Flood Slack Research Articles

Flocculation under combined effects of hydrodynamic and subaqueous biomass in the bottom boundary layer (BBL) of a microtidal estuary

The flocculation dynamics within the bottom boundary layer (BBL) of tidal estuaries constitute a pivotal and intricate aspect entwined with hydrodynamics and morphodynamics. In microtidal estuaries, where saltwater intrusion occurs, the ensuing impacts on ecosystems, biological habitats, and human activities underscore necessity for comprehensive understanding. In pursuit of elucidating flocculation dynamics within estuarine BBLs, an extensive 25-hour survey was conducted throughout a complete tidal-cycle in the Huangmaohai estuary, China. This investigation encompassed the collection of data pertaining hydrodynamics, biochemical characteristics of suspended flocs within the BBL. The observed irregular semidiurnal tide was delineated into six distinct stages: I) Weak flood, II) Flood slack, III) Strong ebb, IV) Ebb slack, V) Strong flood and VI) Flood slack. The amalgamation of empirical findings and theoretical analyses has facilitated the development of conceptual model delineating the intricate processes and interactions of multiple factors within each stage (I-VI) in the BBL of a prototypical micro-tidal estuary. Notably, it reveals biological factors exhibit a significantly higher efficacy in estuarine flocculation dynamics within the BBL compared to the chemical ion attractions, attributable to variations in salinity. Further nuances emerged, indicating that semi-liquid extracellular polymeric substance (EPS) plays a substantial role in the formation of high-density flocs, particularly during periods of heightened turbulent shear conditions in flood and ebb times (I, III, V). Conversely, particulate biological debris predominantly contributes to low-density flocs characterized by a low settling velocity, particularly for large flocs >200 μm during tidal slacks (I, IV), and smaller median-sized flocs (70–200 μm) during flood or ebb times (III, V) due to turbulent induced breakage of bio-particles. This study underscores the significance of quantitative investigations into the biological components within individual flocs under estuarine hydrodynamics as a pivotal step towards comprehending flocculation mechanisms and predicting cohesive sediment transport within the BBLs of estuaries.

Impacts of the Qingcaosha Reservoir on saltwater intrusion in the Changjiang Estuary

The massive Qingcaosha Reservoir (QCSR) is located in the Changjiang Estuary along the northwest coast of Changxing Island. The reservoir significantly narrowed the upper reaches of the North Channel and deepened the channel near the reservoir. These topographical changes inevitably influenced hydrodynamic processes and saltwater intrusion in the estuary. A well-validated model was employed to investigate the influence of the QCSR on saltwater intrusion in the Changjiang Estuary. The model results showed that the narrowed upper reaches of the North Channel decreased the water diversion ratio and thus increased salinity in the North Channel. During the moderate tide after neap tide, the salinity decreased at the water intake of the QCSR because saltwater intrusion was obstructed at flood slack at the surface, while the salinity increase during the moderate tide after spring tide was mainly due to the intensified saltwater intrusion during spring tide. The deepening of the channel near the QCSR resulted in an increased water diversion ratio, and the salinity in the Eastern Chongming Shoal decreased by more than 0.5 psu during spring tide; however, the saltwater intrusion was enhanced due to the strengthened baroclinic force, which is proportional to the water depth. During neap tide, the salinity in the entire North Channel decreased because of a 1.4% increase in the water diversion ratio of the North Channel and the relatively weak tide.

Observation of the sediment trapping during flood season in the deep-water navigational channel of the Changjiang Estuary, China

After the construction of the Deep-water Navigational Channel (DNC) in the North Passage of the Changjiang Estuary, siltation in the channel became severe during the flood season in the navigation channel. To investigate the causes, field observations of tidal current, salinity and suspended sediment concentration (SSC) were conducted by 16 anchored boats in the navigation channel of North Passage during spring tide in the flood season. The across-channel differences of salinity, SSC, and near bottom residual sediment transport at the high siltation area of the DNC were observed and analyzed. The results show that the near bottom SSC presented a south-to-north decreasing trend during the flood tide at the lower reach of the North Passage. During the ebb tide, a suspended sediment cloud of extremely high concentration was detected within the channel. The observed longitudinal sediment trapping was not only shown as the turbidity maximum (TM) trapped within the moving salinity front but also as the longitudinal convergence of the residual sediment transport at the middle reach. The mechanism for the longitudinal sediment trapping was due to the tidal asymmetry in mixing. The observed lateral sediment trapping was prominent at the most severe siltation area of the DNC in the section #2. The lateral sediment trapping at the across-channel section #2 was not only shown as the lateral transport of the sediments from the channel flanks during the flood slack but also as the lateral convergence of the residual sediment transport during the entire spring tidal cycle. The mechanism for the lateral sediment trapping was dominated by the advective transport. The lateral impact on the channel sediment trapping was not negligible in view of the depth-mean lateral tidal straining on the water stratification. The observed convergences of both the longitudinal and the lateral residual sediment transports were responsible for the severe channel siltation in the North Passage.

