In the present study, to build a river-discharge measurement system without floats in upstream, we have attempted to apply a radio current meter and image-processing technique to a flood-flow measurement in Tone River. We conducted eight observations during a typhoon in 6 and 14 October, 2014. We used an ADCP, a radio current meter, a high-definition camera and an aerovane. The observed results indicate that the water-surface velocity measurement using a radio current meter and image-processing technique ware influenced by the wind. However, the measurement accuracy of the velocity using the radio current meter and image-processing technique with the effect of wind into consideration was in good agreement with that of ADCP.

View full abstract

 Various groundwater velocimeters were developed to measure groundwater flow. However, those conventional velocimeters need electricity as well as high cost. Our objective was to develop the low cost and unplugged groundwater velocimeter. In this study, single dot type paper disk groundwater velocimeter was tested. Calibration of the velocimeter was done with calibrating sand vessel. As a result of the study, with 5 minutes measurement, velocity range of 0.3 cm/min-1.5cm/min could be measured. With 60 minutes measurement, velocity range of 0.03cm/min-0.1cm/min could be measured. From the result of the field observation, flow direction of the calorimetry method matched our method for sufficient length of the dot tailing on the paper disc.

View full abstract

 Continuous observation of underflow water in close proximity to a river levee were conducted. The estimated permeability of the ground surrounding a bore hole, calculated from seepage velocities measured in the bore hole, closely matched that of the continuous observations. This series of observations confirmed that the direction of the seepage flow changes in accordance with the hydraulic gradient which is affected by the river water level. These results demonstrate that the system proposed in this study, monitoring underflow water in a bore hole near river levee, can be a viable means of monitoring the river levee leakage. This study requires further investigation from actual data from floods to verify these results.

View full abstract

 In recent years, ultrasonic attenuation spectroscopy has been frequently used for the measurement of suspended sediment concentration and particle size distribution. Sediments are derived from the rock masses at the upper basin and transported downstream owing to floods, and then deposited in the reservoir. The reservoir sediments affect the entire river system because the sediment transport is blocked by dams. For this reason, sediment sluicing has been performed in several rivers. In such cases, it is important to monitor the suspended sediment concentration and particle size distribution. Therefore, we have measured the suspended sediment concentration and particle size distribution using an ultrasonic spectrometer during sediment sluicing by drawdown operation. The applicability of our measurement technology using ultrasonic attenuation spectroscopy for monitoring of suspended sediment concentration and particle size distribution was verified.

View full abstract

 Measuring the bedload transport rate is utmost interest to understand and quantify all sediment transport related phenomena. Three existing bedload measuring systems are 1) the hydrophone, 2) the geophone, and 3) the plate microphone as their modified system. All systems estimate transport rate based on sediment impact, whereas each has respective disadvantages. In this paper, as the other improved system, plate-type vibration sensor is discussed. Calibration experiment is conducted and the system is compared with the other plate hydrophone. In the experiments, impact data is perceived as both vibration pressure and vibration pulse, and analyzed the same as plate microphone. Although it has been clarified that fine sediments lower than 10 mm are not detectable with plate hydrophone, plate-type vibration sensor detected vibration pulses of them clearly. Moreover, detection rate can be linked to the sediment bulk density and the gravel jump length.

View full abstract

 The present study introduces a method with modifying the gradient-based edge detection method for mapping sandbars automatically using satellite-based information. The target area is a part of the mega-sized Jamuna River in which several different sizes of sandbars exist and often change positions. The authors employ threshold and gradient based methods to detect sandbars. The threshold based method (TBM) detects sandbars by computing the Modified Land Soil Water Index (MLSWI) using MODIS 8day composite data. The proposed modified gradient based method (MGBM) produces better results without any threshold using MODIS daily surface reflectance. To verify the results of the proposed method, the authors conducted field surveys traveling along sandbars with GPS and compare with the Landsat images. The results suggest the sandbars' sizes and shapes identified by the proposed method coincide closely with the field investigations and Landsat images.

