Tidal and river discharge forcing upon water and sediment circulation at a rock-bound estuary (Guadiana estuary,Portugal)

The relative impacts of tidal (neap, spring) and river discharge (including a flood event) forcing upon water and sediment circulation have been examined at the rock-bound Guadiana estuary. Near-bed and vertical profiles of current, salinity, turbidity, plus surface suspended sediment concentrations (SSC, at some stations only), were collected at the lower and central/upper estuary during tidal and fortnightly cycles. In addition, vertical salinity and turbidity profiles were collected around high and low water along the estuary. Tidal asymmetry produced faster currents on the ebb than on the flood, especially at the mouth. This pattern of seaward current dominance was enhanced with increasing river flow, due to horizontal advection that was confined within the narrow estuarine channel. The freshwater inputs and, at a degree less, the tidal range controlled the vertical mixing and stratification importance. Well-mixed (spring) and partially stratified (neap) conditions alternated during periods of low river flows, with significant intratidal variations induced by tidal straining (especially at the partially stratified estuary). Highly stratified conditions developed with increasing river discharge. Intratidal variability in the pycnocline depth and thickness resulted from current shear during the ebb. A salt wedge with tidal motion was observed at the lower estuary during the flood event. Depending on the intensity of turbulent mixing, the residual water circulation was dominantly controlled either by tidal asymmetry or gravitational circulation. The SSC was governed by cyclical local processes (resuspension, deposition, mixing, advection) driven by the neap-spring fluctuations in tidal current velocities. More, intratidal variability in stratification indicated the significance of tidal pumping at the partially and highly stratified estuary. The estuary turbidity maximum (ETM) was enhanced with increasing current velocities, and displaced downstream during periods of high river discharge. During the flood event, the ETM was expelled out of the estuary, and the SSC along the estuary was controlled by the sediment load from the drainage basin. Under these highly variable river flow conditions, our observations suggest that sand is exported to the nearshore over the long-term (>years).     

Composition and fluxes of particulate organic matter in a temperate estuary (Winyah Bay, South Carolina, USA) under contrasting physical forcings

To understand the role that physical processes play on the biogeochemical cycles of estuaries, we conducted intense field studies of the turbidity maximum region within a partially mixed estuary (Winyah Bay, SC, USA) under contrasting conditions of river discharge, tides and wind. Water samples and hydrographic data were collected at different depths and locations along the main channel over several tidal cycles during several cruises to Winyah Bay. Tidal variations in current speed, salinity, total suspended solid concentrations were measured within each cruise and were consistent with estuarine circulation processes. Salinity and total suspended solid concentrations ranged from 0 to 32 and from 20 to over 500 mg L⁻¹, respectively, with the highest salinity and total suspended solid values measured during periods of low river discharge. In fact, comparison of tidally averaged salinity and total suspended solid concentrations revealed marked differences among cruises that were negatively correlated to river discharge and SW wind speed. Moreover, significant contrasts in the chemical compositions of suspended particles were evident among periods of contrasting river discharge and wind regime. For example, the weight percent organic carbon content of suspended particles ranged from 1 to over 6% and displayed a positive correlation with river discharge. Similarly, both the molar carbon to nitrogen ratios (10 to 20 mol:mol) and stable carbon isotopic compositions (−25 to −29%) of the suspended organic matter varied significantly as a function of discharge and wind. Such trends indicate that in Winyah Bay low river discharge and steady SW winds promote resuspension of bed sediments from shallow regions of the estuary. These materials contain highly altered organic matter and their incorporation into the water column leads to the observed trends in suspended particle concentrations and compositions. Furthermore, these conditions result in net landward fluxes of salt, sediment and particulate organic matter throughout most of the water column, promoting efficient trapping of materials within the estuary. Our results illustrate the fundamental connection between physical forcings, such as discharge and wind, sediment transport processes and the cycling of biogeochemical materials in estuarine environments.     

