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Yusuke HIRAGA, Yoshihiko ISERI, Michael D. WARNER, Angela M. DUREN, Jo ...
2024 Volume 12 Issue 2 Article ID: 23-16005
Published: 2024
Released on J-STAGE: February 29, 2024
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In basins where disastrous floods are driven by long-duration processes, such as snow accumulation/melt and series of storm events, estimating maximum precipitation (MP) for not a single storm duration but long durations is necessary to estimate maximum flood. Although the model-based methodology to estimate MP for long durations was recently proposed, it is still required to better understand what characteristics of water years lead to higher increases in precipitation depths. To examine this issue, this study performed the model-based maximization of precipitation sequences during winter (Oct-Mar) of the 9 historical water years (94 Atmospheric River (AR) events) for the drainage areas of Bonneville Dam and Libby Dam in the Columbia River Basin. The storm selection based on the thresholds of Integrated Water Vapor Transport (IVT) showed that the use of CFSR resulted in selecting more AR events than the use of 20CRv2c-ensemble mean, implying the importance of reanalysis products in MP estimation. The 1996 water year showed high potential in precipitaton increase, implying that the 1996 Pacific Northwest floods could have been more disastrous. The total duration of AR events during winter in a water year showed high positive correlations (R=0.84 for Bonneville and R=0.97 for Libby) with the precipitation increase rate from historical to maximum in the long-duration precipitation depths. This study found that each AR’s trajectory may be the key to determining the increase rate in winter cumulative precipitation depths. Further studies are needed to examine how the use of different reanalysis products can affect the results and to quantify the potential in precipitation increase using AR trajectories.
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Weijian HUA, Ryosuke AKOH, Kexin LIU, Shiro MAENO
2024 Volume 12 Issue 2 Article ID: 23-16012
Published: 2024
Released on J-STAGE: February 29, 2024
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Because of recent severe flooding events, numerous embankments have been breached, resulting in large amounts of floating litter in residential areas. Understanding the behavior of floating litter during flood events is crucially important. For this study, we improved the comprehensive flood analysis model to reflect drainage from sluiceways and pumps. Moreover, we considered a detailed flood analysis until the floodwaters of the urban area were drained. Furthermore, a simulation of floating litter movement was performed using flood analysis results. Results show that more than half of the floating litter generated in the urban area during flooding remains in the residential area, but about one-fifth of the litter flows out through the breached site and flows into the river. The proportion of litter flowing out is higher in areas closer to the breach locations. Additionally, around 13% of the litter accumulates at drainage gates.
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Pan Ei PHYU, Gubash AZHIKODAN, Katsuhide YOKOYAMA
2024 Volume 12 Issue 2 Article ID: 23-16019
Published: 2024
Released on J-STAGE: February 29, 2024
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Effects of past human activities and recent climate change disasters and their interaction on the riverbed morphology of the Chikugo River estuary, Japan were investigated using the long-term measurement of annual and seasonal topographic surveys. The results showed that human activities caused the extensive riverbed incision of the Chikugo River estuary with a maximum value of about 4 m from 1953 to 1998. After stopping the sediment extraction in 1999, the elevation of the estuary increased by 1.2 m due to the sediments transported by the tide, especially during 1998-2003. Starting from 2009, the occurrence of floods and landslide disasters increased in the upstream of the Chikugo River. Although disasters carried huge sediments, a decrease in elevation was observed between 10-17 km with a value of 1.5 m, which means that sediments supplied by disasters had been deposited in the upstream where sediments were extracted in the past, thus, they couldn’t be able to reach the estuary yet. So increased river flow with insufficient sediments eroded not only the surface silt and clay layer but also the bottom sand layer, resulting in a drastic decrease in the elevation of the estuary. Therefore, this study concludes that the riverbed morphol- ogy of the Chikugo River estuary is affected by both tidally induced sediment transport from downstream and an increase in river flow by recent disasters and human disturbances that were implemented in the past.
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Md. Zakir HASAN, Yuji TODA
2024 Volume 12 Issue 2 Article ID: 23-16021
Published: 2024
Released on J-STAGE: February 29, 2024
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Groynes installation is a popular measure to prevent riverbank erosion in braided rivers. In this study, evaluation of groynes installation has been done at a vulnerable reach of the Jamuna River in Bangladesh. Four groynes (officially termed as cross-bars) have been installed at the erosion prone (right) bank of the river adjacent to Sirajganj town. Due to the unpredictable behavior of the braided river and critical climatic conditions, stability assessment of the groynes is crucial, as well as their impact on surrounding area. To perform the evaluation, 50 km reach of the Jamuna River has been analyzed by numerical model using iRIC Nays2DH software. After successful calibration and verification of the model, the simulations have been done using peak discharges for six different cases. Analysis of the model output reveals that cross-bar 3 is in vulnerable condition among the four based on scouring at the toe of the groyne. Additionally, the impact includes formation of a weak zone at 4.0 km upstream of the cross-bar 1. More than 2.0 km of riverbank becomes susceptible to erosion due to the attack of the stream flow, which shows the impact of the constructed cross-bars in the surrounding area. This study provides insights into the stability assessment of groynes and their impact in braided rivers.