View full abstract

 In recent years, research on the behavior of suspended solids (: SS) in coastal areas has been carried out. However, physical and chemical characteristics of SS have not been understood sufficiently since such SS consists largely of fine cohesive particles. In order to reveal the properties, effects of surface waves, currents and turbulence on SS were investigated by using in-situ data in the neritic region, Isahaya Bay, Ariake Sea which is a typical enclosed sea in Japan. A multiple regression analysis was performed in order to understand the phenomena affecting SS. In this study, turbidity was used for the analysis instead of SS because it is difficult to measure SS continuously. The multiple regression analysis revealed that wind waves in the bay were extremely important for the turbidity, and turbulent energy, sum of each mean square of velocity fluctuation eliminating surface wave components, and mean velocity or mean sea level variation contributing to tidal current were also slightly related to the turbidity.

View full abstract

 To understand difference of density stratification due to freshwater transport from rivers through flood events and tidal range (spring tide or neap tide), four time 25 hours continuous field measurement on tidal current, CTD, turbulence micro-structure, and suspended solid (SS) particle size property were conducted at a point in the northern Ariake Sea in summer and winter in 2014. From the results, the followings were clarified: 1) sub-tidal development and decay process of saline-freshwater stratification during flood season was clearly measured; 2) Turbulent energy dissipation rate was correlated to mean flow velocity in spring tide in winter only, but not in neap tide and in stratified spring tide; and 3) Tempo-vertical profiles of eddy viscosity and dissolved oxygen were related to condition of stratification.

View full abstract

 Coastal marginal seas and estuaries are generally dumpsites for wastewater from sewage and power plants. For an urgent assessment of dilution and dispersal of toxic materials due to accidental leakage, we develop an offline passive tracer model by exploiting oceanic reanalysis products and analyze differences in spatiotemporal variability of the leaked tracers in an estuary and a continental shelf margin on the Pacific side of Japan. In the estuary, the northeastward through flow affects the dispersal, while the month-long transport is dominated rather by inter-seasonal variability of wind-driven Ekman flow and counter-clockwise local circulations associated with the cold-dome density structure. On the marginal coast, the Kuroshio and resultant secondary recirculation readily entrain the tracer, leading to immediate transport mostly in the alongshore direction. The seasonal difference in the tracer dispersal is apparent such as sporadic offshore tracer eruption episodes, depending on the locations of the Kuroshio path.

View full abstract

 Moon jellyfish has many habitat in inner bays and enclosed coastal seas in Japan, which sometimes causes fishery damage and failure of cooling water intake at power plants. It is desired to develop a method to predict an amount of generation and seasonal distribution change of jellyfish, especially in enclosed coastal area. A numerical approach such as a particle tracking method is effective and has been used to a simulation of long-term jellyfish transport. In this study, a particle tracking method is applied in Tokyo Bay to calculate the annual change of jellyfish distribution, in relation with temporal change of water quality such as salinity, DO, etc. An effect of water quality on jellyfish is added as an advection term of particle transport equation in a manner that is proportional to the gradient. Simulated distribution is qualitatively more consistent with existing observed distribution in comparison with the case without water quality effect, which demonstrates the potential of annual simulation of jellyfish distribution in enclosed bay area.

View full abstract

 Recent studies have reported that submarine groundwater discharge (SGD) is not negligible as a nutrient source from land to coastal sea. In order to evaluate SGD rate and its influence on water quality of sea bottom water in Osaka Bay, field surveys and analyses were conducted utilizing ²²²Rn as a natural tracer.
 As the results, ²²²Rn concentration represented the higher value in the bay head than the offshore. Principal component analysis indicated that the water quality of bottom layer in the north port area was affected by SGD. Moreover, numerical analyses showed that ²²²Rn flux from the sea bottom was estimated at 0 to 120 Bq/m²/day and that local flow field and estuary circulation played an important role in ²²²Rn distribution in the bay.

View full abstract

 It is very important to evaluate gas transfer velocity and the surface velocity divergence in the open-channel flows such as artificial flume and natural river. It is however hard to obtain universal form of gas transfer velocity in rough-bottom open-channel flows by using original surface divergence model. The present study focused on the interfacial gas transfer in the open-channel with strip roughness and found that the large surface velocity divergence is induced by shedding vortices around roughness element. Particularly, the gas transfer depends significantly on relative spacing of strip roughness. Furthermore, we examined whether a modified surface divergence model predicts reasonably the gas transfer velocity, irrespective of bottom roughness conditions in open-channel flows.

