Sediment transport in the mouth of the Eden estuary

Observations of water movement and sediment transport were made from a fixed platform at an intertidal location in the mouth of the Eden estuary. The effects of wave activity on water movement and suspended sediment fluxes were evaluated, and an empirical relationship between suspended sediment concentrations and near bed water movement is described. The circulation of water in St Andrews Bay indicates a residual movement onshore near to the bed. Onshore movement of sediment in the mouth of the estuary is indicated by suspended sediment fluxes and bedload transport (determined from bedform migration). The relative roles of suspended sediment and bedload transport are discussed. Morphological changes in the estuary mouth over the three-year survey period are relatively small and indicate a stable environment.     

The effect of bedform dynamics on computing suspended sediment fluxes using optical backscatter sensors and current meters

The sensitivity of the suspended sediment flux is tested with respect to rapid changes in bed-level across the surf zone of a sandy beach. The suspended flux was computed using a fixed instrument array, but bed-level changes due to ripple migration caused the instrument elevations to be significantly changed during the course of the experiment. The nominal elevations of the instruments were adjusted during data processing (using the MOBS array) to maintain a fixed elevation with respect to bed-level changes. The resultant suspended sediment concentrations and fluxes were significantly different from the unadjusted data, and for the present data set O(35%) less when averaged over the tide. The maximum difference between adjusted and unadjusted fluxes may be O(260%). The results indicate that changes in bed-level, particularly those due to bedform migration, must be accounted for when processing OBS data if reliable estimates of suspended sediment transport are to be obtained in the field.     

渤海湾连片开发对湾内水沙通量的影响研究

渤海湾内海岸的连续开发导致岸线 、海床发生较大变化, 同时影响着湾内的水沙通量。根据不同时期的遥感影像、 实测地形和水文泥沙资料, 统计分析了渤海湾岸线 、面积和海床冲淤变化, 构建了渤海潮流泥沙数学模型, 模拟了 1984 年、 2006 年和 2015 年三个时期的水沙分布, 探究了海岸连续开发对水沙分布和通量引起的累积效应。结果表明： 渤海湾岸线和 海湾面积变化主要发生于 2005 年后, 与 1984 年相比, 2020 年的岸线长度增长超过 185%,海湾面积减少近 19%;曹妃甸港 区南侧海域冲刷基本在 2 m 等深线以内, 而近岸和港池水域基本呈现淤积状态, 淤积幅度在 2 m 以内; 海湾的连续开发利用 使得湾内分潮波振幅增大 、传播速度减缓, 近岸海域的余流变化较为明显,南部较北部海域更甚;西北湾顶 0.2 kg/m3 悬沙 分布区域不断缩小, 西南近岸 0. 15 kg/m3 悬沙分布区域向中部海域推进; 悬沙通量变化与潮流通量并不完全一致, 呈外海增 加、近岸整体降低的变化特征, 湾内向外海输移泥沙的能力减弱。     

潮汐河口断面悬沙通量组分模式及其在长江口的应用

断面泥沙通量估算的误差主要来源于计算方法、测点布局等,通量模式应该建立在通量估算统计误差最小的原则基础之上.在断面网格设计中采用统计误差最小的等面积单元网格,在泥沙通量估算中采用泥沙组分浓度,在流速变量插值上垂向采用对数函数插值,横向采用第一边界三次方样条函数插值.这样建立的潮汐河口悬沙断面通量组分模式较以往的任何模式更完善,断面通量估算的误差最小.将该模式应用于长江河口南港断面悬沙通量估算及其输移机制分析,断面泥沙通量表现为大潮期大进大出、大出大于大进;小潮期小进小出、小出大于小进;主要输移机制是拉格朗日输移和潮泵.     

