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1.
A new methodology based on wavelet analysis is used to estimate steep wave statistics under depth-limited conditions and the corresponding high concentration sediment statistics. Steep waves here are defined as wave crests within the wavelet transform exceeding a root mean square derived acceleration threshold. The method is applied to laboratory data obtained in a large-scale wave-flume experiment conducted in 2005 at Oregon State University's O. H. Hinsdale Wave Research Laboratory from an acoustic Doppler velocimeter and a fiber optic backscatter sensor array above a mobile sand bed. The steep wave and high concentration statistical results for the erosive condition suggest that sand suspensions are intermittent when a wave-breaking timescale (the ratio of breaking wave height and rms wave velocity) is used to detect the concurrence among steep wave, high velocity turbulent fluctuations, and sand concentration events near the bed. More importantly, at 1 cm above the bed, though the accretive case has more steep wave events, the erosive case has more steep waves and concurrent high concentration events, suggesting a more intense breaking wave process near the sensors. The use of a longer time window, based on the dominant wave period in the detection process of steep wave and high concentration events at 1 cm above the bed, does not change the resulting statistics for the erosive condition. However, increased percentages of high concentration events correlated with steep wave and high velocity turbulence events for the accretive condition are obtained. These increased percentages are conjectured to be due to advection of non-local turbulent events and sediment concentration peaks from upstream.  相似文献   

2.
The purpose of this study is to investigate the capability of a newly developed process-based model for sediment transport under a wide variety of wave and current conditions.The model is based on the first-order boundary layer equation and the sediment advection-diffusion equation.In particular,a modified low Reynolds number k-e model is coupled to provide the turbulence closure.Detailed model verifications have been performed by simulating a number of laboratory experiments,covering a considerable range of hydrodynamic conditions such as sinusoidal waves,asymmetric waves and wave-current interactions.The model provides satisfactory numerical results which agree well with the measured results,including the time-averaged/dependent sediment concentration profiles and sediment flux profiles,as well as the time series of concentration at given elevations.The observed influences of wave orbital velocity amplitude,wave period and sediment grain size are correctly reproduced,indicating that the fundamental physical mechanisms of those processes are properly represented in the model.It is revealed that the present model is capable of predicting sediment transport under a wide range of wave and current conditions,and can be used to further study the morphodynamic processes in real coastal regions.  相似文献   

3.
This study investigates fluctuations in nitrate concentration at the water table to improve understanding of unsaturated zone processes in the Chalk aquifer. Sampling was conducted using a novel multi‐level sampler during periods of water table rise over 5 years at a vertical resolution of 0.05 m. Nitrate concentration increased as the water table seasonally recovered, with similar inter‐annual trends with depth. The rising water table activated horizontal fractures facilitating the delivery of water elevated by up to 10 mg/l of nitrate with respect to the adjacent groundwater below. These fractures are considered to activate via piston displacement of water from the adjoining matrix. Hydrograph analysis identified 16 events which significantly perturbed the water table within 24–48 h of rainfall. Consistent nitrate concentrations indicate recharge through persistent fracture flow from the surface was not generally the primary driver of the rapid water table response during these events. Instead, the response was attributed to the piston displacement of porewater immediately above the water table. However, a single event in November 2012 delivered relatively dilute recharge indicating rapid persistent fracture flow following rainfall was possible to a depth of 14–15 m. Decreases in porewater nitrate concentration around fracture horizons and the dilution of many groundwater samples with respect to porewaters indicate a fresher source of water at depth. This was considered most likely to be a result of near surface water bypassing the matrix because of widespread mineralization on fracture surfaces, which retard water and solute exchange. Therefore, persistent fracture flow maybe considered a frequent process, operating independently of the matrix, and is not necessarily event driven. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

4.
Summary Internal waves are considered on the hypotheses of small disturbances and the Boussinesq approximation. Consideration is given to the lowest internal mode propagating along athin thermocline, and an approximate calculation is made of the dispersion relation, including an explicit representation of the dominant correction for thermocline thickness. Then, the Stokes drift velocity is calculated (a) approximately for this mode in the thin-thermocline model, and (b) numerically for the first two internal modes associated with a single thick thermocline. Finally, a calculation is made of the mass transport velocity field induced by the interaction between pairs of waves (surface or internal), having thesame frequency but arbitrary directions of propagation.  相似文献   

