Hiroyuki Tosaka

Fully coupled formulation of surface flow with 2-phase subsurface flow for hydrological simulation

With the aim of realizing a surface-subsurface-coupled fluid flow simulation, the authors discuss surface flow models that can be used in the framework of the 2-phase subsurface numerical simulation system. In this paper, three surface flow models, i.e., a kinematic wave model (KW), a diffusion wave model (DW), and a linearized diffusion wave model (LDW), are explained by approximating the momentum equation, and are transformed into a set of nonlinear functions which can be solved simultaneously with the 2-phase subsurface flow. By incorporating these into a 2-phase numerical simulation system, comparison studies are carried out using experimental data and a field scale model. So far as the results of the study are concerned, it is concluded that the DW model gives reasonable performance for any case, the LDW model provides mostly identical results with the DW model with some restrictions, while the KW model is not applicable for the physical modelling of continuous flow through multiple slopes with different angles. Copyright

Cataclysmic Scabland flooding: Insights from a simple depth-averaged numerical model

This work presents numerical assessments of possible routing paths for the reported late Pleistocene Missoula floods, which involved Earths largest known peak discharges of freshwater. For our numerical model, we adopt the diffusion wave approximation of the shallow-water equation with the empirical Manning coefficient. This simplification is required because (1) most previous studies are one-dimensional or analytical studies based on simple equations, so that our two-dimensional expansion is a natural step towards more sophisticated models; (2) a simple model can easily incorporate the effect of complicated topography; and (3) lower computational costs permit a broader exploration of parameters. Important and new insights obtained from this work include: (1) numerically, we confirm the presence of the Cordilleran Ice Sheet at the time of the Missoula floods; (2) floods with relatively small peak discharges cannot inundate some major reaches, including the Cheney Palouse Scabland Tract; (3) deposits indicative of multiple floods are mostly confined in the locations that can be inundated by relatively lower peak flood discharges; and (4) although Lake Missoula glacial lake failure scenario can reasonably reproduce extensive water coverage, the best fit result to the geological evidence of flooding is obtained when the total water volume is three times larger than that estimated for Glacial Lake Missoula. The above results do not negate previous works, but indicate the need for further detailed field investigation and more sophisticated modeling in order to pursue further understanding of the complex behavior of megaflooding in the Channeled Scabland.

The Channeled Scabland: Back to Bretz?: Comment and ReplyCOMMENT

The major controversy regarding the origin of the Channeled Scabland is whether the landforms were formed mainly by multiple periodic floods or by a grand-scale cataclysmic flood from late Pleistocene Glacial Lake Missoula. Evidence for the multiple flood—as many as 40 as estimated by [Waitt (1980

Integrated watershed modeling for simulation of spatiotemporal redistribution of post-fallout radionuclides

Simulation of the watershed-scale fate and transport of radionuclides is required in order to predict the consequences of contamination redistribution. Integrated watershed modeling is a suitable technique for this task, but it requires fully coupled investigation of radionuclide behavior in surface water, suspended sediment and subsurface aquifers. We developed a novel simulator for computing the spatiotemporal redistribution of fallout radionuclides in watersheds. The simulator was applied to an actual reservoir basin contaminated by fallout radionuclides from the Fukushima Dai-Ichi Nuclear Power Plant accident in 2011. As a result, the simulated 137Cs concentration in bottom sediment showed a reasonably close match with the measurement data. The distribution coefficient of 137Cs consistent with the latest measurement data was identified as being at least 400,000?L/kg, and it was estimated that more than 90% of the total 137Cs distributed in the fallout remains in the catchment area. We propose a new simulator to assess environmental impact of fallout radionuclide.Simulation treats surface and subsurface fully coupled hydrological processes.Spatiotemporal variation of the deposited 137Cs can be successfully reproduced.Deposited 137Cs is predominantly transported with the suspended sediment.Results indicate that most of the deposited 137Cs remains within the catchment.

Application of surface-subsurface flow coupled with numerical simulator to runoff analysis in an actual field

A numerical simulator that treats three dimensional, multiphase, surface-subsurface combined flows was applied to actual runoff analysis. A sensitivity study of hydraulic surface/subsurface interaction parameters required in numerical simulation was carried out utilizing short-term data, and a set of parameters that reproduces an actual hydrograph was obtained. To improve reproducibility of the hydrograph, a modification of discretization in expressing riverbed was introduced, so that a more realistic hydrograph is reproduced and a better quantitative estimation about base and side flow is realized. Applying the result of the sensitivity study to long-term runoff analysis, the hydrograph in one year was reproduced satisfactorily with the same hydraulic parameters obtained in the short-term runoff analysis.

Characteristics of radio-cesium transport and discharge between different basins near to the Fukushima Dai-ichi Nuclear Power Plant after heavy rainfall events

This paper describes watershed modeling of catchments surrounding the Fukushima Dai-ichi Nuclear Power Plant to understand radio-cesium redistribution by water flows and sediment transport. We extended our previously developed three-dimensional hydrogeological model of the catchments to calculate the migration of radio-cesium in both sediment-sorbed and dissolved forms. The simulations cover the entirety of 2013, including nine heavy rainfall events, as well as Typhoon Roke in September 2011. Typhoons Man-yi and Wipha were the strongest typhoons in 2013 and had the largest bearing on radio-cesium redistribution. The simulated 137Cs discharge quantities over the nine events in 2013 are in good agreement with field monitoring observations. Deposition mainly occurs on flood plains and points where the river beds broaden in the lower basins, and within dam reservoirs along the rivers. Differences in 137Cs discharge ratios between the five basins are explained by differences in the initial fallout distribution within the basins, the presence of dam reservoirs, and the input supply to watercourses. It is possible to use these simulation results to evaluate future radioactive material distributions in order to support remediation planning.

