Critical Tractive Force Research Articles

Entropy-Based Shear Stress Distribution in Open Channel for All Types of Flow Using Experimental Data.

Korean river design standards set general design standards for rivers and river-related projects in Korea, which systematize the technologies and methods involved in river-related projects. This includes measurement methods for parts necessary for river design, but does not include information on shear stress. Shear stress is one of the factors necessary for river design and operation. Shear stress is one of the most important hydraulic factors used in the fields of water, especially for artificial channel design. Shear stress is calculated from the frictional force caused by viscosity and fluctuating fluid velocity. Current methods are based on past calculations, but factors such as boundary shear stress or energy gradient are difficult to actually measure or estimate. The point velocity throughout the entire cross-section is needed to calculate the velocity gradient. In other words, the current Korean river design standards use tractive force and critical tractive force instead of shear stress because it is more difficult to calculate the shear stress in the current method. However, it is difficult to calculate the exact value due to the limitations of the formula to obtain the river factor called the tractive force. In addition, tractive force has limitations that use an empirically identified base value for use in practice. This paper focuses on the modeling of shear-stress distribution in open channel turbulent flow using entropy theory. In addition, this study suggests a shear stress distribution formula, which can easily be used in practice after calculating the river-specific factor T. The tractive force and critical tractive force in the Korean river design standards should be modified by the shear stress obtained by the proposed shear stress distribution method. The present study therefore focuses on the modeling of shear stress distribution in an open channel turbulent flow using entropy theory. The shear stress distribution model is tested using a wide range of forty-two experimental runs collected from the literature. Then, an error analysis is performed to further evaluate the accuracy of the proposed model. The results reveal a correlation coefficient of approximately 0.95–0.99, indicating that the proposed method can estimate shear-stress distribution accurately. Based on this, the results of the distribution of shear stress after calculating the river-specific factors show a correlation coefficient of about 0.86 to 0.98, which suggests that the equation can be applied in practice.

Measurement of critical tractive force of sponge carrier media in a moving bed biofilm reactor and the application of computational fluid dynamics/discrete element method model simulation

Measurement of critical tractive force of sponge carrier media in a moving bed biofilm reactor and the application of computational fluid dynamics/discrete element method model simulation

호안 제방사면 보강을 위한 지반개량재의 한계소류력 비교

제방을 보호하기 위하여 법면에 설치되는 호안공사는 국내외 설계기준에 의하여 하상과 호안법면에서의 최대 소류력을 산정하여 제방 사면에 평균적인 개념으로 적용하고 있다. 최근에는 기후변화로 인한 극우강우가 많아지고 호안의 안정성을 위한 평가기법이 제시되지 않기 때문에 호안사면의 수리학적 안정성과 지반공학적 파괴거동의 적절한 평가가 필요하다. 본 연구에서는 하천사면을 보강 시, 최대소류력에 견딜 수 있는 강도를 기존재료인 원지반과 지반개량재를 비교하였다. 지반개량재의 다양한 역학적 실험을 통하여 원지반토와 지반개량재의 함유량에 따른 최적의 혼합비율과 적절한 강도의 양생일을 예측할 수 있었다. 세굴률 시험결과를 바탕으로 원지반토와 지반개량재 혼합토의 허용소류력을 비교 분석한 결과, 호안 제방의 비탈면을 보강하기에 충분한 강도증진과 그에 따른 안정성을 확인할 수 있어 하천호안의 세굴과 침식 방지효과가 있으리라 판단되었다. The bank revetment work which is conducted on the slope of river bank to protect against flowing water as a geotechnical structure has been applied as an average value of critical tractive force based on domestic and international standard design. Currently, an appropriate evaluation is needed for the hydraulic stability and geotechnical behavior analysis of bank revetments because of the effects of climate change and ambiguous design criterion. This study has compared the critical tractive forces of soil improvement material and conventional materials used for the bank revetment work. Through various experiments, the shear strength of mixtures with soil improvement material was investigated by curing time and mixture ratio. It was found that the critical tractive force of the mixture obtained from a scour test is suitable to the reinforcement of the slope of river bank which has problems such as seepage erosion and scour.