南黄海辐射沙脊群潮汐水道的悬沙输运特征

根据南黄海辐射沙脊群定点站位的流速和浊度数据,利用通量分解方法,分析了潮汐水道的悬沙输运特征和输运机制。结果表明,辐射沙脊群海域潮汐水道中潮流为往复流,悬沙浓度较高,属于强潮流控制的悬沙浓度相对较高的陆架浅海环境;悬沙输运主要受欧拉余流和潮汐捕捉效应控制,再悬浮的沉积物通过平流作用进行输运。其中,陈家坞槽和西洋水道的悬沙输运以潮汐捕捉效应占优,沉积物向潮汐水道外输运,处于冲刷状态;苦水洋水道以欧拉余流输运为主,水道内的再悬浮的沉积物在强潮流作用下向陆输运,主要堆积在蒋家沙和西洋西侧岸滩等浅滩和潮间带上。由此可见,在陆源物质供应减少背景下,南黄海辐射沙脊群内部的物质充当了新的物源,短期内能够维持近岸潮滩和沙脊的持续增长。     

三门湾的悬沙分布和输移特性

本文根据实测资料,通过悬沙浓度ρ、流速v和水深H的对应分析,得出了三门湾的悬沙浓度分布遵循着ρ∝f(v~2/H)的规律;同时发现湾外海域有着良好的将悬沙输移入湾的动力条件,是三门湾悬沙来源的主要场所;经全潮单宽净输沙量计算表明:进出海域的悬沙净输移量不大,全湾的悬沙输移具有深槽输向边滩、南进北出的趋势,与口外海域的泥沙存在着活跃的交换。通过摩阻流速计算得到三门湾变化的总趋势是:深槽略受冲刷,滩涂缓慢淤涨,入湾通道处于动态平衡中。     

Suspended sediment transport of the Hangzhou Bay and its related hydrodynamic analyses

The 25-h measurements of current speed, flow direction, water depth, suspended sediment concentration and salinity were carried out at six anchored stations in the study area during spring and neap tides in winter of 1987 and summer of 1989. Caculations and analyses of the data obtained show that large amounts of suspended sediments are moved back and forth under the action of tidal current, and the net transport of sediment is small, with its predominance upstream in winter and downstream in summer. These calculations and analyses also suggest that the advective transport of sediment is dominant, while the vertical gravitational circulation of the suspended sediment comes next. Meantime, it is indicated that tidal currents play a major role in the suspended sediment transport, and residual flows have effect on the net transport of the suspended sediment, which is more remarkable during neap tide than during spring tide.     

Observed fluxes of water,salt and suspended sediment in a partly mixed estuary

Observations of the residual fluxes of water, salt and suspended sediment are presented for seven stations along the Tamar Estuary. The data include measurements over single spring and neap tidal cycles, and are generally applicable to medium or high run-off conditions.Surface to bed differences in salinity are typically of the order of several parts per thousand. Gravitational circulation is an important component of residual flow in the deep, lower reaches of the estuary. Here, Stokes drift is insignificant. In the shallow upper reaches, the major residual currents are generated by Stokes drift and freshwater inputs. Data are compared with predictions from Hansen and Rattray's (1966) model of estuarine circulation.Salt fluxes due to tidal pumping and vertical shear are directed up-estuary at spring tides, tidal pumping being dominant. Tidal pumping of salt is also directed up-estuary at neap tides, although it is insignificant in the lower reaches, where vertical shear dominates.Tidal pumping of suspended sediment is directed up-estuary near the head at spring tides, and probably contributes to the formation of the turbidity maximum. The existence of the turbidity maximum is predicted using a simplified model of the transport of water and sediment. The model shows that an additional mechanism for the existence of the turbidity maximum is an up-estuary maximum in the tidal current speeds (and thus resuspension). In the lower reaches, transport of suspended sediment is directed down-estuary at both spring and neap tides, and sediment is essentially flushed to sea with the fresh water.     

江苏大丰潮滩悬沙级配特征及其动力响应

根据2002和2003年夏季在江苏大丰潮滩的现场观测资料,详细分析了悬沙级配的时空分布特征、影响因素及其对再悬浮、沉降和流速的响应.研究结果表明,悬沙颗粒较细,以粉砂为主,悬沙级配在潮周期内的变化模式有两种类型:一是稳定型,悬沙级配的时空(垂向和平面)变化很小;二是双峰型,悬沙级配的时空变化显著,粗细峰高度不断变化.再悬浮、沉降、涨潮时输入潮滩的悬沙和底质级配是影响悬沙级配的重要因子.再悬浮使粗颗粒悬沙的含量增加,悬沙与底质级配不断接近,沉降对悬沙级配的影响与再悬浮相反.再悬浮发生时悬沙级配对流速有明显响应.在没有再悬浮和沉降影响的情况下,潮滩不同部位、不同时间的悬沙级配趋于稳定和相同,对这种状态下的悬沙级配可称为背景悬沙级配,大丰潮滩背景悬沙级配的平均粒径为7μm.     