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Yifan LI, Alexandre CAUQUOIN, Atsushi OKAZAKI, Kei YOSHIMURA
2024 Volume 12 Issue 2 Article ID: 23-16035
Published: 2024
Released on J-STAGE: February 29, 2024
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Stable water isotopes (H216O, H218O and HD16O) are proxies of climate processes occurring in the water cycle, the latter having a crucial role in the Earth’s climate system. For example, water isotopes measured in polar ice cores and Asian speleothems are commonly used to reconstruct past temperature changes in the poles or to study the past dynamics of monsoons, respectively. In this study, we present the ongoing development and first results, for the preindustrial period, of a newly developed isotope-enabled version of the fully coupled Earth System Model MIROC6, called hereafter MIROC6-iso. Our preliminary results are in rather good agreement with isotope observations in precipitation and surface sea water. Some improvements are expected in Antarctica thanks to the coming implementation of water isotopes in the sea ice module of MIROC6-iso. The modeled isotope distribution in the first 1500 meters of the ocean goes also in the right direction compared to observations. For the deeper part, more spin-up time is necessary to reach a quasi-equilibrium state.
Considering the applications using isotope-enabled fully coupled climate models from Europe and the United States for paleoclimate reconstruction, there is a pressing need for Japan to develop such a state-of-the-art model, too. Additionally, it paves the way for the future refinement and development of MIROC6-iso, while also serving as an important point of reference for subsequent analysis of the climate, hydrological cycle, and paleoclimate reconstruction using MIROC6-iso.
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Jose Angelo HOKSON, Shinjiro KANAE
2024 Volume 12 Issue 2 Article ID: 23-16036
Published: 2024
Released on J-STAGE: February 29, 2024
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In the statistical prediction of typhoon-induced rainfall, Fuzzy C Means (FCM) has been widely utilized to identify similar typhoons based on track positions (latitude and longitude). However, despite the potential impact of additional variables on rainfall distribution (e.g., low central pressure and slow movement speed resulting in high mean rainfall), their inclusion in similar typhoon identification using FCM has been largely unexplored. This is particularly relevant given the increasing strength and slowing down of typhoons due to climate change. To address this gap, we conducted a study incorporating these additional variables alongside track positions and evaluated their performance using various statistical measures. Our findings indicate that including central pressure and/or movement speed has little to no impact on the accuracy of rainfall prediction. The statistical measure values do not directly point to the direction of improvement or degradation of accuracy. As we confront the challenges of climate change, new and improved techniques will be necessary for accurate rainfall predictions.
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Akihiko NAKAYAMA, Shu Kai NG
2024 Volume 12 Issue 2 Article ID: 23-16042
Published: 2024
Released on J-STAGE: February 29, 2024
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The Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) has been applied to simulate the flow through a vertical Gravitational Vortex Turbine (GVT) configuration and the motion of the turbine of a small hydropower system. The turbine blades are assumed to be rigid bodies moving with one-degree of freedom, which is the rotation about its shaft due to the hydrodynamic forces of the flow. The intake flow is an open channel flow which falls into a cylindrical basin which houses the turbine and impinges on the turbine blades. The method of simulation is the Weakly Compressible Smoothed Hydrodynamics (WCSPH) with the turbulence models in the equations of motion and the boundary conditions. The results are compared with a laboratory experiments conducted for the same dimension and the configuration. The flow features agree well and the motion of the turbune with a representative mass and a load agrees reasonably with the laboratory experiments.
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Yuexia ZHOU, Yuji TODA
2024 Volume 12 Issue 2 Article ID: 23-16058
Published: 2024
Released on J-STAGE: February 29, 2024
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The destruction of trees in the river (riparian trees) has been mainly analyzed with consideration of flood loading recently, and the decreasing of the critical resistive moment used to evaluate the destruction condition was not analyzed in those studies though the actual resistive moment may decrease with substrate scouring by the heavy flood. In the context of climate change, which increases the likelihood of simultaneous floods and storms, the mechanism of riparian tree destruction under both flood and wind load and the change of critical resistive moment with riverbed erosion should be investigated. We established a mechanical model and analyzed the criteria for the destruction of riparian trees under flood and wind loads. One simple model was also proposed with considerations of root-soil plate radius and erosion depth of river bed to analyze the decreasing of the critical resistive moment. The taller and smaller trees are more vulnerable to be destructed than medium trees, with the consideration of both flood and storm. The critical resistive moment decreased as compared to the previous study, and the usefulness of the established model was verified to some extent.
