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.

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.

Adjustment and extension of the Hansen and Rattray estuarine classification diagram

The 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.

Future salt intrusion climate scenarios: the case of the Po river

<p>A 2-layer Estuary Box Model, named CMCC EBM (Verri et al., 2020), has been devised by the CMCC Foundation to offer a proper representation of the estuarine overturning circulation and mixing processes in a coupled modelling framework with hydrology models and ocean models. The regional to global ocean models reaching the mesoscale cannot solve the estuarine dynamics because they cannot represent the estuary geometry due to their low resolution. Thus, the idea of an estuary box model that gives reasonable values of water volume flux and salinity at the river mouth, which in turn affects the ocean dynamics.</p><p>A further development of the model equations (Verri et al. 2021, under revision) considers the estuary length, i.e.  the length of the salt wedge intrusion, as a model unknown which depends on the competition between the riverine freshwater and the salt ocean water.</p><p>The physical core of the model consists of two conservation equations for volume flux and salt flux both averaged over the diurnal tidal cycle. Moreover, two non-dimensional equations based on the Buckingham theorem have been conceived to provide the estuary length and the along-estuary eddy diffusivity (Verri et al., under revision) as time-variable parameters instead of assuming they are static as most box models do.</p><p>The input fields required by the CMCC EBM are the river runoff at the estuary head and the ocean inflow at the river mouth in terms of both barotropic tidal inflow through the water column and baroclinic inflow at the bottom. The estuary width and depth at the river mouth are the only tunable parameters of the CMCC EBM.</p><p>The model capability to estimate the length of the salt wedge intrusion has been tested and validated. The Po di Goro branch of the Po delta system has been selected as case study. It is representative of the river-dominated estuaries in a micro-tidal sea, the so called “salt wedge estuaries”, with a multiannual average of the salt wedge intrusion around 15 km according to the ArpaE monitoring campaigns.</p><p>Overall the high statistical performance, the short computation time and the minimal calibration encourage to use the CMCC EBM in coupled mode with mesoscale ocean models to produce more realistic operational forecasts and climate scenarios.</p><p>In the framework of the Operandum H2020 project (https://www.operandum-project.eu), the CMCC EBM has been used to provided historical simulations (1981-2010 time window) and mid-term scenarios (2021-2050 time window under RCP 8.5) of both the salt wedge length and the salinity at the Po di Goro mouth. The final aim is to design and develop a site-specific nature-based solution which may address the pressing issue of the salinization of the inland waters. The CMCC EBM results clearly showed a stronger intrusion of saltier ocean water in the middle term. The average, the minimum and the maximum values of salinity at the river mouth provided by the model projections are assumed as reference values to investigate the behaviour of two halophyte species which have been selected to reduce the saline intrusion problem because of their high salinity absorption capacity.</p>

Impacts of biogenic polyunsaturated aldehydes on metabolism and community composition of particle-attached bacteria in coastal hypoxia

Eutrophication-driven coastal hypoxia has been of great interest for decades, though its mechanisms remain not fully understood. Here, we showed elevated concentrations of particulate and dissolved polyunsaturated aldehydes (PUAs) associated with the hypoxic waters in the bottom layer of a salt-wedge estuary. Bacterial respiration within the hypoxic waters was mainly contributed by particle-attached bacteria (PAB) (> 0.8 µm), with free-living bacteria (0.2–0.8 µm) only accounting for 25 %–30 % of the total rate. The concentrations of particle-adsorbed PUAs (∼ 10 µmol L−1) in the hypoxic waters were directly quantified for the first time based on large-volume filtration and subsequent on-site PUA derivation and extraction. PUA-amended incubation experiments for PAB (> 25 µm) associated with sinking or suspended particles retrieved from the low-oxygen waters were also performed to explore the impacts of PUAs on the growth and metabolism of PAB and associated oxygen utilization. We found an increase in cell growth of PAB in response to low-dose PUAs (1 µmol L−1) but an enhanced cell-specific bacterial respiration and production in response to high-dose PUAs (100 µmol L−1). Improved cell-specific metabolism of PAB in response to high-dose PUAs was also accompanied by a shift of PAB community structure with increased dominance of the genus Alteromonas within the Gammaproteobacteria. We thus conclude that a high PUA concentration associated with aggregate particles within the bottom layer may be crucial for some species within Alteromonas to regulate PAB community structure. The change in bacteria community could lead to an enhancement of oxygen utilization during the degradation of particulate organic matter and thus likely contribute to the formation of coastal hypoxia. These findings are potentially important for coastal systems with large river inputs, intense phytoplankton blooms driven by eutrophication, and strong hypoxia developed below the salt-wedge front.

A Unifying Approach to Subtidal Salt Intrusion Modeling in Tidal Estuaries

The salinity structure in estuaries is classically described in terms of the salinity structure as well mixed, partially mixed, or salt wedge. The existing knowledge about the processes that result in such salinity structures comes from highly idealized models that are restricted to either well-mixed and partially mixed cases or subtidal salt wedge estuaries. Hence, there is still little knowledge about the processes driving transitions between these different salinity structures and the estuarine parameters at which such a transition is found. As an important step toward a unified description of the dominant processes driving well-mixed, partially mixed, and salt wedge estuaries, a subtidal width-averaged model applicable to all these salinity structures is developed and systematically analyzed. Using our model, we identify four salinity regimes, resulting from different balances of dominant processes. It is shown that each regime is uniquely determined by two dimensionless parameters: an estuarine Froude and Rayleigh number, representing freshwater discharge and tidal mixing, respectively, resulting in a classification of the regimes in terms of these two parameters. Furthermore, analytical expressions to approximate the salt intrusion length in each regime are developed. These expressions are used to illustrate that the salt intrusion length in different regimes responds in a highly different manner to changes in depth and freshwater discharge. As one of the key results, we show that there are only very weak relations between the process-based regime of an estuary and the salt intrusion length and top–bottom stratification. This implies that the salinity structure of an estuary cannot be uniquely matched to a regime.