실내실험에 의한 혼합사로 구성된 하상 표층에서 지형변동에 따른 유사의 분급 특성 분석

본 연구에서는 혼합사로 구성된 망상하천에서 사주의 변화와 하상토 분급현상을 실내실험을 통하여 파악하였다. 하상경사가 급한 하도에서 유사 유출량은 시간의 변화에 따라 매우 불규칙하지만, 경사가 완만한 하도에서 유사 유출량은 규칙적이고, 일정한 주기를 나타내었다. 또한 2중 퓨리에 해석을 통하여 지배적인 사주의 특성과 진폭을 분석하였다. 하상고가 상승하면 무차원 하상토 입경은 작아졌지만, 하상고가 저하되면 무차원 하상토 입경은 커졌다. 기층에서 무차원 한계소류력과 표층에서 무차원 한계 소류력의 비가 증가하면, 표층에서 평균입경과 무차원 하상토 중앙입경이 작아졌다. 또한 국부적으로 하상경사가 증가하면서, 유사의 선택적 이동에 의하여 하상토가 분급되고, 평면적으로 입경이 불균일하게 분포하는 분급 특성이 나타났다. The development of bars and sediment sorting processes in the braided channels with the mixed grain sizes are investigated experimentally in this study. The sediment in the steep slope channels discharges with highly fluctuation. However, it discharges with relatively periodic cycles in the mild slope channels. The characteristics and amplitudes of the dominant bars are examined by double fourier analysis. The dimensionless sediment particle size decreases as the longitudinal bed elevation increases. However, the size increases as the longitudinal bed elevation decreases. As the dimensionless critical tractive force in the surface layer ratio to the force in the subsurface layer increases, the surface geometric mean size of sediments and the dimensionless sediment particle size decrease. This means that coarse matrix is formed with the dimensionless tractive force by the sediment selective sorting.

수리모형 축척 변화에 따른 SPF공법의 한계능력평가

본 연구는 댐 및 보 하류의 세굴방지 등을 위해 설치되는 SPF(Scouring Protection Form) 공법의 모형 축척 변화에 따른 한계능력을 평가하는 것이 목적이다. 이를 위해 하상보호공의 축소 모형을 제작하여 실험수로에서 수리모형실험을 실시하여 공법의 한계능력을 평가하였다. 실험 결과 파괴 유속값은 1/10인 조건에서는 5.57 m/s, 4.08 m/s로 나타났으며 1/20인 경우 5.55, 4.32 m/s, 1/30인 경우 5.00, 4.31 m/s, 1/50 조건에서는 4.09, 4.65 m/s로 측정되었다. 접근부의 소류력 분석결과 축척비가 클수록 즉 모형의 크기가 작을수록 매트 파괴시의 한계소류력이 크게 나타났다. 실험 및 분석 결과 강한 난류에 버텨야하는 SPF매트와 같은 구조물을 평가할 때 수리학적 상사를 적용할 경우 1/20 이상의 축척비가 필요한 것으로 나타났다. The objective of this paper is to figure out critical velocity of SPF(Scouring Protection Form) method that is installed to prevent dam and weir of scour. Through hydraulic experiments, critical flow of SPF was evaluated by making scale model of bed protection. The destruction flow velocity analyzed 5.57 m/s, 4.08 m/s in actuality rivers manufacturing 1/10 miniature. In case of 1/20 scale, the destruction flow velocity was 5.55 m/s, 4.32 m/s and 5.00 m/s, 4.31 m/s as a scale of 1/30 and 4.09 m/s, 4.65 m/s as a scale of 1/50 respectively. Scale model ratio is directly proportional to critical tractive force and scale model size is inversely proportional to critical tractive force. As a result, the hydraulic test showed to need the hydraulic model experiment over 1/20 scale when evaluating structures such as SPF mat that have to be able to withstand strong turbulence.