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Yogi Sahat Maruli SIMANJUNTAK, Tatsuhiko UCHIDA, Takuya INOUE
2024 Volume 12 Issue 2 Article ID: 23-16082
Published: 2024
Released on J-STAGE: February 29, 2024
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The effects of three-dimensional flow in strongly curve channel with partly emergent vegetation were analyzed numerically by comparing the flow structure of the Bottom Velocity Calculation (BVC) method and conventional two-dimensional calculation (2DC) methods. Numerical calculations result show that BVC method is able to reproduce the flow structure investigated by the experimental dataset, decelerating the surface velocity and accelerating bottom velocity. The presence of vegetation on the inner bank reduces the strength of the secondary flow, while the outer bank vegetation increases the strength of the secondary flow. The secondary flow reduces the streamwise velocity in the non-vegetation region and moves the maximum velocity toward the outer bank. These results show the importance of considering the 3D flow effect for simulating partly vegetation in meandering rivers.
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Aulia Febianda Anwar TINUMBANG, Yutaka ICHIKAWA, Kazuaki YOROZU
2024 Volume 12 Issue 2 Article ID: 23-16093
Published: 2024
Released on J-STAGE: February 29, 2024
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Soil moisture (SM) is a key component in land surface model (LSM). An accurate prediction of SM is therefore important since it affects other variables when estimating water and energy budgets. This study aimed to evaluate the impacts of SM schemes in a simple biosphere model including urban canopy (SiBUC) on water balance and streamflow estimations. SiBUC calculates SM of a three-layers soil model using Richards’ Equation (RE) with an explicit-midpoint method. The default soil parameterizations are based on the Clapp and Hornberger equation, and the van Genuchten (vG) function, popularly used in hydrological models, was newly added here. In SiBUC, the thickness of the first soil layer was 2 cm, whereas the second was several meters thick. Because of the RE’s nonlinear behavior, this coarse partition may not accurately estimate SM because the changes of a hydraulic gradient in near-surface soil may be large, requiring a finer discretization. A reference model called HYDRUS was used to assess SM by SiBUC. For sandy loam and loam soils, the SM by the default schemes in SiBUC was close to that predicted by HYDRUS but overpredicted for clay loam. Applying a finer partition in the near-surface soil and an implicit-midpoint method seem to improve the estimated SM and reduce the overestimated discharge in the Ping River Basin in Thailand. However, because of the relatively large time steps applied in SiBUC, the estimated runoff using vG was less accurate, possibly causing an overestimate of streamflow. Overall, the potential problem of SM using the RE was explored here, which may help the LSMs users to alleviate the numerical errors resulting from the inadequate discretization of the soil layers.
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Do Ngoc KHANH, Alvin C. G. VARQUEZ, Manabu KANDA
2024 Volume 12 Issue 2 Article ID: 23-16120
Published: 2024
Released on J-STAGE: February 29, 2024
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In 2022, intense heat waves occur all around the world. In this study, we project these heat waves into the future with futuristic projection of hourly varying spatially distributed anthropogenic heat for three megacities—Delhi, London, and Tokyo. Different future climate forcing (CMIP5 and CMIP6) was also compared. We found that if similar heat waves occur in the future, they may be 0.8 to 1.5 °C hotter than the past events, on average. Urbanization in Delhi may severely worsen the heat wave, while projected decrease in energy usage in London and Tokyo may make the heat waves less severe. For the concerned heat wave events, urbanization effect was also found to be stronger in nighttime than daytime and exhibits large spatial heterogeneity and dependence on background climate forcing. Difference between CMIP5 and CMIP6 was significant but was much less than the difference between CMIP5/CMIP6 and the present.
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Diani ABEYRATHNE, Ryo KANEKO, Kei YOSHIMURA
2024 Volume 12 Issue 2 Article ID: 23-16127
Published: 2024
Released on J-STAGE: February 29, 2024
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Flood forecasting is one of the most challenging aspects in the field of hydrology. With new developments in computational intelligence, data-driven methods are gradually becoming popular and Long Short-Term Memory (LSTM) approach has shown great potential in accurate river stage forecasting due to its ability to learn long term dependencies. This study investigated methods for further improving accuracy of flood forecasts provided by LSTM models, especially when the prediction lead time is increased. LSTM model simulations were carried out for data obtained from nine river basins in Sri Lanka and forecasts made from 1 hour to 24 hours lead times were analyzed. Different scenarios were used to train the model and the results indicated that using selected data from separate river basins has improved the forecasting accuracy, significantly for longer lead times. A better feature selection method (to be used as input data to train the model) was investigated in this study by evaluating the strength and relationship of the river water level variation among different river gauging stations. Using this feature selection method for selecting optimum water level data combined with rainfall data provided the highest accuracy in the predictions.
