experimental and numerical study of the flow structure around two partially buried objects on a deformed bed abstractthe flow characteristics around two partially buried objects a short cylinder sc and a truncated cone tc were examined experimentally and numerically experiments were conducted in a large tilting flume with vertical walls made of acrylic plastic and steel floor covered by a 24 m long sand pit for each experiment the object was placed on a flat sandy bed the flume was filled to a water depth of 41 cm and a steady unidirectional current was established at high reynolds numbers while sinking of the object was tracked over time velocity records were taken with an acoustic doppler velocimeter adv probe at the final stage of burial and scour hole evolution the flow structure around each object was further investigated using a model that solves the reynolds averaged navier stokes rans equations coupled with a e turbulence closure model and results were compared to the experimental measurements measurements and numerical simulations allowed identification of regions of high and low vel

three dimensional simulation of scour inducing flow at bridge piers presently available formulas for estimating the maximum depth of scour near bridge piers often lead to unreliable results a fully three dimensional hydrodynamic model is therefore used in this study to simulate the flow occurring at the base of a cylindrical bridge pier within a scour hole the results of the numerical simulation are compared with laboratory observations by melville and raudkivi 1977 quantitative and qualitative agreement between the studies is quite good discrepancies between the results of the two studies are of an order that may be attributed to choices in numerical model parameters the simulations may be supplemented by lagrangian particle tracking to estimate the depth of the equilibrium scour condition

experimental study of local scour around subsea caissons in steady currents abstract local scour of sediments around subsea caisson structures was investigated experimentally by carrying out flume tests the horizontal shape of the caissons is rectangular and the incident directions of the flow were 0 45 and 90 with 0 representing flow parallel to the long boundary of the caisson and 90 representing flow parallel to the short boundary of the caisson the study was focused on the low caissons whose vertical dimensions were equal to or less than their horizontal dimensions it was found from the test results that the horseshoe vortex played a less important role compared with the velocity amplification at the sharp corners of the caisson if its height is smaller than its horizontal dimension as the incident angle of flow is either 0 or 90 scour started at the two upstream corners of the caisson while scour at the centre of the upstream boundary did not start until the scour pits at the side corners extended there this was also true for the case with 45 incident angle of the flow the development of the scour depth was fitted according the exponential function and the hyperbolic function it was found that the hyperbolic function fits the experimental data better than the exponential function because its correlation factor is larger

numerical modeling of flow and scour at pipelines a numerical model for the description of fluid flow and suspended and bed load sediment transport is presented density effects are included in the momentum reynolds equations and in the turbulence kappa and epsilon equations changes in bed levels are calculated from sediment continuity and the finite element grid is adapted to the geometry the reynolds equations and the transport equation for suspended sediment are solved numerically using a taylor galerkin finite element method the flow at a surface mounted cylinder in a steady flow is predicted in good agreement with experiments periodic vortex shedding from a cylinder placed above a rigid bed is predicted in good agreement with laboratory experiments provided that sufficiently detailed grids approximately 5 000 nodes are used scour calculations are performed for a cylinder in a steady flow with its underside placed at the level of the original flat bed predicted scour at a pipeline in steady flow is in good agreement with laboratory measurements reported in the literature

frequency and timing of landslide triggered turbidity currents within the agadir basin offshore nw africa are there associations with climate change sea level change and slope sedimentation rates abstract older sequence stratigraphic models suggested that submarine landslide and turbidite activities are greatest during sea level lowstands however growing evidence indicates that many turbidite systems are also active during sea level transgressions and highstands the moroccan turbidite system comprises three depocentres of which agadir basin is closest to the moroccan slope and canary archipelago the very large volumes of sediment transported by individual sediment flows in this system suggest that they are triggered by landslides extensive core coverage and dating control for the agadir basin deposits have provided an excellent opportunity to derive accurate records of turbidite and associated landslide frequency for the last 600 ka previous studies in the more distal madeira abyssal plain depocentre have indicated that large volume 50 km 3 turbidites occurred at oxygen isotope stage ois boundaries this study of agadir basin confirms that two major turbidites beds a5 and a12 occurred during glacial interglacial transitions associated with ois4 and ois6 however this association is based on just two examples and two other large volume turbidites beds a7 and a11 did not occur at a stage boundary the main conclusion of this study is that 90 of turbidites and landslides occurred during rising and high sea level which represents 40 of the total time during the last 600 ka only 10 of the turbidites and landslides occurred during glacials 40 of the time with a paucity of turbidites and landslides at peak glacial lowstands a comparison to sediment accumulation rates in the source area of the turbidite suggests that landslides did not occur preferentially during periods of more rapid sedimentation rate although sedimentation rates in this area only varied from 4 to 6 g cm 2 ka 1

