首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 359 毫秒
1.
Analysis and interpretation of monitoring data for the seabed bathymetry local to offshore windfarm foundations has shown how the scour develops in time and highlighted variations between sites with different seabed sediment characteristics, i.e. sands and clays. Results from European offshore windfarms have generated a unique dataset for comparison with previously published data. Where surficial sediment is underlain by a marine clay the scour (to date) has been limited, whilst those with unconstrained depths of sandy sediments show scour as deep as 1.38 times the monopile diameter. Scour protection has been installed at some sites for structural stability of the foundation or for cable protection. The flow interaction with the protection causes edge scour or secondary scour in the seabed around the protection. In some cases this scour is deeper than the unprotected case. The analysis has resulted in an improved evidence base for scour in the marine environment.  相似文献   

2.
A three-dimensional coupled hydrodynamic-sediment transport model for the Texas-Louisiana continental shelf was developed using the Regional Ocean Modeling System (ROMS) and used to represent fluvial sediment transport and deposition for the year 1993. The model included water and sediment discharge from the Mississippi River and Atchafalaya Bay, seabed resuspension, and suspended transport by currents. Input wave properties were provided by the Simulating WAves Nearshore (SWAN) model so that ROMS could estimate wave-driven bed stresses, critical to shallow-water sediment suspension. The model used temporally variable but spatially uniform winds, spatially variable seabed grain size distributions, and six sediment tracers from rivers and seabed.At the end of the year 1993, much of the modeled fluvial sediment accumulation was localized with deposition focused near sediment sources. Mississippi sediment remained within 20-40 km of the Mississippi Delta. Most Atchafalaya sediment remained landward of the 10-m isobath in the inner-most shelf south of Atchafalaya Bay. Atchafalaya sediment displayed an elongated westward dispersal pattern toward the Chenier Plain, reflecting the importance of wave resuspension and perennially westward depth-averaged currents in the shallow waters (<10 m). Due to relatively high settling velocities assumed for sediment from the Mississippi River as well as the shallowness of the shelf south of Atchafalaya Bay, most sediment traveled only a short distance before initial deposition. Little fluvial sediment could be transported into the vicinity of the “Dead Zone” (low-oxygen area) within a seasonal-annual timeframe. Near the Mississippi Delta and Atchafalaya Bay, alongshore sediment-transport fluxes always exceeded cross-shore fluxes. Estimated cumulative sediment fluxes next to Atchafalaya Bay were episodic and “stepwise-like” compared to the relatively gradual transport around the Mississippi Delta. During a large storm in March 1993, strong winds helped vertically mix the water column over the entire shelf (up to 100-m isobath), and wave shear stress dominated total bed stress. During fair-weather conditions in May 1993, however, the freshwater plumes spread onto a stratified water column, and combined wave-current shear stress only exceeded the threshold for suspending sediment in the inner-most part of the shelf.  相似文献   

3.
A previous model for nearshore orthogonal gradients (Hardisty, 1986) was based upon a simple bedload function and identified feedback relationships between the equilibrium seabed gradient and wave induced flow asymmetry. That model is developed here with the inclusion of suspended load transport, first order shoaling transformations, second order wave induced flows, and a measure of the sediment grain size. The results are compared with some of the general features of prototype geomorphologies.  相似文献   

4.
The erodibility of intertidal sediments is an important factor affecting coastal erosion.In July and October 2008,in situ measurement of erodibility of the surficial sediment were conducted using a recirculating flume at 20 tidal flat experiment sites along the seashore of the Yellow River delta.At the same time,the characteristics of sand ripples and biogenic features on the tidal flat were observed and the physical-mechanical sediment properties such as bulk density,water content,grain size distribution,plasticity,penetration resistance,shear strength and compressibility,were measured.By field measurement,it is obtained that the critical erosion shear stress of the surficial sediment on the tidal flat varies between 0.088 Pa and 0.254 Pa.The factors influencing sediment erodibility are complicated because of physical and biological reworking after the sediment deposited.There’s a positive correlation between shear strength and critical erosion shear stress.The burrowing crabs’ activities changed the sediment microtopography and made the sediment have greater roughness,and that is one possible reason for the higher erodibility.The formation of scour pits on the tidal flat correlates with the heterogeneous erodibility of the surficial sediment.  相似文献   