5.
Fine sediment carrying capacity of combined wave and current flows   总被引:1,自引:0,他引:1  
The so-called fine sediment in many coastal areas and estuaries in China is mostly referred to the mixture of cohesive sediment and non-cohesive sediment. To predict the mixed type time sediment transport, sediment carrying capacity formulae combined with the 2-D suspended sediment transport equation and morphologic equation have been widely used in China. In the present study, the sediment carrying capacity formula suggested by Dou et al. (1995) for wave conditions has been improved and implemented for the prediction of sediment transport in nearshore regions where wave activities are significant. The improvement is based on the wave energy dissipation principle inside and outside the surf zone. In the improved formula, sediment in suspension increases with the magnitude of the wave period and this feature complies with general observations. More than 300 laboratory and field measured data sets have been reviewed and 12 of them have been used to verify and determine the major coefficients in the improved formula. The application of the sediment carrying capacity model in combined wave and current situations shows that the model can faithfully reproduce the cross-shore sediment concentration distributions at the southwest coast of Bohai Bay.  相似文献   

6.
本文利用数值计算和经验公式相结合的方法,获得了在波浪和风生流和波浪共同作用下海底泥沙输移的二维模型.在此基础上,讨论了海底泥沙输移在风速、风向和泥沙粒径不同等三种情况下的变化特征;指出了经验公式对泥沙输移规律的适用范围以及风速、风向对于近岸处泥沙输移重要性.这一研究结果对近岸结构物的选址设计具有实际应用价值.  相似文献   

7.
In the present work, the sediment transport rate in the swash zone is estimated using a Boussinesq model coupled with a porous flow model in order to incorporate the infiltration–exfiltration effects. The transport rate is estimated from a modified Meyer–Peter and Müller formula using different values the multiplier C for uprush and backwash to incorporate the observed higher transporting efficiency of the uprush due to the presence of pre-suspended sediment from the bore collapse. Alternatively, the depth-integrated transport equation for suspended sediment is solved. The comparison between model results and experimental data shows that both approaches are able to give satisfactory results.  相似文献   

8.
利用子午工程海南激光雷达对我国海南地区上空进行持续观测,通过3年的累积观测数据对我国低纬度地区重力波活动的季节分布特性进行研究,依据重力波线性理论对海南地区上空的大气密度扰动规律、空间功率谱及时间频率谱进行分析,并通过选择波长在1 km至8 km范围内具有特定波长以及具有波动周期为60 min至25 min的特定频率的重力波辅助研究大气密度扰动的季节变化规律,总结得出海南地区重力波活动具有夏季大、春秋季小、而冬季依然频繁的季节性分布规律.结合海南地区特殊的地理位置与当地季节性气候特征分析得出海南地区上空重力波活动季节性变化的可能原因为青藏高原地形及我国南海地区存在的热带强对流与赤道潜流共同作用的结果.  相似文献   

9.
Sediment incipient motion is a fundamental issue in sediment transport theory and engineering practice. Whilst Shields curve often is used to determine the threshold of sediment movement under unidirectional current conditions, it is unclear whether it can be directly applied for the wave or combined wave-current conditions. The study developed adaptive criterion curves describing incipient motion of sediment under wave and current conditions based on the flow pattern around the sediment particles. Firstly, the flow pattern law for fixed particles was recognized based on the friction law under various dynamic conditions (wave, current, and their combinations), and the flow pattern demarcations for incipient sediment motion were obtained with the threshold conditions for sediment movement under various dynamic conditions combined. Secondly, the exact shape of the Shields curve in each flow regime was derived under the current condition. By combining the flow pattern demarcations for incipient sediment motion under the wave condition, the criterion curve under the wave condition was derived. By combining the flow pattern demarcations for incipient sediment motion under the combined current-wave condition, the criterion curve for sediment incipient motion under the combined current-wave condition was derived. The results indicated that the flow pattern around incipient particles includes laminar, laminar-rough turbulent transition, and rough turbulent regimes. The criterion curves for sediment incipient motion under various dynamic conditions stayed the same in the laminar and rough turbulent regimes, but different in the transition regime. Depending on the relative strengths of the currents and waves, the shape of the criterion curve under the combined current-wave condition transitions adaptively between the criterion curve under the current condition and the criterion curve under the wave conditions.  相似文献   

