of MP on river banks was larger than that in river water, suggesting that the sources of MP on the river bank were differed from those in river water. Furthermore, the four plastic products (i.e., artificial turf, fiber-based plastic, foamed plastic and waterproof sheet) were dominant in both of river water and sediments. The four items were averagely occupied 39% of all items in both of river water and river bank. Our results suggest that it is essential to strength the management of these four plastic product

 In urban rivers where conbined sewer overflow occurs, scum appears several days after rainfall, causing bad odors and adverse effects on the landscape and ecosystem. To clarify the generation and floating behavior of scum, we conducted the detailed observations of scum using multiple fixed-point cameras and the sediment surveys in the Hirano River, an urban tidal river in Osaka. Organic mud derived from sewage was deposited in two separate areas, of which the upstream was the main source of scum in the river. The floating behavior of scum was mainly dominated by tide, and scum accumulated at the stagnant point at rising tide. Scum was frequently observed at the bend of the river channel, suggesting that the local flow structure had a considerable influence on the behavior of scum over a wide area of the river.

 In dam reservoirs, vertical mixing due to water surface cooling has a significant impact on water environmental problems related to phytoplankton and fine suspended particles. In this study, we motivated the elucidation of the flow structure by water surface cooling, and conducted observation and numerical analysis of lake flow influenced by meteorological conditions for the Isawa Dam in summer, in relation to thermal stratification conditions. The period of the study was about one month from the end of August to September 2016. As a result, large fluctuation of velocity was observed in the epilimnion. This flow regime in the second half of September was presumed to be caused by the convective mixing initiated by surface water cooling. In order to analyze such behavior, a simple plume model was applied to the reservoir stratification condition during the period. Correspondence of time and depth of the horizontal flow between the observation and the analysis indicated the validity of our hypothesis.

 The effects of a bubble circulation system on water temperature stratification in the Gima Dam reservoir in Kume Island, Okinawa were evaluated based on the field surveys and numerical analysis. Approximately 240 NL/min. of compressed air produced by a 2.2 kw compressor is distributed to four diffusers. Approximately 60 NL/min. of compressed air per unit is released from the bottom of the lake at a depth of about 10 meters. As an evaluation of the effects, the bubble plume structure, which is the local flow around a rising bubble, and the stratification in the entire reservoir were studied respectively. In spite of the extremely small air volume, less than one-tenth of the air volume of the existing cases of this countermeasure, the local bubble plume structure was formed even in mid-summer, which is necessary for water quality preservation. Furthermore, the whole reservoir was found to be mixed.

 The hydrological phenomenon and intake characteristics of selective withdrawal system (SWS) with simultaneous operation of multi-gates under multilayered stratification were clarified by thermal fluid analysis based on CFD for Obara Dam reservoir in Japan. First, we constructed a model that can reproduce the outflow water temperature. Then, we examined factors influencing the water intake rate, by sensitivity analysis focusing on the stratified condition in the reservoir, intake rates and intake gates. Finally, we found a new parameter of withdrwal stability to esimate each intake rate for up to three gates. These results will help to further improve the accracy of the water quality simulation in a reservoir for an optimal operation of the simultaneous multi-gate withdrawal.

 As a method for preventing sand deposits at the outlet of the spillway, we investigated a method of flushing by gate operation as an artificial method that does not depend on the flow rate and topographic conditions of floods.We used numerical modeling to investigate a method of clearing sand from a spillway by repeated operation of the regulating gate, using the canal capacity upstream of the gate as a storage area to force flushing. We used a one-dimensional model with calculations of unsteady flow and river bed variation to determine the number of flushes required for sand removal at a given storage volume. We found that maintaining a large difference in water level between the upstream and downstream sides of the gate and increasing the duration of a tractive force was more effective for sand removal than increasing the peak flow rate. Our results showed that sand flushing can be completed with 10 or fewer repetitions even when sand deposits extend for 150 m, making this an effective method of controlling sediment accumulations.

 It is said that the poverty of iron supplied to the coastal areas contributes to the decrease in marine products. The concentration of iron in natural water is low, and it is difficult to clarify the role of iron locally. In this study, an experiment was conducted to develop an iron elution device that uses a microbial fuel cell to elute a sufficient amount of Fe²⁺ from steel slag. At the first stage of the iron elution device development, the balance of the charge in the dissolution of steel slag to the elution of iron ions into the water layer.At the second stage, a prototype was evaluated for site to determine the elution and supply form of iron ions in natural water. As a result, it became clear that the supply of H+ and O₂ to the water layer is indispensable in order to elute sufficient Fe²⁺ ions. Furthermore, the presence of Fe²⁺ was confirmed even in the field where the oxidation of Fe²⁺ to Fe³⁺ was concerned, and the possibility of supplying Fe²⁺ to the tidal flat by this device was found.