Homogeneity and trends in long-term rainfall data, Kelantan River Basin, Malaysia

ABSTRACT The Kelantan River Basin, situated in the northeastern Malaysian Peninsula, suffers serious flood/inundation damage, related to the northeast monsoon season (November–January), every few years. In this river basin, rainfall observation systems have been progressively developed since 1948, and long-term time-series data at distributed rainfall stations have been accumulated. This study firstly investigated the homogeneity of the accumulated time-series data for the purpose of constructing a reliable database for various hydrologic analyses. The homogeneity of rainfall time-series data was established using four absolute homogeneity tests: the Pettitt test, standard normal homogeneity test, Buishand range test, and von Neumann ratio test. It was found that among 50 rainfall stations within the river basin, 9 were flagged by the tests. Of these, inhomogeneous time-series data from four stations were omitted from further analysis. Secondly, using the homogenous time-series rainfall data, a trend analysis of annual rainfall variability was conducted by means of the Mann–Kendall (MK) test. To investigate long- and short-term characteristics of rainfall variability, three sampling methods of the MK test were applied: 30-year, sequentially increased, and 10-year moving segmented sampling. The MK test using the 30-year sampling showed a decreasing trend between 1957 and 1987, and an increasing trend between 1981 and 2011. The MK test using the sequentially increased sampling detected neither significant increasing nor decreasing trends through the same 70-year period. The MK test using 10-year moving segmented sampling clearly showed significant rainfall variability, which corresponded to the El Niño and La Niña events of 1972, 1982, 1988, the mid-1990s, and 2000–2004. Extending the plot, by comparing the turning points with the occurrence of El Niño and La Niña events, the results showed that the influence of the El Niño southern oscillation (ENSO) is the most likely to significantly influence rainfall trends in the Kelantan River Basin.

An estimation of the intrinsic permeability of argillaceous rocks and the effects on long-term fluid migration

Abstract The effect of intrinsic permeability of argillaceous rocks on long-term petroleum migration is discussed from results of both laboratory experiments and numerical sensitivity studies. Results of our experimental studies on one-dimensional mechanical compaction of both muddy slurries and mudstones show that porosity(φ)-permeability(K) relationships are linear on double logarithmic scales, i.e. φ and K satisfy the relationship: K=K0(φ/φ0)a, where K0 is initial permeability, φ0 is initial porosity, and ‘a’ is a parameter depending on the samples used. Comparison with published φ-K relationships of mudstones indicates that the above mentioned relationship could be extrapolated to natural condition when φ>0.3. On the other hand, the relationship becomes deviated from measured data in the low porosity range. Considering the importance of φ-K relationships in φ<0.3 on the timing of expulsion of petroleum from source rocks, this paper discusses the way to find out an appropriate φ-K relationship through trial and error matching of results of basin simulation using different φ-K relationships with observed pore pressure and porosity data. We also present one case study using an imaginary siliciclastic sedimentary basin and discuss the potential of a numerical study to establish one of the appropriate relationships, showing how calculated results become varied by choosing different relationships.

Absolute homogeneity test of Kelantan catchment precipitation series

Along the Kelantan River in north east of Malaysia Peninsular, there are several areas often damaged by flood during north-east monsoon season every year. It is vital to predict the expected behavior of precipitation and river runoff for reducing flood damages of the area under rapid urbanization and future planning. Nevertheless, the accuracy and reliability of any hydrological and climate studies vary based on the quality of the data used. The factors causing variations on these data are the method of gauging and data collection, stations environment, station relocation and the reliability of the measurement tool affect the homogenous precipitation records. Hence in this study, homogeneity of long precipitation data series is checked via the absolute homogeneity test consisting of four methods namely Pettitt test, standard normal homogeneity test (SNHT), Buishand range test and Von Neumann ratio test. For homogeneity test, the annual rainfall amount from the daily precipitation records at stations locat...

Stage discharge curve for Guillemard Bridge streamflow sation based on rating curve method using historical flood event data

The purpose of the stage-discharge curves varies from water quality study, flood modelling study, can be used to project climate change scenarios and so on. As the bed of the river often changes due to the annual monsoon seasons that sometimes cause by massive floods, the capacity of the river will changed causing shifting controlled to happen. This study proposes to use the historical flood event data from 1960 to 2009 in calculating the stage-discharge curve of Guillemard Bridge located in Sg. Kelantan. Regression analysis was done to check the quality of the data and examine the correlation between the two variables, Q and H. The mean values of the two variables then were adopted to find the value of difference between zero gauge height and the level of zero flow, a, K and n to fit into rating curve equation and finally plotting the stage-discharge rating curve. Regression analysis of the historical flood data indicate that 91 percent of the original uncertainty has been explained by the analysis with the standard error of 0.085.