This study presented a method to assess the economical flood damages based on a hydraulic model, which took into account a spatial distribution of extreme precipitation estimated by statistical and meteorological characteristics. In order to estimate variation of precipitation in the future, we used cumulative distribution function of precipitation that was outputted by GCMs. The potential economic damage in 2100 (RCP8.5 scenario) for a flood of 100-year return period was estimated to be 107 billion USD. We evaluated the effect of flood control level using some GCMs with scenarios.

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We attempted to develop a new methodology for flood risk assessment in the Tokyo metropolitan area by considering the effect of precipitation change. By comparing the statistical distribution of the daily precipitation frequency for the whole study period, and those for flood occurrence days, we found that the distributions of the precipitation frequency for the flood occurrence days are corresponding to those for the whole study period. These results indicate that we can estimate flood damage based on frequency of daily precipitation. Based on these results, we estimated the flood damage for Tokyo based on distribution of daily precipitation frequency. We then created a flood-risk curve that represented the relationship between damage and exceeding probability of a flood. By comparing the newly developed flood-risk curve, based on the precipitation frequency, with those in the previous studies, we indicated that a newly developed flood-risk curve could evaluate the potential flood risk with high accuracy.

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The weather warning is important disaster prevention information. Understanding of Internet users for weather warning information was researched based on Internet questionary survey in March 2013. The respondent who understood the correct meaning of a traditional weather warning was only 43%. The traditional warning is, so to speak, "plain text warning information". It is difficult to announce the risk of the disaster only by "plain text warning information". Therefore we asked about graded warning with numerical level. Respondents who chose "When a warning of level 4 was announced, I will evacuate." were 38% of all respondents. On the other hand, respondents who chose "When the super warning was announced, I will evacuate." were only 27%. The "super warning" is "plain text warning" that is equal to a warning of level 4. The answer "graded warning was simply" was 50%. On the other hand, the answer "plain text warning was simply" was 28%. It is possible that graded warning is more effective than plain text warning.

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Damage caused by floods can sometimes be catastrophic. When focusing on lessening the fatalities/victims, knowledge of a flood even just a day in advance could make a difference. Therefore, this research aims to develop a global real time calculation system for flood affected population. A new river model, CaMa-Flood, is introduced into the framework of Today's Earth, a real time calculation system for various hydrological aspects. Gridded population data was then combined to calculate the number of estimated affected population. Validations for the outputs of this system were conducted using multiple observation data. Comparison with floods in 2013 showed that this system was able to detect the occurrence of floods to a certain degree.

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Although it is important to retain the flood water within the river channels by using dikes and reservoirs, these facilities alone cannot cope with excessive floods. In Japan, appropriate combination of facility-based and non-facility-based countermeasures is getting more important. To determine nonfacility-based countermeasures, we need to simulate flood hazardous situation. However, simulation models which include only evacuating on foot are not enough because our society and our lifestyles are based on using cars.
In this study, we developed a model to express the behavior of the car in three simple modes. This enables us to simulate the car's actions in a realistic manner at least. The performances of this model have been tested in the several simulation results in actual flood-plain areas in Japan.

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The flood damage mitigation effects of flood protection forest against inundation flows due to dyke breach as well as the breach discharge by overtopping flows are investigated. The process of breach erosion is reproduced, employing laboratory-scale non-cohesive homogeneous embankments. It is found from laboratory experiments and numerical analysis that dyke breach flows are diverted and diffused by the forest, so that hydrodynamic forces acting on houses located behind the forest are substantially reduced. It is also demonstrated that the dynamic inundation model, which comprises the finite volume method on unstructured grid using FDS technique, is capable of predicting such effects of the forest against the flows with good to reasonable accuracy.

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This study aims to propose a new numerical model for levee breach based on the results of relating experiments. Authors performed large-scale experiments of overtopping levee breach using the Chiyoda experimental flume located on the floodway of an actual river channel. By taking advantage of the scale of the flume, authors monitored the levee breach processes under highly precise hydraulic conditions, and the results showed a correlation between breached volume and shields number. Then authors applied the experimental results to a conventional two-dimensional shallow water with moving bed model, and found some limitations. Next, authors proposed a revised model based on the conventional model by integrating the experimental results into geomechanics, then obtained a fine reproduction result with regard to the levee breach widening during an actual flood of Yabe River in 2013.