1982和2012年枯季长江口最大浑浊带悬浮泥沙和盐度垂向剖面特征对比

2012年1月在长江口北港、北槽和南槽水域纵断面开展枯季多船准同步观测,将获得的大小潮悬浮泥沙和盐度数据与1982年12月同水域调查结果进行对比分析。结果表明:2012年长江口最大浑浊带枯季悬沙浓度比1982年减小了约50%;北港、北槽、南槽相近测点的大潮垂向平均悬沙浓度相较于1982年分别减小了43%、60%和40%,2012年长江口表层平均悬沙浓度与1982年相比减少了约53%。北港断面浑浊带核心与1982年浑浊带核心位置相近;北槽浑浊带核心向内迁移;南槽浑浊带核心位置向外迁移。2012年与1982年枯季遥感反演的长江口同水域表层悬浮泥沙浓度也明显降低。在30年来入海泥沙持续减少背景下,长江口3条入海主汊的最大浑浊带特征依旧显著,径流与潮流的此消彼长、径流的季节分配不同以及口内汊道分流分沙比的变化影响了长江口最大浑浊带核心的移动,浑浊带悬沙浓度最高的地段也是盐度梯度最高的地区。     

Sedimentary processes on an intertidal mudflat in the upper macrotidal Seine estuary, France

In order to understand the hydrodynamic parameters that control the fluvial sediment dynamics on an intertidal mudflat located in a sheltered zone in the upper part (fluvial part) of the macrotidal Seine estuary (France), a two-year field study of high-frequency field measurements was carried out. The bed-level evolution of the mudflat surface was measured from the semi-diurnal period to annual time scales using a high-resolution altimeter. The data showed that the sedimentary patterns on the mudflat were mainly controlled by river flows and tides. During high river flows in winter, sedimentation dominated; suspended particulate matter concentrations were higher, submersion was constant and at semi-diurnal scale, sedimentation duration was more important than erosion due to an asymmetrical tide. By contrast during low river flows in summer, erosion dominated mainly as a result of immersion/emersion of tidal flats during semi-diurnal cycle. From this annual sedimentation–erosion cycle we identify a temporary storage of 10–30% of the fine-grained (<63 μm) river-borne particles on mudflats in the upper section of the fluvial Seine estuary during high river flows.River-related sediment fluxes were estimated from the measurement of fine-grained sedimentation zones in the fluvial part of the estuary. The erosion/sedimentation processes were perennial, and the amounts of contributing sediments were directly related to the solid river load. Our results indicate that mudflats in the fluvial part of the Seine estuary play an important role in the downstream transfer of fine-grained suspended particulate matter (SPM) towards the turbidity maximum and the Rouen docks particularly during low river flows, when roughly 30–50% of the SPM originates from the eroded intertidal flats.     

椒江河口层化动力特性研究

基于椒江河口大、小潮期间水位、流速、盐度和悬沙浓度观测数据,研究了椒江河口主潮汐通道的水动力、盐度和悬沙浓度的时空变化特征,解释了高浊度强潮作用下的层化物理机制。椒江河口大潮期悬沙浓度和盐度均大于小潮期,主潮汐通道区域落潮期悬沙浓度大于涨潮期;盐度随潮变化,盐水锋面出现在S2测站,锋面附近出现最大浑浊带;自陆向海,悬沙浓度递减,盐度递增;随水深增加,悬沙浓度与盐度递增。Richardson数与混合参数显示,盐度和悬沙引起的层化现象,是随着潮汐的变化而变化,涨潮时的层化均强于落潮,小潮时的层化持续时间最长,区域更广。混合参数随潮周期变化,大潮期高于临界值1.0,小潮期低于临界值1.0。小潮期水体层化强于大潮期;潮汐应变项是影响势能差异变化率的重要因素;落潮期间层化向混合状态转化,涨潮相反。     

黄河口悬浮泥沙时空动态及其驱动机制

受径流输沙、风浪、潮汐等影响,河口近岸海域悬浮泥沙具有显著的时空变化。本文基于小时分辨率的GOCI遥感影像,利用最优遥感反演算法,结合空间分析和统计方法,深入研究黄河口及邻近海域悬沙时空动态特征及驱动机制。结果表明,径流输沙对悬沙浓度的影响以河口近岸区为主,高径流输沙对悬沙浓度分布影响可达距岸约20 km,并向孤东近岸方向扩散。大风可引起清水沟老河口区泥沙强烈再悬浮,形成高浓度悬沙区。涨落潮对小时尺度的悬沙浓度影响显著,并影响悬沙的南北扩散。大潮悬沙浓度变化和扩散范围均大于小潮,潮流流速不同是造成该差异的主要原因。水深与悬沙浓度之间存在较明显的负相关关系,根据不同驱动因素的差异,悬沙浓度随着水深的增加呈现出指数型、幂函数型、线性3种关系。     

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.     