three dimensional calculation of scour around cylinders the flow of water around a cylinder was calculated by a numerical model the model used a finite volume method to solve the nontransient navier stokes equations for three dimensions on a general nonorthogonal grid the k e turbulence model was used to solve the reynolds stress term the flow field gave the shear stress on the boundaries which was used to calculate sediment concentration for the bed elements the diffusion convection equation for the sediment concentration was solved and continuity for the sediment concentration gave changes in the bed elevation using the modified bed a new flow field was calculated and also a new sediment concentration this sequence was repeated until the depth of the calculated scour hole was identical to the depth of the scour hole from a physical model study the rest of the geometry of the resulting scour hole compared well with the measured scour hole from the experimental study also other flow characteristics given by the numerical model were found to be in ag

numerical simulation of scour and backfilling processes around a circular pile in waves abstract this study continues the investigation of flow and scour around a vertical pile reported by roulund et al 2005 flow and scour backfilling around a vertical pile exposed to waves are investigated by using a three dimensional numerical model based on incompressible reynolds averaged navier stokes equations the model incorporates 1 k turbulence closure 2 vortex shedding processes 3 sediment transport both bed and suspended load as well as 4 bed morphology the numerical simulations are carried out for a selected set of test conditions of the laboratory experiments of sumer et al 1997 2013a and the numerical results are compared with those of the latter experiments the simulations are carried out for two kinds of beds rigid bed and sediment bed the rigid bed simulations indicate that the vortex shedding for waves around the pile occurs in a one cell fashion with a uniform shedding frequency over the height of the cylinder unlike the case for steady current where a two cell structure prevails the rigid bed simulations further show that the horseshoe vortex flow also undergoes substantial changes in waves the amplification of the bed shear stress around the pile including the areas under the horseshoe vortex and the lee wake region is obtained for various values of the keulegan carpenter number the principal parameter governing the flow around the pile in waves the present model incorporated with the morphology component is applied to several scenarios of scour and backfilling around a pile exposed to waves in the backfilling simulations the initial scour hole is generated either by a steady current or by waves the present simulations indicate that the scour and backfilling in waves are solely governed by the lee wake flow in agreement with observations the numerical model has proven successful in predicting the backfilling of scour holes exposed to waves the results of the numerical tests indicate that the equilibrium depth of scour holes is the same for both the scour and the backfilling for a given keulegan carpenter number in full agreement with observations

characteristics of horseshoe vortex in developing scour holes at piers the outcome of an experimental study on the turbulent horseshoe vortex flow within the developing intermediate stages and equilibrium scour holes at cylindrical piers measured by an acoustic doppler velocimeter adv are presented since the primary objective was to analyze the evolution of the turbulent flow characteristics of a horseshoe vortex within a developing scour hole the flow zone downstream of the pier was beyond the scope of the investigation experiments were conducted for the approaching flow having undisturbed flow depth 0 25 m greater than twice the pier diameter and the depth averaged approaching flow velocity 0 357 m s about 95 of the critical velocity of the uniform bed sand that had a median diameter of 0 81 mm the flow measurements by the adv were taken within the intermediate having depths of 0 25 0 5 and 0 75 times the equilibrium scour depth and equilibrium scour holes frozen by spraying glue at a circular pier of diameter 0 12 m in order to have a comparative study the adv measurements within an equilibrium scour hole at a square pier side facing the approaching flow of sides equaling the diameter of the circular pier were also taken the contours of the time averaged velocities turbulence intensities and reynolds stresses at different azimuthal planes 0 45 and 90 are presented vector plots of the flow field at azimuthal planes reveal the evolution of the characteristics of the horseshoe vortex flow associated with a downflow from intermediate stages to equilibrium condition of scour holes the bed shear stresses are determined from the reynolds stress distributions the flow characteristics of the horseshoe vortex are discussed from the point of view of the similarity with the velocity and turbulence characteristic scales the imperative observation is that the flow and turbulence intensities in the horseshoe vortex flow in a developing scour hole are reasonably similar

