Influences of River Discharge Variation and Tidal Asymmetry on the Spatial Evolution of the Turbidity Maximum Zone in Yangtze Estuary

2020 ◽  
Author(s):  
Ping Dong ◽  
Huabin Shi
Keyword(s):  
River Discharge ◽  
Yangtze Estuary ◽  
Turbidity Maximum ◽  
Tidal Range ◽  
Tidal Asymmetry ◽  
Interannual Variations ◽  
The South ◽  
Turbidity Maximum Zone ◽  
Averaged Model ◽  
Seasonal And Interannual Variations

<p>The Yangtze estuary is characterized by its extremely high suspended sediment concentration (SSC) and the extensive turbidity maximum zone (TMZ). The estuary is physically forced by an upstream river discharge seasonally varying in a wide range of 6000 – 92000 m3/s and semidiurnal-diurnal mixed tides with the tidal range up to 5 m. The influences of the seasonal and interannual variations in the upstream river discharge and the tidal asymmetry on the location of the Yangtze TMZ are numerically investigated with a two-dimensional depth-averaged model. Sensitivities of SSC and hence the location of TMZ to the bottom shear stress, bed erodibility, and the sediment settling velocity are studied. The spatial and temporal evolutions of the TMZ position in the cases of various upstream river discharges with different monthly, seasonal and interannual variations are simulated and discussed. The effects of the M2/M4-induce tidal asymmetry on the TMZ position and those of the interactions between the eight main astronomical tides (M2, S2, N2, K2, K1, O1, P1, and Q1) are compared. It is shown that the M2/M4-induce tidal asymmetry plays a critical role in the formulation of TMZ in the downstream of the South Branch of Yangtze estuary, while the interactions between the eight main astronomical tides have more significant effects on the TMZ location in other areas of Yangtze estuary such as the South and the North Passages.</p>

Salt intrusion in an estuarine network, a study with an exploratory model

2020 ◽  
Author(s):  
Huib E. de Swart ◽  
Inge van Tongeren
Keyword(s):  
River Water ◽  
Fresh Water ◽  
River Discharge ◽  
Yangtze Estuary ◽  
Strong Impact ◽  
The South ◽  
The Yangtze Estuary ◽  
Salt Intrusion ◽  
The North ◽  
Averaged Model

<p>Many estuarine systems experience increased salt intrusion, which is harmful for ecology and agriculture and may cause problems for fresh water supply to cities. Some causes of salt intrusion are extraction of fresh water in the upper reaches of the estuary and climate change. Besides, anthropogenic measures, like deepening of channels, are known to have a strong impact on the salt balance.</p><p>This contribution focuses on salt intrusion in estuarine networks, which consist of multiple connected channels. The motivation of the study arose from observations in the Yangtze estuary that reveal frequent overspill of salt between its different channels. To understand the underlying physics of such behaviour, an exploratory, width- and tidally averaged model has been developed and analysed. This model describes the competition between export of salt by river flow and import of salt by density-driven flow and horizontal diffusion. Its key new aspect is that it generalises an earlier model MacCready (2004) from a single channel to estuarine networks. The new model calculates the distribution of salt in, and salt exchange between the channels, as well as the distribution of river water over the different channels.  </p><p>Here, results will be presented for a simplified estuarine network consisting of the South Channel, South Passage and North Passage of the Yangtze Estuary. It will be shown that, for the present-day situation, dry season and spring tide, salt intrusion is larger in the South Passage than in the North Passage. As will be explained, this is mainly due to the different geometry of the two channels. Furthermore, it will be shown that there is slightly more river water transport through the South Passage than through the North Passage, except during high river discharge and neap tide. These results agree with field data and results from numerical studies.</p><p>Other results that will be presented are the sensitivity of salinity intrusion length and distribution of river water over the different channels to changes in, respectively, upstream river discharge, tidal currents and human interventions. Specifically, the effects of the creation of a Deepwater Navigation Channel in the North Passage on salt dynamics will be shown and discussed.</p><p>Reference:<br>MacCready, P. 2004. Toward a unified theory of tidally-averaged estuarine salinity structure. Estuaries 27, 561-570.</p>