10.
《国际泥沙研究》2022,37(6):715-728
Rainfall-induced floods may trigger intense sediment transport on erodible catchments, especially on the Loess Plateau in China, which in turn modifies the floods. However, the role of sediment transport in modifying floods has to date remained poorly understood. Concurrently, traditional hydrodynamic models for rainfall-induced floods typically ignore sediment transport, which may lead to inaccurate results for highly erodible catchments. Here, a two-dimensional (2D) coupled shallow water hydro-sediment-morphodynamic (SHSM) model, based on the Finite Volume Method on unstructured meshes and parallel computing, is proposed and applied to simulate rainfall-induced floods in the Zhidan catchment on the Loess Plateau, Shaanxi Province, China. For six historical floods of return periods up to 2 years, the numerical results compare well with observations of discharge hydrographs at the catchment outlet. The computed runoff-sediment yield relation is quantitatively reasonable as compared with other catchments under similar geographical conditions. It is revealed that neglecting sediment transport leads to underestimation of peak discharge of the flood by 14%–45%, whilst its effect on the timing of the peak discharge varies for different flood events. For 18 design floods with return periods of 10–500 years, sediment transport may lead to higher peak discharge by around 9%–15%. The temporal pattern of concentrated rainfall in a short period may lead to a larger exponent value of the power function for the runoff-sediment yield relation. The current finding leads us to propose that incorporating sediment transport in rainfall-induced flood modeling is warranted. The SHSM model is applicable to flood and sediment modeling at the catchment scale in support of risk management and water and soil management.  相似文献   

11.
Gravel beaches are common throughout the high latitudes, but few studies have examined gravel transport rates, in particular at high energy levels, and no studies have quantified gravel transport around headlands. Here, we present the first complete sediment budget, including supra-, inter- and sub-tidal regions of the beach, across multiple headland-separated gravel embayments, combined with hydrodynamic observations, over an extreme storm sequence, representing at least a 1-in-50-year event. Unprecedented erosion was observed (~400 m3 m−1, −6 m vertical), with alongshore flux of 2 × 105 m3, equivalent to annual rates. Total system volume change was determined to the depth of closure and then used to calculate alongshore flux rates. Alongshore wave power was obtained from a wave transformation model. For an open section of coastline, we derive a transport coefficient (CERC formula) of KHs = 0.255 ± 0.05, exceeding estimates in lower-energy conditions by a factor of 5 or more. We apply this coefficient to rocky segments of the shoreline, determining rates of headland bypass from 0 to 31% of potential flux, controlled by headland extent and toe depth. Our results support the hypothesis that gravel is transported more efficiently at higher energy levels and that a variable rate or threshold approach may be required. Complete coverage and varying morphology make this dataset uniquely suited to improving model predictions of gravel shoreline change. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.  相似文献   

12.
Summary Recently, a new theory of the subsidence of rock masses has been advanced byLitwiniszyn and coworkers, which is based on the notion that the individual rock particles perform random walks. It is shown in the present paper that the final equations of the random walk theory can be justified from general principles of statistical mechanics in which no specific theoretical model of particle behaviour has to be made. The theory appears to account appropriately for subsidence phenomena.The research reported in this paper was supported by the Petroleum Research Fund of the American Chemical Society. Acknowledgment is made to the donors of the Petroleum Research Fund administered by the American Chemical Society for support of this reseach.  相似文献   

