Deltas are home to billions of people and are often highly developed and engineered systems. Extreme weather events such as droughts are a threat to deltas worldwide. During droughts salt can intrude far inland and threaten the drinking, agricultural and industrial water supply of many people. Under climate change the frequency of extreme events is expected to increase and the threat of salt intrusion may intensify. Here we use data and models to explore salt intrusion in the Rhine-Meuse Delta (RMD) during the severe European drought in the summer of 2022. The RMD is one of the most highly managed deltas in the world, with numerous interconnected waterways and an open connection to the sea at the mouth of the Rotterdam Waterway. The outflow of the Rhine River through the Rotterdam Waterway generates the strongly stratified Rhine River plume. Under normal conditions a salt wedge intrudes about 16-18 km inland on every tide. In contrast, under drought conditions in summer 2022, observations show salt intruding over 42 km inland and the Rhine River plume diminished in size. We explore the changes in estuarine dynamics during the drought using velocity, salinity and temperature data from various field campaigns near the mouth of the Rotterdam Waterway and within the delta, together with numerical models. We also compare drought condition observations with data from prior field campaigns during normal discharge conditions. Shifts in the relative strength of the dominant mechanisms of landward salt flux throughout the drought are explored and linked to the changes in estuarine response.@en

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.

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Around the world, estuaries have been partially or completely closed-off from the sea and their number may increase with rising sea levels. Concurrently, there is a trend to reintroduce seawater inflow into enclosed former estuaries for ecosystem improvement. This is also the case in the Haringvliet, a former estuary in the Rhine-Meuse Delta, closed-off in 1970 with floodgates blocking seawater inflow and regulating outflow. As the reintroduced salt water inflow can threaten fresh water intake, inflow, flushing and dispersion need to be well understood and carefully managed. Here we investigate stratification, flow circulation and salt transport in the Haringvliet by analyzing ADCP data collected in two former tidal channels, together with salinity time series and profiles. The profiles show that the incoming water reaches the deeper parts and that the system tends to be strongly stratified. Over time, the interface levels deepen in steps, mainly coinciding with floodgate discharge events, which are strongly correlated with the primary current velocities in the channels. However, even floodgate discharges for above average Rhine discharge conditions were insufficient to quickly flush or mix the salt out of the channels. This is consistent with calculated gradient Richardson numbers, which barely get in the range of critical values. For closed floodgates with no outward discharge, we found considerable depth-averaged upwind currents in the channels for axial winds. This reveals a dominant horizontal circulation, with downwind currents over the shallow parts and upwind currents over the deep parts of the system, explained by a local imbalance between the wind stress and pressure gradient force at both shoals and channels. This horizontal circulation is an important driver for inland salt transport, as increased salinity values were found at landward locations for seaward wind. This implies this is a condition with increased risks for fresh water availability. Analytical calculations confirmed the upwind currents in the channels can become sufficiently strong to transport salt mixed up at one side of the system to the other within the duration of a wind event. However, the current-related shear is likely not strong enough to induce interfacial mixing directly above the deep parts, and we hypothesize mixing mostly occurs when salt water reaches less deep areas after tilting of the pycnocline. The insights from this study are relevant for other formerly enclosed estuaries for which reintroduction of seawater inflow is considered, as well as presently open systems for which (partial) closure is discussed.@en

Tidal river plumes dominate many shelf seas, transporting freshwater, sediment, nutrients, pollutants and larvae downstream. The Rhine River Plume is one of the largest in Europe, under typical discharge conditions it is dominated by tidal plume fronts in the near to mid-field plume and by tidal straining in the mid- to far field plume. Moreover, in agreement with other tidal river plumes discharging onto the shelf, internal waves generated ahead of tidal plume fronts are an important source of mixing in the river plume. We compare field data collected downstream of the mouth of the Rhine River in 2013 and 2014 under typical discharge conditions, with data collected in the near field plume during 2022 during a major drought. Together with numerical models we explore how extreme variations in freshwater discharge impact both tidal straining and the formation and strength of tidal plume fronts. Furthermore we explore how in turn, this influences the structure and mixing of the near to far-field Rhine River Plume. We use a 3D hydrostatic model of the Rhine River Plume and a potential energy anomaly analysis to explore changes in the mixing. We explore how the river plume adjusts to extremely low discharge conditions and discuss the possible impact on the transport of freshwater, tracers, larvae and fine sediment.@en

We investigate the changes in surface water salinity intrusion lengths for estuaries around the world under influence of climate change. To do this, we make use of information from global data sets on present river geometry and present and predicted future river discharges, mean sea levels and tidal ranges, which we combine with various models for salt intrusion lengths. The used predictions are based on the RCP8.5 climate scenario and we use 2050 as time horizon, with the 10-percentile lowest discharge as representative value used as input in the intrusion length calculations. The salt intrusion models are two parametric descriptions and a semi-analytical model. With this, we calculate absolute and relative changes in salt intrusion length for a selection of estuaries around the world, to eventually scale up the analysis and develop a global map of changes in salt intrusion around the world under influence of climate change. The results so far indicate that many estuaries may be expected to experience a relative increase of salt intrusion length of over 10%. We also investigate which of the changing forcings most strongly affects the intrusion lengths and what type of estuary is most sensitive to changes. For most systems, the changes in river discharge characteristics are the most influential change, exceeding the influence of sea level rise. This study highlights the importance of studying the effect of climate change on estuarine salt intrusion in more detail, both in global analyses as in system specific detailed studies.@en

Tidally 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.

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In field observations from a sinuous estuary, the drag coefficient (Formula presented.) based on the momentum balance was in the range of (Formula presented.), much greater than expected from bottom friction alone. (Formula presented.) also varied at tidal and seasonal timescales. (Formula presented.) was greater during flood tides than ebbs, most notably during spring tides. The ebb tide (Formula presented.) was negatively correlated with river discharge, while the flood tide (Formula presented.) showed no dependence on discharge. The large values of (Formula presented.) are explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of (Formula presented.), consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated with (Formula presented.) during flood tides. While (Formula presented.) generally increased with water depth, (Formula presented.) decreased for the highest water levels that corresponded with overbank flow. The decrease in (Formula presented.) may be due to the inhibition of flow separation with flow over the vegetated marsh. The dependence of (Formula presented.) during ebbs on discharge corresponds with the inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature-induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source.@en