scholarly journals Study on hydraulic resistance of erodible bed at the Chiyoda experimental flume

2014 ◽  
Vol 39 ◽  
pp. 81-87
Author(s):  
T. Kakinuma ◽  
T. Inoue ◽  
R. Akahori ◽  
A. Takeda
Keyword(s):  
Large Scale ◽  
Flow Distribution ◽  
Longitudinal Survey ◽  
Sand Wave ◽  
Vertical Flow ◽  
Sand Waves ◽  
Wave Development ◽  
Logarithmic Distribution ◽  
Steady Flow Condition ◽  
Experimental Flume

Abstract. The authors made erodible bed experiments under steady flow condition at the Chiyoda Experimental Flume, a large-scale facility constructed on the floodplain of the Tokachi River, and observed sand waves on the bed of the flume. In this study, the characteristics of the sand waves are examined along the longitudinal survey lines and confirmed to be dunes. Next, the authors estimated Manning's roughness coefficients from the observed hydraulic values and assumed that the rise of the coefficients attributed to the sand wave development. Finally, vertical flow distribution on the sand waves are examined, and observed velocity distribution on the crest of waves found to be explained by the logarithmic distribution theory.

Study of sand wave migration over five years as observed in two windfarm development areas, and the implications for building on moving substrates in the North Sea

2014 ◽  
Vol 105 (4) ◽  
pp. 241-249 ◽  
Author(s):  
Ken P. Games ◽  
David I. Gordon
Keyword(s):  
Case Studies ◽  
Large Scale ◽  
Oil And Gas ◽  
Sand Wave ◽  
Sand Waves ◽  
Migration Rates ◽  
The North ◽  
The North Sea ◽  
Wave Migration ◽  
The Impact

ABSTRACTSand waves are well known indicators of a mobile seabed. What do we expect of these features in terms of migration rates and seabed scour? We discuss these effects on seabed structures, both for the Oil and Gas and the Windfarm Industries, and consider how these impact on turbines and buried cables. Two case studies are presented. The first concerns a windfarm with a five-year gap between the planning survey and a subsequent cable route and environmental assessment survey. This revealed large-scale movements of sand waves, with the displacement of an isolated feature of 155 m in five years. Secondly, another windfarm development involved a re-survey, again over a five-year period, but after the turbines had been installed. This showed movements of sand waves of ∼50 m in five years. Observations of the scour effects on the turbines are discussed. Both sites revealed the presence of barchans. Whilst these have been extensively studied on land, there are few examples of how they behave in the marine environment. The two case studies presented show that mass transport is potentially much greater than expected and that this has implications for choosing turbine locations, the effect of scour, and the impact these sediment movements are likely to have on power cables.

scholarly journals SHORELINE SAND WAVES AND BEACH NOURISHMENTS

2011 ◽  
Vol 1 (32) ◽  
pp. 102
Author(s):  
N. Van den Berg ◽  
A. Falqués ◽  
F. Ribas
Keyword(s):  
Large Scale ◽  
Incidence Angle ◽  
Wave Train ◽  
Shoreline Change ◽  
Sand Wave ◽  
High Angle ◽  
Wave Instability ◽  
Littoral Drift ◽  
Sand Waves ◽  
Change Models

The effects of the feedback between the changing coastal morphology and the wavefield on the generation and propagation of large scale (O(1-10 km)) shoreline sand waves is examined with a quasi-2D morphodynamic model. Traditional shoreline change models do not include this feedback and are only able to describe diffusion of shoreline sand waves and furthermore they are unable to describe migration. It is found with the present model that if there is a dominant littoral drift, the feedback causes downdrift migration of coastline features no matter if they grow or decay. Consistently with previous studies, simulations show that a rectilinear coastline becomes unstable and sand waves tend to grow spontaneously from random perturbations, if the wave incidence angle is larger then about 42o (θc) at the depth of closure (high angle wave instability). The initial wavelengths at which the sand waves develop are 2-3 km and this is similar to previous linear stability analysis. The implications of high angle wave instability for beach nourishments are investigated. The nourished shoreline retreats initially due to cross-shore transport because the nourished profile is steeper than the equilibrium profile. When a dominant littoral drift is present, the nourishment also migrates downdrift. If the wave angle at the depth of closure is below θc the alongshore transport contributes to the diffusion of the nourishment. However, if the angle is above θc (constant high angle wave conditions) the diffusion is reversed and the nourishment can trigger the formation of a shoreline sand wave train. Numerical experiments changing the proportion of ‘high angle waves’ and ‘low angle waves’ in the wave climate show that relatively small proportions of low angle waves slow down the growth of sand waves. These simulations with more realistic wave climates show shoreline sand waves that migrate downdrift maintaining more or less the same amplitude for years.