13.
Shelf-to-canyon suspended sediment transport during major storms was studied at the southwestern end of the Gulf of Lions. Waves, near-bottom currents, temperature and water turbidity were measured on the inner shelf at 28-m water depth and in the Cap de Creus submarine canyon head at 300 m depth from November 2003 to March 2004. Two major storm events producing waves Hs>6 m coming from the E–SE sector took place, the first on 3–4 December 2003 (max Hs: 8.4 m) and the second on 20–22 February 2004 (max Hs: 7 m). During these events, shelf water flowed downcanyon producing strong near-bottom currents on the canyon head due to storm-induced downwelling, which was enhanced by dense shelf water cascading in February 2004. These processes generated different pulses of downcanyon suspended sediment transport. During the peak of both storms, the highest waves and the increasing near-bottom currents resuspended sediment on the canyon head and the adjacent outer shelf causing the first downcanyon sediment transport pulses. The December event ended just after these first pulses, when the induced downwelling finished suddenly due to restoration of shelf water stratification. This event was too short to allow the sediment resuspended on the shallow shelf to reach the canyon head. In contrast, the February event, reinforced by dense shelf water cascading, was long enough to transfer resuspended sediment from shallow shelf areas to the canyon head in two different pulses at the end of the event. The downcanyon transport during these last two pulses was one order of magnitude higher than those during the December event and during the first pulses of the February event and accounted for more than half of the total downcanyon sediment transport during the fall 2003 and winter 2004 period. Major storm events, especially during winter vertical mixing periods, produce major episodes of shelf-to-canyon sediment transport at the southwestern end of the Gulf of Lions. Hydrographic structure and storm duration are important factors controlling off-shelf sediment transport during these events.  相似文献   

14.
The western coast of India (Kachchh) has ubiquitous preservations of Quaternary carbonate deposits popularly known as “miliolites”. Field-based detail documentation of the nature and distribution of miliolites supported by sedimentology and scanning electron microscopy (SEM) microtextural studies indicates that the miliolites were primarily deposited by wind as aeolinites and are termed “primary miliolites (PM)”. These were subsequently fluvially eroded and deposited and such deposits are termed “secondary miliolites (SM)”. The PM are dated between 28 ka to 16 ka largely clustering around the Last Glacial Maxima (LGM). In comparison, the fluvially reworked SM occurs as valley-fill deposits and were deposited episodically during 23 ka to 10 ka before present. Climatically, the deposition of PMs suggests enhanced aridity whereas SMs represent episodic strengthening of the Indian Summer Monsoon (ISM). The bedding attitudes of the PMs indicate that they were deposited predominantly by the northeasterly winds whereas the presence of the deflation lag deposits suggests downwind migration of sands during the equilibrium condition (reduced sediment supply). The aeolian parameters (based on the grain size and wind velocities) for PMs indicate high shear velocity (~23 to 48 cm−1) and high transport rate (< 0.1 to 0.2 g/cm s). The short average flying distance implies that the biogenic sands were transported in multiple pulses (from coast to inland) at rates varying from ~200 m/h to ~1 km/h. Comparing our data with related occurrences along the biologically productive coasts in the mid-latitudinal belt indicates that the regional/global aridity during the lowered sea level was responsible for inland occurrences of aeolinites, particularly during the LGM. The strengthened northeast monsoon winds during the LGM was conducive to upwelling and production of biogenic carbonates along the coasts. The subsequent reworking of the aeolinites were mainly the results of local variability in precipitation.  相似文献   

15.
Flow, sediment transport and bed deformation in alluvial rivers normally exhibit multiple time scales. Enhanced knowledge of the time scales can facilitate better approaches to the understanding of the fluvial processes. Yet prior studies of the time scales are based upon the concept of sediment transport capacity at low concentrations, which however is not generally applicable. This paper presents new formulations of the time scales of fluvial flow, suspended sediment transport and bed deformation, under the framework of shallow water hydrodynamics, non-capacity sediment transport and the theory of characteristics for the hyperbolic governing equations. The time scale of bed deformation in relation to that of flow depth is demonstrated to delimit the applicability region of mathematical river models, and the time scale of suspended sediment transport relative to that of the pertinent flow information is analyzed to address if the concept of sediment transport capacity is applicable. For shallow flows with high sediment concentrations, bed deformation may considerably affect the flow and a fully coupled model is normally required. In contrast, for deep flows at low sediment concentrations, a decoupled model is mostly justified. This pilot study of the time scales delivers a new theoretical basis, on which the interaction between flow, suspended sediment transport and bed deformation can be potentially better characterized.  相似文献   

