土木工学

The key to the study of an alluvial process is to elucidate interactions between flow properties, sediment movements and bed or channel configurations. For this reason, any physical model of sediment transport is required to describe reasonably the non-equilibrium state of an alluvial process. From this point of view, the sediment model has been recently reinvestigated on the basis of knowledge accumulated on the mechanism of sediment motion. In this paper. the latest research on development of sediment models is reviewed. Particularly, importance is attached to unified treatment of bed material load, taking into account the transition from bed load motion to suspension both through incipient motion and transport process, and a physical model for description of non-equilibrium situation is proposed by applying a stochastic approach. Further in order to predict the scouring process of river banks, formation of armor coat, incipience of debris flow, etc., the effects of channel boundaries such as side slope, sand mixtures and permeable bed on sediment transport process are discussed.

This paper was presented at the general meeting for establishment of the society as an invited memorial lecture. The paper involves a review of multiphase flow appearing in civil engineering field.First of all, the phenomena of multiphase flow in civil engineering are classified in Table 1, and especially features of mudflow in mountains and extremely high concentration flow in Yellow River are introduced as peculiar phenomena of one directional two-phase flows with water and sediment. Secondly sediment transport on the beach and beach deformation by waves are discussed as a typical two-phase flow problem in coastal engineering, and in addition, a pneumatic breakwater using air bubble curtain flow is introduced as an application of two-phase flow with air and water.

Regulations controlling the quality of water discharged from reclaimed areas are becoming more and more strict for reclamation projects which use cutter suction dredgers. These regulations seek to prevent the damaging effects of spilled water on the coastline. For this reason, it is necessary to skillfully manage the growing estimates of water that is discharged along with fine soil particles. Reclamation projects which use cutter section dredgers dredge the sea bed, sucking up soil as well as water, and transport everything to the reclaimed area through a system of distribution pipes. These pipes are also connected to the project's outlet system for discharging water and fine soil particles. The conditions at reclaimed areas under construction change daily, and we must be able to precisely predict these changes. Although several methods have been proposed, one must be selected. This paperwill introduce the actual problems of density and the growing estimates of discharged water and fine soil particles.

河川流域の地形は侵食・堆積現象を通じて絶えず変動している。これが急激に起こる場合にはときには大災害を引き起こし、緩やかな場合にもそれに伴う河床変動によって治水上の問題が引き起こされる。このような地形の変動は、すべて流砂の不平衡によってもたらされている。ここに、流砂研究が河川工学の中心課題の一つであるゆえんがある。流砂現象は固液混相流の代表的なものの一つであるが、それを統一的に扱う支配方程式が確立されていると云える段階にはない。本講座では、紙面を通じて識者の議論の種を蒔くことを目的として、1. 土石流の力学、2. 掃流状集合流動、3. 掃流砂の流れと土石流の力学の相似性、および4. 固液混相流の支配方程式の課題等について解説したい。

第1回目は土石流の力学に関する研究成果について解説した。ここでは、それに引き続き掃流状集合流動あるいは土砂流と呼ばれている流れについて解説する。土石流の解説 [1] において勾配がある限界値よりも大きい場合には、砂礫と水との混合物の流れは、巨視的には水と砂礫とは一体になって流れるが、勾配が限界値よりも緩くなると、自由水面近傍には砂礫を含まない水の流れが形成される。このような流れを土砂流あるいは掃流状集合流動と呼んでいる。したがって、一流体モデルによる解析においては、上層の流れは通常の清水流の応力モデルが適用でき、下層の流れには、土石流のものが直接適用できる。なお、以下の議論は、あくまで平均流に関するものである。

沖積地河川には、大小さまざまのスケールの流路・河床変動が現れる。これらは、流砂輸送の不均衡によって引き起こされる。流砂のうち、河床を転動、滑動、跳躍しながら輸送されるものが掃流砂である。ここでは、前回に引き続き、流砂に関連する諸問題のうち、掃流砂に関する輸送量の推定式がどんな経緯で導かれてきたかを紹介するとともに、それらと土石流の力学との相違点と類似点について紹介する。

これまで3回にわたって土石流、掃流状集合流動、掃流砂を含む流れについて解説してきた。まず、土石流は典型的な固液混相流のひとつであり、それを単一の連続体で近似する場合の流れの力学的なフレームとその解析法について解説した。さらに、その理論を掃流砂にまで応用し、その結果と従来の掃流砂に関する解析法との比較を行った。両者には確かに類似の関係式を見いだすことができるものの、その基となっている力学的なフレームに関する理解はかなり異なっていることを示した。このことは、流砂現象一般を固液混相流として統一的にとらえることの可能性を示唆するとともに、今後、流砂現象に対して、固液混相流としての支配方程式がどのように構築されるべきかについて十分考察する必要があることを示している。そこで流砂現象がどのような支配方程式に基づいて説明されてきたかを整理概観し、その課題について若干言及してみたい。