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Manoj KHANIYA, Yasuto TACHIKAWA, Takahiro SAYAMA
2024 Volume 12 Issue 2 Article ID: 23-16129
Published: 2024
Released on J-STAGE: February 29, 2024
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This paper presents a study on the performance of the ensemble optimal interpolation (EnOI) scheme with stationary covariance matrices for assimilation of synthetic water level observations into the rainfall-runoff-inundation (RRI) model. Five state and observation error covariance matrices are picked from the ensemble Kalman filter (EnKF) simulation of a flood event and then used during the update stage for Kalman gain calculation for (i) state and (ii) parameter estimation with the EnOI scheme. The stationary EnOI improves state estimation at randomly selected validation locations (along with most of the river grids) for all five covariance matrices, but improvement throughout the basin is not guaranteed as performance is degraded at some unobserved locations. The method also has the potential to be used in real applications as the results improve not only for the same flood, but also for a different event from which the matrices are not extracted. Model parameters can also be successfully estimated with the EnOI scheme, but the estimation may be compromised if the parameters interfere with each other. Independently updating, or only updating the most sensitive parameter (Manning’s roughness coefficient for river in this case), leads to better estimation.
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Baixin CHI, Shinichiro YANO, Akito MATSUYAMA, Lin HAO
2024 Volume 12 Issue 2 Article ID: 23-16138
Published: 2024
Released on J-STAGE: February 29, 2024
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After Minamata disease occurred in Kumamoto Prefecture, the Minamata Bay Pollution Prevention Project on Minamata Bay area was started to deal with mercury pollution. Beneficial from the project, the mercury content in the bay area has been significantly reduced. However, many studies have shown that the residual trace mercury content around the bay is still high. Furthermore, mercury has migrated from Minamata Bay to the Yatsushiro Sea. Therefore, continuous research on the distribution and content of mercury in this region is required.
In this study, using sediment classification, the relationship between sediment particle size, particle specific surface area, and T-Hg (total mercury) concentration was investigated. Based on the numerical simulation, the particle size effect on the migration of mercury-containing sediments and the T-Hg distribution in the Yatsushiro Sea were studied. The result showed that the larger the particle size, the lower the T-Hg concentration in the sediment. Meanwhile, the larger the particle specific surface area, the higher the T-Hg concentration. Moreover, the smaller the particle size of the mercury-containing sediment, the higher the migration speed as well as the more comprehensive migration range. From the numerical simulation results, it was found that mercury was mainly distributed in the southwest and northeast directions from Minamata Bay. Finally, the simulated mercury distribution, which considers the relationship between particle size and mercury concentration, shows a high agreement with the past measurements of mercury, suggesting the importance of particle size effect in mercury migration.
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Nay Oo HLAING, Gubash AZHIKODAN, Katsuhide YOKOYAMA
2024 Volume 12 Issue 2 Article ID: 23-16140
Published: 2024
Released on J-STAGE: February 29, 2024
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The impact of topography and tidal forcing on counter-current flow formation in monsoon-affected multi-channel macrotidal river estuaries is seldom studied. This research investigates the occurrence of counter-current flow during the monsoon season, characterized by strong semi-diurnal tidal currents, at the confluence area of the complex multi-channel macrotidal Tanintharyi River estuary (TRE) in Myanmar. Continuous measurements of velocity and discharge, along with surveys of topography and water quality (salinity, turbidity), were conducted during a 12-hour intensive survey (30-minute intervals) at the upstream junction area of the main channel and the branch during the spring tide. The main channel’s width, approximately four to five times larger than the branch’s width, experiences counter-current flow during ebb tide when water levels between the two channels are similar. Despite the freshwater environment and stable turbidity indicated by vertical profiles, tidal effects still influence counter-current flow formation. The imbalance in area and discharge ratios between the main channel and branch can be attributed to another important factor: downstream topography. The main channel with wide (1.3 km), straight (SI: 1.1), and short (28 km) topography, is highly sensitive to tides compared to branch with the narrow (180 m), meandering (SI: 2.1), and long (40 km) topography. The difference in downstream topography between the main channel and branch, combined with tidal forcing, plays a crucial role in counter-current flow formation in the upstream junction area of the TRE.