16.
Ferromagnetic iron sulfides have been identified by thermomagnetic, X-ray crystallographic and reflection-microscopic analyses of deep-sea sediment cores taken from the Sea of Japan. Occurrence of iron sulfides at certain depths of the cores seems to indicate that the paleoenvironment of the deep basin in the Sea of Japan was very stagnant, probably due to the enclosure by lands caused by the lowering of the sea level during the glacial periods. Oxidation of sulfides as well as sulfuration of oxides in the sedimentary layer may account for the abnormally high heat flow in the deep basin.  相似文献   

17.
Summary Considerations concerning the distribution of artificial radioactive substances in the troposphere point to the occurrence of stratospheric-tropospheric exchange in the region of the discontinuity between the tropical and middle latitude tropopause. Computations of the horizontal flux of ozone in the lower stratosphere and examination of the distribution of the tracer tungsten 185 in the stratosphere suggest that the trace substances are transported northwards from the tropical stratosphere by the action of transient eddy processes in which northward moving parcels of air are sinking and southward moving parcels are rising. The amount of ozone transported varies seasonally and is a maximum in the late winter and spring. The total transport appears sufficient to account for the observed spring build-up of ozone in middle and high latitudes. The eddies invoked are shown to be consistent with the observed countergradient transport of heat in the lower stratosphere, the pattern of stratospheric isentropes and the covariance values found from meridional and vertical velocities. The combined results indicate a new view concerning the mechanics of stratospheric motions as contrasted with the classical ideas of mean meridional motions.
Zusammenfassung Erwägungen im Hinblick auf die Verteilung künstlicher radio-aktiver Bestandteile in der Troposphäre deuten auf das Auftreten von stratosphärischem-troposphärischem Austausch in der Diskontinuäts-Zone zwischen der tropischen und mittleren Breite in der «Tropopause». Berechnungen des horizontalen Einfliessens des Ozons in der niedrigeren Stratosphäre und Untersuchung der Verteilung des Spurmaterials Wolfram 185 in der Stratosphäre deuten darauf hin, dass die Spuren-Bestandteile nordwärts transportiert werden von der tropischen Stratosphäre durch die Wirkung vorübergehender Wirbel-Prozesse; die Luftmassen, die sich in nördlicher Richtung bewegen, sinken, die in südlicher Bewegung, steigen. Der transportierte Ozon-Gehalt wechselt mit der Jahreszeit, ist maximal spät in Winter und Frühjahr. Der gesamte Transport erscheint genügend, um den beobachteten Frühjahrs-Zuwachs von Ozon in den mittleren und höheren Breitegraden zu rechtfertingen. Die in Betracht kommenden Wirbel scheinen in Übereinstimmung zu sein mit dem beobachtetem Hitze-Transport in der niedrigen Stratosphäre entgegen dem Aufstieg, mit dem üblichen Verlauf der stratosphärischen Isotropen und den Werten, die für die Co-Varianten gefunden wurden von meridionalen und vertikalen Geschwindigkeiten. Die kombinierten Resultate geben einen neuen Einblick in den Mechanismus der strato-sphärischen Strömungen in Kontrast zu den klassischen Ideen einer durchschnittlichen meridionalen Strömung.

Résumé Des considérations concernant la distribution des substances radio-actives artificielles dans la troposphère indiquent l'occurrence d'un échange strato-sphérique-troposphérique dans la région de discontinuité entre les latitudes tropicales et centrales de la tropopause. Des computations du flux horizontal d'ozone dans la basse stratosphère et l'examen de la distribution du traceur tungstène 185 dans la stratosphère suggèrent que les trace-substances soient transportées vers le nord de la stratosphère tropicale par l'action des procès de tourbillon transitoire dans lequel les parcelles d'air mouvant vers le nord s'enfoncent et les parcelles mouvant vers le sud s'élèvent. Le total d'ozone transporté varie periodiquement et devient maximum pendant la fin de l'hiver et du printemps. Le transport total paraît suffisant à justifier l'élévation de l'ozone observée au printemps aux latitudes moyennes et supérieures. Les tourbillons invoqués se montrent consistant au contregradient du transport de chaleur observé dans la basse stratosphère, le modèle des isentropes et les valeurs covariées se trouvent dependre des vitesses méridionales et verticales. Les résultats combinés indiquent une nouvelle vue sur les mécaniques des mouvements stratosphériques contrastant des idées classiques aux mouvements moyens méridionaux.