Vertical profile of suspended sediment concentration in the turbidity maximum zone of the partially stratified Changjiang Estuary

2020 ◽  
Author(s):  
Zhanhai Li
Keyword(s):  
Suspended Sediment ◽  
Vertical Profile ◽  
Suspended Sediment Concentration ◽  
Layer Structure ◽  
Sediment Concentration ◽  
Changjiang Estuary ◽  
Turbidity Maximum ◽  
The South ◽  
Turbidity Maximum Zone ◽  
Linear Profiles

<p>In order to study the vertical profile of suspended sediment concentration (SSC) and its temporal variation in the partially stratified estuaries, the profile of SSC, as well as the profiles of current and salinity were measured over a neap-spring cycle for 16 tidal cycles in April 2012 in the turbidity maximum zone (TMZ) of the North Passage in the Changjiang Estuary. The observations revealed that the TMZ was characterized by high SSCs, strong current velocities and remarkable saltwater intrusion. Both salinity and SSCs could yield strong density stratification which would exert important influence on the shape of SSC profile by damping sediment diffusion. The vertical profiles of SSCs mainly exhibited three typical types, i.e., two-layer structure profile, exponential profile and linear profile, and had significant flood-ebb and neap-spring variation patterns. In a tidal cycle, the two-layer structure profile mainly occurred during the strong stratification periods, and the exponential and linear profiles mainly occurred in the weak stratification periods. About 60% observed SSC profiles belonged to the two-layer structure profile, and 40% observed SSC profiles belonged to the exponential and linear profiles. The formation of the two-layer structure profiles during the latter half of floods and early half of ebbs was attributed to the bottom lateral currents, because it could drive the higher SSC and higher salinity in the deep channel to the south shoal through the bottom water layer. Two new empirical equations for the SSC profiles are proposed. They can predict the linear and exponential profiles accurately, and predict the two-layer structure profile reasonably. Both the exponential and linear SSC profiles had constant diffusion coefficient in the water column, and they can be delineated by a unified equation. Additionally, the bottom lateral currents directed to the south flank during 87% of the survey period, and could enhance the SSC, salinity and water exchanges between the channel and the shoal.</p>

scholarly journals Seasonal and interannual variations of MODIS Aqua chlorophyll-a (2003–2017) in the Upper Gulf of Thailand influenced by Asian monsoons

2021 ◽  
Author(s):  
Jutarak Luang-on ◽  
Joji Ishizaka ◽  
Anukul Buranapratheprat ◽  
Jitraporn Phaksopa ◽  
Joaquim I. Goes ◽  
...  
Keyword(s):  
El Niño ◽  
River Discharge ◽  
El Nino ◽  
Gulf Of Thailand ◽  
Interannual Variations ◽  
La Nina ◽  
Seasonal And Interannual Variations ◽  
East West ◽  
Low Precipitation ◽  
Upper Gulf Of Thailand