An evaluation of two alongshore transport equations with field measurements

The performance of two well-known equations to predict the depth-averaged alongshore suspended sediment flux [Van Rijn, L.C., 1984. Sediment transport, part II: suspended load transport. Journal of Hydraulic Engineering 110, 1613–1641; and Bailard, J.A., 1981. An energetics total load sediment transport model for a plane sloping beach. Journal of Geophysical Research 86, 10938–10954] was assessed by comparing predictions with 2306 field estimates based on a vertical stack of three optical backscatter sensors and a single electromagnetic flow meter. The observations were collected at four cross-shore positions on the intertidal beach of Egmond aan Zee, the Netherlands, during calm to storm conditions, with the offshore significant wave height peaking at 3.7 m. Measured hydrodynamics were employed in the computations of both models. Also, default parameter values were used without calibration to the data. We found that both models underpredicted the observations. Overall, the Van Rijn model outperformed the Bailard model, with about 70% of the model prediction lying between 1/5 to 5 of the observations under energetic conditions. For the Bailard model this was only about 20%. The performance of the Van Rijn model is, however, sensitive to the wave-related roughness, one of its highly uncertain free parameters. This may allow for an easy calibration when estimates of the depth-averaged alongshore sediment flux are available but may lead to serious errors in situations without data to constrain the predictions. We suspect that the discrepancy between the observations and model predictions is due to an overestimation of the observed fluxes (high turbidity, air bubbles) and an underestimation of the modeled fluxes because of missing physics related primarily to breaking waves.     

Measurements and modelling of the advection of suspended sediment in the swash zone by solitary waves

Novel laboratory experiments and numerical modelling have been performed to study the advection scales of suspended sediment in the swash zone. An experiment was designed specifically to measure only the sediment picked up seaward of the swash zone and during bore collapse. The advection scales and settling of this sediment were measured during the uprush along a rigid sediment-free beach face by a sediment trap located at varying cross-shore positions. Measurements were made using a number of repeated solitary broken waves or bores. Approximately 25% of the pre-suspended sediment picked up by the bores reaches the mid-swash zone (50% of the horizontal run-up distance), indicating the importance of the sediment advection in the lower swash zone. The pre-suspended sediment is sourced from a region seaward of the shoreline (still water line) which has a width of about 20% of the run-up distance. An Eulerian–Lagrangian numerical model is used to model the advection scales of the suspended sediment. The model resolves the hydrodynamics by solving the non-linear shallow water equations in an Eulerian framework and then solves the advection–diffusion equation for turbulence and suspended sediment in a Lagrangian framework. The model provides good estimates of the measured mass and distribution of sediment advected up the beach face. The results suggest that the correct modelling of turbulence generation prior to and during bore collapse and the advection of the turbulent kinetic energy into the lower swash is important in resolving the contribution of pre-suspended sediment to the net sediment transport in the swash zone.     

青岛近海悬沙输运的三维数值模拟

研究近海海域水流、悬沙运动规律,运用基于COHERENS发展的水动力悬沙模型COHERENS-SED,结合当地一般波浪条件,模拟了青岛近海悬浮泥沙输运情况,并验证及分析了水动力环境及悬沙输运的模拟结果。结果表明：在波流合作用下,近岸掀沙明显,大潮期间该海域近岸悬沙浓度值最高可达50mg／L。     