5.
The morphodynamics of shallow, vertically well-mixed estuaries, characterised by tidal flats and deeper channels, have been investigated. This paper examines what contributes to flood/ebb-dominant sediment transport in localised regions through a 2D model study (using the TELEMAC modelling system). The Dyfi Estuary in Wales, UK has been used as a case study and, together with idealised estuary shapes, shows that shallow water depths lead to flood dominance in the inner estuary whilst tidal flats and deep channels cause ebb dominance in the outer estuary. For medium sands and with an artificially ‘flattened’ bathymetry (i.e. no tidal flats), the net sediment transport switches from ebb-dominant to flood-dominant where the parameter a/h (local tidal amplitude ÷ local tidally averaged water depth) exceeds 1.2. Sea level rise will reduce this critical value of a/h and also reduce the ebb-directed sediment transport significantly, leading to a flood-dominated estuarine system. A similar pattern, albeit with greater transport, was simulated with tidal flats included and also with a reduced grain size. This suggests that analogous classifications for flood/ebb asymmetry of the tide in estuaries as a whole may not represent the local sediment transport in sufficient detail. Through the Dyfi simulations, the above criterion involving a/h is shown to be complicated further by augmented flow past a spit at the estuary mouth which gives rise to a self-maintaining scour hole. Simulations of one year of bed evolution in an idealised flat-bottomed estuary, including tidal flow past a spit, recreate the flood/ebb dominance on either side of the spit and the formation of a scour hole in between. The erosion rate at the centre of the hole is reduced as the hole deepens, suggesting the establishment of a self-maintaining equilibrium state.  相似文献   

6.
Sandy shallow seas, like the North Sea, are very dynamic. Several morphological features are present on the bed, from small ripples to sand waves and large tidal sandbanks. The larger patterns induce significant depth variations that have an impact on human activities taking place in this area. Therefore, it is important to know where these large-scale features occur, what their natural behaviour is and how they interact with human activities. Here, we extend earlier research that compares the results of an idealized model of large-scale seabed patterns with data of seabed patterns in the North Sea. The idealized model is extended with a grain size dependency. The adaptations lead to more accurate predictions of the occurrence of large-scale bed forms in the North Sea. Therefore, grain size dependency and, in particular, critical shear stress are important to explain the occurrence of sand waves and sandbanks in the North Sea. Responsible Editor: Alejandro Souza  相似文献   

7.
Local scour around cylinders in a side-by-side or tandem arrangement under clear-water conditions is investigated numerically. Large eddy simulations with a Smagorinsky subgrid model are combined with a ghost-cell immersed boundary method, and details of the bed scouring are realized with sophisticated sediment and morphodynamic models. The scour patterns and depths in the two-cylinder cases are shown to be significantly influenced by the cylinder spacing. The features of the scour evolution, depth, and flow fields for a range of cylinder spacings are discussed. The maximum scour depth in the side-by-side cylinder cases increases as the distance between the cylinders decreases, whereas in the tandem cases, it tends to initially increase with increasing distance between the cylinders, after which it gradually decreases beyond the peak point. The maximum scour depths and trends computed using the present model show good agreement with the measured data in the literature.  相似文献   

8.
The current study proposes a novel framework for the numerical model for estimating the temporal scour considering unsteady sediment inflow and the sediment sorting process. The framework was applied to local scour upstream of a slit weir. The scour model is based on an ordinary nonlinear differential equation derived from sediment continuity and scour rate equations. A one-dimensional(1-D)Boussinesq-type model coupled with nonequilibrium sediment transport was incorporated in the scour model to...  相似文献   

9.
Spur dikes are river training structures that have been extensively used worldwide for towards enhancing flood control and the stability of embankments and riverbanks.However,scour around spur dikes can be a major problem affecting their stability and hydraulic performance.The precise computation of temporal scour depth at spur dikes is very important for the design of economical and safe spur dikes.This study focuses on experimentally assessing the temporal variation of scour depth around a vertical wall spur dike and identifying the parameters,which mostly influence spur dike performance for a channel bed surface comprised of sand-gravel mixtures.In the current study,the authors did physical experiments in a flume based study to obtain new data,aimed at deriving a new predictive model for spur dike scour and comparing its performance to others found in the literature.It was found that the dimensionless temporal scour depth variation increases with an increase in(i)the threshold velocity ratio,(ii)the densimetric Froude number of the bed surface sediment mixture,(iii)the flow shallowness(defined as the ratio of the approach flow depth,y,to the spur dike’s transverse length,l),and(iv)the flow depth-particle size ratio.It is also concluded that the temporal scour depth variation in the sediment mixture is influenced by the non-uniformity of sediment and decreases with an increase in the non-uniformity of the sediment mixture.A new mathematical model is derived for the estimation of temporal scour depths in sand-gravel sediment mixtures.The proposed equation has been calibrated and validated with the experimental data,demonstrating a good predictive capacity for the estimation of temporal scour depth evolution.  相似文献   