AbstractSeasonal and interannual variations of chlorophyll-a (chl-a) in the upper Gulf of Thailand (uGoT) were obtained using new regionally tuned algorithms applied to Moderate Resolution Imaging Spectroradiometer-Aqua. This long time-series (2003–2017) data were analyzed in the context of variations in environmental conditions associated with the Southeast Asian Monsoon. Chl-a distribution patterns were distinct for the non-monsoon (NOM), southwest-monsoon (SWM), and northeast-monsoon (NEM) seasons. During the SWM/NEM, high/low chl-a concentrations were associated with high/low precipitation and river discharge. During the NOM chl-a concentrations were generally low, because of low precipitation. In general, chl-a variability was tightly coupled to discharge from the Chao Phraya and Tha Chin rivers. Chl-a concentrations were generally higher in the north, but chl-a accumulation in the east/west of the uGoT could be linked to piling of freshwater to the east/west during the SWM/NEM caused by changes in wind direction and the reversal of currents. Interannual changes in chl-a were attributed to El Niño-Southern Oscillation (ENSO) rather than Indian Ocean Dipole (IOD) driven changes in precipitation, river discharge, and wind patterns. During the SWM, positive/negative chl-a anomalies coincided with high/low precipitation and river discharge during La Niña/El Niño. During the NEM, positive/negative chl-a anomaly coincided with high/low river discharge and strong/weak wind during La Niña/El Niño. Meanwhile, during NOM, positive chl-a anomaly could be attributed to anomalous high wind speed and precipitation during El Niño.

Dynamic sedimentation process of the turbidity maximum zone in the Yangtze River Estuary under the influence of human activities

2020 ◽  
Author(s):  
Weihua Li ◽  
Xiaohe Zhang ◽  
Zhanhai Li ◽  
Jiufa Li
Keyword(s):  
River Estuary ◽  
River Mouth ◽  
Turbidity Maximum ◽  
Yangtze River Estuary ◽  
The Yangtze River ◽  
The South ◽  
Turbidity Maximum Zone ◽  
The North ◽  
The Yangtze River Estuary ◽  
Mouth Bar

<p>Due to the impact of the Three Gorges Dam on water and sediment storage, the sediment flux into the Yangtze River Estuary has dropped sharply by 70%, and the suspended sediment concentration in the estuary has responded accordingly. From the comparison of the measured suspended sediment concentration data of the Yangtze River estuary for many years, it is known that the suspended sediment concentration in the South Passage has been reduced by about 60% recently, and that in the middle and upper reaches of the North Channel and the South Channel has been reduced by about 40%. On the other hand, A series of artificial engineering has been completed in the past 20 years, such as the 12.5m Deep-Waterway Regulation Engineering, the Nanhui Shoal Slush-enclosure Engineering, and the Hengsha Shoal Slush-enclosure Engineering, etc. These engineering have significantly changed the original water and sediment transport pattern of the Yangtze River Estuary. It resulted in a significant change of the estuarine turbidity maximum zone and the corresponding river mouth bar topography. This paper intends to discuss the impact of human activities on the dynamic sedimentation process of the maximum turbidity zone in the Yangtze River Estuary based on field measured data. Results are as follows:</p><p>(1) Compared to two decades ago, the suspended sediment concentration in the North Passage, the South Passage and the North Channel, and the middle and lower reaches of the North Branch is still high, which is related to the existence of the turbidity maximum zone and river mouth bar in these river sections.</p><p>(2) The implementation of man-made engineering such as the submerged diversion dike between the North Passage and the South Passage and the Nanhui Shoal Slush-enclosure Engineering changed the flow structure in the upper section of the South Passage, leading to the turbidity maximum zone and the corresponding river mouth bar have completely disappeared.</p><p>(3) Affected by the 12.5m Deep-Waterway Regulation Engineering, the turbidity maximum zone and the corresponding river mouth bar originally located at the upper section of the North Passage have also disappeared.</p><p>(4) The longitudinal circulation flow structure, salt water wedges, and stagnation points in the middle and lower sections of the North Passage and the South Passage still exist. The positions of the turbidity maximum zone and the corresponding river mouth bar topography are not significantly affected by the engineering. And the core area of ​​the obvious turbidity maximum zone and the river mouth bar (only in the South Passage) still exist. Due to the artificial dredging of the navigation channel in the North Passage, it actually appeared as an invisible river mouth bar that has been dredged by continuous dredging.</p><p> (5) The drastic reduction of sediment flux from the basin has caused seabed erosion adjacent to the Yangtze River Estuary, and the corresponding eroded sediment has become one of the main sediment budget sources of the turbidity maximum zone.</p>

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