View full abstract

 In the Higashiyokobori and Dotonbori rivers, water quality is controlled by inflowing outside non-polluted river water from upstream gate and outflowing inside water from downstream one. However, there is hypoxic water mass during high-temperature period. In the present study, field surveys, laboratory experiments, and numerical simulations were conducted to evaluate the water-quality improvement effect of exchanging water mass with outer rivers through the gates and to propose more effective methods. The results showed that main factor of hypoxia was DO consumption process in the waters and that more than 2.0 m³/s discharge through the gates is needed to maintain a good DO environment in summer.

View full abstract

 Marine debris coming from inlands has become a huge environmental issue in coastal areas recently. However, there is very little information of actual behavior of floating litters in rivers available. Thus, we examined Lagrangian behavior of floating litters in rivers using floats with GPS sensers and mobile phones. Field surveys were conducted in Edo and Mogami Rivers for flood events. The results indicated that the travel distance of floats during a flood increases with its magnitude. The results also indicated the positions of floats in the river were mainly in leeward side and outer bank in Edo and Mogami Rivers respectively. The different position of the floats is mainly due to the differences in flow and wind fields, which are appreciably influenced by the channel alignment and morphology respectively.

View full abstract

 In this study, a new assessment of river environment using benthic fauna is proposed. Field observations for the samplings of water quality and benthic fauna were conducted at 5~12 points in the ten first-class rivers of Chugoku district. As a result, there wasn't any significant correlation relationship among the water quality (T-N) and the stable isotope ratios of nitrogen (δ¹⁵ N) in benthic fauna. However, δ¹⁵ N in benthic fauna have a clear relation with the land use ratio of urban and farm area in the catchment for each sampling point which was estimated using GIS analysis. This result indicates that the δ¹⁵ N in benthic fauna can be an indicator of the river environment.

View full abstract

 This paper suggests a global model for environmental flow requirements (EFR) focusing on the regional differences in richness and vulnerability of fluvial ecosystems. The EFR in this study consists of base flow and disturbance. Base flow is estimated by trophic levels and vulnerability while rate of disturbance is defined by hydro-climatic characteristics. The EFR is set as a percentage of mean annual discharge. In comparison with the existing global EFR which is estimated by flow regime only, this model suggests 20-50% higher EFR in monsoonal and tropical savannah regions with high ecological richness as well as large rivers and lakes in higher latitude where primary productivity is low and ecological structure is vulnerable.

View full abstract

 Development of methods to predict the changes in fish biomass due to the changes in flow field is very important to evaluate the impact of river management on the ecosystem. In this study, we developed a method to predict fish biomass in all parts of river's main stream. The weight of each fish species was calculated by visual observation and sampling and then re-classified into four categories: ‘amphidromous-necton’, ‘amphidromous-benthos’, ‘freshwater-necton’, and ‘freshwater-benthos’ to set objective variables of generalized linear models (GLMs). The results of GLMs analysis showed that the predicting models of biomass can be constructed for all four categories by using hydraulic characteristics. The method developed in this study can be applied in river management to evaluate the relationships between environmental changes and biomass of the entire basin.

View full abstract

 In this study, we constructed a prediction and evaluation model using PHABSIM to target ayu spawning in the Takatsu River. In addition, we examined the experimental excavation shape to improve the spawning environment. A test drilling was carried out on site to verify the validity of the prior predictions based on the monitoring results from before and after drilling. Furthermore, based on the monitoring results, it was proposed to improve the prediction and evaluation models for the improvement of prediction accuracy.

View full abstract

 A numerical simulation model was developed to describe the habitat of benthic macroinvertebrates, which includes two types of riverbed structure simulation and two types of habitat evaluation for the benthos in gravel-bed. The model was applied to an actual river around a dam for model verification, in which the composition rates of bed material and the community structures of the benthos were compared with field data. The result of the numerical simulation shows that the non-equilibrium simulation for riverbed structure can reproduce the spatial change of the composition rates of bed material at observation sites. Comparing between the results of the numerical simulation and the field data, the composition rate of Bed-residence types at the downstream of the tributary confluence has shown a tendency of rehabilitation toward the composition at the upstream by supply of fine sediment from the tributary.