 Secondary flow that arises in the normal direction of the along-channel flow in the meander is generated as a result of the interaction between centrifugal acceleration and barotropic pressure gradient. In a saline environment, the baroclinic pressure gradient also comes into play, which induces more complex dynamics of secondary flow. However, the introduction of a flood control structure could generate further complexion on the structure of secondary flow as well as its governing mechanisms. In order to get a proper understanding, a 3D numerical model, known as Delft3D, is used to simulate the secondary flow and salinity structure in the Ota Diversion channel. The numerical simulation is distinguished into two different scenarios based on the existence of a flood gate. In the meantime, a field observation that takes advantage of the Acoustic Doppler Current Profiler (ADCP), has been also done to validate the numerical simulation results. The simulation yields that the existence of a flood gate indeed affects the secondary flow structure. The gate gives resistance to water flow, thus limiting the streamwise velocity. This limitation induces a smaller centrifugal force, which results in the dominance of the barotropic pressure gradient and baroclinic pressure gradient over other terms. The interaction between these two terms generates a strong secondary flow in the meandering section of the Ota Diversion Channel.

 In recent years, affected by global warming, overturning is suppressed in large lakes such as Lake Biwa and the similar phenomenon is likely to occur in the future. In order to prevent such phenomenon, it is very important to reproduce the stratified lakes with high accuracy. However, the past research based on the hydrostatic pressure approximation did not take the loss of potential energy (𝐸𝑝) into consideration, and the analysis of 𝐸𝑝 is larger than reality. Therefore, this study is focused on internal solitary waves and tries to model the energy loss under hydrostatic pressure approximation. As a result, the energy loss of internal solitary waves can be modelled under hydrostatic pressure approximation successfully by using frictiontype energy loss and highly accurate analysis of the stratified lakes under hydrostatic conditions becomes possible.

 To clarify the effect of saltwater intrusion on the freshwater distribution at the X-shaped confluence point of the Chikugo River estuary, cross-sectional flow velocity and salinity distribution were observed for one diurnal tidal cycle. This study made the following conclusions: (1) The estuary was well-mixed during spring tide and that the flow rate ratio and cross-sectional area ratio between the tributary and main channels are not significantly different; (2) Vertical distributions of velocity and salinity confirm saltwater retention at the confluence point of Chikugo and Hayatsue Rivers at the start of ebb tide; (3) These findings can be generally explained by the difference in river lengths considering this as the limiting morphometric parameter.

 The Segara Anakan Lagoon is a two-inlet lagoon with a high sedimentation rate in the western part. Tides from the two inlets can be the main factor that leads to the sedimentation pattern. A two-dimensional hydrodynamic model was used to investigate the tidal dynamics in the Segara Anakan Lagoon (SAL). The model results of water elevation and water discharge were acceptable compared with the observations. Spatial computation of tidal duration asymmetry, peak discharge asymmetry, and slack water asymmetry is used to identify the tidal dynamics characteristics of the SAL. The results of the computation show that the tidal wave propagates to the lagoon from both inlets and amasses in the center of the lagoon. The type of tides in SAL is a mixed semi-diurnal tide with around 2 m in maximum range. Analysis of the results of the water elevation model and discharge show that the tidal duration and slack water asymmetry are ebb dominance in both inlets, while the peak discharge is ebb dominance in the western lagoon and flood dominance in the eastern lagoon. However, the tidal duration, peak discharge and slack water asymmetry show flood a dominance in amsassing area. The convergence of tidal waves in the center of the lagoon results in abrupt changes in the tidal range and near-zero discharge in the amassing area. The tidal asymmetry pattern might have implication to the sedimentation pattern in the Segara Anakan Lagoon.

 In the estuary area of the Sagami River in Japan, current geomorphic feature of the sandbar is different from the one in 1992 due to increased height and longitudinal length of the sandbar, resulting that sandbar flushing is unlikely to occur by a small flood. Sandbar flushing at the estuary area play an important role as a sediment provider in formulating the Chigasaki Coast. The objective of this study is to analyze sandbar flushing mechanisms and discuss projections for promoting Integrated Sediment Management at estuary area of the Sagami River Basin. Firstly, this study analyzes processes of sandbar flushing during recent floods by observation data, satellite images, and numerical simulations, namely quasi-three-dimensional flow and two-dimensional bed variation model. The results indicate that local high-flows near the left banks be alleviated and overflows on the sandbar be occurred in order to facilitate sandbar flushing even by a small flood. Secondly, based on the results, desirable geomorphic features of sandbars are set and evaluated by comparing flow velocity and direction, occurrence of sediment flushing, and amount of sediment transported to estuary area. Thirdly, this study discusses projections for promoting integrated sediment management, by proposing an alternative approach, in addition to current sediment augmentation from dams, to making use of sandbars as an important input to formulating the Chigasaki Coast.