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Worameth CHINCHUTHAKUN, David WINDERL, Alvin C.G. VARQUEZ, Yukihiko YA ...
2024 Volume 12 Issue 2 Article ID: 23-16151
Published: 2024
Released on J-STAGE: February 29, 2024
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Historical land cover data is crucial for understanding urbanization dynamics, climate modeling, and monitoring water resources. Following recent advancements in deep learning for processing Landsat archive data, prior studies have released high-resolution historical land cover maps on a global scale. However, these works often present prediction results limited to specific periods of coverage, which hinders their utility in conducting time series analysis across different urban agglomerations. To address this issue, we propose deep-learning models for land cover classification from Landsat images at a 30-meter spatial resolution. Our models are specifically designed for urban areas and are trained to be compatible with the sensors used in the Landsat series from 1972 to the present. Experimental results demonstrate that our models are highly effective in predicting land cover maps in new cities, particularly in built-up land and water regions. Our research provides pretrained models for land cover classification, facilitating future studies in related fields.
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Sunmin KIM, Masaharu SHIBATA, Yasuto TACHIKAWA
2024 Volume 12 Issue 2 Article ID: 23-16152
Published: 2024
Released on J-STAGE: February 29, 2024
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This study presents a downscaling method that utilizes a machine learning algorithm to improve the regional analysis capabilities of general circulation models (GCMs) in hydrological prediction. The U-Net algorithm, a representative approach based on Convolutional Neural Networks (CNNs), is employed to emulate the dynamic downscaling process of regional circulation models (RCMs). In this process, the GCM output is utilized as the input, while the output from the regional climate model (RCM) acts as the label data. The accuracy of the model is evaluated by considering various factors, including the input datasets, model structures, and output formats. The input datasets consist of essential atmospheric variables such as precipitation, temperature, and humidity, which are examined in both two-dimensional and three-dimensional formats. The model structure is assessed by testing different hyperparameters and analyzing their impact on the accuracy of the predictions. Furthermore, the effect of the label data is also investigated. The results of the experiments demonstrate that the U-Net algorithm successfully emulates the dynamic downscaling process, even in the absence of precipitation input data. Increasing the number of convolutions in the model structure contributes to improved accuracy; however, enlarging the receptive filter size does not necessarily yield better results. Moreover, incorporating temperature data along with precipitation in the labeled dataset enhances the accuracy of the predictions compared to using precipitation alone.
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Ying-Hsin WU, Akihiko YAMAJI, Eiichi NAKAKITA
2024 Volume 12 Issue 2 Article ID: 23-16153
Published: 2024
Released on J-STAGE: February 29, 2024
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In this study, as a first attempt, we utilize the method of Radial-Basis Function Network (RBFN) to examine hourly rainfall intensity and soil-water index obtained from climate projections for investigating features of future rainfall under climate change influences. Our study area is focused on Rokko Mountain in Hyogo prefecture. The 1-km MLIT 3rd-mesh system is adopted in our analysis. The 2-km and 5-km Non-Hydrostatic Regional Climate Models (NHRCM) under RCP2.6 and RCP8.5 scenarios are used as future climate projections. A transformation matrix is used to quantify geometrical changes of RBFN contours. The characteristics of cumulative rainfall amount and rainfall duration of hazardous rainfall events are also examined. The analysis results show that soil-water index is much enlarged while hourly rainfall is a bit enlarged under RCP2.6, but, reversely, soil-water index remains unchanged while hourly rainfall is much intensified on the mountain side facing Osaka Bay under RCP8.5 in 2-km NHRCM. The hazardous rainfall events in 2-km NHRCM have shorter durations and smaller cumulative amount comparing to the ones in 5-km NHRCM.
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Badri Bhakta SHRESTHA, Mohamed RASMY, Takafumi SHINYA
2024 Volume 12 Issue 2 Article ID: 23-16158
Published: 2024
Released on J-STAGE: February 29, 2024
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Developing a quantitative flood damage estimation method is important to plan and implement effective adaptation and preventive measures for reducing flood damage to residential areas. We thus developed a flood damage assessment method by integrating hydrologic-hydraulic simulation model outputs and a flood damage model capable of considering house characteristics, and conducted the quantitative assessment of flood damage to houses and household contents, including the effectiveness analysis of flood damage reduction by the use of existing dams for flood control and elevating the plinth level of houses. We also developed average flood damage curves as a function of flood depth for the houses considering house types and also for household contents, using secondary data sources. Selecting the Solo River basin of Indonesia as the study area, we computed flood characteristics using a water and energy budget-based rainfall-runoff-inundation model and estimated flood damage using the developed flood damage curves based on house characteristics and the exposed property value. The findings show that the use of the dam for flood control in the basin can reduce the expected annual damage (i.e., the average total expected damage per year from all flood probabilities) to residential areas by 21%. Flood damage to households can be reduced by more than 45% by elevating the plinth level height.