View full abstract

 Since 2002, the catch of Corbicula japonica has decreased in Lake Ogawara, which is located in the southeast of Aomori prefecture. The lake is one of the representative brackish lakes. The cause of the decrease until 2005 is the catch limit by Ogawarako fisheries cooperative association. The catch limit were due to decreases of the stock biomass of Corbicula japonica in the lake. After that, in 2006 and 2007, the stock biomass increased. In recent years, the commercial size is over 18.5mm in length, which has not increase. The purpose of this study is to investigate the cause of the decrease in the biomass over 18.5mm in length in recent years, and to consider the relationship between the bottom sediment, water quality and the stock biomass of Corbicula japonica in the lake. The results show that the habitat for Corbicula japonica have become small, according to the values of dissolved oxygen, the mud content in sand and the ignition loss.

View full abstract

 Recently, sludge purification of organic matter enriched river bottom using granulated coal ash (GCA) have been used in Hiroshima Otagawa delta areas. It has been proved that GCA can absorb the compounds of phosphorus, nitrogen and sulfur. By previous study, the material properties of GCA for biological affinity have been clarified. On the other hands, a characteristic as the inhabiting basis of bivalves is not sufficiently clarified. The purpose of this study is to evaluate GCA covering method utilized to habitat basis of Corbicula japonica. Laboratory experiments and field investigation were carried out to compare GCA (covering material) for habitat basis with control materials (e.g. natural sand, natural stone and organic mud) about a sneaking act by Corbicula japonica. From our findings, it can be concluded that GCA can be used as a covering material to improve biological environment and utilized to habitat basis of Corbicula japonica in organic matter enriched river bottom.

View full abstract

 Recently, sludge purification of organic matter enriched sediments using granulated coal ash(GCA) have been used in Hiroshima delta areas. It has been proved that GCA can absorb the compounds of phosphorus, nitrogen and sulfur. However, the material properties of GCA for soil reactivity and sediment stabilization have not been clarified. The purpose of this study is to evaluate the sediment stabilization mechanism and the performance of sediments improvement by GCA utilized as covering material. Laboratory experiments were carried out to evaluate the soil reactivity (e.g. chemical properties, CEC, phosphate absorption coefficient) and sediments stabilization mechanism by GCA covering method. And, field investigations were also carried out to estimate the stabilization ability by GCA. From our findings, it is concluded that GCA can be used as a covering material to improve sediments stabilization in organic matter enriched sediments.

View full abstract

 The radioactive substance, such as ¹³⁴Cs and ¹³⁷Cs from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), were transported with finer soil particles. In order to simulate the radiocesium transport, it is necessary to understand well the behavior of soil particle in hydrological systems. In this paper, we conducted the soil erosion experiment by using rainfall simulator and applied the existing erosion model to experimental results. Then we focused on the soil erosion phenomenon of each soil particle size. According to the experimental result, erosion rate reduces with time and flow rate become constant. The particle size distribution of eroded sediment and original soil surface are similar results. The existing erosion model reasonably reproduced different characteristic of each soil type and each particle size.

View full abstract

 This study proposes an improvement to the method for evaluating suspended sediment (SS) transport from different regions of a catchment to the downstream area using X-ray fluorescence (XRF) analysis. The target area, the Oromushi River basin in Hokkaido, has SS yields among the highest in Japan due to the predominance of soils originated from volcanic rock and high contents of silt. The geochemical composition of soil and deposited sediment samples collected from 12 land groups, each with an area of 2.45 km², was analyzed by XRF analysis. Kruskal-Wallis H test results pointed out that among the detected geochemical components by XRF only Na₂O, SiO₂, Al₂O₃, CaO, Fe₂O₃, MgO, P₂O₅, K₂O, MnO and TiO₂ were statistically suitable to conduct SS estimations. A modified Neural Network Analysis (NNA) was applied to estimate the SS yields from the different land groups. The results suggested that land groups G1, G3, G11 and G12 representing 33.45% of the total area produce about 64.70% of the total SS yield.