 To clarify the impact of the Kurobe River on the surface layer of Toyama Bay, six oceanographic surveys were conducted at the mouth of the Kurobe River since 2018. As a result of CTD and XCTD observations of water temperature, salinity and density, a three-layered structure was observed near the mouth of the Kurobe River, with Tsushima Warm Current in the upper layer of deep water (unique water of the Sea of Japan) and coastal surface water due to river water inflow at the surface. In summer, the salinity of the river estuary was decreased due to the increase in groundwater level and the spring water. In addition, depending on the scale of the river flow, a decrease in salinity up to 2 m in the surface layer was also observed 0.7 km offshore from the river mouth. Finally, the results of bathymetric surveying confirmed a microtopography of the river channel in the past. The seafloor gradient from the mouth of the river to 0.8 km was found to be extremely steep, with a slope of 1/4.

 The present study evaluated the climate change impacts on freshwater discharge and pollutant loads from river basins to Seto Inland Sea based on the long-term simulations using a distributed runoff model under present and future RCP8.5 scenario. Comparing our simulated results during 20 years between the present and future climates, the whole annual river runoff in the future climate hardly changed, but the daily mean runoff in extremely large rainfalls increased in the future climate, leading to 1.2-1.7 times increases in the annual SS loadings. The annual COD, TN, and TP loadings were not significantly different between both climates. These results were attributed to the dominant point source loads although the non-point source loads slightly increased in response to the precipitation in the future climate.

 Destructive tsunamis can damage vegetation and produce tsunami induced large driftwood (TLD). Recent studies have elucidated mechanisms to control the damage of TLD. These methods include inland forests to trap TLD, and the use of an embankment and or moat structures, to reduce the impact velocity of the TLD. This study further investigated the function of hybrid designs and an inland forest in retaining and trapping of TLD. An embankment-moat design, downstream of a vegetation patch showed retention of TLD as well as rotation of TLD. This rotation encourages trapping of TLD by a downstream forest. This benefit in the rotation is especially noticed in tall driftwood models. Embankment-moat design, considering a long period wave, typical to that of a tsunami, then shows the ability to control TLD related damage.

 Okinawa coral reef areas have their own characteristics caused by the peculiar topography.Many reseachers have already revealed various phenomena on site in the Okinawa coral reefs from the viewpoint of the environmental aspects, such as the runoff of red clay and global warming. On the other hand, there are little basic knowledges regarding the characteristics of the currents in this area. In this paper, we performed numerical experiments exerted only tide in the ideal simple geography. The results indicated that the change of the outer reef height had great influence on the structure of residual currents, however, the change of average sea level height had not so much influence on the structure of residual currents.

 The prediction of approximate flood waste amount before and immediately after flood occurrence is an essential element in designing a treatment plan. However, the methodology for predicting the amount of waste produced in large-scale flood, until recent, remains unexamined. In this study, we attempt to develop a method to estimate the amount of flood waste, which can be applied to a large-scale flood in recent years and future. The developed method only requires the statistical information on housing in the damaged area and housing damage information, and either of which can be obtained even in the early stages of flood disasters. The method is mainly based on the actual data of Western Japan Heavy Rain (2018) and Joso flood (2015), and furthermore, it is under the assumption that most of damaged buildings are wooden houses, the dismantle rate differs depending on the extent of damage, and non-wooden houses which are not officially recognized as damaged houses also generate waste. The result of prediction using the developed method was also validated with the actual data and compared with the conventional method. The method developed in this study can estimate the amount of flood waste with a more agreeable result with the actual amount compared to the conventional one.

 To mitigate flood damage caused by outflows of bed load and debris wood in the lower reaches of rivers, improvements to existing sediment basin design are required. To determine how best to improve the transport and capture of sediment and debris wood in sediment basins, we were experimented for quantifying changes to sediment basin structure during debris flows using a sediment basin scale model. To demonstrate the utility of this method, we show the effects of flow conditions within the sediment basin on the sediment transport function.

