欧陽 懋

Abstract. Controlling weather is an outstanding and pioneering challenge for researchers around the world, due to the chaotic features of the complexatmosphere. A control simulation experiment (CSE) on the Lorenz-63 model, which consists of positive and negative regimes represented by the statesof variable x, demonstrated that the variables can be controlled to stay in the target regime by adding perturbations with a constant magnitude toan independent model run (Miyoshi and Sun, 2022). The current study tries to reduce the input manipulation of the CSE, including the total controltimes and magnitudes of perturbations, by investigating how controls affect the instability of systems. For that purpose, we first explored theinstability properties of Lorenz-63 models without and under control. Experiments show that the maximum growth rate of the singular vector (SV) reduceswhen the variable x was controlled in the target regime. Subsequently, this research proposes to update the magnitude of perturbationsadaptively based on the maximum growth rate of SV; consequently, the times to control will also change. The proposed method successfully reducesaround 40 % of total control times and around 20 % of total magnitudes of perturbations compared to the case with a constant magnitude.Results of this research suggest that investigating the impacts of control on instability would be beneficial for designing methods to control thecomplex atmosphere with feasible manipulations.

Internal erosion has been frequently reported and has caused failures and instabilities of geotechnical structures. A plane strain erosion apparatus is developed in this study to allow the subsequent conduction of drained compression test after seepage test and the microscopic observation of particle movement through a transparent window. A drained compression test preceded by a seepage test is performed on specimens containing the same initial fines contents to investigate the mechanical consequence impacts of seepage-induced internal erosion. Experimental results reveal that, compared with uneroded soils, internally eroded soils show a larger secant stiffness at a small strain level (-1 %). At medium strain level (-15 %), the soils with erosion show smaller deviator stress comparing with soils without erosion. The analysis of images recorded by the microscope proves that the fines contacted with coarse particles possibly transferring the load are distinct between the soils with and without internal erosion at both small and medium strain levels during the drained compression test, which indicates that the soil fabric could affect the mechanical behaviors of soils subjected to internal erosion. Our designed equipment and microscopic observation could throw some light on the research of internal erosion from the view of particle scale.

数値気象予測において，観測情報は予報誤差低減に重要な役割を果たす一方，観測位置を最適化する手法は未確立である．近年，PODモードと観測値を用いて場を復元する方法や，復元される場のスパース性に着目し，フィッシャー情報行列Fを最大化するスパースセンサ位置最適化(SSP)手法が提案されている．本論文では大次元力学系に適したSSP手法を検討し，SSTを用いた数値実験により提案手法の妥当性を考察した．場の復元にデータ同化を新たに応用し，精度改善に成功した．更にSSPにモードのみを用いた場合と，モードと特異値を考慮した場合を比較し，特異値を考慮した観測位置決定では上位モードが優先されることを示した．Fの最大化の解釈として，Fの行列式最大化とFの逆行列対角和の最小化を検討し，後者が適することを示した．

雨量計は降水量の時空間分布推定に大きな役割を果たし，効率的に雨量計を配置することが重要である．本研究では，情報科学分野で提案されているスパースセンサ位置最適化手法（SSP）を雨量計配置の問題に適用し，時空間分布推定に有効な位置を推定する．更に少ない観測情報から全体の場を復元する手法として，新たに局所アンサンブル変換カルマンフィルタ（LETKF）を適用する．気象庁の解析雨量を使用して，雨量計配置と場の復元を北海道を対象に行った．AMeDASの観測位置情報から場の復元手法をLETKFと最近傍法とで行い，LETKFがより高精度であることを確認した．更にSSPによる雨量計配置とAMeDASの観測位置からLETKFによる場の復元をそれぞれ行い，SSPによる雨量計配置が有効に機能することを確認した．

Anomalous precipitation (more than 360 mm/d) caused severe flood inundation in the Kujukuri plain, Japan, on October 25, 2019. In this contribution, we employed a hydrological and hydraulic model to reproduce this flood and to examine the possible factors affecting the disaster. The simulated results showed consistent areal extents of inundation compared with the field investigation, suggesting potential application of numerical approaches for assessing the flood hazard. Details of the simulation results obtained from the flood on October 25, 2019, together with those on October 8, 2004 and October 15, 2013, revealed that the local geomorphological and geohydrological features significantly influenced the flood hazard. The low elevation lands located near the confluence zones of the rivers and enclosed by higher elevation terraces were found to be quite vulnerable to floods. Numerical simulation suggested that the study area showed both direct groundwater recharge from the inundated surface and indirect recharge from the stream leakage. Groundwater stored during the flood fed the river after the peaks of the precipitation, causing the delay of river water recession process in the stations situated inside the inundated areas. Our results showed that considering local geomorphological and geohydrological characteristics are crucial in evaluating flood hazard.