View full abstract

 In recent years, the excessive growth of trees such as Salix species after river channel excavation has posed negative effects on flood control and the river ecosystem, creating another challenge for river management. In this study, we investigated how Salix chaenomeloides may invade and take root in the Ibi River by conducting genetic and flow regime analysis, based on data and information accumulated for ten years after the excavation and research on the current conditions of vegetation in the river channel. The study found that fast flow velocity due to high water levels in the river may allow Salix chaenomeloides at farther upstream points to travel to excavation points.

View full abstract

 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. The results of Maxent show that the area under the curve (AUC) exceeded 0.7 and the sensitivity based on the vegetation map of the River Environment Database exceeded 0.9 in all cases. Thus, these results provided adequate accuracy that can explain the potential habitat of willows. In addition, in the case of the Saba River, density of the weir was higher in the area of actual than in the area of potential willow habitat.

View full abstract

 Gravel-bed rivers in Japan are now facing a multitude of problems relating to fluvial environment and flood control. These problems include the expansion of river beds with less disturbance by flooding and the growth of the riverine trees and the decrease of high-quality gravel-bed bars and the immobilization of low-flow channel. In particular, the gravel bed river that dominated by vigorous growth of Willows and Robinia pseudo-acacia, sometimes increase the riverine trees by vegetative propagation under the flood disturbance. In addition, expansion of the overgrowth density is likely to occur as the young riverine trees that easily fallen by flood disturbance. In this study, to target the gravel bed of Watarase river that Robinia pseudo-acacia, etc. grows in the river, evaluated the dynamics of young riverine trees after felling, and proposed the excavation shape of gravel-bed bar for forestation suppressing by using flood disturbance.

View full abstract

 In the middle reaches of the Asahi River, gravel bars were disappeared and were mostly covered by vegetation on the bars in the recent years. At a sand bar of the Gion area which is one of the fairly vegetated bars, gravel bar restoration work was carried out by removal of vegetation and deposited sand in 2005. After that the gravel bar have been maintained there in several years, and aggradation of severely scoured riverbed at low-water channel was confirmed. Numerical simulation using a vegetation growth and disappearance model proves that the conducted gravel bar restoration work have an effect on an expansion of gravel bar and river bed aggradation at local scour area. Furthermore, an expansion of spawning area of ayu by gravel bar restoration was clarified quantitatively using PHABSIM assessment.

View full abstract

 Forestation on the bars in the middle reaches of the Asahi River has fairly progressed in recent years. The forestation reduces flood flow capacity and increases the water level during flooding. As countermeasure works of the forestation, tree's trimming and removal of deposited sand were conducted at the bar of the Gion area. Although the restored gravel bar was maintained for several years, reforestation has been confirmed there. In this study, the effect of a combination of two countermeasure works as sand removal of the bar and backfilling sand of low-water channel was evaluated using annual scale flood flow analysis over the next several years. Numerical results show that the the combination of works is more effective against the reforestation.

View full abstract

 Inverse estimation of the flow resistance (or bed roughness) because of thick vegetation in the flooded rivers was conducted using variational and sequential data assimilation methods. An adjoint method and a particle filtering technique were effectively used with the shallow-water model for searching optimum values of distributed Manning's roughness coefficients in both space and time. These data assimilation methods were applied to the flooded Asahi River which occurred in September 2011. The assimilated data were water levels that were longitudinally observed in the target reach using 18 water-stage recorders. Results showed that the optimum roughness coefficients analyzed by two methods present the similar trend in both space and time. They also showed such a trend is closely related with the establishment of type of dominant vegetation and vegetation wash-out phenomena during flooding.

View full abstract

 In this study, we developed the vegetation prevalence model. This model takes into account the existence/nonexistence of vegetation and the resistance of willows above ground and their root systems to flowing water and sediment transport at different tree growth stages. The growth increment regarding various parts of a willow tree is estimated on the basis of the tree age. In the model, willows' resistance to flowing water and sediment transport is varied with time to simulate each growth stage. The vegetation prevalence model was incorporated into a riverbed evolution analysis model for analyzing the mechanism and the contributing factors of thick and extensive vegetation growth.

