• Title/Summary/Keyword: Shields Parameter

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Development of Transverse Bed Slope Model for Nonuniform Sand Bed at River Bend (만곡부 혼합입경 하상횡경사 모형의 개발)

  • 최종인;고재웅
    • Water for future
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    • v.29 no.4
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    • pp.177-186
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    • 1996
  • The analytical approach to determine transverse sand bed slope at river bend are based on two phases that the flow is considered as fully developed flow and the bed is fluvial having bed load. All existing methods are theoretically derived from the initiation of motion of the particles at river bed. They assume that the Shields parameter has a constant value of 0.06. In this study, the variability of Shields parameter due to the differences of shape of grain size distribution is considered. Therefore the parameter is not a constant, 0.06, but depends on the shape of the grain size distribution. This result gives good agreement to estimate transverse bed slope with actual field data at river bend.

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A Case Study of Sediment Transport on Trenched Backfill Granular and Cohesive Material due to Wave and Current

  • Choi, Byoung-Yeol;Lee, Sang-Gil;Kim, Jin-Kwang;Oh, Jin-Soo
    • Journal of Advanced Research in Ocean Engineering
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    • v.2 no.2
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    • pp.86-98
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    • 2016
  • In this study, after the installation of a subsea pipeline, backfilling was performed in the trenched area. During these operations, a stability problem in the subsea pipeline occurred. The pipeline was directly impacted by environmental loading such as waves and currents that were caused by backfill material when scouring or sediment transport and siltation was carried out. Therefore, this study reviewed whether trenching was necessary, and conducted research into an indigenous seabed property that contains granular soil. A study of cohesive soil was also conducted in order to cross-correlate after calculating the values of the critical Shields parameter relevant to elements of the external environment such as waves and current, and the shear Shields parameter that depends on the actual shearing stress. In case of 1), sedimentation or erosion does not occur. In the case of 2), partial sedimentation or erosion occurs. If the case is 3), full sedimentation or erosion occurs. Therefore, in the cases of 1) or 2), problems in structural subsea pipeline stability will not occur even if partial sedimentation or erosion occurs. This should be reflected particularly in cases with granular and cohesive soil when a reduction in shear strength occurs by cyclic currents and waves. In addition, since backfilling material does not affect the original seabed shear strength, a set-up factor should be considered to use a reduced of the shear strength in the original seabed.

The Local Scour around Submarine Pipelines in the Interaction Region Combined with Waves and Currents (파랑과 정상흐름의 공존역에서 해저관로 주변의 국부세굴)

  • Kim, Kyoung-Ho;Lee, Ho-Jin;Kim, Wan-Shik
    • Journal of Korean Society of Coastal and Ocean Engineers
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    • v.20 no.5
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    • pp.510-521
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    • 2008
  • In the study, experiments are performed in the interaction region combined with wave and current to investigate the characteristics of local scour around submarine pipelines. Wave generator and current generator are used for the experiments and two current directions were used; co-direction and counter direction to the wave. The local scour depths around the pipeline are obtained according to the various pipe diameters(D), wave periods(T), wave heights(H), and current velocities(V). The experiments show that the maximum equilibrium local scour depth increases with pipe diameter, wave period, wave height, and current velocity. Using the experimental results, the correlations of scour depth and parameters such as Shields parameter($\theta$), Froude number(Fr), period parameter, Keulegan-Carpenter number(KC), Ursell number($U_R$), modified Ursell number($U_{RP}$) and ratio of velocities($U_{c}/(U_{c}+U_{m})$) are analyzed. In the interaction region combined with waves and currents, Froude number and Shields parameter are found the main parameters to cause the local scour around the submarine pipelines and this means that current governs the scour within any limits of the currents.

Numerical Analysis on the Turbulence Patterns in The Scour Hole at The Downstream of Bed Protection (하상보호공 직하류부 세굴공의 난류양상에 관한 수치해석적 연구)

  • Lee, Jaelyong;Park, Sung Won;Yeom, Seongil;Ahn, Jungkyu
    • Journal of the Korea Academia-Industrial cooperation Society
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    • v.20 no.5
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    • pp.20-26
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    • 2019
  • Where hydraulic structures are to be installed over the entire width of a river or stream, usually a bed protection structure is to be installed. However, a local scour occurs in which the river bed downstream of the river protection system is eroded due to the influence of the upstream flow characteristics. This local scour is dominant in the flow and turbulence characteristics at the boundary of the flow direction and in the material of the bed materials, and may gradually become dangerous over time. Therefore, in this study, we compared the turbulence patterns in the local scour hole at the downstream of the river bed protection with the results of the analysis of the mobile bed experiment, and compared with the application of OpenFoam, a three dimensional numerical analysis model. The distribution of depth-averaged relative turbulence intensities along the flow direction was analyzed. In addition to this result, the stabilization of scour hole was compared with the bed shear stress and Shields parameter, and the results were compared by changing the initial turbulent flow conditions. From the results, it was confirmed that the maximum depth of generation of the three-stage was dominantly developed by the magnitude of depth-averaged relative turbulence intensity rather than the mean flow velocity. This result also suggests that design, construction or gate control are needed to control the depth-averaged relative turbulence intensities in order to reduce or prevent the local scour faults that may occur in the downstream part of the bed protection.