长江分汊河口涨、落潮悬沙不对称特征及季节性差异

入海河口由于径流的存在以及河口地貌形态的影响,存在涨、落潮水动力、悬沙以及盐度分布等不对称现象,同时这一不对称现象还存在显著的区域性和季节性差异。根据2013年7月和2014年1月洪、枯季长江口定点准同步水文泥沙调查结果,发现长江口分汊型河槽悬沙浓度在时间上存在洪枯季、大小潮不对称特征,在空间上存在东西向沿程分布、南北向横向分布以及垂向上表底层分布不对称特征。河势演变形成南、北支河口涨、落潮悬沙浓度不对称分布的整体格局;洪、枯季变化影响河口涨、落潮悬沙分布的再分配过程;大潮涨、落潮过程对悬沙分布不对称影响显著大于小潮;季节性风浪作用影响河口最大浑浊带涨、落潮悬沙不对称南北差异;底部高含沙浓度对口门段涨、落潮悬沙不对称性贡献显著。     

A modelling study of residence time in a macro-tidal estuary

This paper outlines a numerical modelling study to predict the average residence time of a conservative tracer in a macro-tidal estuary, namely the Mersey Estuary, UK. An integrated hydrodynamic-dispersion model was used to predict the average residence time in the estuary for various tidal level and freshwater discharge conditions. The numerical model was verified against six sets of field measured hydrodynamic data, with the model-predicted water elevations and salinity levels generally agreeing well with the field measurements. The numerical model results show that in the Mersey Estuary both the tidal level and river discharge affect significantly the predicted average residence time. The value of the average residence time is also shown to be closely linked to the intensity of the residual tidal current. This is due to the fact that a large proportion of the Upper and Inner Estuary dries out during low tides, thus a significant amount of the tracer material is transported through the deep channels. An increase in the freshwater discharge causes a considerable increase in the intensity of the residual current along the main channels and thus a reduction in the average residence time. The predicted overall tracer residence time for the whole estuary is relatively short for a relatively large estuary, ranging from less than 1 day to 4 days for various tidal level and freshwater flow combinations. When the tidal range and freshwater discharge are both small, then the local tracer residence time in the upper part of the estuary can be significantly longer than the values predicted for the middle and lower reaches of the estuary.     

Organic matter dynamics and budgets in the turbidity maximum zone of the Seine Estuary (France)

Organic matter was studied in the turbidity maximum zone (TMZ) of the Seine Estuary during 8 tidal cycles from April to October in 2001, 2002 and 2003, covering a salinity range from 0 to 27. The hydrological conditions were quite varied (extremely wet in 2001, unusually dry in 2003). A particularly striking feature is the high organic matter content in the suspended solids (SS) of the Seine estuary (4–5%).     

Suspended sediment transport from the Gironde estuary (France) onto the adjacent continental shelf

Current velocity and suspended sediment concentration measurements at anchor stations in the downstream extremity of the Gironde estuary indicate that during periods of high river discharge, a significant amount of suspended sediment is transported out of the estuary onto the adjacent continental shelf. The vertical profile of the residual (non-tidal) suspended sediment flux is similar to that of the residual current velocity, with a net upstream flux near the bottom and an overlying seaward-directed transport. The overall, depth-integrated result is a net seaward transport of suspended sediment out of the estuary. It appears that this net seaward transport varies directly with tidal amplitude.Aerial photography and water sampling indicate that during high river inflow, the downstream extremity of the turbidity maximum extends onto the continental shelf at ebb tide. The tidal and coastal current patterns of the inlet and inner shelf induce a northward transport of the turbid estuarine water, and at each tidal cycle, a certain amount of suspended sediment leaves the estuary; part of this sediment is deposited in a silt and clay zone on the continental shelf.     

辽河口海区悬浮体运移扩散动力特征

控制着辽河口海区悬浮体运移的主要动力是河流迳流、潮流和余流．它们对最大浑浊带中河水与海水的混合，密度流循环以及悬浮物的沉降和运移共同起作用．悬浮物向海运移与河口区最大浑浊带有关，即与河流迳流、潮流和余流作用所控制的悬浮物数量有关。悬浮物向海运移似乎遵循两个周期：１．小潮—大湖周期；２．河流迳流丰枯周期．最大运移发生在大潮期和河流迳流丰水期。     