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Hideto YOSHIMURA, Yoshiro OMORI, Ichiro FUJITA
2024 Volume 12 Issue 2 Article ID: 23-16165
Published: 2024
Released on J-STAGE: February 29, 2024
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A numerical simulation of a uniform flow in an open channel under wind stress conditions was performed to investigate the wind effect on the surface velocity and the sub-surface velocity profile, which is of interest in river-discharge measurements. The results of the open-channel flow show that the surface velocity and the sub-surface profile are more sensitive to wind stress as the bed friction velocity is smaller and the water depth is larger. It is also found that the magnitude of the surface drift velocity in the open channel increases monotonically with the ratio of the wind stress to the bed shear stress and reaches a maximum of 3–4% of the wind speed. This fact suggests that the wind speed correction factor in the discharge measurement should be estimated considering hydraulic and wind conditions rather than a constant value. The surface drift velocity obtained by several river observations may include not only the drift current but also other unknown effects.
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Qiyun PANG, Shinichiro ONDA
2024 Volume 12 Issue 2 Article ID: 23-16175
Published: 2024
Released on J-STAGE: February 29, 2024
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Levee breaching due to overtopping flows in rivers has become increasingly frequent, particularly during torrential rains and flood events. For the purpose of risk management, it holds significant importance to gain a comprehensive understanding of the mechanisms underlying dike failure and accurately predict this process. In this study, a three-dimensional RANS model which can simultaneously simulate surface, seepage flows and sediment transport in a curvilinear coordinate system is developed. The numerical model is applied to bar formation in a meandering channel and levee breaching to verify the model's performance. The results demonstrate that the flows and bed deformation in the meandering channel and the temporal process of dike breaching is accurately reproduced in the simulation.
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Heli YU, Kenji KAWAIKE, Kazuki YAMANOI, Takahiro KOSHIBA
2024 Volume 12 Issue 2 Article ID: 23-16178
Published: 2024
Released on J-STAGE: February 29, 2024
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The spacing is an important consideration in spur dikes design. In meandering channels, the flow field and sediment transport are affected by both the spur dikes and channel sinuosity. The paper employed a 3D model to investigate the effect of the spacing between spur dikes on bed deformation and flow in meandering channels under different sinuosity. The results showed that severe local scour occurred near both the most upstream spur dike and spur dikes downstream of the meander apex. As the spacing increased, complete scour holes emerged starting from the downstream spur dikes, the potential bank erosion became larger, and the flow near spur tips became more turbulent with a large velocity gradient. The maximum local scour depth increased with the spacing and decreased with the channel sinuosity in most cases. The non-uniform spacing could be used to lower the construction cost, and different sinuosity required different spacing arrangements. Based on the simulation results, the performance of different spacing was quantitatively evaluated simply by considering four factors of bank protection, structural stability, construction cost, and aquatic habitat.
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Juiche CHANG, Tomohiro TANAKA, Yasuto TACHIKAWA
2024 Volume 12 Issue 2 Article ID: 23-16188
Published: 2024
Released on J-STAGE: February 29, 2024
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Fluvial floods have been considered a lower risk to society in Taiwan since the implementation of well-protection works. However, given the impact of climate change on water-related disasters, it is necessary to re-evaluate the capability of existing facilities in Taiwan to adapt to a changing climate. By utilizing the d4PDF dataset in the rainfall-runoff simulation, we estimated potential changes in peak discharge in the Choshui River basin in central Taiwan to identify the impact of future extremes on runoff characteristics. Comparing the results with the design discharge of downstream Choshui River (100-year return period), the d4PDF +4K ensemble experiments indicate a 9.9%-27.9% increase in peak discharge, highlighting a heightened risk of fluvial flooding. Impact assessments of overflow were conducted by a 1-D river flow and a 2-D inundation model to simulate the fluvial flood inundation. To solve the intrinsic limitation of the 1-D river channel routing with mesh size for large-scale rivers, this research proposes a new scheme to automatically build up the connection between the river, floodplain, and inland cells to consist of the simulation with the actual channel boundary. This procedure successfully progresses the model simulation to be flexible to fit different shapes of river channels. Applied to the model, we provide several case studies of fluvial inundation by incorporating discharge derived from d4PDF simulation input. The results of the inundation simulation show that coastal areas around the Choshui River basin expose to a high risk of fluvial floods under climate change, highlighting the insufficiency of existing flood prevention works. Also, we demonstrate the potential risk areas for future fluvial floods, alone the Choshui River.
