Salt Wedge Research Articles

Assessment of Shatt Al-Arab Water Quality Using CCME/WQI Analysis in Basrah City of South Iraq

The Water Quality Index (WQI) is an important parameter in describing the water resources' suitability for human uses and is one of the most effective methods of describing water quality and indicative of assessing water quality and suitability for human utilization and the health of ecosystems. WQI of the Canadian Council of Ministers of the Environment (CCME) was used in the study to describe the Shatt al-Arab water quality in Basrah Southern Iraq, and its suitability for drinking use. The data for analyzing river water samples were adopted from five stations along the river every month during the years from 2014 to 2018 by the Iraqi Ministry of Environment, as it included the measurement of acidity function PH, Dissolved Oxygen DO, phosphorous PO4, nitrate NO3, Calcium Ca, potassium K, magnesium Mg, sodium Na, Total Hardness TH, sulfates SO4, chlorides Cl, Electrical Conductivity EC, Total Soluble Salts TDS, and Alkalinity ALK. The distributions of water quality index values along the river were mapped by GIS techniques. Study results were referring to illustrate the poor level of the water quality status of Shatt al-Arab in all study periods at all monitoring stations. The reason for this is due to the deterioration of fresh water quality drained from the Tigris and Euphrates, and the provision of salt wedges from the Arabian Gulf. However, with the continuous discharge of sewage water, industrial and oil effluents and sewage discharged from urban areas, the water quality of Shatt al-Arab was declined during the study period.

The Impact of Tidal Straining and Advection on the Stratification in a Partially Mixed Estuary

Stratification and mixing of the water column is an important dynamic process in the estuary, which plays a significant role in the estuarine circulation, mass transport and energy exchange. Based on the multi-station synchronous observation data from 26 February to 6 March in 2011 during dry season in the North Channel of the Changjiang Estuary, the Richardson number, the Simpson number and the potential energy anomaly of water were calculated to analyze the tidal variation of the mixing and stratification processes. The roles of the depth-mean straining, longitudinal advection, non-mean straining and tidal stirring in the processes of mixing and stratification of the water column were analyzed by calculating the contribution terms of the time-derivative of potential energy anomaly. The results show that the mixing and stratification of the water column in the North Channel have significant spatiotemporal variation. Stability of the stratification gradually decreases from neap tide to spring tide. In the reaches of salt wedge migration, permanent stratification develops during neap and mean tide, with stability increasing on the flood and decreasing on the ebb, which is dominated by longitudinal advection. During spring tide, periodic stratification develops, with development of stratification on the flood and its breakdown on the ebb, which is dominated by longitudinal advection and tidal stirring. In the main reaches of saltwater intrusion, permanent stratification develops during neap tide, with stability increasing on the ebb and decreasing on the flood, which is dominated by depth-mean tidal straining. During mean and spring tide, periodic stratification occurs, with development of stratification on the ebb and its breakdown on the flood, which is controlled by depth-mean tidal straining and assisted by tidal stirring. In the North Channel, tidal advection is the main stratifying agent in the salt wedge migration reaches, and tidal straining is the main stratifying agent in the main reaches of saltwater intrusion.

Using CFD Modeling to Simulate the Control of the Propagation of Salt Wedge using Inclined Roughness Elements