View full abstract

 The objective of this study is to observe the characteristics of lotic seeds distribution on bare sandbars, and to confirm the differences of bed morphology change without vegetation and with vegetation patches on sandbars by means of laboratory experiments and numerical analysis. In the experiments, after sandbars were formed, enough amounts of alfalfa seeds were supplied from the inlet section of the flume. The result of experimental flume tests showed that alfalfa seeds, transported by flow, were settled at the front edge of sandbars during the recession stage of the discharge. This is because, the flow at most downstream areas of sandbars and the front edge of sandbars decreased earlier than that of middle area of sandbars during decreasing of discharge, as shown with the numerical results. This flow causes the specific seeds deposition areas. Those results imply that invading vegetation has specific settling characteristics and make vegetation patches on sandbars.

View full abstract

 Influence of hydraulic conditions on the bed-level fluctuation of mangrove forest during flood was clarified at the Okukubi River in Okinawa Island, Japan. Bed-level changes were monitored every one month from June 2012 to July 2015 at 18 stations. While non-dimensional shear stress during a flood was estimated by two-dimensional bed deformation analysis. Then relationship between observed bed-level change and non-dimensional shear stress was evaluated from correlation analysis. Bed-level change negatively correlates with non-dimensional shear stress during large flood. While during small flood, bed-level change doesn't correlates with non-dimensional shear stress. These results represent that bed-level fluctuation is occurred at sites where large non-dimensional shear stress is keeping during large floods. This condition can apply to selecting priority site in order to restoration of degraded mangrove forest.

View full abstract

 The lower reach of Gounokawa River has a lower river improvement security level than the upper reaches. It will take long time for dike construction to be completed and therefore the rapid improvement of the river improvement security level cannot be expected. On the other hand, bamboos planted as flood reduction measures have been still left as flood defense forests. However, the expansion of the growth range and having withered on the stalk of the bamboos become the problem for the river improvement. Therefore it is important to maintain flood defense forests in consideration of the river improvement effect. In this study, I evaluated positive and negative aspects of the flood defense forests and examined an improvement method using a 2-D shallow-water flow model.

View full abstract

 Unmanned Aerial Vehicles (UAVs) are a new type of platform for high-resolution aerial photogrammetry. Despite of their growing applications there has been little assessment of their performance. This study reports the results from two field experiments designed to evaluate the accuracy of digital surface model (DSM) derived from the imagery acquired with a low-cost digital camera onboard an UAV. Concurrently the survey was carried out using RTK-GPS and Total Station. The first experiment was performed to evaluate the highest measurement precision at a parking lot at Hiroshima University. Another experiment was conducted in the reaches of the Jyoge River. Comparisons between the DSM and the survey data have pointed out that the number of Ground Control Point and the quality of photographic lens are important factors for high precision measurement and that the r.m.s. errors at altitude of 100 m are less than 2 cm, suggesting that UAV photogrammetry has high applicability to river morphological mapping.

View full abstract

 Interspersed weirs in river basin are not only affected on the flow regime and riverbed deformation but also on the river ecosystem. In this study, the effect of weirs on river environment is discussed using river ecological model. This ecological model consists of one-dimensional unsteady flow, one-dimensional calculation of bed variation, heat balance, water quality and biological model. This model is applied to Saba River, where many weirs are installed. In the calculation with weirs (present condition), the water level and intake discharge at the weirs are taken into account. On the contrary, in the calculation without weirs, the water discharge without the intake are assumed. The comparison between these calculations revealed that the intake at the weirs causes the imperceptible change of water depth and temperature, however amount of aquatic livings in stable state remain unchanged.

View full abstract

 Submerged aquatic plants are found in many rivers and have a significant impact on water ecosystems. Detailed field measurements of the biomass flux of specific aquatic plant species during floods may provide powerful support for understanding the removal, transport, and settlement of the focused aquatic plants during floods. Quantitative knowledge of the response of aquatic plant biomass to flood impacts is essential for establishing a predictive model of aquatic plant biomass change. This study aims at making clear how and how much aquatic plant is washed away by flushing flows. The field surveys were conducted during flushing flows implemented every in March from 2013 to 2015. A budget of aquatic plant biomass as a response to flushing flow impact was estimated.