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The numerical simulations of inundation flows due to dyke breach are carried out by using a distributed hydrological model and a dynamic inundation model, which comprises the finite volume method on unstructured grid using FDS technique. The model is applied to simulate inundation flows due to hypothetical dyke breach in the Onga river basin, and the study is directed towards the examination of effects of breach process as well as flood damage mitigation by flood protection forest in the basin. It is found from numerical simulations that dyke breach flows are diverted and diffused by the forest, so that hydrodynamic forces acting on houses located behind the forest are substantially reduced.

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Since hydraulic conditions of actual rivers vary over a wide range, phenomenon analysis of experiments with a wide range of hydraulic conditions is indispensable for analyzing levee breach phenomenon. In this study, results of the preliminary experiment conducted for a levee breach experiment using experimental flume were organized and characteristics of levee breach progress were discussed based on the results of the experiment with varied river width and riverbed slope. As a result of the small scale levee breach experiment, it was found out that the levee breach speed was fast and flood volume kept increasing without drawdown of river channel in the cases of wide river width and high-angled riverbed slope. The topographic changes of the small scale levee breach experiments showed the similar tendency with the full-scale experiment. The estimated formula of collapse volume also showed the similar tendency.

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A new flood control concept called 'Cascade method' which permits a dam to overflow from an emergency spillway around an upstream region with multiple dams constructed in series in a river basin has been suggested. In this study, mechanism of Cascade method is investigated using dry dams for various inflow flood hydrographs. Cascade method has the following two effects to control flood. First, overflowing at upstream dams in Cascade method suddenly increase each inflow discharge when the water level in the following dam is still not so high. Second, a conventional method in upstream dams performs useless flood control in the second half of a flood. On the other hand, each dam in Cascade method efficiently reduces peak outflow discharge to the design high water discharge in the downstream dam which is the target outflow discharge for the river basin. In Cascade method the second effect is significant compared with the first one.

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In coastal farmlands, farm roads and farmland serve to dissipate the energy of the tsunami in stages. The role of drainage channels in spreading and draining a tsunami flood should also be considered. The energy dissipation characteristics of a drainage channel and sidewall behind a coastal levee were evaluated in this paper. The sidewall was installed either on the coastal side or on the inland side of the channel bank. A tsunami that overflowed the coastal levee washed the back slope of the levee by supercritical flow. The tsunami that reached the beginning of the inland zone generated a hydraulic jump, the location of which moved to the coastal levee. A comparison of the distribution of the bottom velocity in case of the drainage channel with or without sidewall yielded the following results: 1) In the region of small volume of overflowed tsunami, the drainage channel without sidewall dissipates the energy of the tsunami. However, a drainage channel with a wide cross section is required for a large volume. 2) The sidewall effectively dissipates tsunami energy, and the effect varies with the location of the sidewall.

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A field test was conducted in a tsunami-affected paddy field in Natori city, Miyagi prefecture to understand temporal changes in soil salinity over the past 27 months and to collect information about the desalinization in the near future. Vertical profiles of the soil salinity and water content were obtained by TDR and four-electrode sensors installed down to a depth of 0.6m below the paddy soil surface near our micrometeorological station. Salt mass in the soil up to 200mm from the soil surface varied greatly due to eluviation of salt from the soil by rainfall and due to re-adsorption by the drying of the soil surface. Salt movement in the soil deeper than 200mm was, however, hardly observed for about one month to two years. Cl- concentration in the soil was reduced to below the allowable concentration of rice planting about 400 days after the tsunami attack by runoff from the paddy field due to rainfall. It is seen from the above results that the salt removal of saline soil surface layer of 200mm is effective for the desalinization.

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Coagulation treatment is one of countermeasures for turbid water in a reservoir that is brought by lowering of surface level or sediment transport in a reservoir. We have taken volcanic ash coagulants which contain allophane and investigated their coagulation, dispersion characteristics and electrokinetic phenomena. Moreover, we conducted site experiment of coagulation treatment by using volcanic ash coagulants. Through this study, coagulation was observed when pH of suspension was adjusted to 7 to 9. Zeta potential of coagulants were raised by adding ultrasonic dispersion. High coagulation performance was obtained in site experiment, on the other hand, reduction of coagulants and effective dispersion method were desired for the practical use of volcanic ash coagulants.