河控型河口盐度层化对悬沙的捕集机制

河控型河口盐度混合和层化是控制悬沙输移扩散的重要动力机制。以珠江磨刀门河口为研究对象,基于2017年洪季三船同步大、小潮水文泥沙观测数据,分析河控型河口水体盐度层化结构的时空变化对悬沙分布的影响机制。结果表明:受径潮动力耦合时空变化影响,河口盐度垂向分布表现出时空差异,即受径流主导的M1站(挂锭角),河口盐度在涨落潮周期内垂向混合均匀,受径潮控制的M2站(口门)在整个潮周期内盐度层化结构明显,口门外侧的M3站,潮动力作用较强,盐度垂向分布随涨落潮变化而变化;悬沙空间分布与盐度分布关系密切,盐度混合均匀利于悬沙垂向均匀分布,而盐度层化则使悬沙倾向于滞留在底层水体中,且在盐度层结界面之下出现高悬沙浓度,悬沙浓度垂向分布曲线呈L字型或抛线型,纵向上表现为高浓度悬沙团抑制在盐水楔前端,盐度层化对悬沙的捕集效应明显。通过对比水体标准化分层系数与水流垂向扩散强度系数发现,两者呈现负相关关系,即标准化分层系数愈大,垂向扩散强度愈小,表明水体层化抑制悬沙垂向扩散强度,而且水体层化程度越高,悬沙垂向扩散抑制程度越大,进而促进了河口水体盐度层化对悬沙捕集作用。本研究有助于揭示河口细颗粒泥沙运动机制及河口拦门沙演变机制,并为磨刀门河口拦门沙治理提供科学依据。     

椒江河口高混浊水混合过程分析

根据１９９１年洪季的实测资料分析了高度浑浊的椒江河口的混合过程,并探讨了水动力学和沉积动力学因素对河口混合的重要作用,调查研究表明,椒江河口最大浑浊带下的高浑浊水－浮泥层厚达１ｍ,高浑浊水－浮泥层与上覆水之间是泥跃层,泥跃层与高混浊水－浮泥层对水体稳定的作用比同期观测到的盐跃层大１７倍以上,当高浑浊水－浮泥层被侵蚀时,在高浑浊水－浮泥层中的低盐水体又增加了水体的垂向混合能力。     

长江口悬沙动力特征与输运模式

本项研究用ADCP在长江河口进行高频、高分辨率三维流速和声学浊度的定点观测,通过对定点站位潮周期内的悬沙浓度、流速和盐度的分析,计算悬沙输运率;悬沙输运机制分析表明平流作用、斯托克斯漂移效应在悬沙输运中占据主导地位.此外,从河口内向河口外,潮周期内的水动力特征与悬沙净输运具有明显的地域性差异,主要表现在悬沙输送的贡献因子、盐度的垂向混合和分布特征、垂向流速等方面.在拦门沙下游和口外地区,悬沙均向西、北方向输送,而拦门沙上游则向东、南方向输送.这种悬沙输运格局,对于长江口拦门沙及附近最大浑浊带的形成有着重要的作用.     

长江口南汇边滩冲淤变化规律与机制

根据1842—2004年海图资料分析发现,南汇边滩存在近百年尺度的强烈冲刷—淤积旋回。长江主泓走南港或北港是造成冲刷期"北滩、东滩淤积,南滩、过渡带冲刷"或淤积期冲淤态势反相的主要原因;冲刷期内风暴强度和频数明显多于淤积期,造成冲刷期滩面叠置记忆的是暴风浪成因的"高滩冲刷、低滩淤积"的冲淤态势,而淤积期保存的是弱风浪成因的"高滩淤积、低滩冲刷"叠置增强的剖面特征。尽管三角洲整体冲淤态势的转变主要受流域来沙量的控制,但不同岸段受河口河势分水分沙作用、潮流和波浪等共同作用,明显存在此冲彼淤、冲淤动态调整等特征。已有的入海泥沙含量阈值研究以点代面或以局部代整体,这是造成阈值估算偏高的主要原因。2003年三峡水库开始蓄水后平均年输沙量154 Mt/a已低于低阈值184 Mt/a,但三角洲尚未如预测那样发生由净淤积向净侵蚀的转变。已有的河口水文观察资料显示,水体含沙量也未发生明显下降,这可能是潮控型三角洲潮流对泥沙在河口的再分配起主导作用,并可能由此延长三角洲冲淤转变对入海泥沙量减少的滞后。今后需进一步加强潮控型河口复杂过程的综合研究,提高对泥沙含量阈值估算和应对可能面临的海岸侵蚀及其相关的环境地质灾害的能力。     

Prediction of salinity intrusion in the sheltered estuary of Terengganu River in Malaysia using 1-D empirical intrusion model