This research aims to simulate and use Computational Fluid Dynamics, CFD, to control the propagation of salt wedges. The flume has a cross-section of 25 cm in height and 7.5 cm in width with a length of 6 m and a zero slope. The simulated model is used to investigate the use of inclined roughness elements to control the propagation of salt wedges. The elements are blocks of 2 cm by 3 cm cross-sections having an inclined face in the direction of the flow, with a length of 2 and 3 cm. These elements were installed in two rows at both sides of the bed of the flume with variable spacing between them in the direction of the flow, and their centerline is inclined by an angle of 30 degrees in the direction of the flow. The simulation model was validated by comparing its output with a published laboratory experiment. Ten CFD model runs were conducted under two different discharges and five different spacing between the inclined roughness elements. The used discharges are 30 l/min and 45.3 l/min, and the spacing between elements was 3, 6, 9, 12, and 15 cm. The results demonstrated a good relationship between the obtained model runs and the observations of the laboratory experience under the same conditions. The result showed that when no roughness elements were used, the propagation of the salt wedge extended to 3.9 and 3.1 m at a discharge of 30 l/min and 45.31 l/min, respectively. The propagation of the salt wedge is reduced as the spacing increases to some limit and then starts to decrease when you use roughness elements. At the maximum applied discharge of 45.3 l/min, the propagation of the salt wedge was reduced by 82, 84, and 85% when the spacing between the blocks is 3, 6, and 9 cm, respectively. The percentage of reduction in the propagation of the salt wedge starts to reduce to 79% and 75% as the spacing between the blocks is increased to 12 and 15 cm, respectively. When the discharge is 30 l/min, the propagation of the salt wedge is reduced by 76, 74, and 78% at a spacing of 3, 6, and 9 cm, respectively. At the same time, the propagation is reduced by 58% at 12 cm and 53% as the spacing is increased to 12 and 15 cm.

Salt Transport Regimes Caused by Tidal and Subtidal Processes in Narrow Estuaries

AbstractTidally averaged transport of salt in estuaries is controlled by various subtidal and tidal processes. In this study, we show the relative importance of various subtidal and tidal transport processes in a width‐averaged sense. This is done for a large range of forcing and geometric parameters, which describe well‐mixed to salt wedge estuaries. To this end, we develop a width‐averaged process‐based model aimed at conducting and analyzing a large number of experiments (∼40,000). We find that the salt transport is dominated by one of seven salt transport balances, or regimes. Four of these regimes are dominated by subtidal processes, while the other three are dominated by tidal processes. Which regime occurs in a part of an estuary depends on four dimensionless parameters, representing local geometry, and forcing conditions. One of the regimes features salt import by correlations between the depth‐averaged tidal velocity and salinity. While this mechanism was previously only associated with along‐channel geometric variations, we find it can also be a dominant mechanism in a significant part of the parameter space due to river‐induced tidal asymmetry, independent of river geometry. We apply our classification to a case study of part of the Dutch Rhine delta and compare to decomposition results of a fully realistic three‐dimensional model. We find the estuary features two regimes, with import dominated by subtidal shear transport in the seaward part of the estuary and by depth‐averaged tidal correlations in the landward part of the estuary.

The effect of waves in hydrodynamics, stratification, and salt wedge intrusion in a microtidal estuary

Waves have been found to modulate circulation, stratification, and sediment dynamics in several estuaries, mainly near the mouth. This study analyzes the effects of waves on the hydrodynamics, stratification process, and dynamics of the salt wedge in an estuary with a microtidal range, high fluvial and sediment discharges, and dominated by waves: the Magdalena River estuary (MRE). It is, under low flow conditions, a highly stratified, salt wedge type. Field measurements and the MOHID 3D modeling system, 2D coupled with the SWAN model, were used for this purpose. The low flow seasons of 2018 (February-March) and 2020 (March) were taken as case studies. Results show that when considering wave effects in the numerical simulations, more realistic conditions are reproduced in the circulation patterns and salinity distribution in the outer estuary. Variations in velocity patterns and salinity distribution are found between the mouth and 2 km upstream of the mouth when comparing the simulations with and without waves, especially in the mixing layer. These variations in hydrodynamics and stratification may be associated with increased wave-induced bed shear stress, variations in barotropic and baroclinic acceleration, and increased vertical mixing. At 2 km into the river channel, the reduction in wave height energy of 95% and changes in salinity distribution are already lower than 2%. In addition, it was observed that waves do not generate significant changes in the dynamics of the salt wedge, which is mainly affected by the diurnal tidal cycle, presenting variations in the length of the intrusion of up to 1 km, and in the magnitude of the longitudinal salinity gradient at the salt front, presenting low salinities at high tide when the wedge enters, and high salinities at low tide, in its retreat.