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Lake Ogawara is a brackish lake, which is located in southeast side of Aomori prefecture. In past researches, it is known that the water depth of the halocline in the lake has become shallow. The volume of anoxic water has increased in the lake, and the boundary area has increased in proportion. Therefore, they have influenced the habitat of fish, shellfish and eutrophication of the lake. In this study, it was discussed that the relationship between halocline and water quality in the lake. Furthermore, it was tried to clarify the relationship between the water quality and the vertical position of halocline by the vertical distribution of water quality. According to the vertical distribution of salinity, the depth of halocline has become shallow. The vertical distribution of DO shows that the values in August begin to decrease from the depth of deeper than thermocline. In December, the values of DO begin to decrease from the depth of deeper than halocline. The values of COD increase gradually in the lower layer.

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Distribution of settling velocity and characteristics of particle-water interface were discussed by using organic mud in Lake Abashiri located in a cold region, east Hokkaido in Japan. The distributions of settling velocity were estimated from temporal variations measured by laser diffraction analyzer. Since the settling velocity of cray mineral sample having low cohesiveness with dispersing agent agreed well with theoritical value, the analytical method were well verified. Comparing with the model consdering organic substances in floc, floc structure varies with floc size in lake water less than in distilled water. Zeta potential representing interface characteristics of organic mud is closely related to dissolved divalent-ion derived from sea water and humic-like organic substance.

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Two-dimensional coherent flow structure (2DCS) of shallow water jet was investigated by the laboratory experiments changing water depth and buoyancy of the outlet. Experiments with several water depth indicated that the shallower case tend to prompt 2DCS flow characterized complicated movement such as enlargement , unification , separation, etc. of plane eddies triggered at near the outlet of released jet. On the other hand, buoyant effects which were caused by the temperature difference between water temperature of outlet and ambient water were revealed to weaken the coherent flow structure. The effects of 2DCS and buoyancy against spreadings of the jets were also studied in detail and indicated that the spreading mechanism is different in the strong buoyancy condition and the buoyancy controls the 2DCS development.

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Expeditious processing is required for oil spill accidents in water area since they can cause enormous damage to the local habitat and environment. In cold snowy regions, there are frequent heating oil spill incidents in winter. In addition, oil spill under river-ice cover and its process can hardly be observed. In this study, a series of hydraulic experiments using ice model is performed to investigate oil spill phenomena under ice cover. As the result, it is clarified that spilling speeds become slower and the oil layer becomes thicker compared with the cases with open channel. Moreover, simulation by one dimensional two-layer unsteady flow model is performed and consequently it is revealed that oil spill speed is reproducible with appropriate shear stress at the interface and upstream end conditions.

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The purpose of this study is to develop a simple method for estimating evaporation rates of water using oxygen isotopic ratio. The proposed method is just to use only δ¹⁸O_liq of water and δ¹⁸O_vap of vapor and humidity. Under low humidity condition such as less than 60% humidity, variation of the change in δ¹⁸O_liq per unit change of the evaporation rate was small. Therefore evaporation rate can be calculated from the change in δ¹⁸O_liq per unit change of the evaporation rate and δ¹⁸O_liq of evaporated water without measuring of daily temperature and humidity. However, under more than 60% humidity, the change in δ¹⁸O_liq per unit change of the evaporation rate was variable and then evaporation rate cannot be calculated.

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Lake Ogawara is becoming eutrophicated very rapidly due to the contaminant upwelling from the anaerobic saline water layer. The authors have already developed a practical simulation model to predict the saline water intruding in winter. In this paper, the previous model is improved by adding the effect of thermal stratification and the current in saline layer. Then, year-round calculation was conducted to investigate the expansion of the anaerobic saline water layer in the recent ten years. The model reasonably reproduced the field measurement results on seasonal change and long-term expansion of saline water layer. The simulation result confirms that the expansion of saline water layer and the saline water entrainment are the primary reasons for the eutrophication in surface layer. It also suggests these events occur mainly in winter, when seawater intrudes frequently and thermal stratification is absent.