scholarly journals A CLASSIFICATION METHOD FOR THE PRESENCE OF TIDAL SAND WAVES AND MAINTENANCE DREDGING DESIGN

2018 ◽  
pp. 48
Author(s):  
Rick De Koning ◽  
Jaap van Thiel De Vries ◽  
Bas Borsje
Keyword(s):  
Large Scale ◽  
Tidal Flow ◽  
Narrow Channel ◽  
Sand Wave ◽  
Maintenance Strategies ◽  
Sand Waves ◽  
Recirculating Flow ◽  
The North ◽  
Tidal Sand ◽  
And Migration

The study into sand wave dynamics in South Channel commenced after large dune forms were observed in monitoring campaigns following the channel deepening project of the Port of Melbourne. The project involved deepening of the harbor berths and channels, but more importantly, it involved the deepening of South Channel in Port Phillip Bay. South Channel, the main shipping channel, crosses the bay over ≈20km. The growth of bedforms at various locations in South Channel now threatens to impede marine traffic. The dimensions and migration rate of the bedforms in the channel are remarkable, especially in the harsh flow conditions in the narrow channel. Therefore, the bedforms in South Channel cannot be given an obvious classification. In this paper it is shown that the bedforms in South Channel can be classified as a tidal sand wave type with a method that requires only insight in water depth, tidal flow velocity and grain size. Tidal sand waves are large-scale bedforms generated by recirculating flow cells that drive sediment to the top of a crest and are commonly observed on shallow coastal seas such as the North Sea. The bedform concern in the channel illustrates the necessity of an evaluation of the present, and alternative, channel maintenance strategies. A numerical model in Delft3D software is applied, along with a probabilistic calculation that combines insights from the simulations and survey data, to assess different maintenance strategies.

Mud drapes in sand-wave deposits: a physical model with application to the Folkestone beds (early cretaceous, southeast England)

1982 ◽  
Vol 306 (1493) ◽  
pp. 291-345 ◽  
Keyword(s):  
Early Cretaceous ◽  
Large Scale ◽  
Tidal Currents ◽  
Sand Wave ◽  
Sand Waves ◽  
Tidal Regime ◽  
Cross Bedding ◽  
Marine Sand ◽  
Depositional Cycles ◽  
The Cross

Large-scale cross-bedded units with mud-draped bottomsets and foresets occur in several shallow-marine sand formations attributed to tidal sand waves. The deposition and preservation of mud drapes on sand waves are favoured by a large sand-wave asymmetry, a high bottom concentration of suspended mud, large timevelocity asymmetry and low strength of tidal currents, and a high eccentricity of the tidal-current ellipse. The deposits formed on a strongly asymmetrical sand wave beneath a strongly asymmetrical current during one semidiurnal or diurnal tidal cycle will be a distinctive couplet composed of (i) a compound mud drape, with an internal silt-sand parting formed by the subordinate tidal stream, overlain by (ii) a group of sandy foresets and bottomsets deposited by the dominant stream. As the tides wax from neaps towards springs, and subsequently wane toward the next neaps, the spacing of drapes between sandy foresets will at first increase and then decline, whence a bundling or clustering of mud layers, and a periodicity in the streamwise arrangement of drapes and sandy foresets, will appear within the cross-bedding set. Tidal regime and the bed-material erodibility determine the character of these spring-neap depositional cycles, or bundles. The number of sand layers, their accumulated thickness, and their range in thickness within a spring-neap depositional cycle all increase as the tidal currents grow in strength relative to the threshold speed for sand erosion. Nontidal factors may modify the tidally dependent spring-neap pattern of drapes and foresets, among which wave action seems most important. Mud deposition is suppressed at times of heightened wave-activity, with the result that spring-neap depositional cycles become abbreviated in the number of identifiable sedimentary episodes while acquiring an exaggeratedly large range in drape spacing. Long term changes of tidal regime, such as occur between equinoxes and solstices, should be detectable as gradual changes through a long sequence of spring-neap bundles. The Folkestone Beds of the western and northeastern Weald include many thick cross-bedded units with mud drapes often visibly bundled. At three western sites, the sands are fine to medium grained, with some coarse-grade and even pebbly material. The drapes there, consisting of fine- to very-fine-grained kaolinitic silt, range in thickness mainly between about 0.002 mm and 0.02 m. The spacing between groups of sandy foresets and bottomsets changes in an orderly way along the cross-bedding sets, varying from as little as about 0.01 m to several metres. With reference to the model, and with the help of time-series and Fourier analysis, the character of the drapes themselves, and the nature of the depositional cycles to which they contribute, it seems likely that the Folkestone Beds were deposited from diurnal tidal currents of spatially changing strength assisted by a strong unidirectional current. The limitation of drapes to western and northeastern areas is consistent with the restriction of the more eccentric tidal currents to nearshore areas, even though the currents seem to have been strongest nearest to shore. The length of the depositional cycles in the Folkestone Beds - proposed to record spring-neap tidal cycles-is consistent with the slightly longer year (in terms of solar days) inferred for early Cretaceous times on various independent grounds.