The fate of microplastics in estuary: A quantitative simulation approach

Microplastics pollution is an emerging environmental concern. However, there are almost no MPs numerical simulation studies in the Yangtze Estuary which is considered as the largest plastic export in the world and quantitative simulation is not carried out in the existing models. Therefore, completing quantitative simulation and exploring different patterns of MPs transport are the main objectives of this study. In addition, the concentration distribution and risk of MPs are also analyzed. Mass-Number method is proposed to quantitatively simulate microplastics concentration in Feb. and May with errors of less than 18%. Compared with sediment flocculation and settling transport, independent floating transport is more susceptible to surface currents resulting in increased beaching and more inhomogeneous concentration distribution. Meanwhile, under the influence of current, local topography and salt wedge, the MPs perform linear motion and clockwise spiral motion inside and outside the estuary and rapidly form a "hot spot" on the southeastern part of Chongming Island and 57% to 90% of MPs are beached or settled inside the estuary, especially on the north shore. Therefore, MPs risk in some sensitive targets should be concerned according to risk assessment results. Our results break the space-time limit and explore the fate of MPs in the Yangtze Estuary and provide new idea and concern of MPs numerical simulation.

Lateral Variation of Tidal Mixing Asymmetry and Its Impact on the Longitudinal Sediment Transport in Turbidity Maximum Zone of Salt Wedge Estuary

The lateral bathymetry in the estuary results in different degrees of tidal mixing asymmetry, which has significant impacts on the longitudinal sediment transport by changing the temporal variation of vertical eddy diffusion. This study focus on the lateral variation of tidal mixing asymmetry and longitudinal sediment transport at the landward boundary of turbidity maximum zone in the North Channel of Yangtze estuary, which is a typical time-dependent salt wedge estuary. A transect survey was carried out in December, 2018; five vertical profiles of flow velocity, salinity and suspended sediment concentration were simultaneously measured covering a spring tidal cycle. Analysis of the data revealed that, after the maximum ebb, the stratification in the main and secondary channel was stronger than that on the shoal. In the channel, during ebb tide, the stronger stratification restrained the turbulent mixing induced by vertical shear, vertical mixing during the flood tide was stronger than that during ebb tide and vertical mixing coefficients ranged from 0.06 to 0.12, showing regular tidal mixing asymmetry over a flood–ebb tidal cycle. Therefore, stronger eddy diffusion caused by vertical mixing resulted in higher suspended sediment concentrations during flood tide, the larger landward tidally averaged sediment transport rate was induced by tidal pumping with the transportation of flood tidal current and the net sediment transport over a flood–ebb tidal cycle in the channel was landward. Meanwhile, on the shoal, under the effect of vertical shear, the vertical mixing during flood tide was weaker than that during ebb tide; vertical mixing coefficients ranged from −0.27 to −0.02, showing the reversed tidal mixing asymmetry. Higher suspended sediment concentration was transported seaward by the ebb current, the tidally averaged sediment transport rate by both tidal pumping and advection was seaward and the net sediment transport was seaward. Furthermore, large river discharge increased the seaward advection sediment flux on the surface layer in the main channel, resulting in the seaward tidally averaged sediment flux. Strong resuspension increased the seaward advection sediment flux near the bottom in the main and secondary channel, resulting in the seaward tidally averaged sediment flux.

Impact of Estuarine Dams on the Estuarine Parameter Space and Sediment Flux Decomposition: Idealized Numerical Modeling Study

AbstractEstuarine dams are constructed for securing freshwater resources, flood control, and improving upstream navigability. However, their impact on estuarine currents, stratification, and sediment fluxes is not well understood. To develop a general understanding, an idealized modeling study was carried out using the COAWST modeling system. Idealized estuarine geometry was based on 10 estuaries with estuarine dams located near their mouth. Tide and river forcing were varied to produce strongly stratified, partially mixed, periodically stratified, and well‐mixed estuaries. Each model ran for 1 year. Next, the models were subject to the construction of an estuarine dam and run for another year. Then, the predam and postdam conditions were compared. Results showed that estuarine dams can amplify the tidal range and reduce the tidal currents. The postdam estuaries tended to be salt wedge, strongly stratified, partially mixed, or periodically stratified types during freshwater discharge, but during no freshwater discharge they became fjord, bay, periodically stratified, or well‐mixed types based on the estuarine parameter space. For all estuaries, the estuarine turbidity maximum moved seaward, and the suspended sediment concentrations tended to decrease. While the depth changes depended on the estuary type, the surficial sediment texture shifted to being muddier for all types. In terms of sediment flux mechanisms, the estuarine dam increased the seaward river runoff for cases with strong river, and increased the landward tidal pumping for cases with strong tides. This study is one of the first to generalize the effect of estuarine dams to a range of estuarine types.

