Understanding Salinity Intrusion and Residence Times in a Small-Scale Bar-Built Estuary under Drought Scenarios: The Maipo River Estuary, Central Chile
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The Maipo River estuary is a low-inflow bar-built estuary that includes a protected wetland, which harbors a rich ecosystem. The estuary and wetland have been threatened by a persistent drought for more than a decade, which has resulted in greater salinity intrusion and increased residence times. Previous studies have described salinity and pollutants in estuaries; however, almost all have focused on deeper and/or wider estuaries with dimensions much larger than those of the small-scale Maipo River estuary. In this study, we used the numerical model FVCOM to simulate the dynamics of the Maipo River estuary under drought scenarios and explored the interactions between river discharge and tides in terms of saline intrusion and particle dispersal. The model was validated against observations collected during a field campaign near the river mouth. The simulations successfully reproduced the water surface elevation but underestimated salinity values, such that the vertical salinity structure observed in the field was not captured by the model in this shallow and morphologically complex estuary. Consequently, our model results provide qualitative insight related to salinity and baroclinic dynamics. Results of maximum saline intrusion showed an exponential decay with increasing river discharge, and the analysis of salinity intrusion time series revealed that droughts may cause permanent non-zero salinity levels in the estuary, potentially affecting ecological cycles. The incorporation of passive tracers showed that decreasing river discharge increases the residence time of particles by allowing the tracers to re-enter the estuary. Model results showed the formation of accumulation zones (hotspots) in the shallower zones of the estuary.Keywords:
Freshwater inflow
Saltwater intrusion
Discharge
Meander (mathematics)
Chunnambar estuary situated about 8 km from
Puducherry receives its freshwater inflow during
monsoon season from Varahanadhi, a tributary of
Palar River and meets the Bay of Bengal. During
the observation period, the water temperature
and salinity in this shallow estuary ranged from
25 – 33 OC and salinity 20 – 28 ppt respectively.
BENGAL
Freshwater inflow
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Estuaries are among the most densely populated and heavily utilised regions in the world, where crucial functions—e.g., freshwater availability and water safety—strongly relate to the natural dynamics of the system. When developing nature-based solutions to safeguard these essential functions, a thorough understanding of estuarine dynamics is required. This study describes an elaborate sensitivity analysis on the salt intrusion length using an idealised estuary, which is parametrically designed using key estuary-scale parameters—e.g., river discharge and tidal flats—to cover a wide range of estuary classes. We were able to systematically investigate such a wide range of estuary classes due to the combination of (1) state-of-the-art hydrodynamic modelling software, (2) high performance computing, and (3) reduction and analysis techniques using machine learning. The results show that the extent of the estuarine salt intrusion length is largely determined by four estuarine features: (1) river discharge; (2) cross-sectional area (especially water depth); (3) tidal damping/amplification; and (4) tidal asymmetry. In general, the salt intrusion length shows clear correlations with (a combination of) estuary-scale parameters, which all put an upper limit on the salt intrusion length. These relations provide crucial insights for successful development of nature-based solutions to mitigate salt intrusion in estuarine environments.
Saltwater intrusion
Salt marsh
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We present a new concept and techniques for processing flow and salinity data and calibrating a box model designed to estimate water residence times in Charlotte Harbor, a large estuary in southwestern Florida. The new concept is that, over many tidal cycles, the tidally averaged “flow” ( Q g ) of water from the Gulf of Mexico with a salinity of 35‰ can be treated as a constant at any point in the estuary. This flow is used in a simple mixing equation to predict salinity in the estuary at different river inflows, and the predicted salinities are used to compute residence times for water in the estuary. The techniques developed to achieve optimal precision in the relation between river inflow and salinity include a newly derived equation to fit Q g by a least‐squares method and a procedure to determine the optimal averaging period for river inflow. Results from Charlotte Harbor indicate that, under average (70 m 3 s −1 ) river inflow, 95% of the original water present in the harbor flushes into the gulf in 130 d.