a hydraulic box model study of the mediterranean response to postglacial sea level rise 1 this paper quantifies the role of changing sea level in affecting mediterranean stratification using a new model of the strait basin system which allows for explicit time dependence during the last deglaciation fairbank s meltwater peak 1b leads to rapidly rising global sea levels and a freshening of the oceans and hence increasing influx of ever freshening atlantic water into the mediterranean basin owing to long residence times the salinity of the deep and intermediate water reservoirs would have decreased more slowly so that the basin was more stably stratified for a period with possible implications for sapropel formation our model is used to estimate the size of this reservoir effect in comparison to the freshwater influx from the opening of the black sea at the bosphorus it is found that this mechanism affects the stratification to a similar degree as the black sea opening other climatic mechanisms have also affected the freshwater budget of the mediterranean at this time but their sizes are more difficult to quantify

numerical investigation of local scour beneath a sagging subsea pipeline in steady currents abstract two dimensional 2 d numerical simulations are conducted to predict scour below the midsection of the suspended span of a pipeline that sags naturally into a scour pit the deflection of the pipeline is calculated based on the suspension length or span length of the pipeline which is predicted by an empirical formula for the length of the scour pit the general properties of the scour process are investigated for various pipeline properties pipeline embedment depths and shields parameters the effect of each of these parameters on pipeline sagging are combined into one non dimensional sagging parameter a touchdown time is defined as the time when the pipeline touches the bottom of the sand pit empirical formulae are proposed for predicting the final scour depth at touchdown and the time scale required for touchdown it is found that the final sagging depth is a function of the sagging parameter and the shields parameter whilst the touchdown time is only a function of the sagging parameter a formula is also developed for predicting the time history of scour depth prior to touchdown collectively the empirical formulae and numerical simulations are found to be in reasonable agreement with an existing theoretical model available in the literature

local scour around two pipelines in tandem in steady current abstract in this study local scour around two identical pipelines in a tandem arrangement are investigated numerically the two dimensional reynolds averaged navier stokes rans equations together with the conservation equation of the sediment mass are solved to simulate the flow around two pipelines and the evolution of the local scour profile below the pipelines the gap ratios between the two pipelines g d range from 0 5 to 5 with an interval of 0 5 where g is the gap between the two pipelines and d is the diameter of the pipelines the focus of the study is to investigate the effects of the gap ratio on the scour depth and scour time scale it is found that the scour depth below the downstream pipeline is greater than that below the upstream pipeline under both clear water and live bed conditions because of the effect of the vortex shedding from the upstream pipeline the maximum scour depth below the downstream pipeline occurs at g d 2 5 under both clear water and live bed conditions which is about the critical gap ratio beyond which vortex shedding from the upstream pipeline occurs as the gap ratio exceeds 2 5 the scour depth below the downstream pipeline decreases with increasing gap ratio due to the weakening effect of the vortex shedding from the upstream pipeline on the scour the scour under the clear water condition is simulated at a velocity 25 lower than the critical velocity for live bed scour the variations of the scour depths below the two pipelines in the clear water and live bed conditions are found to be similar to each other strong positive mean lift coefficient on the downstream pipeline was observed during the scour process which is believed to have a negative effect on the stability of downstream pipeline the rms lift coefficient on the downstream pipeline increases significantly when the vortex shedding from the upstream pipeline occurs

experimental and numerical investigation of local scour around a submerged vertical circular cylinder in steady currents abstract local scour around a submerged vertical circular cylinder in steady currents was studied both experimentally and numerically the physical experiments were conducted for two different cylinder diameters with a range of cylinder height to diameter ratios transient scour depth at the stagnation point upstream edge of the cylinder was measured using the so called conductivity scour probes three dimensional 3d seabed topography around each model cylinder was measured using a laser profiler the effect of the height to diameter ratio on the scour depth was investigated the experimental results show that the scour depth at the stagnation point is independent on cylinder height to diameter ratio when the later is smaller than 2 the increase rate of equilibrium scour depth with cylinder height increases with an increase in shields parameter a three dimensional finite element numerical model is developed for simulating local scour around submerged vertical cylinders steady flow around the cylinder is simulated by solving the reynolds averaged navier stokes rans equations with a k turbulence closure both suspended load and bed load sediment transport rates were included in the model efforts are made to reduce the computational time associated with three dimensional morphological modelling including the use of wall function to avoid resolving the near wall boundary layers large morphological time step and parallel computing techniques scour around a submerged vertical circular cylinder founded on the seabed is simulated and the numerical results are validated against the test data the scour mechanisms around the submerged cylinder are investigated using the numerical model