Generally one dimensional(1-D) empirical salinity intrusion model is limited to natural alluvial estuary. However,this study attempts to investigate its ability to model a sheltered alluvial estuary of the Terengganu River in Malaysia. The constructed breakwater at the mouth of the river shelters the estuary from direct influence of the open sea. The salinity density along the estuary was collected during the wet and dry seasons for scenarios before and after the constructed breakwater. Moreover, the freshwater discharges, tidal elevations and bathymetry data were also measured as model inputs. A good fit was demonstrated between simulated and observed variables,namely salinity distribution and intrusion length for both scenarios. Thus, the results show that 1-D empirical salinity model can be utilized for sheltered estuarine condition at the Terengganu Estuary, but with an appropriate determination of an initial point. Furthermore, it was observed that the salinity intrusion in the study area is largely dependent on the freshwater discharge rather than tidal elevation fluctuations. The scale of the salinity intrusion length in the study area is proportional to the river discharge of the –1/2 power. It was appeared that the two lines of the 1-D empirical salinity model and discharge power based equation fitted well to each other, with the average predicted minimum freshwater discharge of 150 m3/s is going to be required to maintain acceptable salinity levels during high water slack(HWS) near the water intake station, which is located at 10.63 km from river mouth.     

Geochemistry of rare earth elements in the dissolved,acid-soluble and residual phases in surface waters of the Changjiang Estuary

The concentrations of rare earth elements in the dissolved, acid-soluble and residual phases in surface waters of the Changjiang Estuary were determined using ICP-MS. The main purposes of the study are to understand the estuarine geochemistry of rare earth elements and to explore water-particle interactions in the Changjiang estuarine mixing zone. The results show that there are two distinct processes operating on dissolved rare earth elements in the estuary: large scale removal at low salinities due to salt-induced coagulation and remarkable release at mid to high salinities. These processes result in modification of the effective river water flux and the systematical fractionation of the dissolved rare earth elements toward the East China Sea. The increase in concentration of dissolved rare earth elements in the mid to high salinity waters of the Changjiang Estuary suggests a sediment source in the mixing zone of the estuary, which is located over a shallow, broad shelf where there is extensive physical contact between bottom sediment and estuarine waters. Acid-soluble rare earth elements, the concentrations of which also dropped sharply in the low salinity region, appear to be controlled by salt-induced coagulation process and intense deposition of suspended particulate matter in the low salinity region. In the mid to high salinities, all acid-soluble rare earth element concentrations increase slightly with increasing salinity, suggesting that resuspension of sediments occurred. In contrast, the residual rare earth element concentrations are relatively constant with salinity variation in the Changjiang estuarine surface waters.     

Sedimentation in mangroves and coral reefs in a wet tropical island, Pohnpei, Micronesia

A six-month-long study was conducted of the fate of turbid river plumes from the Enipein watershed in Pohnpei, Federated States of Micronesia. Pohnpei is one of the wettest places on earth, with a mean annual rainfall exceeding 4 m in the lowlands and 8 m in the highlands. The river waters were clear of sediment except after major storms with rainfall exceeding 5 cm day⁻¹. Following a storm, the river plume spread in the mangrove fringed estuary and in the coral reef lagoon. The waters were highly stratified in temperature, salinity, and suspended sediment concentration. The brackish water was flushed out in four days, while the suspended sediment all settled out in the estuary, in the mangroves, and in the lagoon including on the coral reefs, in less than one day. The mean rate of sedimentation exceeded 35 mg cm⁻² d⁻¹ both over the mangroves and on the adjacent coral reefs. While this leads to no detrimental effects on the mangroves, sediment smothers corals and leads to substantial coral mortality in the lagoon. The mud is not flushed out from the lagoon because there are no strong currents from waves or tides. This high sedimentation rate is attributable to poor farming and land-use practices on the upland areas.     

Suspended-sediment response to pulses in river flow and semidiurnal and fortnightly tidal variations in a mesotidal estuary

The effects of the tidal regime and high river flows on the suspended sediment concentration field have been separated and the retention time of fluvial sediment has been determined. The study was carried out in the upper Sado estuary, Portugal, and included surveys at high and low tide in different semidiurnal tide ranges every month for 1 year, including contrasting freshwater discharge. DDT and its metabolites (DDD and DDE) were determined in the suspended sediment and in the whole soft tissues of oysters, and they have been used as tracers of the derived flood material in the upper estuary. The tide is responsible for the relatively slow relaxation time following a winter runoff event.