scholarly journals Experimental Investigation on Characteristics of Sand Waves with Fine Sand under Waves and Currents

Water ◽  
2019 ◽  
Vol 11 (3) ◽  
pp. 612 ◽  
Author(s):  
Zhenlu Wang ◽  
Bingchen Liang ◽  
Guoxiang Wu
Keyword(s):  
Large Scale ◽  
Wave Length ◽  
Wave Theory ◽  
Fine Sand ◽  
Sand Wave ◽  
Wave Steepness ◽  
Sand Waves ◽  
Wave Characteristics ◽  
Waves And Currents ◽  
Wave Nonlinearity

A series of physical experiments was conducted to study the geometry characteristics and evolution of sand waves under waves and currents. Large scale bedforms denoted as sand waves and small bedforms represented by ripples were both formed under the experimental hydrodynamic conditions. Combining the experimental data with those from previous research, the characteristics of waves and currents and measured sand waves were listed. Small amplitude wave theory and Cnoidal wave theory were used to calculate the wave characteristics depending on different Ursell numbers, respectively. The results show good agreement between the dimensionless characteristics of sand waves and the dimensionless wave characteristics with a smaller wave steepness. When the wave steepness is large, the results seem rather scattered which may be affected by the wave nonlinearity. Sand wave steepness hardly changed with bed shear stress. A simple linear relationship can be found between sand wave length and wave steepness. It is easy to evaluate the sand wave characteristics from the measured wave data.

Physical Distancing and Social Media Use in Emerging and Older Adults During COVID-19: Large Scale Cross-Sectional and Longitudinal Survey Study (Preprint)

2021 ◽  
Author(s):  
Thabo J van Woudenberg ◽  
Roy Hendrikx ◽  
Moniek Buijzen ◽  
Julia CM van Weert ◽  
Bas van den Putte ◽  
...  
Keyword(s):  
Older Adults ◽  
Social Media ◽  
Emerging Adults ◽  
Large Scale ◽  
Media Use ◽  
Longitudinal Survey ◽  
Social Media Use ◽  
Random Intercept ◽  
Social Media Platforms

BACKGROUND Although emerging adults play a role in the spread of COVID-19, they are less likely to develop severe symptoms after infection. Emerging adults’ relatively high use of social media as source of information raises concerns regarding COVID-19 related behavioral compliance (i.e., physical distancing) in this age group. OBJECTIVE Therefore, the current study investigated physical distancing in emerging adults in comparison to older adults and looked at the role of using social media for COVID-19 news and information in this regard. In addition, this study explored the relation between physical distancing and different social media platforms and sources. METHODS Secondary data of a large-scale national longitudinal survey (N = 123,848, 34.% male) between April and November 2020 were used. Participants indicated, ranging for one to eight waves, how often they were successful in keeping 1.5 meters distance on a 7-point Likert scale. Participants between 18 and 24 years old were considered young adults and older participants were identified as older adults. Also, a dummy variable was created to indicate per wave whether participants used social media for COVID-19 news and information. A subset received follow-up questions asking participants to indicate which platforms they have used and what sources of news and information they had seen on social media. All preregistered hypotheses were tested with Linear Mixed-Effects Models and Random Intercept Cross-Lagged Panel Models. RESULTS Emerging adults reported less physical distancing behaviors than older adults (b = -.08, t(86213.83) = -26.79, p < .001). Also, emerging adults were more likely to use social media for COVID-19 news and information (b = 2.48, SE = .11, Wald = 23.66, p = <.001), which mediated the association with physical distancing, but only to a small extend (indirect effect: b = -0.03, 95% CI = [-0.04; -0.02]). Opposed to our hypothesis, the longitudinal Random Intercept Cross-Lagged Panel Model showed no evidence that physical distancing was predicted by social media use of the previous wave. However, we did find evidence that using social media affected subsequent physical distancing behavior. Moreover, additional analyses showed that most social media platforms (i.e., YouTube, Facebook and Instagram) and interpersonal communication showed negative associations with physical distancing while others platforms (i.e. LinkedIn and Twitter) and Governmental messages showed no to a slightly positive associations with physical distancing. CONCLUSIONS In conclusion, we should be vigilant for physical distancing of emerging adults, but this study give no reason the to worry about the role of social media for COVID-19 news and information. However, as some social media platforms and sources showed negative associations, future studies should more carefully look into these factors to better understand the associations between social media use for news and information, and behavioral interventions in times of crisis.