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Yutaka HAYASHI, Masashi KIYOMOTO, Go TANGE, Keishi NOGUCHI, Satoshi HA ...
2024 Volume 12 Issue 2 Article ID: 24-17054
Published: 2024
Released on J-STAGE: November 01, 2024
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On November 22, 2016, a magnitude (MJMA) 7.4 intraplate normal-fault earthquake with the tension axis in the northwest–southeast direction occurred off Fukushima Prefecture, Japan (2016 off-Fukushima earthquake). However, north–south oriented reverse fault-type earthquakes were assumed for this region by the tsunami database, which contains the precalculated tsunami data used for real-time tsunami forecasting by the Japan Meteorological Agency (JMA). According to our previous study, among the differences in the earthquake mechanism between that assumed in the tsunami database and that of the 2016 off-Fukushima earthquake, while the misforecast between the normal and reverse faults did not affect the real-time tsunami forecast, the difference in the strike angle of the fault plane was the main cause of the underestimated height forecast for Miyagi Prefecture. Based on the concept that an efficient scenario arrangement can be achieved when differences in forecasts from adjacent scenarios in the tsunami database are similar, and the results from sensitivity analyses of the tsunami height distribution to fault parameters for the 2016 off-Fukushima earthquake, it is reasonable to add tsunami scenarios with different fault plane strikes angles and the same simulation point near the epicentral area. We compared the centroid moment tensor solutions for earthquakes of moment magnitude 5.5 or more that occurred in coastal areas around Japan with the fault parameters assumed in the tsunami database. We then added scenarios to the tsunami database for areas where the difference in strike angles was 30° or more. Our performance tests using past scenarios indicate that the underestimation of tsunami warnings for the 2016 off-Fukushima earthquake can be avoided by using the modified database and that new scenarios will be frequently referenced in future tsunami warning operations. The modified tsunami database has been used for real-time tsunami warning operation by JMA.
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Annisa Farida HAYUNINGSIH, Takayuki SUZUKI, Martin MÄLL, Masayuki BANN ...
2024 Volume 12 Issue 2 Article ID: 24-17162
Published: 2024
Released on J-STAGE: November 01, 2024
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Mega cusps are critical for risk management due to their connection with rip currents and erosion in the swash zone. This study aims to explore the effectiveness of aerial photos and object detection algorithms in capturing mega cusp shapes, focusing on both edge-based and object-based methods. The research area focused on the Hasaki Coast in Kamisu City, Ibaraki Prefecture, and data were gathered from aerial photos and topographic data sources on June 6th, 2023. Four algorithms—Random Forest, Otsu's, Canny Edge, and Laplacian detection—were utilized to detect mega cusp shapes, extending their use beyond identifying sharp edges of objects such as buildings and roads to extracting smooth features that represent mega cusp shape lines. The Random Forest algorithm closely aligns with topographic data, detecting the mega cusp shape extraction in the area between wet and dry sand after classifying sand pixel colors. Meanwhile, the Otsu’s, Canny Edge, and Laplacian algorithms focus on the sand berm object, which has edges. Edge-based algorithms, such as Canny Edge and Laplacian, concentrate on the top edge of the sand berm, whereas Otsu’s algorithm focuses on the base of the berm sand. Elevation data extracted through Digital Terrain Models (DTM) and GIS tools highlight the importance of detailed examination for understanding mega cusp shape elevations. The Random Forest algorithm provides an average elevation of 1.26 m, similar to the high tide level, with 0.01 standard error of elevation, while Otsu’s, Canny Edge, and Laplacian algorithms yield higher elevations ranging from 2.03 m, 2.25 m, and 2.20 m, respectively, with a larger standard error of position. In conclusion, the study recommends the Random Forest algorithm, a machine learning-based approach, emphasizing its ability to detect the mega cusp object using a training dataset during a neap tide.
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Aye Nyein MON, Mohammad TABASI, Jun MITSUI, Shin-ichi KUBOTA
2024 Volume 12 Issue 2 Article ID: 24-17179
Published: 2024
Released on J-STAGE: November 01, 2024
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This paper aims to examine how the slope angle of wave dissipating work influences the hydraulic performance of horizontally-composite breakwaters under irregular wave action. The hydraulic performance, in terms of reflection coefficient, transmission coefficient and energy dissipation rate, is assessed through a numerical tool of CADMAS-SURF/2D (V5.1). When the slope of wave dissipating work became gentler, its reflection coefficient tended to become lower, and its transmission coefficient experienced only a slight reduction, while an increase in the energy dissipation rate was observed. The numerical results were verified using not only hydraulic model experimental results but also empirical formulas found in the literature.