10.
In order to simulate the dynamics of fine sediments in short tidal basins, like the Wadden Sea basins, a 1D cross-sectional averaged model is constructed to simulate tidal flow, depth-limited waves, and fine sediment transport. The key for this 1D model lies in the definition of the geometry (width and depth as function of the streamwise coordinate). The geometry is computed by implementing the water level and flow data, from a 2D flow simulation, and the hypsometric curve in the continuity equation. By means of a finite volume method, the shallow-water equations and sediment transport equations are solved. The bed shear stress consists of the sum of shear stresses by waves and flow, in which the waves are computed with a depth-limited growth equation for wave height and wave frequency. A new formulation for erosion of fines from a sandy bed is proposed in the transport equation for fine sediment. It is shown by comparison with 2D simulations and field measurements that a 1D schematization gives a proper representation of the dynamics in short tidal basins.  相似文献   

11.
The Kuroshio inflow northeast of Taiwan Island plays an important role in the heat and nutrient balances over the East China Sea(ECS). Based on merged satellite altimeter data and the PCM-1 mooring observation at the East Taiwan Channel(ETC), the study employs a correlation iteration scheme to find the optimal transport index for the Kuroshio inflow. The sea level difference with the highest correlation to the ETC transport is across the ECS shelf break rather than along the PCM-1 line. The counter-intuitive result is caused by large signal noise and poor track coverage of altimeters near the Taiwan coast. The optimal altimetric index is highly correlated with the two-year in-situ measurements as well as the ten-year output of the global assimilation model. It serves as a better estimator of Kuroshio inflow than those using tidal gauge data, and helps pinpoint a 5 cm mismatch of mean sea level in the Keelung tidal record. The mean transport of Kuroshio inflow based on the twenty-year altimetric index is 20.55 Sv with a standard deviation of 3.05 Sv. Wavelet spectrum of the index reveals that semi-annual period dominates the Kuroshio variation northeast of Taiwan Island.  相似文献   

12.
This paper describes a computer simulation model which is designed to predict the selective shore-normal sorting of grain sizes in the nearshore environment. The model simulates wave shoaling, wave height attenuation due to frictional losses and breaking, using linear theory up to the break point and a breaker decay model in the surf zone. Peak horizontal orbital velocities at the bed are calculated from Stokes second-order wave theory. The peak onshore and offshore velocities are used with the threshold expression of Komar and Miller (1975) to generate a spatial pattern of size variation of threshold grain diameter along a profile normal to the shore from deep water to the swash zone. The predicted grain size is used in an hydraulic interpretation of grain size distribution on the intertidal profile, based on the hydrodynamic variations over a tidal cycle on a macrotidal beach. The model is successful in predicting the broad pattern of increasing grain size in the onshore direction which has been observed in nature. Comparisons between measured and predicted grain size distributions indicate that the predictions of the model are better than those of previous models, but the model is more successful at predicting sediment size distributions than at predicting mean sizes on a beach profile.  相似文献   

13.
The results of studying the scour of bed composed of fine and coarse sand around models of large-diameter vertical cylinder under the effect of regular waves are presented. Comparison of the results of experiments with fine and coarse sand showed essential qualitative and quantitative distinctions between the two cases. The results of experiments with coarse sand were used to derive, for the first time, a formula for the estimation of the score depth, including two similarity criteria—the sediment motion factor (SM-factor), and Keulegan-Carpenter number. This formula is applicable to both the engineering analysis of the scour depth of gravel bed or protective riprap by crashed rock or fine stone and the approximate prediction of the scour depth of sand bed around structures under real storm conditions. The conditions for correct modeling of wave scour around structures with large horizontal dimensions are considered and the irrelevance of modeling such scour with the use of fine-sand models is demonstrated.  相似文献   