Coupling Hydrodynamic and Energy Production Models for Salinity Gradient Energy Assessment in a Salt-Wedge Estuary (Strymon River, Northern Greece)

Salinity gradient energy (SGE) plants generate power from the mixing of salt water and fresh water using advanced membrane systems. In the Strymon River, under low-flow conditions, a salt wedge is formed, developing a two-layer stratified system, which could be used to extract SGE. In this paper, a novel study was implemented by coupling a 3D hydrodynamic model simulating the salt wedge flow, with the SGE model which assesses the net energy produced by a 1 MW SGE plant. Two scenarios were followed: (a) the optimal scenario, operating throughout the year by mixing salt water from the sea (38.1 g/L) and fresh water (0.1 g/L) from the river to produce 4.15 GWh/yr, and (b) the seasonal scenario, utilizing the salinity difference of the salt wedge. Results show that the daily net SGE production varies between 0.30 and 10.90 MWh/day, in accordance with the salinity difference (ΔSsw ~15–30 g/L). Additionally, a retrospective assessment (from 1981 to 2010) of the annual and seasonal net energy production was conducted. This analysis illustrates that the salt-wedge formation (spring to late summer) coincides with the period of increased regional electricity demand. In the future, the emerging SGE could serve as a decentralized renewable energy source, enhancing energy security in the region.

Asymmetric Holmboe instabilities in arrested salt-wedge flows

The asymmetric Holmboe instabilities that form on an arrested salt wedge are investigated in the laboratory. The flow is characterized by three regions. Near the tip of the salt wedge, there is little wave activity; immediately downstream, mostly positive waves form; further downstream, both positive and negative waves are present. The appearance of these regions is determined by the spatial variation in the thickness of the saline layer, the shear layer thickness, and the offset between the density interface and velocity interface. We predict the growth of the instabilities by applying linear stability theory to the mean flow field. The predicted initial growth rate is consistent with the laboratory measurements until the Holmboe wave reaches a steepness ratio of 5–7 %. Then, the growth of the Holmboe wave is nonlinear. We found that the Holmboe wavelength increases downstream along the salt wedge. This wave stretching is related to the increase in the shear layer thickness and is supported by the gradual acceleration of the upper layer fluid. Eventually, the growth of the wave amplitude and the wave stretching balance a maximum wave steepness ratio of 10%. Although the wave is nonlinear, the predicted wave speed and wavelength are consistent with the laboratory measurements.

Layered seawater intrusion and melt under grounded ice

Abstract. Increasing melt of ice sheets at their floating or vertical interfaces with the ocean is a major driver of marine ice sheet retreat and sea level rise. However, the extent to which warm, salty seawater may drive melting under the grounded portions of ice sheets is still not well understood. Previous work has explored the possibility that dense seawater intrudes beneath relatively light subglacial freshwater discharge, similar to the “salt wedge” observed in many estuarine systems. In this study, we develop a generalized theory of layered seawater intrusion under grounded ice, including where subglacial hydrology occurs as a macroporous water sheet over impermeable beds or as microporous Darcy flow through permeable till. Using predictions from this theory, we show that seawater intrusion over flat or reverse-sloping impermeable beds may feasibly occur up to tens of kilometers upstream of a glacier terminus or grounding line. On the other hand, seawater is unlikely to intrude more than tens of meters through permeable till. Simulations using the Ice-sheet and Sea-level System Model (ISSM) show that even just a few hundred meters of basal melt caused by seawater intrusion upstream of marine ice sheet grounding lines can cause projections of marine ice sheet volume loss to be 10 %–50 % higher. Kilometers of intrusion-induced basal melt can cause projected ice sheet volume loss to more than double. These results suggest that further observational, experimental and numerical investigations are needed to determine the conditions under which seawater intrusion occurs and whether it will indeed drive rapid marine ice sheet retreat and sea level rise in the future.