Inflow
Freshwater inflow
Box model
Flushing
Meander (mathematics)
Discharge
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For sustaining the ecosystem health of estuaries,it is critical to understand how instream flows affect riverine ecosystem health.In this paper,the relationship between freshwater inflows and Net Ecosystem Productivity(NEP) in the Yellow River Estuary was investigated.NEP was calculated by the open-water dissolved oxygen curve method,and correlation analysis was conducted to find out which attribute of the freshwater inflows acts on NEP most.In the Yellow River Estuary as a whole,NEP ranged from-5.32 to 4.84 mg·L-1·d-1 between April and September.Examination of the specific spatiotemporal variation of NEP further revealed that,the up-estuary and the middle-estuary were more heterotrophic than the far-estuary;meanwhile,the whole estuary was relatively highly heterotrophic between August and September(wet period) and but lower between May and June(dry season),and more heterotrophic after water and sediment regulation in the Yellow River.On the whole,the Yellow River estuary was entirely heterotrophic in 2009,and its respiration was greater than its photosynthesis,indicating that it could be a huge source of CO2.Among a variety of variables characterizing the freshwater inflow,turbidity,salinity and COD were identified as significant correlation factors for NEP.These are the significant factors influencing variation of NEP and the turbidity was to a great degree determined by freshwater inflow in Yellow River Estuary.
Freshwater inflow
Turbidity
Dry season
Seasonality
Wet season
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Along with the rapid economic development of Shanghai City,the freshwater resources cannot meet the demands and new freshwater resources are needed to be developed.Shanghai City is located at the estuary of the Yangtze River and the river can provide abundant freshwater for the city,but the salt-tide intrusion affects its quality.In order to establish a salt-tide controlling and supporting system,the feature and law of saltwater intrusion in the Yangtze River Estuary should be further studied,with a combination of research requirement,the monitoring contents about salt-tide are put forward and the layout of monitoring station network is determined initially based on the present monitoring status.After that,the construction of saltwater intrusion information system is discussed deeply.
Saltwater intrusion
Yangtze river
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ABSTRACT: A study was initiated to examine the effects of wind speed, wind direction, freshwater inflow, and tide height on suspended solid concentration and distribution in the Loxahatchee estuary, Florida. Recent efforts to increase freshwater flows in this system raised concerns that suspended solid concentrations would increase as well, which might result in negative impacts for the estuary. The data indicated that total suspended solids (TSS) in the estuary are derived primarily from the inlet and not from freshwater tributaries. In addition, total suspended solids and volatile suspended solids were correlated strongly with salinity, suggesting that suspended sediments act conservatively throughout this system. No one environmental factor had an overwhelming influence on suspended solid concentration throughout the estuary; different regions of the estuary were influenced by different factors. Freshwater inflow was negatively related to TSS in the upper reaches of the estuary but was positively related to TSS in the central embayment region of the estuary. We attribute this latter finding to the fact that extremely high inflows both prevented the normal transport upstream of tidal borne suspended sediments and promoted mixing when the freshwater front moving downstream confronted the tidal front moving upstream. Wind speed, wind direction, and tide height had relatively small effects on TSS concentration but were most influential in reaches upstream of the central embayment, where tidal velocity begins to diminish.
Freshwater inflow
Total suspended solids
Tidal irrigation
Discharge
Inflow
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Saltwater intrusion
Discharge
Dry season
Tidal irrigation
Wet season
Water discharge
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Flushing
Freshwater inflow
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Understanding how long water is retained in an estuary and how quickly it is completely flushed is essential to estimate an estuary’s health in areas with significant pollutant loadings. The present study analyses the effect of five different Tagus River discharge scenarios ranging from low to extreme on residence time (RT), exposure time (ET) and integrated water fractions inside pre-established Tagus estuary areas, to identify its most vulnerable areas to pollution. The 3D version of the MOHID hydrodynamic model coupled to a lagrangian tool was used. The increase of the river discharge generated high current velocities which, in turn, led to an increased rate of tracers leaving the estuary. As a consequence, RT and ET decreased from 59 to 3.5 days under a low and extreme river discharge scenario, respectively. Under a low river discharge, significant differences were observed between RT and ET in the areas located in the main body of the estuary and in the bays. As river discharge increased, RT and ET decreased in all areas of the estuary and those differences faded, with the greatest differences observed in the areas situated along the south margin. In general, results showed that with high river discharges the tracers released in the upper estuary are spread throughout the estuary, but mainly in downstream areas. However, when the river discharge reached exceptionally high values, local eddies were formed, leading to the retention of the tracers in the estuary’s south margin and inner bays. The results in this study allowed to identify the most vulnerable areas within the estuary as a function of the river discharge.
Discharge
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