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Hydrodynamic structure was measured in a reservoir equipped with bubble-diffuser type destratification system. Water quality of the reservoir is much eutrophic, so that the system of destratification is operated during summer to prevent algal bloom. However, appropriate operation of the system requires substantial understanding on hydraulic structure in the reservoir. In this study, two kinds of flow measurements were conducted: vertical profile of velocity at a fixed point for two weeks during stratification season, and spatial flow pattern in the reservoir. The former elucidated three-layered structure of current which is likely to be made by not only the destratification system but also internal seiche and the seasonal stratification. The latter illustrated detailed spatial flow pattern under operation of bubble diffusers.

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In order to prevent algal blooms in the Ogouchi reservoir, a pump system was deployed to transfer surface water containing phytoplankton and to release it in the bottom of the reservoir. Field measurements were conducted to investigate the movement of the released water. Turbid water was poured into the pump system, and the vertical profiles of temperature, turbidity, and chlorophyll-a around the pump outlet were determined.The released turbid water was not confirmed near the outlet and was not found in upper depth; whereas, chlorophyll-a was distributed horizontally at depths of 7 m and 11 m. Horizontal movement of the reservoir water was estimated from the measured current profile and vertical migration of released water was analyzed using plume theory. Based on the two analyses, we conclude that the peak concentration observed at a depth of 7 m was a natural propagation of phytoplankton, and the phytoplankton contained in the released surface water caused the peak concentration observed at a depth of 11 m.

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Water temperature in water bodies, such as rivers, lakes, and reservoirs, is one of the most fundamental aquatic environmental factors that affect water quality and ecosystem. Lakes located in high altitude area are considered to be sensitive to global warming. In this study, we carried out detailed field measurements on water temperature and meteorological condition in the Tuni Reservoir in Bolivia Cordillera. Heat balance in this area is different from that in low altitude areas like Japan because the low atmospheric pressure decreases sensible heat transfer between the water body and atmosphere. By the analysis of the detailed field data, water temperature changes in the reservoir were successfully reproduced. The river water temperature in the Tuni reservoir catchment was also analyzed, and the transition of river water temperature along the river channel was discussed.

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A 3-D coupled hydrodynamic and ecological model of ELCOM-CAEDYM was applied to Urayama Reservoir to study the effects of climate change on the thermal structure and water quality in the reservoir as well as those downstream. We evaluated the differences between present period (2002-2010) and future period (2062-2070). Scenarios were estimated using JRA25 and GFDL-CM3 with CO₂ emission scenario of RCP4.5 by downscaling of WRF model and a hydrological model of YHyM/BTOPMC. Future surface and hypolimnetic water temperatures were predicted to rise by 2.9°C and 1.6°C, respectively. As a result, a stronger thermal stratification was expected. In the reservoir, significant increase of future SS concentrations by more frequent and bigger floods and decrease of phytoplankton biomasses by high water temperature, hydrologic condition and light limitation were also predicted. In the downstream of the reservoir, less frequent cold-water release and more frequent warm-water release were both predicted. Moreover, more frequent and high turbid water releases were predicted due to more frequent and bigger floods in the future.

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We evaluated effects of selective withdrawal system (SWS) for water quality improvement adapted for climate change in the Urayama Reservoir. We applied a 3-D coupled hydrodynamic and ecological model of ELCOM-CAEDYM with four SWS operation scenarios under future period (2062-2070). In the reservoir, the future period simulations showed that SWS operation using a surface intake could increase water temperature and deepen the thermocline compared with the present period. On the other hand, SWS operation of intaking at 10-12 m depth in the future period increased the thickness of turbid water intrusions by deeping the thermocline and decreased phytoplankton biomasses by higher surface water temperature than the present period. Moreover the increase of cold-water release and the decrease of warm-water release were expected by the surface intake operation in the future period.

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This manuscript in Journal of Japan Society of Civil Engineers, Ser.B1 (Hydraulic Engineering), Vol. 68, No. 4, I_103-I_108, 2012 was withdrawn by the Editor on October 3, 2013 at the request of the authors.

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