The research reported in this article was sponsored by the Atomic Energy Commission under Contract No. AT (30-1) 2241 and by the Geophysics Research Directorate, Air Force Cambridge Research Center, under Contract No. AF 19 (604) 5223.  相似文献   

18.
A cross-sectional nonhydrostatic model using idealized sill topography is used to examine the influence of bottom friction upon unsteady lee wave generation and flow in the region of a sill. The implications of changes in shear and lee wave intensity in terms of local mixing are also considered. Motion is induced by a barotropic tidal flow which produces a hydraulic transition, associated with which are convective overturning cells, wave breaking, and unsteady lee waves that give rise to mixing on the lee side of the sill. Calculations show that, as bottom friction is increased, current profiles on the shallow sill crest develop a highly sheared bottom boundary layer. This enhanced current shear changes the downwelling of isotherms downstream of the sill with an associated increase in the hydraulic transition, wave breaking, and convective mixing in the upper part of the water column. Both short and longer time calculations with wide and narrow sills for a number of sill depths and buoyancy frequencies confirm that increasing bottom friction modifies the flow and unsteady lee wave distribution on the downstream side of a sill. Associated with this increase in bottom friction coefficient, there is increased mixing in the upper part of the water column with an associated decrease in the vertical temperature gradient. However, this increase in mixing and decrease in temperature gradient in the upper part of the water column is very different from the conventional change in near-bed temperature gradient produced by increased bottom mixing that occurs in shallow sea regions as the bottom drag coefficient is increased.  相似文献   

19.
Event sediment transport and yield were studied for 45 events in the upstream part of the 260 km2 agricultural Koga catchment that drains to an irrigation reservoir. Discharge and turbidity data were collected over a period of more than a year, accompanied by grab sampling. Turbidity was very well correlated with the sediment concentrations from the samples (r = 0.99), which allowed us to estimate the temporal patterns of sediment concentrations within events. The hysteresis patterns between discharge and sediment concentrations were analysed to provide insight into the different sediment sources. Anticlockwise patterns are the dominant hysteresis patterns in the area, suggesting smaller contributions of suspended sediment from the river channels than from the hillslopes and agricultural areas. Complicated types of hysteresis patterns were mostly observed for long events with multiple peaks. For a given discharge, sediment yields in August and September, when the catchment was almost completely covered with vegetation, were much smaller than during the rest of the rainy season. The hysteresis patterns and timing suggest that the sediment availability from the agricultural areas and hillslopes affects sediment yields more strongly than does peak discharge. Two distinct types of sediment rating curves were observed for the season when the agricultural land was covered with vegetation and when it was not, indicating the dominating contribution of land use/cover to sediment yields in the catchment. The rate of suspended sediment transport in the area was estimated as 25.6 t year?1 ha?1. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

20.
It was observed that in some closed inland lakes sediment transport was dominated by wind-induced currents, and the sediment resuspension was primarily driven by wind-induced waves. This paper presents the development and application of a three-dimensional numerical model for simulating cohesive sediment transport in water bodies where wind-induced currents and waves are important. In the model, the bottom shear stresses induced by currents and waves were calculated, and the processes of resuspension (erosion), deposition, settling, etc. were considered. This model was first verified by a simple test case consisting of the movement of a non-conservative tracer in a prismatic channel with uniform flow, and the model output agreed well with the analytical solution. Then it was applied to Deep Hollow Lake, a small oxbow lake in Mississippi. Simulated sediment concentrations were compared with available field observations, with generally good agreement. The transport and resuspension processes of cohesive sediment due to wind-induced current and wave in Deep Hollow Lake were also discussed.  相似文献   

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