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Mohammad TABASI, Aye Nyein MON, Jun MITSUI, Shin-ichi KUBOTA
2024 Volume 12 Issue 2 Article ID: 24-17197
Published: 2024
Released on J-STAGE: November 01, 2024
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This study investigates the impact of different seaside slope angles on breakwater stability through hydraulic model experiments conducted at a 1/50 scale. The cross-section of the breakwater was varied with slope angles represented as cot α = 1, 1.2, 4/3, 1.5, 2, where ߙ denotes the slope angle. Each trial subjected the breakwater to 1000 hits of irregular waves generated by a piston-type wave generator, with significant wave heights ranging from 11 to 17 cm and wave periods of 1.6 and 2.1 s. Stability coefficients (KD) were estimated based on Hudson’s stability equation. For a total damage rate of 1%, (KD) values were computed as 15.6, 11.3, 11.4, 8.9, and 8.4, corresponding to cot α = 1, 1.2, 4/3, 1.5, and 2, respectively. All computed (KD) values surpassed the threshold of 8.3 proposed in prior research, indicating its validity for breakwater design. However, as the slope becomes gentler, a decrease in the required mass is expected, suggesting that the Hudson equation may slightly overestimate the level of stability improvement for a gentler slope. Furthermore, the stability number (NS) was calculated using an equation that incorporates various parameters such as relative damage level, and constant values represented by ܽ and ܾ. Given the relative damage level of 0.3, 2.32 was adopted for ܽ based on previous studies, regardless of slope angles. Consequently, different β values were derived for cot α = 1, 1.2, 4/3, 1.5, and 2, yielding 1.75, 1.54, 1.60, 1.54, and 1.76, respectively. These estimated ܾ values exceeded those proposed in prior studies for all slope angles, affirming the conservative nature of the calculations.
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Yawen SONG, Satoquo SEINO
2024 Volume 12 Issue 2 Article ID: 24-17241
Published: 2024
Released on J-STAGE: November 01, 2024
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Coastal plant communities act as natural barriers to capture or intercept marine debris on the coast, thus reducing the risk of recurrent interior pollution. This study also examines whether these communities, known for capturing flying sand, can similarly intercept marine debris, reinforcing their role as natural barriers. Findings indicate that coastal plant communities more effectively capture Styrofoam debris, particularly particles ranging from 0.1 to 0.5 cm in size, commonly found in the Carex kobomugi community. The results of this study can inform coastal ecological restoration projects and guide the cultivation of plants on artificial coasts. Additionally, they offer insights into mitigating the detrimental effects of aesthetically displeasing marine litter on the tourism value of beaches.
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Calvin SANDI, Nobuhito MORI, Tomoya SHIMURA, Takuya MIYASHITA
2024 Volume 12 Issue 2 Article ID: 24-17268
Published: 2024
Released on J-STAGE: November 01, 2024
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The rising sea level poses a serious threat to Southeast Asia, endangering low-lying islands and coastal areas. Climate events like El Niño significantly influence the sea level changes and variability. However, Southeast Asia has a series of complex island structures located between the Indian and Pacific oceans, creating unique characteristics of Sea Level Anomaly (SLA) yet complex correlation with these climate events. This study investigates the possible teleconnection pattern of climate variability modes on SLA. A gridded SLA dataset derived from satellite altimetry (CMEMS) for 1993-2022 was utilized. Eleven potential climate indices to represent the Pacific and Indian Oceans were analyzed. While ENSO primarily influences the Pacific Ocean, the best index to capture ENSO impact depends on the region and the variable studied. Hence, four ENSO-related climate indices (Niño 3.4, ONI, MEI, and SOI) were included to identify the most appropriate index to describe SLA relation to ENSO in Southeast Asia. ONI was subsequently excluded due to its high correlation with Niño 3.4 (R = 0.99). Initially, the correlation coefficient of each index and SLA were calculated. Six indices exhibited high correlation, indicated by statistically significant areas exceeding 50% at a 95% confidence level. Multiple regression analysis was then conducted along with Relative Weight Analysis (RWA) to identify every index's contribution (importance) level. PWP emerged as the most important index, with a significant area of 95.24% and a contribution level of 35.60%. MEI was also identified as the best ENSO index with the highest contribution level. Furthermore, this study explored the long-term spatiotemporal variability of SLA. On the interdecadal scale, SLA in Southeast Asia is increasing from one interdecadal to another with a decreasing speed (trend). The anomaly is higher and more fluctuative during DJF. Moreover, variability is higher in the open ocean compared to the semi-enclosed ocean.
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