CFD Simulation Model of Salt Wedge Propagation

This study aims to numerically simulate the flow of the salt wedge by using computational fluid dynamics, CFD. The accuracy of the numerical simulation model was assessed against published laboratory data. Twelve CFD model runs were conducted under the same laboratory conditions. The results showed that the propagation of the salt wedge is inversely proportional to the applied freshwater discharge and the bed slope of the flume. The maximum propagation is obtained at the lowest discharge value and the minimum slope of the flume. The comparison between the published laboratory results and numerical simulation shows a good agreement. The range of the relative error varies between 0 and 16% with an average of 2% and a root mean square error of 0.18. Accordingly, the CFD software is quite valid to simulate the propagation of the salt wedge.

Interannual and seasonal variability of stratification and mixing in a high-discharge micro-tidal delta: Magdalena River

The spatio-temporal variability of stratification and mixing at the mouth of the Magdalena River Delta (MMRD) was studied for the warm (El Niño), cold (La Niña), and neutral phases of El Niño–Southern Oscillation (ENSO), using the Richardson number (RL), potential energy anomaly (φ), buoyancy frequency (β), turbulent kinetic energy production (P), and bottom shear stress (τxz). These parameters were obtained from the MOHID numerical model outputs, which were implemented and calibrated for the MMRD. Analysis of the spatio-temporal variability of φ and β indicated stratification at different mouth zones, and the spatio-temporal variability of RL and P indicated a mixing process. Results show that during the dry season, the mixing zone of the MMRD presents its maximum intra-annual variability, and during El Niño conditions, it has its highest interannual variability. Further, the maximum penetration of the salt wedge occurs during El Niño - dry season conditions. On the other hand, during the wet season, the salt wedge is located at the mouth entrance, independent of the ENSO phase. During La Niña conditions, the mixing zone restricts its intra-annual variability to around 0 km of the MMRD.

Spatial and temporal variation of salinity stratification in a tropical estuary

Longitudinal and vertical distribution of salinity field in the Vellar River Estuary (VRE) was investigated across the lower, mid and upper estuarine zone, monthly from January to December 2011. The hydrological survey of VRE revealed shallow bathymetry with a complex topography and a mean tidal amplitude of around 0.9 to 1 m. The depth-averaged tidal velocity had a magnitude that was observed with a maximum value of 0.56 m/s at the estuarine mouth and was indirectly proportional to the increasing distance from the mouth. The wind speed fluctuated from 1 to 4.1 km h-1 with a mean of 2.5±0.82 km h-1. The temperature ranged from 25.32 °C to 32.93 °C with a mean of 28.89±2.26 °C while salinity varied between 0 to 34 psu with a mean of 13±12.77 psu and negatively correlated with rainfall (r = -0.69; p = 0.04). A higher stratification number, i.e. ns ~ 1 at the lower estuarine zone indicated the formation of the salt wedge in the monsoon while during the rest of the year the estuarine water column was observed to be homogeneous. Wave height, tide and currents were estimated through 2D hydrodynamic models and they were significantly correlated with in-situ data. Recorded seasonal variation in salinity stratification of VRE from the present work reflected on shaping the primary governing factors on estuarine circulation at spatial and temporal scales.