View full abstract

 Monitoring the thickness of the alien aquatic weed Egeria densa overgrowth and identifying the environmental conditions that affect its growth are important factors for its management in rivers. In this study, unmanned aerial vehicle (UAV) was used to establish a method to quantify the cover degree of E. densa. In addition, environmental conditions that affect the cover degree of E. densa were explored by using fluid models, geographic information system (GIS), and generalized linear models (GLM). The results of this study showed that the cover degree of E. densa can be easily quantified by using images taken by UAV and GIS. Furthermore, the results of GLM showed that the distance from weir located downstream is the most effective parameter for the cover degree of E. densa.

View full abstract

 In order to understand the swimming characteristics and swimming ability of demersal fish, swimming experiments were conducted using Goby (Gymnogobius urotaenia sp.) during the upstream river migration period. Our observations were as follows: 1) at a flow velocity of 17 cm/s and higher, Gobies adhered to the bottom using the suction cups on their ventral fins, and showed repeated advancement and adherence; 2) the body length of Gobies is between 3-4 cm, and their average swimming speed was found to be 56-107 cm/s, which is 16-31 times the body length; 3) a swimming-curve-formula defining the relationship between swimming speed and swimming time was obtained; and 4) at flow velocities of 44 cm/s and 53 cm/s, we frequently observed Gobies flowing down in exhaustion during advancement.

View full abstract

 It is important to keep suitable area by boulder for fish in pool-and-weir fishway to make the migration rates high. In this study, migrating behaviors of oikawa by putting boulder on pool bed in pool-and-weir fishway was investigated. The migration rate of oikawa was obtained with the aid of two sets of digital video cameras. In addition, swimming position, swimming direction and migration route was investigated. It was found that migrating behaviors are difference between bed with boulder and flat bed. This is because oikawa swim slow flow area caused by boulder on pool bed. Further, swimming direction of oikawa in the case of flat bed comparatively match with each other, whereas that of it in the case of bed with boulder don't match.

View full abstract

 Fish sometimes enter into an intake gate of weirs and dams, so that a lot of released larval ayu (Plecoglossus altivelis altivelis) are lost. Some techniques with air bubbles, electricity and transmitted light have been developed to avoid fish from the intake gate. However, a lot of fisheries co-operative associations pointed out that the most of such techniques have no effect. It is not known that fish avoid the area which is included air bubbles or not. In this study, the air bubbles are injected from the bottom of channel with changing the flow velocity, and trajectories of fish are recorded. It was found that air bubbles almost effect on the swimming behavior of ayu. The swimming distance and the swimming speed of ayu are decreased in air bubble area.

View full abstract

 This paper verifies a control-theoretic approach for assessing upstream migration of individual fishes in 1-D open channels. This approach uses a solution to a Hamilton-Jacobi-Bellman equation (HJBE) for the optimal swimming velocity of individual fishes as the minimizer of the cost to migrate upstream. With the help of an analytically derived formula from the solution to the HJBE, the cost function, which is the core of the control-theoretic approach, is identified from the observed data sets of swimming speed of migratory fishes in laboratorial and natural open channels. Both advantages and disadvantages of using the identified cost function are discussed and its sensitivity analysis is carried out. Applicability of the presented mathematical model with the identified cost function to practical problems is also mentioned to indicate its potential applicability to practical problems.

View full abstract

 The prevalence of exotic species has been a major ecological problem all over the world, and Japan is no exception. This study modelled distributions of five exotic fish species (channel catfish, bluegill, largemouth bass, smallmouth bass and mosquitofish) in major rivers in Kanto region by using classification techniques: classification and regression trees (CART) and random forest (RF). National Census on River Environments was used as a response and predictor variables. Both models showed high prediction accuracy for all exotic species, and RF outperformed CART. Suitable habitat ranges of the fish estimated by RF were well accorded with the ranges reported based on observations. Additionally, the result was presented also as the species distribution map as an example of its application. Overall, this research demonstrated the importance of the combination between advanced statistical approach (CART and RF) and detailed environmental data.

View full abstract

 For the purpose of mitigating flood damage by overflow from a levee breached, the authors conducted an experiment using the full-scale levee at the Chiyoda Experimental Channel on the Tokachi River. The scale of the channel is 1300m long and 30m wide, accompanying a flood area of 80m width. In the experiment works, a group of blocks were installed on the slope of the levee 10 and 20 m downstream from the artificial breach, and water was let flow from upstream. After the water was let flow, when the breach progressed downstream to the concrete armor blocks, the blocks protected the failure levee. The downstream progress of breach was expected to be arrested by the concrete blocks which were expected to weaken the inundation flow from the channel. The control effects by the concrete blocks were examined.
 The authors concluded that placing concrete blocks on a downstream slope of levees can control levee breaches effectively.