numerical modeling of flow and scour below a pipeline in currents part ii scour simulation abstract a vertical two dimensional 2d numerical model for time dependent local scour below offshore pipelines subject to unidirectional steady flow is developed the governing equations for the flow and sediment transport are solved by using finite difference method in a general curvilinear coordinate system the performance of two turbulence models the standard k model and smagorinsky subgrid scale sgs model on modeling time dependent scour processes is examined both suspended load and bed load are considered in the scour model the suspended load model is verified against two channel sediment transport cases the change of bed level is calculated from the continuity equation of total sediment transport a new time marching scheme and a sand slide scheme are proposed for the scour calculation it is found that the proposed time marching scheme and sand slide model work well for both clear water and live bed scour situations and the standard k turbulence closure is more preferable than the sgs model in the 2d scour model developed in this study

numerical and experimental investigation of flow and scour around a circular pile the flow around a vertical circular pile exposed to a steady current is studied numerically and experimentally the numerical model is a three dimensional model the model validation was achieved against new experimental data which include two component laser doppler anemometry lda flow measurements and the hot film bed shear stress measurements and reported in the present paper and the data of others and a k w turbulence model was used for closure the model does not have a free surface facility and therefore is applicable only to cases where the froude number is small fr 0 0 2 the flow model was used to study the horseshoe vortex and lee wake vortex flow processes around the pile the influence on the horseshoe vortex of three parameters namely the boundary layer thickness the reynolds number and the bed roughness was investigated in the latter investigation the steady solution of the model was chosen a study of the influence of the unsteady solution on the previously mentioned flow processes was also carried out the ranges of the parameters covered in the numerical simulations are the boundary layer thickness to pile diameter ratio is varied from 2 x 10 2 to 10 2 the pile reynolds number from 10 2 to 2 10 6 and the pile diameter to roughness ratio from 2 to about 10 3 the amplification of the bed shear stress around the pile including the areas under the horseshoe vortex and the lee wake region was obtained for various values of the previously mentioned parameters the steady state flow model was coupled with a morphologic model to calculate scour around a vertical circular pile exposed to a steady current in the case of non cohesive sediment the morphologic model includes i a two dimensional bed load sediment transport description and ii a description of surface layer sand slides for bed slopes exceeding the angle of repose the results show that the present numerical simulation captures all the main features of the scour process the equilibrium scour depth obtained from the simulation agrees well with the experiments for the upstream scour hole some discrepancy up to 30 was observed however for the downstream scour hole the calculations show that the amplification of the bed shear stress around the pile in the equilibrium state of the scour process is reduced considerably with respect to that experienced at the initial stage where the bed is plane

edge scour at scour protections around piles in the marine environment laboratory and field investigation abstract when building offshore wind turbines with monopile foundations scour protection typically is placed to avoid scouring of the soil close to the monopile an important aspect is that the scour protection itself causes erosion inflicted by the local increase in current and or wave velocities and in turn increased bed shear stresses scour of the edge material alongside the scour protection may cause deformations and failure of the scour protection of offshore wind turbine foundations this can reduce the stability of the stone layer and cause exposure of cables running between the monopiles where they go from buried to the transition piece on the foundation although much information is available on the design of scour protection systems around monopiles little is known on the mechanisms causing edge scour and the equilibrium stages of the edge scour process in steady current waves and combined waves and current this paper presents an extensive experimental campaign to explain the edge scour process in current and combined irregular waves and current as well as tidal current the three dimensional flow field around the pile and scour protection is resolved by particle image velocimetry and bed shear stress measurements showing a local increase in the flow velocities and bed shear stresses leading to increased sediment transport and scour the governing process in steady current is a pair of symmetrical counter rotating vortices emerging in the near bed region in the wake of the pile and scour protection causing a significant downstream scour hole it is found that the equilibrium scour hole depth and length scales with the pile diameter and the ratio between the thickness and the width of the scour protection in the second part of the present paper the results from the experimental campaign are compared with the edge scour experienced in practice outlined by a survey program of the offshore wind park egmond aan zee and a published field investigation of scroby sands owf by whitehouse et al 2011

experimental investigation of local scour around submerged piles in steady current abstract submerged vertical piles are an analogue for a range of subsea structures in this paper scour around submerged piles with both circular and square cross section are explored experimentally in steady currents a wide range of cylinder height to diameter ratio h d 0 1 8 is considered for flow intensities spanning clear water to sheet flow conditions results are presented in terms of the equilibrium scour depth and the time scale of the scour process for the cross sectional shapes considered the experimental results indicate that the equilibrium scour depth reduces with aspect ratio when h d 4 whilst for larger aspect ratio the scour depth is similar to that for an infinitely tall cylinder the amount of reduction in scour depth with aspect ratio for short piles is found to be dependent on the flow intensity such that the effects of aspect ratio and flow intensity on scour depth are not separable with respect to the time scale the experiments indicate that for a flow intensity corresponding to live bed conditions a maximum time scale occurs for an intermediate aspect ratio for aspect ratios below this value the time scale reduces because the equilibrium scour depth reduces whilst for larger aspect ratios the time scale reduces because the local transport rate is increased alternatively in clear water conditions the time scale reduces continuously with increasing aspect ratio empirical equations are derived based on the experimental results so as to better account for aspect ratio in scour design