River restoration is a current topic in river engineering, where multiple functions of rivers such as the safety against flood, water resources utilization, amenity of human life and ecological system conservation are desired to be satisfied simultaneously. For this purpose, the technique to control the interactions among flow, sediment transport, vegetation and river morphology is strongly required. It is supported by fluvial hydraulics, hydraulics of flow with vegetation and habitat hydraulics, and the research from the view point of multiphase-flow study is expected to contribute them. In this paper, the new understanding of river restoration is explained by using the concept of “river landscape”, and it is discussed how the recent development of study on fluvial hydraulics related to riparian vegetation contributes to it. Since the mechanics of sediment transport and modeling of flow with vegetation are reviewed by the other authors of this volume, the studies on fractional transport of graded sediment with sorting process and the riarian morphology related to vegetation growth are focussed on herein.

The turbulent flow structure and the transports of heat and substances in and above vegetation layer were studied by the field observation and the laboratory test.At the field observation, fluctuations of wind velocity, temperature, carbon dioxide and vapor were measured by using sophisticated devices.The vortices were observed to be composed of strong sweep and weak ejection, and the temporal variation of sensible heat showed ramp pattern. Transports of heat, fluid momentum and vapor are found to be closely correlated with organized vortices. Twodimensional horizontal structure of vortices was detected by thermography. The instantaneous three-dimensional structure of the organized vortex was measured by employing PIV and MASCON model in the laboratory flume.The shape of the organized vortex is elliptical, and the vortex is inclined downward toward the front, a profile of which is effective in yielding Reynolds stress.

Recently hydraulics in open-channel flow with vegetated zone becomes important increasingly for river restoration. Vegetated zone can be regarded as a kind of porous region and momentum sink due to the form drag. Flows inside and outside vegetation zones bring about strong shear layer and cause momentum exchange and additional resistance between them. In this paper, the recent studies of numerical calculation on fully developed turbulent flow in open-channel with vegetation zone are outlined:(1) modeling of vegetation for river hydraulics, (2) flow over vegetation, (3) compound channel flow with vegetation, (4) flow with vegetation zone.

The grouting method newly developed in the present study is called Balloon grouting method. This method is effective as a countermeasure against liquefaction of ground beneath existing structures.

Groundwater is one of the most important water resource for many country, and adequate management is required to use it sustainably. Modeling and simulation are the strong tool for groundwater management. However, hydraulic conductivity distribution of the flow field has been ig.nored in the most of simulations. In this report, the outline of the model for spatial distribution of hydraulic conductivity is described, and some examples of the numerical simulations in the non-uniform flow field are introduced.

During the Tohoku earthquake, the tsunami had created a large scale seabed scour at the entrance of the port of Hachinohe in Japan. This damage would pose a problem to the safety of the navigation ships. So it was necessary to carry out a backfill work with dredged sand to ensure the safety of the navigation ships. Hence we introduced the construction method of dredging and filling by using cutter suction dredger and sand filling vessel. This method was carried out using a filled sand which was dredged by cutter suction dredger and then delivered via discharge pipe to the sand filling vessel. It was then placed on the designated area using tremie pipe which was equipped on the sand filling vessel.

The present article overviews some granular/multiphase flows observed in civil engineering fields and discusses their grain-scale mechanics. In particular, solid/fluid phase transition behaviors are highlighted to characterize the mechanics of granular materials. In liquefaction phenomena, uni-directional shear causes solidification of the system, while cyclic shear leads to liquefaction. This behavior can be understood as an induced anisotropy of granular packing structure. In rapid flows, kinetic stress increases with increasing shear rate, which may result in drastic reduction of shear resistance of the flow. Emphasis is also put on the significance of the analysis of long-term geological formation whose micro-mechanism is governed by the grain/fluid interaction during erosion, transformation and sedimentation process.

This study aims to evaluate evacuation safety by using several crowd dynamics models based on self-driven particles. In this paper, we implemented two models, namely, the social force model and the RVO model. The social force model is a dynamics model that solves a motion equation. The RVO model is an expanded model of velocity obstacles for collision avoidance among moving objects. When we try to understand the characteristics of those models, the results are as follows. If the desired speed of particles is over 3.5 m/s, the flow coefficient of the social force model declines because of increasing the frequency of clogging. On the other hand, the flow coefficient of the RVO model does not decline. As regards an application to evacuation from a building, the flow of a room declines because of the crowded corridor.