Controlling the Salt Wedge Intrusion in Shatt Al-Arab River by a Barrage

Shatt Al-Arab River in Al Basrah, Iraq, has recently recorded massive levels of TDS values (Total Dissolved Solids) in the water as a result of reduced fresh water discharge from sources, causing the river to become salinized due to salt wedge intrusion. Therefore, a block dam in the south reach is required to salt intrusion prevention. The main objective of this research is to simulate the hydraulic impact of a suggested barrage in Ras Al Besha on the Shatt Al-Arab River. The HEC-RAS (5.0.7) model was used to develop a one-dimensional unsteady model to gaining an understanding of the proposed barrage's influence on river behaviour. The daily discharges of the Tigris River provided as the upstream boundary conditions, while the hourly water levels of the Shatt Al-Arab River provided as the downstream boundary conditions. The model was initially run on the basis of daily discharges in Aug 2018 and March 2020 for the model's calibration and verification. Then, a model was run with a proposed barrage, Four cases of discharge were chosen which were the low and moderate discharge that equal to (20-50-100 and 250) m3/s with adopted spring tide cycle. The operation scenarios were examined under the influence of three cases of barrage gates (fully opened, 50% open and programmed opening). The results indicate that the investigated discharges will cause a significant problems in navigation depths, especially in the case of the programming of gates opening where the stages drop range between 2.01-3.3m comparing with the normal case. Furthermore, the velocity indicators show that the significant reduction in velocity upstream the barrage led to more sedimentation in the river reach.

The double structure of the Estuarine Turbidity Maximum in the Cam-Nam Trieu mesotidal tropical estuary, Vietnam

The characteristic parameters of the Estuarine Turbidity Maxima (ETM), their aggregates and their tidal variations were investigated within the Cam-Nam Trieu mesotidal estuary (Red River and Thai Binh River system, North Vietnam) at spring tides during the early wet season, wet season and dry season. The structure observed repeatedly highlights the separation of two distinct zones of high turbidity around the salt wedge, separated by a zone of lower turbidity. The upstream part developed at very low salinity (from 0.1 psu) on a quasi-homogeneous water column (at Simpson parameter between ~0.65 and 7–10); tidal pumping associated with salinity-induced flocculation are likely responsible for its formation. A second maximum was observed in the lower layer of stratified waters (at Simpson parameters >10), i.e. in the salt wedge, at higher salinities up to ~12–15 psu; in this downstream part, governed by the gravitational circulation, settling is also seasonally enhanced by the presence of transparent exopolymeric particles (TEP) for salinity >10 psu. Turbidity was higher in the upper part of ETM in the dry season and higher in the lower part of ETM in the wet season. Floc size, excess of density and settling velocity were the highest during dry season. Stronger ETMs occur in dry season than in early wet season and wet season, when the tidal-induced ETM is maximum. The locations of both ETM parts changed seasonally, moving upstream in the dry season and downstream in the wet season. Their length depended on the longitudinal salinity gradient and was highest at low tide than at high tide. The intermediate zone of lower turbidity between these two parts was longer in the wet season.

Responses of Hydrodynamics and Saline Water Intrusion to Typhoon Fongwong in the North Branch of the Yangtze River Estuary

The typhoon impact on an estuarine environment is complex and systematic. A three-dimensional hydrodynamic and salinity transport model with a high-resolution, unstructured mesh and a spatially varying bottom roughness, is applied to investigate the effects of a historical typhoon, Fongwong, which affected Shanghai, on the hydrodynamics and saline water intrusion in the North Branch (NB) of the Yangtze River Estuary (YRE). The model is well validated through observation data of the tidal level, current velocity and direction, and salinity. The numerical results of this typhoon event show that: (1) the tidal level and its range increase toward the upstream part of the NB due to the combined effects of the funnel-shaped plane geometry of the NB and the typhoon; (2) the current velocity and the flow spilt ratio of the NB varies with the tides, with a maximum increase by 0.13 m/s and 26.61% during the flood tide and a maximum decrease by 0.12 m/s and 83.33% during the ebb tide, i.e., the typhoon enhances the flood current and weakens the ebb current; (3) the salinity value increases in the NB to a maximum of 1.40 psu and water is well-mixed in the vertical direction in the typhoon’s stable and falling period. The salinity distribution gradually recovered to the normal salt wedge pattern in 3 days following the typhoon. Although this study is based on a site-specific model, the findings will provide valuable insights into saline water intrusion under typhoon events, and thus assist in implementing more efficient estuarine management strategies for drinking water safety.

