Shinji Egashira

Experimental study on the entrainment of bed material into debris flow

The present study describes entraining characteristics of bed material into debris flow, based on flume tests, numerical and dimensional analyses. Flume tests are conducted to investigate influences of bed sediment size on erosion rate by supplying debris flows having unsaturated sediment concentration onto erodible beds. Experimental results show that the relative erosion rate, E/E0, decreases monotonically with increase of relative sediment size, d/d0, although E/E0 changes slightly with sediment concentration of debris flow. Herein, E is the erosion rate of bed sediment of size, d, E0 is the erosion rate when solid particle size, d0, of debris flow, are the same of the erodible bed material. According to the relation between E/E0 and d/d0, erosion rate, E, can be estimated by using Egashiras formula for E0. Therefore, the validity of erosion rate formula for E0 is tested by solving numerically for debris flow characteristics in terms of governing equations. In addition, critical size of bed sediment entrainment is discussed by introducing non-dimensional effective bed shear stress which is formulated by using fluid shear stress (total shear stress minus yield stress), bed sediment size, d, specific weight of sediment particle in water and acceleration due to gravity, and it is found that the critical non-dimensional effective shear stress takes a value similar to critical Shields parameter for bed load movement.

Role of suspended-sediment particle size in modifying velocity profiles in open channel flows

Previous experimental and analytical studies have revealed that suspended particles can attenuate or enhance turbulence, depending on the particle size in relation to turbulence scales. Incorporating this mechanism, an empirical turbulent eddy viscosity-based closure model is proposed for the mean velocity structure of suspended sediment-laden flow in open channels. The model integrates the sediment particle Stokes number, the ratio of particle-size-to-turbulence microscale, the ratio of particle settling velocity to bed shear velocity, and local sediment concentration. Its good performance is demonstrated in comparison with available laboratory observations. It is characterized that single-phase turbulence closure models can be adapted for sediment-laden flows by implementing sediment particle size effects.

Transition Mechanism of Debris Flows Over Rigid Bed to Over Erodible Bed

Abstract In debris flows over erodible beds, the kinematic conditions at the bed surface, such as velocity and velocity gradient, are determined by the dynamic condition that the driving force must be equal to the yield stress at the bed surface. In debris flows over rigid beds, on the other hand, kinematic quantities depend on such conditions as bed slope, sediment discharge rate, static friction angle of sediment and friction angle of grain to the bed surface. In the present study, the differences between debris flows over erodible and rigid beds as well as the transition between the two are analyzed theoretically by solving for velocity and sediment concentration profiles. An important difference between the two lies in the shear stress distributions near the bed. The velocity gradient takes a finite value in the case of a rigid bed, and zero in the case of an erodible bed, which causes several different features in the profiles of velocity, and sediment concentration, and correspondingly in the flow resistance. The theoretical results are verified by flume data.

Carbon to chlorophyll-a ratio in modeling long-term eutrophication phenomena

Carbon to chlorophyll-a ratio (CCHL) is formulated based on the assumption that adaptive changes in carbon to chlorophyll occur so as to maximize the specific growth rate for ambient conditions, including solar radiation and water temperature. With the dynamic CCHL, an unsteady two-layered, two-dimensional eutrophication numerical model for density stratified coastal waters has been developed. Saturated light intensity (IS) is determined as weighted average of the light intensity for previous three days to incorporate light acclimation by phytoplankton. The bottom water anoxic condition during summer in Tolo Harbour, Hong Kong is successfully reproduced by the present method. Otherwise, the simulation with a constant CCHL gave a wrong result.

Mechanics of Sediment Transport and Sediment Laden Flow. 4. Governing Equations for Sediment Laden Flows.

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。

Mechanics of Sediment Transport and Sediment Laden Flows. 2. Sediment Gravity Flow.

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。

Mechanics of Sediment Transport and Sediment Laden Flows. 1. Mechanics of Debris Flows.

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。

SEDIMENT RUNOFF DUE TO EROSION OF DEBRIS PRODUCED BY LANDSLIDE

Sediment runoff process in an actual small basin is discussed, based on the results of field survey and numerical analysis. In Nigorisawa river, a tributary of R. Mogami, a very active sediment runoff took place in June 1993 due to the erosion of debris produced by landslide. Numerical simulation is conducted in order to specify the sediment runoff and the characteristics of sediment movements along the river reach. The numerical results show that the flow discharge increases rapidly to a magnitude of several ten times of supplied water discharge due to the occurrence of debris flow, and the sediment transport rate and mode change from place to place according to the variation of bed slope and flow width.

Bed Variation in a Gradually Widening Channel

In braided channels, channels vary spatially and temporally, which results in the big variation of the sediment dischrge. It is important to know the channels pattern and process of stream channels variation. Channel patterns can be predict by the length and width of each stream channel. In this paper, two dimensional simulation model are employed to clarify the process of sand bar formation and channel diverging phenomenon in a gradually widening channel, in order to investigate the length of a stream channel. The results of the calculation are compared with the data obtained from flume tests to verify the numerical model.