테트라포드 사방댐의 적용성 평가 -산청군 홍계리 지역을 대상으로-

In order to connect eco-corridor and to harmonize with natural environment, it is necessary to construct small debris barrier. Therefore, we will evaluate applicability of small debris barrier which made of tetrapod utilized in construction of seashore protection facilities and breakwater. From result compared with the adaptation processes in fields, tetrapod were stabilized to critical tractive force. Stability evaluation method on sliding and overturn of tetrapod debris barrier was developed, and hereafter could be applied to similar debris barriers. According to results analyzed to continuity on sediment supply in debris barrier, boulders were deposited in upstream and gravels were deposited in downstream. Therefore, tetrapod debris barrier will be able to prevent in the long-term the erosion of river bed in down-stream, and the maintenance is easy because dredging is not necessary. Current were formed in between the tetrapod provided eco-corridors for fishes and shellfishes.

Modeling reservoir sedimentation associated with an extreme flood and sediment flux in a mountainous granitoid catchment, Japan

Extreme events, such as large storms and floods, have a major effect on the transport of sediment and contaminants and, hence, on water quality. To be realistic, models designed to predict the short-term transport and fate of sediment and contaminants must include large floods and storms, an idea not incorporated into existing models based on uniform sediment flux. Sediment transport in and to river channels in mountainous terrain is strongly influenced by precipitation, particularly when heavy precipitation triggers a surface landslide in the catchment area. This study provides a quantitative measure of such sediment transport. The Yahagi Dam watershed was modeled using a 50-m grid-based digital elevation model (DEM), and flow directions were determined for each of the slopes present. The study objective was to develop a general algorithm for simulating water flow and sediment movement in a dammed mountain basin using water and sediment dynamics modeling. The nonuniform flow model used for stream channels can incorporate energy loss around dam reservoirs, and the resulting algorithm can estimate all phenomena related to sediment movement. The new method was verified by applying it to the Tokai rainfall disaster and the Yahagi Dam basin, Japan. In this basin, the volume of bedload sediment deposited is 993 × 10 3 m 3/y, suspended-load sediment is 669 × 10 3 m 3/y, and wash load sediment is 900 × 10 3 m 3/y, as determined from sediment production in each tributary of the Yahagi River. Most sediment is probably introduced during extreme flood events owing to lithological factors, and thus extreme rain events that now accompany climate change are very important in sediment supply. During extreme flood events, the river's effective tractive force ( τ* e, d90) exceeds the critical tractive force ( τ* c), especially upstream of the Yahagi Dam. Rainfall-related sediment disasters can be accurately predicted using water- and sediment-dynamics models that are founded on realistic geomorphological characteristics and sediment flux of rivers.

小粒“れき”の限界掃流力に関する実験的研究

小粒“れき”の限界掃流力に関する実験的研究

STUDY ON EXPANSION OF BUR CUCUMBER COLONY BY LARGE FLOODS-BY EXPERIMENT AND NUMERICAL ANALYSIS

Bur cucumbers which are annual weeds have grown thickly in Chikuma River since 1994. We presumed that this phenomenon is accelerated by large floods that may spread the seeds of them. We have estimated the critical tractive force of seeds by a waterway experiment and analyzed the places where transported seeds can accumulate along the river and the destruction force of bur cucumber colonies by numerical simulation. Consequently, critical tractive force (τ*) of the seed was determined as 0.04. When the friction speed (u*) that works at the river bed exceeds about 17cm/s, the destruction of bur cucumber colonies seem to begin. However, bur cucumber colonies are not destroyed when they are accompanied with false acasia even if the friction speed exceeds that level.