Understanding the circulation in the deep, micro-tidal and strongly stratified Congo River estuary

The Congo River estuary is characterised by a deep canyon that connects the river to the deep ocean by cutting through the continental shelf. Its estuary is influenced by high river discharge and micro-tidal conditions, with a large depth and limited vertical mixing. This restricts the supply of oxygen from the surface waters to the more saline bottom waters, leading to hypoxic and anoxic zones. We study the dynamics of the Congo River estuary by applying the multi-scale baroclinic coastal ocean model SLIM 3D (www.slim-ocean.be) to this topographically challenging environment. By allowing a high degree of flexibility in the representation of both the complex geometry and the strong stratification, SLIM 3D is able to simulate riverine, tidal and gravitational processes that drive the estuarine circulation. Model results compare favourably with in-situ data in the estuary, suggesting that the exchange flow is correctly simulated. The latter is characterised by a two-layer dynamics. The combination of the large river discharge, the strong stratification and the large depth results in a moderate freshwater Froude number and a very small mixing number. It makes the Congo River an outlier in state-of-the-art estuarine classifications, closer to fjords than salt wedge estuaries. Furthermore, using the age as a diagnosis sheds light on the spatial variability of the estuarine waters ventilation. Local maximum of renewing water age located just below the pycnocline is exceeded by old dense oceanic waters which stagnate at the bottom of the canyon for more than two months due to the small vertical mixing. It helps explain the hypoxic and anoxic conditions observed at the bottom of the submarine canyon.

Adjustment and extension of the Hansen and Rattray estuarine classification diagram

AbstractThe classification diagram developed by Hansen and Rattray (1966, Limnol. Oceanogr.) is one of the classic papers on classification of estuarine salinity dynamics. However, we found several inconsistencies in both their stratification-circulation and estuarine classification diagrams. These findings considerably change the interpretation of their work. Furthermore, while their classification includes salt wedge estuaries, the model used to derive this is only applicable to well-mixed and partially mixed estuaries. Here, we identify and solve these inconsistencies, and we propose new adjusted and extended stratification-circulation and classification diagrams. To this end, we summarise the model of Hansen and Rattray and extend their work to find analytical model solutions and an adjusted stratification-circulation diagram. Using this new diagram, it is shown that Hansen and Rattray incorrectly discussed the behaviour of dispersion dominated estuaries and that several parts of the diagram correspond to physically unrealistic model solutions. This is then used to demonstrate that several estuarine classes identified by Hansen and Rattray correspond to physically unrealistic model solutions and can therefore not be interpreted. A new and extended classification is proposed by using a recently developed model that extends the work of Hansen and Rattray to salt wedge estuaries. This results in an extended estuarine classification including examples of the location of 12 estuaries in this new diagram.

An Experimental Study on Investigating and Controlling Salt Wedge Propagation

This study aims to investigate the variation of profiles and propagation of salt wedge. It also aims to investigate the use of air and water curtains in controlling its propagation. A laboratory flume system was prepared to simulate the propagation of the salt wedge. Relations between the profiles and propagation of salt wedge and the discharge of fresh water, longitudinal slope of bed of flume, the concentration of the salt, and the depth of salt water were obtained and analysed. Seventy-seven experiment runs were carried out to investigate the propagation of salt wedge. Moreover, a set of sixteen experimental runs were conducted to investigate controlling the salt wedge using air or water curtains. It was found that by increasing the discharge of fresh water or the slope of the flume bed leads to a reduction in the length of the salt wedge. This reduction reached a maximum percentage of 95%. The propagation of the salt wedge varies proportionally and linearly with the concentration of salt water and its variation with depth of salt water follows a power function. Furthermore, a maximum reduction of 87% was achieved in the salt wedge propagation when the EC is reduced by about 49%. The salt wedge propagation was reduced by 95.3%% when the salt water depth reduced by 40%. It was found that both of the air and water curtains are efficient methods in controlling the propagation of salt wedge. A minimum discharge of air or water curtains to prevent the propagation of the salt wedge was obtained.