Modeling lateral circulation and its influence on the along-channel flow in a branched estuary

A numerical modeling study of the influence of the lateral flow on the estuarine exchange flow was conducted in the north passage of the Changjiang estuary. The lateral flows show substantial variabilities within a flood-ebb tidal cycle. The strong lateral flow occurring during flood tide is caused primarily by the unique cross-shoal flow that induces a strong northward (looking upstream) barotropic force near the surface and advects saltier water toward the northern part of the channel, resulting in a southward baroclinic force caused by the lateral density gradient. Thus, a two-layer structure of lateral flows is produced during the flood tide. The lateral flows are vigorous near the flood slack and the magnitude can exceed that of the along-channel tidal flow during that period. The strong vertical shear of the lateral flows and the salinity gradient in lateral direction generate lateral tidal straining, which are out of phase with the along-channel tidal straining. Consequently, stratification is enhanced at the early stage of the ebb tide. In contrast, strong along-channel straining is apparent during the late ebb tide. The vertical mixing disrupts the vertical density gradient, thus suppressing stratification. The impact of lateral straining on stratification during spring tide is more pronounced than that of along-channel straining during late flood and early ebb tides. The momentum balance along the estuary suggests that lateral flow can augment the residual exchange flow. The advection of lateral flows brings low-energy water from the shoal to the deep channel during the flood tide, whereas the energetic water is moved to the shoal via lateral advection during the ebb tide. The impact of lateral flow on estuarine circulation of this multiple-channel estuary is different from single-channel estuary. A model simulation by blocking the cross-shoal flow shows that the magnitudes of lateral flows and tidal straining are reduced. Moreover, the reduced lateral tidal straining results in a decrease in vertical stratification from the late flood to early ebb tides during the spring tide. By contrast, the along-channel tidal straining becomes dominant. The model results illustrate the important dynamic linkage between lateral flows and estuarine dynamics in the Changjiang estuary.

颍河上游褚河段古洪水事件及其沉积响应

通过对颍河上游禇河镇段一处典型的古洪水滞流沉积层剖面(SWD)的粒度、磁化率等指标进行综合研究，并结合野外特征描述，分析了该剖面理化特征，探讨了该地区发生洪水的次数。研究结果表明：1) 在YH-CH剖面粒度组成中以粉砂为优势粒径，含量介于47%~85%之间，平均含量为64.3%；粘粒含量较少，含量波动于3.2%~9.5%之间，平均为5.3%；砂级颗粒平均含量为17%；在累计频率曲线图上，B、C、D、E等层位偏左，SWD1和SWD2偏右；SWD1和SWD2分选较好且沉积颗粒较粗。2) 该剖面低频磁化率(X-lf)和高频磁化率(X-hf)具有相似的变化趋势，都呈现出B、C、D、E磁化率值相对较高，而SWD1与SWD2磁化率值相对较低，说明了SWD1和SWD2为典型的古洪水滞流沉积物。综合研究表明，YH-CH剖面记录两期古洪水事件，对应的剖面深度分别为60~80 cm、180~200 cm，其中第一期洪水较第二期洪水能量较高，携带的沉积物颗粒较粗。同时这些特征也为颍河上游的古洪水事件的识别标志提供参考。 Based on the test of grain size and magnetic ability from typical flood slack water deposits (SWD) in the Chuhe site, in upstream reaches of the Ying River, coupled with filed description, we analyze its physicochemistry features and discuss the sedimentary characteristics when floods happen. The results show that: 1) in the grain size of YH-CH profile, silt that varies from 47% - 85% is the main constituent, and the average level is 64.3%; clay content is less, between 3.2% and 9.5%, and its average level is 5.3%. Average sand-size particles’ content is 17%; on the cumulative frequency cure, B, C, D, and E incline to the left, and however, SWD1 and SWD2 incline to the right. It is obvious that SWD1 and SWD2 are special ancient flood slack water sediments; 2) the section of low-frequency magnetic susceptibility (X-lf) is in line with high-frequency magnetic susceptibility (X-hf) broadly in a changing trend; the magnetic susceptibility value of B, C, D, and E is relatively high, and the magnetic susceptibility value of SWD1 and SWD2 is relatively low. According to the experimental analysis, we conclude that the profile recorded two kinds of flood. And the depth of the corresponding profiles respectively is from 60 to 80 cm and 180 to 200 cm; the energy of the first ancient flood that carried a lot of coarse matters is higher than that of the second ancient flood. In addition, the study of sedimentary characteristics provides the reference for the identification of ancient floods in upstream reaches of the Ying River.