基于小波变换的西、北江水沙关系特征分析

对珠江高要、石角站近50年(1957-2000年)流量、含沙量和输沙量月均时间序列利用复Morlet小波变换,分析水沙序列的多时间尺度变化特征,并在流量的特征时间尺度上分段建立相应的水沙关系,结果显示:(1)两站水沙时间序列具有多时间尺度特征,年内洪枯季周期最为显著,水沙丰枯变化不同期.流量的长周期振荡特征较含沙量与输沙量明显,中时间尺度振荡在20世纪70年代有明显的突变点,石角站突变点略滞后于高要站.高要站输沙量明显多于石角站,在20世纪80年代之后石角站输沙量明显减少;(2)特征时间尺度上水沙关系存在丰枯变化.水沙关系公式Cs=aQb中,系数a值变化反映出少水期泥沙的供给量较小,指数b值显示少水期水流的挟沙能力强.两个多水时段(1992-2000年和1957-1974年)水沙关系近似平行下移,说明水流的挟沙能力相近,但泥沙供给量存在一定差别,前一个多水时段泥沙供应量小于后一多水时段,明显的人类活动是主要原因.整体时段的水沙关系曲线只能代表多水期的水流挟沙特征,而不能代表少水期的水沙关系.     

Grain-size distribution patterns of suspended sediment in response to hydrodynamics on the Dafeng intertidal flat, Jiangsu, China

1Introduction The sediment dynamics of intertidal flats in-volves erosion, transport, and settling processes. A-mong these, the mechanisms of sediment transportare a key question. Progress has been made in awide range of associated areas, including sedime…     

Hydrodynamic control of the supply of reworked terrigenous sediment to coral reefs in the Bay of Banten (NW Java, Indonesia)

The Bay of Banten is an example of a shallow-water reef environment adjacent to an eroding delta system, where corals survive in turbid conditions. This study investigated the hydrodynamic controls over residual fluxes of terrigenous sediment in the bay. Observations of wind, waves, currents and suspended sediment concentrations in 1998 and 1999 revealed contrasting dynamic turbidity conditions influenced by tides, monsoon-driven flows and locally generated waves. The most sediment-rich suspensions originate from the shallow coastal margin of an inactive delta, where waves resuspend sediment and small creeks discharge freshwater and sediment. Because of coupling between monsoonal wind, throughflow in the bay and wave height, when wave-induced resuspension peaks seasonally during the northwest monsoon, an eastward throughflow prevents the nearshore reefs in the bay from being directly exposed to turbid water masses generated in the eroding delta. In inshore waters, tidal asymmetry and the spatial variation of current amplitudes cause residual sediment transport, largely depending on the availability of erodible sediment. In the center of the bay, where there is a patch reef complex, the tidal and subtidal currents are weak and therefore here there is a zone where sediment accumulates.     

强潮环境下悬沙对底部边界层的影响

对杭州湾金山深槽附近两个定点站位的大潮期间同步水文、泥沙观测结果进行了粉砂分布区再悬浮效应的分析,提出了根据再悬浮过程反演底床切应力的新方法。研究结果表明,在强潮动力、高悬沙质量浓度环境下,即使无密度成层性,悬沙质量浓度对底部边界层的影响仍然存在,表现为底部切应力的减小。在这一条件下,如仍然采用卡门-普朗特模型(κ=0.4),则估算的底部切应力将大大高于实际的数值。     

Deep slope currents and suspended particle fluxes in and around the Foix submarine canyon (NW Mediterranean)

Deep slope currents and particulate matter concentrations were studied on the Barcelona continental margin in and around the Foix submarine canyon from May 1993 to April 1994. This year-long moored experiment revealed that near-bottom slope currents are strongly influenced by the bottom topography, being oriented along isobaths and along the canyon axis. The deep slope current fluctuations are controlled by the local inertial motion (18.3 h) and also by low-frequency oscillations at periods of 6–10 days, related to the passage of atmospheric pressure cells. Particulate matter concentrations recorded during the experiment do not show a clear seasonal variability, except outside the canyon, where significant peaks of particulate matter concentrations were recorded only during the winter-fall deployment. In addition, the temporal evolution of suspended particulate matter concentration is not linked to changes in the cross-slope or along-slope current components and did not show a clear relationship with river avenues or wave storm events. This suggests that suspended particulate matter exported from the shelf is dispersed on the slope by advective processes, which attenuate the signal of the shelf-slope sediment transfer. Mean particulate matter concentrations differed among sampling sites, but the magnitude of the mean horizontal suspended particle flux reflects a quite similar value in the whole study area, ranging from 2.53 to 4.05 mg m⁻² s⁻¹. These horizontal suspended particle fluxes are 27 (canyon head) to 360 (open slope) times higher than the settling particle fluxes measured at the same sampling sites, indicating that the suspended particulate transport on the Barcelona continental slope dominates over the settling particle fluxes, even inside the Foix submarine canyon.     