Simulation des écoulements diphasiques dans les stations de pompage

The purpose of the present study is to develop numerical tools for two-phase flow ploblems in pumping stations. Reliable simulation methods of two-phase flow are inevitable in nuclear engineering field because computer programs are important design tools for nuclear reactors which are exposed to severe ambient conditions of high temperature, high pressure and gamma ray irradiation. Mock-up tests of nuclear thermal-hydraulics are very difficult. Hitachi Ltd. has created advanced computer programs for multi-dimensional two-phase flow to analize reactor systems and components. The numerical methods enable accurate and stable interface calculation in spite of large density ratio between the phases, because non-staggered finite volume method, which is suitable for treatment of discontinuity, and minimum numerical diffusion are adopted. The multi-dimensional computer programs employ two types of two-phase flow model. The first one is the extended two-fluid model, which has capability of both interface tracking and ordinary two-fluid model calculations. This model is appropriate for the present purpose because it can treat free surface of channels and entrained bubbly mixture due to breaking waves. The second one is the improved VOF (Volume of Fluid) method in which PLIC (Piecewise Linear Interface Calculation) technique was employed. The interface state calculation is simplified by approximating cubic calculation cells to spherical ones. These computer program were applied to hydraulic studies in Hitachi Industries Co., Ltd., a vender of fluid machinery and pumping systems. It is necessary to evacuate air downstream from siphon outlets as bubble entrainment by water flow in order to operate the plant effectively after pumping station start-up. Experiments were carried out by using a reduced scale test-section. The test section was made of transparent acryl and the cross-section was rectangular. Free surface of water layer on the inclined wall and entrained bubbly mixture into the water pool due to water layer collision with free surface were observed by a video camera. The extended two-fluid model results of transient water level and pressures in the simulated siphon outlets were in good agreement with the experimental data. The predicted pressure histories at the crest and the pump site were in good agreement with the experimental data. The sedimentation in a inlet channel of a pump station was carried out. Three dimensional free surface flow was calculated by the improved VOF method and behavior of suspended sediment in the flow field is calculated. The surface wave velocity in the shallow channel was in good agreement with the theoretical one. Sediment particles had size distribution and drag force and gravity and buoyancy was taken into consideration. Critical tractive force of the sediment on floor was estimated from Schields’ correlation. The predictions of sedimentation area and suspended fraction agreed well with experimental data. These results showed that the present computer programs were able to treat key two-phase flow phenomena in pumping stations.

STUDY ON THE DESTRUCTION CRITERION OF RIPARIAN TREES IN A GRAVEL-BED RIVER CAUSED BY FLOOD DISTERBANCE

Recently riparian trees (Robinia pseudo-acacia) in the course of gravel bed-river have extremely increased and enlarged their covered-area over bars and flood plain. Such a situation brings about problems of river management for the environmental aspects as well as the safety of rivers against flood.In this paper, the fall down process and the damage conditions of the riparian trees caused by the 1999, 2001 floods were examined in the segment-1 reach of Watarase River. Especially, from the field surveies after floods, it was clear that the damage, which the floods gave to the trees, could be classified by the scale of the flood external force. Based on the field study and numerical calculation with 2D-flow model, the critical tractive force to be a key index of the scale of flood disturbance was evaluated and results showed that the fall down process of riparian trees is based on the degradation of movable river-bed

STUDY ON THE EXPANSION OF ROBINIA PSEUDO-ACACIA IN GRAVEL-BED RIVER CAUSED BY FLOOD DISTERBANCE

Recently riverine trees in the course of gravel bed-river have extremely increased and enlarged their covered-areaover bars and flood plain. Such a situation brings about problems of river management for the environmental aspectsas well as the safety of rivers against flood. In order to clarify such problems, we should get the knowledge abouthow the instream vegetation grows up and expands their covered zones in the gravel-bed river.In this paper, the cause of the excessive growth of Robinia pseudo-acacia was examined in the segment-1 reach ofWatarase River. Especially, from the investigation of the annual rings of tree, it was clear that some forests of Robinia pseudo-acacia in bars were formed right after the main flood. We can consider that the flood caused changeof generation of the trees and the damage which the flood gave to the trees can be classified by the scale of the floodexternal force. Based on the field study and numerical analysis with 2D- flow model, the critical tractive force to bean index of the scale of flood disturbance was evaluated and results showed that rapidly expansion by thegermination from fallen trees makes the excessive growth of riverine trees.

SECONDARY FLOW AND TRACTIVE FORCE IN COMPOUND SINUOUS CHANNEL

Over-bank flow in a straight channel with a meandering main channel is discussed based on experimental results. The flow structure is characterized by secondary flow and fluid mixing between the main channel and flood plain flow. The secondary flow was directly visualized by a submergible video camera. The secondary flow shows spiral motions and affects the distribution of boundary shear stress and sediment transport. The relation between these structure and the traction process of bed materials is discussed on the basis of the distribution of tractive force visualized by a new method. In the experiments, we used light materials whose critical tractive force is equivalent to the mean shear stress on the bed.