View full abstract

 Riverbank failures by overflow erosion are frequently occurred by extreme floods. However, studies of the relationship between hydraulic conditions, material conditions and levee failures by overflow erosion is insufficient, specially, the relationship between overflow duration and levee failures by overflow erosion. In the present study, we estimated overflow duration that is difficult to measure in the field surveys, by the one-dimensional simulation of unsteady flow. From that, a simplified estimation method of overflow duration is proposed. The results indicated that the simulation of this study can reproduce overflow duration in extreme flood. Furthermore the simplified estimation method of overflow duration is valid.

View full abstract

 The spatial depth distribution of flood inundation, caused by 2015.09 Kanto-Tohoku heavy rainfall, was estimated at the lower part of the Kinu River basin. The presented procedure follows 1) to measure the elevations of flood marks (i.e. flood water levels) with a high spec GPS (at 35 locations in this study), 2) to spatially interpolate the measured water levels and 3) extract the topographic elevation from the interpolated water level. The estimation result suggested that paddy field in the southern part of Joso city had the deepest depths with about 3.8 m, while the average depths in the entire area was estimated to be 1.44 m. Average depths on natural levees (= 0.87 m), typically used for residential areas, was about 50 % smaller than that on other flat areas (= 1.75 m). The presented approach is an efficient method to estimate the spatial distribution of flood water depths over large areas.

View full abstract

 This study is concerned with lateral exchange of flood water between the flood plain, where a banking structure exists, and river channel, due to breach flows from river channel as well as return flows of flood water. The study is directed toward the examination of lateral exchange of flood water simultaneously occurring in a plurality of river channel. It is demonstrated that a proposed numerical model, that comprises the finite volume method based on unstructured grid using flux-difference splitting (FDS) technique and numerical method to solve simultaneously flows in a channel and inundation flows in a flood plain along with appropriate boundary conditions is effective to simulate the above mentioned complex lateral exchange of flood water.

View full abstract

 Large floods occurred frequently in the upper Ping River basin and their potential damages are increasing due to the growth of cities and economical activities. Therefore, the information for flood protection is needed to reduce the damages, especially for city areas. This study attempts to establish a model to simulate flood and inundation against extreme rainfalls in the upper Ping River basin in Thailand for analyzing causal factors of inundation and proposing countermeasures. CASC2D model, a two dimensional hydrologic model was applied for this purpose. The model was calibrated and verified with flood discharge and inundated areas observed during the devastating floods in 2011. The simulation results were evaluated using two indices: Nash-Sutcliffe efficiency (NSE) and coefficient of determination (r²) to show acceptable model performance. The model performance of inundated areas was improved by modifying topographic conditions. It showed predicted areas of local inundation along rivers and in the agricultural fields after storm events.

View full abstract

 In Japan it has been difficult to construct new dams because of environmental and financial restrictions. So it is important to maintain existing dams suitably. Especially reservoir sedimentation is one of the most important problems for securing long term functions of dams. It is, therefore, necessary to plan and carry out efficient and economically feasible asset management for existing dam reservoirs.
 In this study, we studied efficient asset management measure for several dams in the same river basin. We analyzed the several risks regarding flood control and water utilization based on sedimentation progress, and proposed how to prioritize several dams for sediment management. As the result, we showed the advantage of asset management measure to deal with dams in a group.

View full abstract

 In recent years, rainfall forecasting technologies have been advanced remarkably and some forecasts with high accuracy have been applied to dam operations practically.
 In this study, a new proposed weather and flood forecasting method, which combines the efficient formulation of GSM (Japan Meteorological Agency's Global Spectral Model) and the information about typhoons, have been applied to Ikehara and Kazeya hydroelectric power dam at Kumano river basin and evaluated. The results showed that the proposed method had the applicability to the dam operations in terms of the reductions of dam flood discharge.

View full abstract