秋季东海内陆架悬浮体分布特征

秋季是处于夏、冬季之交的过渡季节,夏季风减弱、冬季风增强的背景下海洋水文泥沙环境有独特特征,关于该时期悬浮体分布特征的研究较少.本文基于东海内陆架2016、2017与2018年秋季悬浮体及温盐数据,分析该海域秋季悬浮体浓度的水平和垂向分布特征.结果表明:悬浮体浓度近岸高于远岸,浓度等值线大致平行于等深线,近底层悬浮体浓...     

Tidal asymmetry in suspended sand transport on a macrotidal intermediate beach

Time-series of nearbed horizontal flow velocities and suspended sediment concentrations obtained from a colocated electromagnetic current meter (EMCM) and optical backscatter sensor (OBS), respectively, are used to examine the relative importance of steady and fluctuating components to the total sediment transport over a full tidal cycle on a macrotidal, intermediate beach (Spurn Head, UK). Fluctuating sediment fluxes are decomposed into gravity and infragravity contributions using co-spectral techniques. The relative importance of the oscillatory (gravity and infragravity) and steady (mean) transport components to the total sediment transport is analysed throughout the tidal cycle.

A continuum of 34 discrete suspended sediment-cross-shore velocity co-spectra are computed over a full tidal cycle for the OBS and EMCM measurements 0.10 m above the bed. These net transport spectra vary greatly both with cross-shore location and tidal state. In particular, a marked asymmetry in transport processes is evident between the flood and ebb tides, with high levels of sediment resuspension and transport occurring on the ebbing tide approximately two hours after high water (just seaward of the breakpoint). At this time the dominant transport was directed offshore (co-spectral peak, 0.04 kg/m²/s) at incident wave frequency.

Typical patterns are observed in transport spectra outside the surf zone and within the inner surf zone. Outside the narrow surf zone cross-shore transport spectra show weak offshore transport (co-spectral peak = 0.002 kg/m²/s) associated with bound long waves and stronger onshore transport (co-spectral peak = 0.006 kg/m²/s) at incident wave frequencies. Conversely, co-spectra computed within the inner surf zone show the offshore sediment fluxes (spectral peak = 0.010 kg/m²/s) at infragravity frequencies to be greater in magnitude than the corresponding onshore transport (co-spectral peak = 0.008 kg/m²/s) occurring at incident wave frequencies.     

Particle flux across the mid-European continental margin

Results are presented from particle flux studies using sediment trap and current meter moorings along a transect at the European continental margin at 49°N within the EU-funded Ocean Margin Exchange (OMEX) project. Two moorings were placed, at the mid- and outer slope in water depths of 1500 and 3660 m, with traps at 600 and 1050 m and at 580, 1440 and 3220 m, respectively. Residual currents at the mid-slope follow the slope contour, whereas seasonal off-slope flow was registered at the outer slope. At 600 m on the slope fluxes are similar to those in the abyssal North Atlantic. The flux of all components (bulk dry weight, particulate organic and inorganic carbon, lithogenic matter and opal) increased with water depth. Highest fluxes were recorded at 1440 m at the outer slope, where off-slope residual currents mediate particle export. The injection of biogenic and lithogenic particles below the depth of winter mixing results in the export of particles from shallower waters. Calculated lateral fluxes of particulate organic carbon exceed the primary flux by over a factor of 2 at 1440 m on the outer slope. Estimated lateral fluxes of suspended particulate matter in the water column and intermediate nepheloid layers at the outer slope are potentially large compared to sinking fluxes measured by sediment traps. A comparison is made of particle flux at three continental margin sites and two sites in the adjacent open North Atlantic, from which it is seen that bulk and organic matter flux increases exponentially with proximity to the shelf break. The percentage contribution of particulate organic carbon to biogenic fluxes increases from a mean of 5.7% in the abyssal N. Atlantic to 13.9% at the continental margins.