Sediment Transport Calculation Considering Cohesive Effects and Its Application to Wave-Induced Topographic Change (점착력을 고려한 표사유동 수치모델의 제안과 파랑에 의한 지형변동의 적용성 검토)

  • Cho, Yong Hwan;Nakamura, Tomoaki;Mizutani, Norimi;Lee, Kwang-Ho
    • Journal of Korean Society of Coastal and Ocean Engineers
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    • v.25 no.6
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    • pp.405-411
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    • 2013
  • A sediment transport calculation considering cohesive force is proposed to deal with the transport phenomena of cohesive sediment. In the proposed calculation, each sand particle is assumed to be surrounded by a thin layer of mud. The critical Shields parameter and bed-load sediment transport rate are modified to include the cohesive force acting on the sand particle. The proposed calculation is incorporated into a two-way coupled fluid-structure-sediment interaction model, and applied to wave-induced topographic change of artificial shallows. Numerical results show that an increase in the content ratio of the mud, cohesive resistance force per unit surface area and water content cause increases in the critical Shields parameter and decreases in the bed-load sediment transport rate, reducing the topographic change of the shallow without changing its trend. This suggests that mixing mud in the pores of the sand particles can reduce the topographic change of shallows.

The Local Scour around a Slender Pile in Combined Waves and Current (파랑과 흐름이 결합된 공존역에서 파일 주변의 국부세굴)

  • Park, Jong-Hwan;Kim, Kyoung-Ho
    • Journal of Korean Society of Coastal and Ocean Engineers
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    • v.22 no.6
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    • pp.405-414
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    • 2010
  • In the study, experiments are performed in the mixing region combined wave and current to investigate the characteristics of local scour around a slender pile. Wave generator and current generator are used for the experiments and currents are co-directions with the waves. The local scour depths around the pipeline are obtained according to the various pipe diameters, wave periods, wave heights, and current velocities. The experiments show that the maximum equilibrium local scour depth increases with pipe diameter, wave period, wave height, and current velocity. Using the experimental results, the correlations of scour depth and parameters such as Shields parameter ($\theta$), Froude number (Fr), Keulegan-Carpenter number (KC), Ursell number ($U_R$), modified Ursell number ($U_{RP}$) and ratio of velocities ($U_c/U_c+U_m$) are analyzed. In the mixing region combined with waves and currents, The Froude number of single parameters is the main parameter to cause the local scour around a slender pile due to waves and current and this means that current governs the scour within any limits of the currents.

Suspension of Sediment over Swash Zone (Swash대역에서의 해빈표사 부유거동에 관한 연구)

  • Cho, Yong Jun;Kim, Kwon Soo;Ryu, Ha Sang
    • KSCE Journal of Civil and Environmental Engineering Research
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    • v.28 no.1B
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    • pp.95-109
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    • 2008
  • We numerically analyzed the nonlinear shoaling, a plunging breaker and its accompanying energetic suspension of sediment at a bed, and a redistribution of suspended sediments by a down rush of preceding waves and the following plunger using SPH with a Gaussian kernel function, Lagrangian Dynamic Smagorinsky model (LDS), Van Rijn's pick up function. In that process, we came to the conclusion that the conventional model for the tractive force at a bottom like a quadratic law can not accurately describe the rapidly accelerating flow over a swash zone, and propose new methodology to accurately estimate the bottom tractive force. Using newly proposed wave model in this study, we can successfully duplicate severely deformed water surface profile, free falling water particles, a queuing splash after the landing of water particles on the free surface and a wave finger due to the structured vortex on a rear side of wave crest (Narayanaswamy and Dalrymple, 2002), a circulation of suspended sediments over a swash zone, net transfer of sediments clouds suspended over a swash zone toward the offshore, which so far have been regarded very difficult features to mimic in the computational fluid mechanics.

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A Case Study of Sediment Transport on the Seabed due to Wave and Current Velocities

  • Choi, Byoung-Yeol;Lee, Sang-Gil;Kim, Jin-Kwang;Oh, Jin-Soo
    • Journal of Advanced Research in Ocean Engineering
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    • v.2 no.3
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    • pp.99-111
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    • 2016
  • Seabed affected by scouring, sedimentation, and siltation occurrences often cause exposure, which induces risks to existing structures or crude oil or gas pipeline buried subsea. In order to prevent possible risks, more economical structure installation methodology is proposed in this study by predicting and managing the risk. Also, the seabed does not only consist of sandy material, but clayey soil is also widespread, and the effect of undrained shear strength should be considered, and by cyclic environmental load, pore water pressure will occur in the seabed, which reduces shear strength and allows particles to move easily. Based on previous research regarding sedimentation or erosion, the average value of external environmental loads should be applied; for scouring, a 100-year period of environmental conditions should be applied. Also, sedimentation and erosion are mainly categorized by the bed load and suspended load; also, they are calculated as the sum of bed load and suspended load, which can be obtained from the movement of particles caused by sedimentation or erosion.

Comparative Analyses on Hydraulic Stability Formulae of Riprap (사석의 수리적 안정식의 비교분석)

  • Choi, Hung-Sik;Lee, Min-Ho
    • Journal of the Korean Society of Hazard Mitigation
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    • v.8 no.3
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    • pp.149-155
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    • 2008
  • To evaluate the formulae for the stability of riprap, the formulae of Isbash, California division of highway, Netherlands, ASCE, Pilarczk, and Maynord are comparatively analysed with the experimental results. The critical velocity which initiates the motion of riprap is increased with the weight and the size in diameter and the riprap size with water depth, Froude number, shear velocity with mean velocity, Shields parameter have great correlation with them. The results by 6 formulae are overestimated in riprap size in diameter and the result by Maynord formula proposed by U.S. Army Corps of Engineers estimates rather correct. The results by Isbash, Netherlands, and Pilarczk are overestimated in riprap weight but the result by California division of highway formula coincides with experimental result. In the experimental results of model riprap artificially made by gypsum with light weight density, the critical velocity is increased with shape factors. The critical velocity appears greater in regular arrangement of model riprap than in random arrangement of it. Therefore the shape factor and the degree of interlocking are an important parameters in riprap stability.