The Albert Shields Story

In the vast literature on sediment transport, rivers, and related topics, few if any names are more frequently cited than Albert F. Shields (1908–1974). Yet all of these citations are to a single publication: his doctoral thesis submitted to the Technischen Hochschule Berlin in 1936 in which he developed his ideas on Aenlichkeitsmechanik (similarity mechanics) for application to riverine sediment transport, ripple formation, and initiation of motion. Shields' most famous results are his graph for critical tractive force (initiation of particle motion), and secondarily, his sediment-transport formula. Presented here is the story of the many difficulties Shields encountered in conducting his research in Nazi Germany; his inability to find employment in hydraulics following his return to his native United States; the chance encounter with and promulgation of his work by Rouse; and his eventual relinquishment of hydraulics for a long and successful career in machine design.

Saltaion of Blown Sand Particles and Wind-Velocity Distribution

Saltation motion of blown sand is analyzed by coupling of solving equation of motion and the model fore irregular repulsion on the bed surface. Furthermore, the suppression of the Reynolds stress due to the additional force by which the blown sand is accelerated is taken into account to evaluate the wind velocity distribution. Under equilibrium, by using the fact that the Reynolds stress at the bed should equal the critical tractive force, the equilibrium transport rate and wind velocity profile with blown sand are predicted reasonably.

Lateral bedload transport for bank materials with wide distribution of size considering the slope effect

Lateral bedload function for fractional particle sizes on the tilting side slope was developed by following Parker's method. On the slope with near angle of repose, tangential component of the gravity force acts on the surface particles in equivalent intensity comparing with the tractive force. Thus the full non-linear equation for a particle motion was solved numerically. Thickness of the bedload layer was given from extending Ashida Michiue equation to consider the gravity effect and to use Egiazaroff equation for critical tractive force of fractional particles. Summation of the lateral sediment discharge for each fractional particle size calculated from the product of the moving particle velocity and bedload layer fairly well agree with the observed data obtained from large scale bank erosion experiments using volcanic ashes for the bank material.

Geomorphic Assessment of the Effects of Flow Diversion on Anadromous Fish Spawning Habitat: Newhalem Creek, Washington∗

A simple model for estimating streamflow competence is used to assess the impact of water diversion on the retention of spawning gravels in Newhalem Creek, a small watershed in the Cascade Mountains. Surveyed stream cross sections and discharge estimates are employed to characterize the routine tractive force conditions of the streambed. A conservative estimation procedure results in tractive force values of 120–390 Nm−2for one-year, five-year, and 10-year recurrence interval events. Theoretical and empirical estimates of critical tractive force for particle sizes suggest a routine competence of 100–450 mm diameter for these flows. Substantial accumulations of bed material in the range of 1–100 mm diameter is desirable for anadromous fish spawning redds. As a result, Newhalem Creek is naturally poor habitat for spawning. Steep channel gradients and a “flashy” hydrograph conspire to routinely flush the main channel of spawning-size substrate. In this instance, the modest water diversion is immaterial to th...

Erosion of cohesive bottom sediments: The influence of benthos

The effect of benthic organisms (Oligochaeta and Chironomidae) on the erosion of cohesive sediments was studied in a laboratory flume. The critical erosive velocities were shown to be 2-3 times smaller than that of the control sediments. The effect depends strongly on the population density of benthos and temperature of sediments. It was also shown that micro-organisms (bacteria and protozoa), attached to small particles have a great impact on the transportation of suspended sediments due to the decrease in the settling velocity and the critical tractive force.

CRITICAL TRACTIVE FORCE AND BED-LOAD DISCHARGE IN GRAVEL-BED CHANNEL WITH STEEP SLOPE

CRITICAL TRACTIVE FORCE AND BED-LOAD DISCHARGE IN GRAVEL-BED CHANNEL WITH STEEP SLOPE

Experimental study on critical tractive force in a steep slope channel

Experimental study on critical tractive force in a steep slope channel