Intertidal mudflats are important for land conservation, flooding risk alleviation and biological environment. Mudflat profiles have been measuring monthly along some cross-shore lines exceeding 1,000 m in the vicinity of a river mouth since December 2000. For the mud profile evolution, the annual accretion rate was 4.2 cm/year and 1.6 cm/year on the right and left lines, the seasonal variation in the range -5 cm and 5 cm, and the maximum episodic variation 12.1 cm and 22.0 cm. The wind waves could play many contributions for deposition and erosion of sediment. Sediment budgets based on the monthly bed level and net sediment flux monitoring were estimated to examine the relative contributions of tides and river discharge to sediment transport. The estimated sediment budgets are interpreted using the water mass balance equation and the horizontal tidal current pattern. The intertidal flat accreted during normal discharge conditions are primarily attributable to the alongshore sediment flux toward the river mouth. However, the flat was eroded when the large offshore suspended sediment transport occurred on the flat during the large river discharge. The net alongshore tidal current causes alongshore sediment fluxes toward the river mouth on this intertidal flat adjacent to the river mouth. In observation periods, there were two times of a significant flood occurred on July 12, 2012 with the discharge water rates of 2,300 m3/s and on June 21, 2016 with 1,600 m3/s. Also two earthquakes scaled of 7 on the Japanese scale struck Kumamoto Prefecture on April 14 and 16, 2016, and then about the 0.4 m ground subside was occurred at the present field site. A large amount of sediment from Shirakawa River was discharged into the delta due to the flood. The clinoform along the water route approximately propagated 150 m in the offshore direction and the bed level increased by 1.5 m. The estimated sediment discharge was 1.1x105 m3/yr./km2 during 2014 to 2016, corresponds four times during 1978 to 1997.

Field measurements were carried out in the port of Niigata to elucidate sediment transport processes around navigation channel and turning basin in the port at river mouth. The measurements include salinity, suspended sediment concentration and in-situ mud density at several monitoring points along the navigation channel in the port. The survey successfully captured the formation of three layers structure during a river flood condition with high turbid fresh water in the upper layer and low turbid sea water in the middle layer. Furthermore, high concentrated mud or fluid mud layer was also found in the lowest layer at the deeper dredged channel area. Physical model experiments were also carried out with a circulating flume to elucidate the fluid mud transport on the slope at the edge of step-like bathymetry. The experiments indicate the behavior of near bed turbid water depends on the overlying flow conditions.

Building energy simulation (BES) is commonly conducted in architecture design process to evaluate building energy performance. The coupling between computational fluid dynamics (CFD) and BES not only improves energy simulation accuracy but also makes it possible to simultaneously consider energy consumption and indoor environment. Nevertheless, to conduct high fidelity CFD simulation is generally time-consumed and thus it is almost impractical to carry out a long-term coupled simulation that requires multiple CFD executions. A fast and accurate prediction method is therefore required to serve as a surrogate for CFD in the coupled simulation. Inspired by successful applications of deep learning neural networks (NNs) in various fields due to the high computation speed and prediction accuracy, the authors proposed a deep learning-based prediction method to achieve fast and accurate prediction of indoor air distributions. This paper provides a general introduction to the proposed prediction method with regard to its principle, implementation, performance, and application. Predictions of two-dimensional isothermal flow and three-dimensional non-isothermal flow are demonstrated. The results confirmed the feasibility of NN models for fast and accurate indoor airflow prediction. Meanwhile, as an example of practical application, the NN model is coupled with a BES tool to implement a coupled simulation framework for fast energy simulation considering non-uniform indoor environment. The coupling scheme is introduced and validation of the crucial functionality of the framework is presented.

Smoothed Particle Hydrodynamics (SPH), which is a Lagrangian meshless method, is known as very useful method for large deformation problems, such as movement of wave absorbing blocks and sliding of a caisson breakwater. This study aims to simulate the movement of tsunami boulders and storm boulders, comparing with the hydraulic experiment, and evaluating the transport characteristics. The study also aims to clarify the scattering process of armour blocks on the additional rubble mound behind breakwaters against tsunami overflows. The series of simulation show the difference of movement characteristics between tsunami and storm boulders are caused by the difference of acting direction of tsunami and random waves on boulders, and hydrodynamic forces acting on the block differed depending on the block shape and initial water level conditions.