Effects of a coastal power plant thermal discharge on phytoplankton community structure in Zhanjiang Bay, China

The effects of a thermal discharge from a coastal power plant on phytoplankton were determined in Zhanjiang Bay. Monthly cruises were undertaken at four tide times during April–October 2011. There were significant differences for dominant species among seven sampling months and four sampling tides. Species diversity (H′) and Evenness showed a distinct increasing gradient from the heated water source to the control zone and fluctuated during four tides with no visible patterns. Species richness, cell count and Chl a at mixed and control zones were significantly higher than heated zones, and showed tidal changes with no obvious patterns. The threshold temperature of phytoplankton species can be regarded as that of phytoplankton community at ebb slack. The average threshold temperature over phytoplankton species, cell count and Chl a, and the threshold temperature of cell count can be regarded as that of phytoplankton community at flood slack during spring and neap respectively.

Longitudinal Transportation Mechanism of Sallinity in West Channel of Lingding Bay in Dry Season, China

Estuary is the transition zone,where fresh water and salty water mixes strongly.In dry season,because Discharge of the Pearl River water decreases,salt water intrusion become serious in the Pearl River delta,and affect the safety of fresh water supply.So,salinity transport mechanism is a very important topic.In this paper,according to the measured data in November 3-4,2005,calculate and analyze the longitudinal flow characteristics,salinity distribution and transport mechanisms. Research results show that the velocity varieties with ebb and flood processes,the surface velocity is larger than that of bottom and middle layer.In ebb slack and flood slack period,there is a zero velocity point,and its movement distance is up to 10km.Vertical residual circulation is obvious, surface layer is to sea,and bottom layer is to land.Net salinity transport direction is from sea to land.salinity vertical stratification is evident,and varieties obviously with tide changes.Surface salinity gradient is larger In flood period,and bottom salinity gradient is larger in ebb period.In ebb slack time,salinity is vertical uniform distribution.

CARACTERIZAÇÃO HIDRODINÂMICA DO ESTUÁRIO DO RIO CUBATÃO, JOINVILLE.

Este trabalho apresenta uma caracterização preliminar da hidrodinâmica do estuário do Rio Cubatão baseada em dados hidrográficos obtidos durante uma campanha de treze horas (um ciclo completo de maré) em condições de maré de sizígia. Dados de velocidade e direção de corrente, salinidade, temperatura e turbidez foram obtidos à 1,5 m abaixo da superfície, próximo da desembocadura do estuário com o fundeio de um correntógrafo acústico. Perfis verticais de salinidade e temperatura foram realizados próximos ao fundeio a cada hora com um CTD para fornecer a estrutura vertical. Perfis de CTD também foram realizados ao longo do estuário durante a preamar. Os resultados mostram que o estuário do Rio Cubatão apresenta-se verticalmente homogêneo com períodos menores de estratificação durante a vazante. O regime de maré mostra uma assimetria acentuada, com correntes de vazante muito mais intensas do que as correntes de enchente.

Tidal Time-scale Changes in the Composition of Particulate Organic Matter Within the Estuarine Turbidity Maximum Zone in the Macrotidal Seine Estuary, France: the Use of Fatty Acid and Sterol Biomarkers

During low flow and spring tide conditions of September 1995, the organic character of suspended particulate matter (SPM) was investigated in the estuarine turbidity maximum (ETM) zone located within the inner Seine River estuary, France.The appearance of the ETM was observed 2–4h before low tide, during ebb resuspension into the water column. During the flood resuspension, a second ETM was observed but only in bottom waters. This suggests separation of the surface (fresh waters) from the bottom layer (brackish/marine waters) at maximum flood velocities. Suspended particulate matter composition was relatively constant in terms of organic carbon (2%) and only a slight decrease was observed during the high SPM load.Lipids (i.e. fatty acids and sterols) were used as biomarkers to assess different sources of organic matter including terrestrial inputs, phytoplankton, bacteria and domestic sewage. Given specific assumptions, source-specific biomarker concentrations were used to trace quantitatively temporal changes in the relative and absolute importance of source-derived organic material. Terrestrial matter was a very important part of ETM organic matter [35–58% particulate organic carbon (POC) in ebb ETM at low tide; 10–20% POC at high slack tide], whereas sewage organic matter was a relatively minor (<4% of POC) component. Although, the proportion of phytoplankton matter in the high SPM load was low, phytoplankton was a predominant component of SPM during the flood slack (12–31% of POC). This prominence of SPM was associated with the oxygen saturation due to the diatom bloom in the estuary.