14.
《国际泥沙研究》2020,35(3):295-306
  相似文献   

15.
Understanding sediment sorting and bedding dynamics has high value to unravelling the mechanisms underlying geomorphological, geological, ecological and environmental imprints of tidal wetlands and hence to predicting their future changes. Using the Nanhui tidal flat on the Changjiang (Yangtze) Delta, China, as a reference site, this study establishes a schematized morphodynamic model coupling flow, sediment dynamics and bed level change to explore the processes that govern sediment sorting and bedding phenomena. Model results indicate an overall agreement with field data in terms of tidal current velocities, suspended sediment concentrations (SSCs), deposition thicknesses and sedimentary structures. Depending on the variation of tidal current strength, sand-dominated layers (SDLs) and mud-dominated layers (MDLs) tend to form during spring and neap tides, respectively. Thinner tidal couplets are developed during daily scale flood–ebb variations. A larger tidal level variation during a spring–neap tidal cycle, associated with a stronger tidal current variation, favours the formation of SDLs and tidal couplets. A larger boundary sediment supply generally promotes the formation of tidal bedding, though the bedding detail is partially dependent on the SSC composition of different sediment types. Sediment properties, including for example grain size and settling velocity, are also found to influence sediment sorting and bedding characteristics. In particular, finer and coarser sediment respond differently to spring and neap tides. During neap tides, relatively small flow velocities favour the deposition of finer sediment, with limited coarser sediment being transported to the upper tidal flat because of the larger settling velocity. During spring tides, larger flow velocities transport more coarser sediment to the upper tidal flat, accounting for distinct lamination formation. Model results are qualitatively consistent with field observations, but the role of waves, biological processes and alongshore currents needs to be included in further studies to establish a more complete understanding.  相似文献   

16.
The interactions between overland flow hydraulics and sediment yield were studied in flume experiments on erodible soil surfaces covered by rock fragments. The high erodibility of a non-cohesive fine sediment (D50 + 0·09mm) permitted the effects of local turbulence and scour on sediment yield to be examined. Overland flow hydraulics and sediment yield were compared for experiments with pebble (D50 + 1·5cm) and cobble (D50 + 8·6cm) rock fragment covers. Cover percentages range from 0 to 99 per cent. Rock fragment size strongly affects the relations between flow hydraulics and rock fragment cover. For pebbles spatially-averaged hydraulic parameters (flow velocity, flow depth, effective flow width, unit discharge, total shear stress, Darcy-Weisbach friction factor, percentage grain friction and grain shear stress) vary most rapidly within cover percentages at low covers (power functions). In contrast, for cobbles these parameters vary most rapidly within cover percentages at high covers (exponential functions). As the type of the function that describes the relation between flow hydraulics and cover percentage can be deduced from the ratio of rock fragment height to flow depth, the continuity equation can be employed to determine the actual coefficients of the functions, provided the regression of one hydraulic parameter (e.g. flow velocity) with cover percentage is known and a good estimate exists for two values of another hydraulic variable for a low and a high cover percentage. The variation of sediment yield with cover percentage is also strongly dependent on rock fragment size, but neither the convex-upward relation for pebbles, nor the positive relation for cobbles can be solely attributed to the spatially averaged hydraulics of sheet-flow. Rock fragments induce local turbulence that leads to scour hole development on the stoss side of the rock fragments while deposition commonly occurs in the wake. This local scour and deposition substantially affects sediment yield. However, scour dimensions cannot be predicted by spatially averaged flow hydraulics. An adjustment of existing scour formulas that predict scour around bridge piers is suggested. Sediment yield from non-cohesive soils might then be estimated by a combination of sediment transport and scour formulas.  相似文献   