live bed pier scour in a covered flow abstractresults are presented for a novel experimental investigation of live bed scour at a cylindrical pier in covered flow the experimental campaign also comprehended a reference scour test in c

stability of cover stones around a vertical cylinder under the influence of waves and current abstract incipient motion of cover stones in scour protections around piles in the marine environment is studied in waves current and co direction waves and current the flow mechanisms in the immediate surroundings of the pile such as the horseshoe vortex lee wake vortices and contraction and acceleration of flow have been identified as reason for agitating and stirring up the cover stones in current these flow mechanisms and the threshold for incipient motion of cover stones are governed by the shields parameter the water depth to pile size ratio and the stone size to pile size ratio for shields parameters slightly above the threshold of incipient motion in current the mobilized stones are ultimately only moved within the lee wake zone of the pile and only few relocations of cover stones were observed for larger shields parameters the horseshoe vortex mobilizes the cover stones and immediately causes severe damage to the scour protection as cover stones are moved outside the scour protection in the case of waves similar effects are observed and the wave motion in terms of the keulegan carpenter number k c number is the governing parameter of incipient motion design diagrams are presented for the threshold of incipient motion of cover stone in waves current and combined waves and current

northwest african continental margin history of sediment accumulation landslide deposits and hiatuses as revealed by drilling the madeira abyssal plain 1 odp drill sites in the madeira abyssal plain reveal sequences of organic rich turbidites derived from the northwest african margin in which each turbidite has a volume of tens to hundreds of cubic kilometers the frequency of turbidite emplacement has been combined with core and seismic data to show the volume of redeposited sediment the basin began to fill about 22 ma with numerous small turbidites up to 100 per million years each with volumes of a few cubic kilometers the total volume of turbidites deposited increased between 16 and 11 ma as did their individual volumes and then declined to 7 ma at 7 ma there was a dramatic increase in the amount of turbidite input to 768 km3 myr and a rise in the average volume of each unit to 59 km3 these high values have been maintained to the present day the variations in the amount of redeposited sediment most likely reflect the rates of sedimentation on the northwest african margin since high sedimentation leads to oversteepening of the slopes and eventual mass wasting the dramatic changes at about 7 ma may be due to a large increase in upwelling off northwest africa caused by circulation changes associated with increased glaciation of the poles up to 20 of sediment may be remobilized by landslides with each event leaving a hiatus each of these hiatuses extends over an average area of 4800 km2 and represents removal of sediment layers several tens of meters thick and of several hundred thousand years duration

data analysis of bed concentration of suspended sediment an empirical relation for the bed concentration of suspended sediment cb defined at a few grain diameters above the bed is developed its derivation is based on the experimental data sets of guy et al in 1966 the values of the near bed concentration were obtained by first isolating the suspended load transport from the observed rates of total load transport by use of a bed load formula the suspended load was then equated to einstein s formulation which allowed the determination of cb the influence of large concentrations of suspended sediment and of the reference level for cb on the calculated values of bed concentration has been analyzed the proposed relation compares satisfactorily to values of near bed concentration obtained from independent data sets the procedure used to derive cb provides a simple method for the calculation of the total transport rate of noncohesive sediment

source mechanism of the bartin earthquake of september 3 1968 in northwestern turkey evidence for active thrust faulting at the southern black sea margin abstract the bartin earthquake of september 3 1968 is the strongest instrumentally recorded earthquake to occur along the black sea margin in northwestern turkey the source mechanism of the bartin earthquake as determined in this study from long period p wave first motions and from the inversion of p and sh waves indicates thrust faulting with a strike of 28 dip of 38 and rake of 80 the average centroid depth is 4 km and the seismic moment is 3 9 10 25 dyn cm the bartin earthquake provides the first evidence for active thrust faulting at the southern margin of the black sea the black sea has probably evolved as a back arc basin resulting from the northward subduction of tethys seismicity fault plane solutions and other geophysical data suggest that at the present time the black sea is closing under north south compression