Numerical Simulation of the Flow Field for Rotor Ventilation System of Large-Scale Nuclear Power Turbo-Generator

2012 ◽  
Vol 152-154 ◽  
pp. 1498-1504 ◽  
Author(s):  
Xiao Hu Zhang ◽  
Lei Hu ◽  
Jian Hua Yuan ◽  
Yi Chao Yuan
Keyword(s):  
Flow Field ◽  
Nuclear Power ◽  
Large Scale ◽  
Ventilation System ◽  
Flow Distribution ◽  
Basic Unit ◽  
Turbo Generator ◽  
Key Issues ◽  
Computational Fluid Dynamics Cfd ◽  
And Performance

The nuclear power turbo-generator with large capacity is a basic unit of nuclear power plant, while the cooling technology becomes one of the key issues which affect its design and operation deeply. Axial-radial ventilation structure for rotor is commonly used in large nuclear power generator. In this article, according to the basic principles of computational fluid dynamics (CFD), ventilation’s structure and performance is analyzed, 3D flow model is also established. After the boundary conditions are determined, the numerical calculation and analysis is finished. And then, the rules of flow distribution is obtained, the flow field and the static pressure character of the gap is also computed, which could be very important to the ventilation system of the whole generator.

Sand Wave Migration and its Factors on Giant Sand Ridge in Taiwan Strait

2020 ◽  
Author(s):  
Yin-Hsuan Liao ◽  
Ho-Han Hsu ◽  
Jyun-Nai Wu ◽  
Tzu-Ting Chen ◽  
Eason Yi-Cheng Yang ◽  
...  
Keyword(s):  
High Resolution ◽  
Taiwan Strait ◽  
Sand Wave ◽  
Sand Ridge ◽  
Elevation Change ◽  
Seismic Profiles ◽  
Migration Patterns ◽  
Sand Waves ◽  
And Migration ◽  
Wave Migration

&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; Submarine sand waves are known to be induced by tidal currents and their migration has become an important issue since it may affect seafloor installations. In Taiwan Strait, widely spreading sand waves have been recognized on the Changyun Ridge, a tide-dominated giant sand ridge offshore western Taiwan. However, due to lacking of high-resolution and repeated geophysical surveys before, detailed characteristics and migrating features of the sand waves in Taiwan Strait were poorly understood. As new multibeam bathymetric and seismic data were collected repeatedly during 2016 - 2018 for offshore wind farm projects, we can now advance the understanding of sand wave characteristics and migration patterns in the study area. We apply a geostatistical analysis method on bathymetry data to reveal distribution and spatial characteristics of the sand waves, and estimate its migration pattern by using an updated spatial cross-correlation method. Then, sedimentary features, internal structures and thicknesses of sand waves are observed and estimated on high-resolution seismic profiles. Our results show that the study area is mostly superimposed by multi-scaled sandy rhythmic bed forms. However, the geomorphological and migrating characteristics of the sand waves are complicated. Their wavelengths range from 80 to 200 m, heights range from 1.5 to 8 m, and crests are generally oriented in the WNW-ESE direction. Obvious sand wave migration was detected from repeated high-resolution multi-beam data between 2016 and 2018, and migration distances can be up to ~150 m in 15 months. The average elevation change of the seafloor over the whole survey area is ~3.0 m, with a maximum value of 6.9 m. Moreover, the sand waves can migrate over 30 m with ~2.5 m elevation change in 2 months and migrate over 5 m with ~1 m elevation change in 15 days. The results also show that the orientation of wave movement can be reversed even within a small distance. By identifying the base of sand wave on seismic profiles, the thicknesses of sand waves are found ranging from 1 to 10 meters. The base of wave structure become slightly deeper from nearshore to offshore. Our results indicate that the thickness of sand waves increases with degree of asymmetry and migration rate. By bathymetric and reflection seismic data analyses, systematic spatial information of sand waves in the study area are established, and we suggest that not only tidal currents can affect sand wave migration patterns, but also wave structures and thicknesses play important roles in sand wave migrating processes and related geomorphological changes.&lt;/p&gt;

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