17.
From May to June 2014, the geochemical characteristics of dissolved barium(Ba) in sea water and its influx from the Kuroshio into the East China Sea(ECS) were studied by investigation of the Kuroshio mainstream east of Taiwan Island and the adjacent ECS. This allowed for the scope and extent of the Kuroshio incursion to be quantitatively described for the first time by using Ba as a tracer. The concentration of Ba in the Kuroshio mainstream increased gradually downward from the surface in the range 4.91–19.2 μg L.1. In the surface layer of the ECS, the Ba concentration was highest in coastal water and gradually decreased seaward, while it was higher in coastal and offshore water but lowest in middle shelf for bottom layer. The influx of Ba from Kuroshio into the ECS during May to October was calculated to be 2.19×108 kg by a water exchange model, in which the subsurface layer had the largest portion. The distribution of Ba indicated that Kuroshio upwelled in the sea area northeast of Taiwan Island. The north-flowing water in the Taiwan Strait restrained the incursion of Kuroshio surface water onto the ECS shelf, while Kuroshio subsurface water gradually affected the bottom of the ECS from outside. The results of end member calculation, using Ba as a parameter, showed that the Kuroshio surface water had little impact on the ECS, while the Kuroshio subsurface water formed an intrusion current by climbing northwest along the bottom of the middle shelf from the sea area northeast of Taiwan Island into the Qiantang Estuary, of which the volume of Kuroshio water was nearly 65%. Kuroshio water was the predominant part of the water on the outer shelf bottom and its proportion in areas deeper than the 100 m isobath could reach more than 95%. In the DH9 section(north of Taiwan Island), Kuroshio subsurface water intruded westward along the bottom from the shelf edge and then rose upward(in lower proportion). Kuroshio water accounted for 95% of the ocean volume could reach as far as 122°E. Ba was able to provide detailed tracing of the Kuroshio incursion into the ECS owing to its geochemical characteristics and became an effective tracer for revealing quantitative interactions between the Kuroshio and the ECS.  相似文献   

18.

In order to simulate the dynamics of fine sediments in short tidal basins, like the Wadden Sea basins, a 1D cross-sectional averaged model is constructed to simulate tidal flow, depth-limited waves, and fine sediment transport. The key for this 1D model lies in the definition of the geometry (width and depth as function of the streamwise coordinate). The geometry is computed by implementing the water level and flow data, from a 2D flow simulation, and the hypsometric curve in the continuity equation. By means of a finite volume method, the shallow-water equations and sediment transport equations are solved. The bed shear stress consists of the sum of shear stresses by waves and flow, in which the waves are computed with a depth-limited growth equation for wave height and wave frequency. A new formulation for erosion of fines from a sandy bed is proposed in the transport equation for fine sediment. It is shown by comparison with 2D simulations and field measurements that a 1D schematization gives a proper representation of the dynamics in short tidal basins.

  相似文献   

19.
A forward modeling approach is proposed to simulate the preservation potential of tidal flat deposits. The preservation potential is expressed as a function of net deposition rate and a factor that represents the vertical flux of suspended load, or seabed lowering during erosion periods associated with bedload transport. The model takes into account a number of geometric parameters of a tidal flat sediment system and sediment dynamic processes. The former includes high water level, total sediment supply, the annual rate of the supply, the ratio of mud to bilk sediment in the supply, the bed slope of the tidal flat profile, as well as the slope of the stratigraphic boundary; the latter includes spring-neap cycles of tidal water level changes, boundary layer processes, resuspension of fine-grained sediments, bedload transport due to tidal currents, and bed elevation changes in response to sediment movement. Using this model, numerical experiments are carried out for a tidal flat system on the Jiangsu coast, eastern China, with the input data being derived from literature and from a series of sediment cores collected along an onshore–offshore transect. The results show that the preservation potential is highest over the upper part of the inter-tidal zone and in the lower part of the sub-tidal zone, and lowest near mean sea level and at low water on springs. The preservation potential tends to decrease with the advancement of the shoreline. The bed slope, tidal current direction and resuspension intensity influence the spatial distributions of the preservation potential. An implication of these results is that the temporal resolution of the tidal flat record depends upon the location and depth within the deposit; this should be taken into account in the interpretation of sedimentary records. Further studies are required to improve the model, on the hydrodynamic processes associated with extremely shallow water depths, sediment dynamic modeling of bed slope and profile shape, and the combined action of tides and waves for sediment transport on tidal flats.  相似文献   

20.
An integrated field data-modelling approach is employed to investigate relationships between the wavelength of tidal sand waves and four environmental parameters: tidal current amplitude, water depth, tidal ellipticity and median grain size. From echo sounder data at 23 locations on the Dutch continental shelf, the average wavelengths of observed sand waves are determined and compared with the wavelengths obtained with a process-based model. The latter describes the initial formation of these bedforms due to feedbacks between the tidal current and the erodible bed and uses environmental parameters for the 23 locations as input. Good agreement between observed and modelled wavelengths is found if the bottom stress experienced by tidal currents is adequately quantified. Model results show that the wavelength of sand waves increases with increasing water depth, tidal ellipticity and grain size (coarse sand), whilst it decreases with increasing tidal current amplitude and grain size (fine sand). Due to the limited number of stations and the fact that all four parameters change from location to location, the modelled relationships are only partly supported by the field observations.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号