 Sediment transport maps are used in dam sediment management. Maps are created for each particle size group and show the sediment transport at the observation point, the sediment volume in the section, and the sediment balance. Since sediment transport is difficult to measure by observation, it is generally calculated using sediment and suspended sediment formulas. The sediment volume is measured by sediment surveying. In this paper, sediment transport maps are created from suspended sediment data measured using an ultrasonic attenuation spectrometer and section sediment volume measured by a reservoir sedimentation survey. The maps are used to evaluate the sluicing and flushing operation at the Setoishi Dam on the Kumagawa River in Japan.

 Dam sluicing operation is expected to promote the sediment outflow from dams. Yamasubaru-dam, Saigo-dam, and Ohuchibaru-dam in Mimi river are being planned to integrated dam sluicing operation. The operation of Saigo-dam and Ohuchibaru-dam was conducted from 2017. We observed turbidity, fine and coarse suspended solid (FSS and CSS) at the Ohuchibaru-dam in eight floods to estimate of the effect of dam sluicing on FSS and CSS loadings. Estimators of FSS and CSS were built using the linear mixed model (LMM) which predictors are discharge, turbidity, and water level at Ohuchibaru-dam. Differences of rainfall condition, hydro graph and so on between floods were considered by random effect in LMM. As a result, LMM containing random effect from differences between floods improves the estimation error. Thereafter, accumulated loading of FSS and CSS in dam sluicing operation and virtual operation (conventional operation) were calculated using the estimators. The results showed the dam sluicing promotes the outflow of FSS and CSS from Ohuchibaru-dam. Especially, dam sluicing more increased in outflow of CSS than FSS.

 The UAV-based SST(Sea Surface Temperature) observing system equipped thermal camera has been developed and evaluated its practicability and applicability for the use of environmental survey. At first, the air-sea condition effect to measurement of UAV system was investigated through the observation in the sea off Onjuku by comparing with the moored thermometer. The UAV system underestimated the SST about 1.8ºC to 3.1ºC in the condition of the significant wave height about 1m to 2m comparing with the thermometer at 0.1m depth, which was caused due to the variability of the sea surface emissivity and increased discrepancy between skin-temperature and bulk-temperature associated with wave turbulence. Next, the wide area SST observation was conducted with two UAV systems in the Saru River mouth. It was clarified that the relative SST map was obtained by correcting system noise between thermal cameras and the absolute SST map estimated by using the calibration data by ship-based SST could capture the the spatial and temporal features of the river plume.

 This study shows characteristics of parametric models for atmospheric pressure, wind speed and wave height in a typhoon (or hurricane). The Myers expression and extended ones such as the Holland expression including a shape parameter B and an elliptic type distribution for typhoon-induced pressure distribution are used and a gradient wind model associated with any of these expressions is applied to wind speed estimation. Discernible difference is seen among pressure patterns changing from central pressure toward far-field pressure obtained by the models. As for wind speed, increase of B yields reinforcement of the maximum value and reduction of strong wind area. Additionally, introduction of either Super Gradient Wind (SGW) concept or an elliplic pressure distribution produces its strengthening. In cases where central pressure difference ∆Pc-dependent relation is used for the estimation of typhoon radius R, the maximum wave height in a typhoon calculated by either the Ijima et al. relation or the Young et al. relation is given by a function of both ∆Pc and moving velocity V and its highest value appears around the value of V = 45 km/h in ∆Pc and V space. SWAN provides a wave height pattern with the maximum value near the maximum wind speed area in a typhoon.

 This paper shows the relationship between onshore-offshore movements of the Kuroshio and water temperature variation at an observation tower. Field observation has been carried out at at the observation tower of Shirahama Oceanographic Observatory (SOO), DPRI, Kyoto University, and measured water temperature data from 2015 to 2019 include both periods of straignt and meandering paths of the Kuroshio. When the Kuroshio has straight path, the axis of the Kuroshio exists close to the land, and the bifurcation current called "Furiwakeshio" often occurs. On the other hand, when the Kuroshio has meandering path, the axis of the Kuroshio aparts from the land.

 From the results of frequency analysis, variations of water temperature with about 30 days interval exist regardless of the axis positions of Kuroshio. This result indicates that some factors other than the onshore-offshore movements of the Kuroshio lay behind the water temperature variations at the observation site.

 Water temperature variations in the winter of 2017, 2018 and 2019 are also influenced by onshore-offshore movements of the Kuroshio. In 2017, the Kuroshio exists close to the land, and the existence of Kuroshio has a dominant influence on the water temperature variations. When the axis of Kuroshio is apart from the land in 2018 and 2019, water temperature variation at the observation site is affected primarily by the water temperature distribution in adjacent area. Drop in air temperature and movement of cold water mass act as triggers for an event of colder water around the observation site in 2018.