Current estimates of flood hazards are often based on the assumption that topography is static. When tectonic and/or anthropogenic processes change the land surface elevation, the spatial patterns of floods might also change. Here, we employ the hydrological and hydraulic modeling to simulate floods in the Kujukuri Plain, Japan, in the years 2004 and 2013, when two severe floods occurred. In between the two floods, land surface elevations were changed by the 2011 Tohoku-Oki earthquake. The effects of land surface elevation changes on inundation areas were quantified by changing input topographies. Our results showed that, without taking into account land surface elevation changes, around 10% of inundation areas were underestimated at the time of flood events in the year 2013. The spatial distribution of inundation locations varied with local topographical features, for example, the areas with backmarsh and valley fill deposits were sensitive to the extent of inundation by land surface elevation changes. The sub-watershed near the coastal shoreline having below-zero meter elevation areas showed that the earthquake-induced land surface elevation changes exacerbated an additional 22% inundation area. This study suggests that the inundation areas will increase in catchments suffering severe settlements, which highlights the necessity of taking into account the spatio-temporal changes of land surface elevations on the assessment of flood hazards.

Flood-inundation hazard maps are generally made based on the anticipated flood patterns under the configurations such as current topography and land use/land cover situations. These maps have not taken into account the possible significance of land subsidence and surface environmental changes. The Kujukuri Plain, Japan, the site of this research, has experienced severe land subsidence due to 2011 Tohoku earthquake. This paper aims to quantify the effects of local land subsidence on flood hazards under heavy rainfalls in the Kujukuri Plain. The high-resolution LiDAR data at two different periods (before and after the Tohoku earthquake) were obtained and used as the input data for the physically-based hydrological model. Through the comparison of simulated inundation areas of the scenarios with the same precipitation pattern but different topographies, the effect of land subsidence was discussed. The maps estimating the inundation areas by only considering rainfalls, i.e., without taking into account the effect of land subsidence, underestimated the inundation areas by around 10% compared with the ones that accounted for the local land subsidence. The results of this study highlight the importance of taking into account the temporal changes of elevations and other parameters in flood hazard assessments.

Seepage-induced internal erosion often happens in earth structures. This paper presents experimental investigations on the influence of initial fines content on fabric of soils subjected to internal erosion. The tested materials were the binary mixtures of silica No. 3 and silica No. 8, which correspond to the coarse and fine fractions, respectively. One group of specimens was prepared with initial fines contents of 0, 15%, 25%, and 35% by weight. The undrained monotonic compression tests were performed on this group to examine the influence of fines content on the undrained behavior. The other group was prepared with initial fines contents of 15%, 25%, and 35% by weight, on which the seepage tests and subsequent undrained compression tests were carried out to demonstrate the mechanical influence of the internal erosion. The undrained behavior of the first group of specimens reveals that the presence of fines would decrease the peak and residual strengths. A comparison between the undrained behavior of soils with erosion and that of soils without erosion shows that the soils become less contractive after the internal erosion. When the axial strain is less than 0.4%, the undrained secant stiffness of soils with erosion is larger than that without erosion at the same axial strain. Meanwhile, the undrained peak strength and residual strength are larger for soils with erosion than that for soils without erosion. The less amount of excess pore-water pressure is generated during the undrained compression for the eroded soils comparing to those of the uneroded soils. Furthermore, the eroded soils show a wider instability zone than that of the uneroded soils, which suggests that the instability zone be enlarged by the internal erosion. Besides, one-dimensional upward seepage tests were performed to investigate the change of fabric of the mixed sand with 15%, 25%, and 35% fines contents due to internal erosion. The recorded microscopic images of soils before and after erosion reveal that the fabric is altered by the internal erosion.

Suffosion is a seepage-induced instability phenomenon whereby fines are eroded away through the constrictions formed by coarse particles, resulting in a reduction in soil volume and a change in hydraulic conductivity. In this study, upward seepage tests were performed on gap-graded soil containing coloured fines in a plane strain apparatus. A series of images was recorded through a transparent window to quantify the features of suffosion. The estimated cumulative eroded soil mass from image analysis was found to generally agree with independent macroscopic observations, indicating that optical analysis allows an easy identification of suffosion characteristics. For the tested soil, the fines were prone to be transported within an instant period of increasing hydraulic gradient, with few of them moving during the constant flow. Furthermore, the volume of the specimen was reduced due to suffosion, leading to an alternation of preferred coarse particle orientations in the observation field.

This paper investigates the effects of internal erosion on soils by conducting seepage and compression tests in triaxial apparatus. The tested materials were the mixtures of Silica no. 3 and no. 8.Two groups of specimens were prepared with initial fines content of 15, 25 and 35% by weight, respectively. The conventional undrained monotonic compression tests were performed on one group of soils to examine the influence of initial fines content on the soil undrained behavior. To demonstrate the mechanical influence of internal erosion, the seepage tests and subsequent undrained compression tests were carried out on the other group of soils. It is found that the soil with 35% initial fines content shows the largest cumulative eroded soil mass during the seepage tests. Results of monotonic compression tests indicate that peak strength and residual strength of the soils with erosion is larger than those of the soils without erosion if the initial fines content of those soils are the same.

Suffusion, defined as the phenomenon whereby the fines gradually migrate through the voids of coarse fractions in a soil, has been widely detected in natural deposits and artificial earth structures. The occurrence of suffusion may chronically loosen the soil structure, increasing the vulnerability against large deformation and soil failure. In this paper, experimental studies on volume change of saturated gap-graded cohesionless soil during suffusion and its mechanical influence on undrained behavior are presented. Test results reveal that because of the loss of large amounts of fines during suffusion, volume of tested soil decreases and void ratio increases. Correspondingly, a distinctive undrained behavior of suffusional soil is noted from that of the reference soil without suffusion.