Tidal variations in the movement of organic carbon in New Jersey salt marshes

The movement of organic carbon was assessed by statistical and simulation modeling analyses in two marsh types in New Jersey;each marsh contained three water-drainage systems in which three tidal cycles were sampled in May and June 1973. Hourly water samples were obtained and filtered through a gelman Type A glass filter for separation into dissolved (DOC) and particulate (POC) organic carbon components of the total organic carbon (TOC). Simulation data showed that individual creeks and marshes functioned differently on the sampled tidal cycles in regard to net movement of water and organic carbon components. Organic carbon components exhibited similar tidal variations, with significantly lower concentrations at flood slack than at ebb slack. Mid-ebb concentrations were significantly higher than mid-flood concentrations for TOC and POC. Individual marshes showed significantly different concentrations in the latter segments of the tidal cycle for TOC's and POC's. Results indicate that individual creeks, marshes and tidal cycles are not representative of the total movement of organic carbon in estuaries.

Processus de sédimentation des vases dans l'estuaire de la Gironde Contribution d'un traceur radioactif pour l'étude du déplacement des vases

The hydraulic and sedimentological processes operating in the Gironde estuary (SW France) are linked to the interaction between the river discharge (mean values between 800-1,000 m3/s), tides (ranges from 2 to 6 m), the suspended sediment influx from the rivers (2 to 3 million tons per year), and the general morphology of the estuary, consisting in two channel systems separated by a series of islets, shoals and bars. The fluvial discharge undergoes important seasonal variations, and the length of the salinity intrusion varies correspondingly : during periods of low river flow (summer-fall) it extends above Bec d'Ambes (73 km from the inlet), and during high river flow, below Pauillac (45 km from the inlet). The residual circulation pattern is linked to the intensity of the density gradients, and also varies seasonally, following changes in the ratio of river flow to tidal prism. Generally speaking, in the zone of the saline intrusion the residual currents are stratified : upstream on the bottom, and downstream toward the surface. The channel and shoal topography can locally disrupt the residual currents by amplifying the flood or the ebb flow. The principal null point, or zone of convergence of the bottom residual currents upstream of which the bottom residuals are always oriented downstream, generaly coincides with the upstream extremity of the salinity intrusion, and follows the same seasonal migrations. There is a marked lateral salinity gradient in the lower estuary, with less saline water flowing along the right bank (looking downstream). A well marked forms at the upstream limit of the salinity intrusion, in the zone of bottom residual current convergence. This turbidity maximum migrates seasonally, in response to the migration of the salinity intrusion and null point. The concentration of suspended sediment in the turbidity maximum is very high (0.1 to 10 g/l). The deposition of suspended sediment in the main estuarine channel occurs through the formation of pools of extremely turbid bottom water (100-300 g/l). Practically no movement is discernible in these pools of and intense bottom deposition by settling occurs. These pools are formed in the turbid core of the turbidity maximum, in zones of motionless lenses, on the bottom occuring at flood slack by the amplification of the velocity time assymetry in the salinity intrusion. This fluid mud follows a cycle of erosion and deposition linked to the monthly cycle in tidal ranges. During neap tides the mud accumulates, and during spring tides, it is eroded and resuspended into the overlying turbidity maximum. It appears, from suspended sediment flux measurements in the lower estuary, and analysis of the plume of turbid estuarian water escaping out on to the continental shelf during river floods and spring tides, that a significant amount of alluvial suspended sediment bypasses the estuary and is deposited in a silt and olay zone on the inner continental shelf. Possibly part of this estuarine water is recirculated in an estuarine and lagoonal system further north. In order to study and assess the diffusion of the fluid mud into the turbidity maximum during spring tides, and also to follow the upstream migration of these fluid mud pools during a period of decreasing river discharge, a radioactive tracer study was made of the fluid mud. This study showed that a very slow movement, on the order of 0.1-0.2 cm/s occurs in the fluid mud, and appears to be directed downstream. Also it was shown that at each cycle of erosion and deposition of the fluid mud, which reformed further upstream at each cycle, the same sediment appears to be recycled.