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The expected increase in rates of sea level rise during the 21st century and beyond may cause barrier islands to drown. Barrier drowning occurs due to a sediment imbalance induced by sea level rise, causing inlets to open and expand. It is still unclear how fast barrier islands can drown. To gain insight into the morphodynamics of barrier systems subject to sea level rise, we here present results obtained with a novel barrier island exploratory model, BarrieR Inlet Environment‐Drowning, that considers inlet expansion beyond equilibrium size. We quantify how much of a barrier island chain is drowned by calculating the fraction of its length that is below mean sea level due to sea level rise. Results show that barrier drowning is mostly sensitive to the wave height and the rate of sea level rise. In the model, it takes 100s of years for barrier islands to start drowning in response to high rates of sea level rise (more than 5 mm/yr, for a typical coastal environment). This lag in barrier response is caused by a gradual decrease in the sand volume of the barrier. Higher rates of sea level rise cause earlier and more severe barrier drowning. Modeled barrier systems that face higher waves undergo more frequent inlet closures that lower the rate of drowning, but they also have a deeper shoreface that increases the rate of drowning. In model simulations, the latter process dominates over the former when sea level rise rates exceed 5 mm/yr. Model results fairly agree with available field data. Plain Language Summary: In extreme sea level rise scenarios (like those predicted during the 21st century and beyond) barrier islands may drown. Barrier drowning occurs due to a lack of sediment induced by sea level rise, which causes submergence of (parts of) the barrier chain. It remains difficult to predict when and under which conditions drowning may occur. In this study we investigated the dynamics of drowning barrier islands with an exploratory numerical model. A key finding from our model is that high rates of sea level rise (higher than 5 mm/yr), but also high waves (higher than 1.5 m) result in barrier drowning. However, even under model simulations with high rates of sea level rise, it takes a long time for the sand in the barrier island to erode. Barrier drowning and disappearance therefore might take 100s of years. The model results are consistent with available field data, but more observations are needed to achieve a full model verification. Key Points: Idealized morphological model simulations show that tidal inlets expand until barrier islands disappear under rapid sea level riseModeled barrier drowning lags sea level rise acceleration by 100s of yearsLag in modeled barrier drowning is explained by a decrease in the sand volume of the barrier [ABSTRACT FROM AUTHOR]

Determining the statistical properties of salt intrusion in estuaries on sub‐tidal time scales is a substantial challenge in environmental modeling. To study these properties, we here extend an idealized deterministic salt intrusion model to a stochastic one by including a stochastic model of the river discharge. In the river discharge model, two types of stochastic forcing are used: one independent (additive noise) and one dependent (multiplicative noise) on the river discharge state. Each type of forcing results in a non‐Gaussian response in the salt intrusion length, which we consider here as the distance of the 2 psu isohaline contour to the estuary mouth. The salt intrusion model including both types of stochastic forcing in the river discharge provides a satisfactory explanation of the multi‐year statistics of observed salt intrusion lengths in the San Francisco Bay estuary, in particular for the skewness of its probability density function. Key Points: A stochastic dynamical model is developed to explain the statistics of salt intrusion in estuariesAdditive (multiplicative) noise in river discharge induces a positive (negative) skewness in the salt intrusion length statisticsBoth additive and multiplicative noise in river discharge are important forthe salt intrusion statistics in the San Francisco Bay estuary [ABSTRACT FROM AUTHOR]

Salt overspill, defined as the net salt transport from a channel of an estuarine network through a junction to another channel, can be a major contributor to salt intrusion. Here, an idealized subtidal model is constructed of a network consisting of one river channel and two sea channels, and used to investigate the sensitivity of overspill to different values of river discharge, tidal current, width, and depth of the channels. Two prototype systems are considered: the North and South Passage of the Yangtze Estuary and the Modaomen and Hongwan Channel of the Pearl River Estuary. Model results indicate that in both systems, increasing river discharge decreases the amount of salt overspill, except in the regime of weak river discharge in the Yangtze Estuary. Increasing the strength of the tidal current increases the overspill in the Yangtze Estuary, but it decreases the overspill in the Modaomen Estuary. Analysis of the model results shows that salt overspill is linearly related to the salinity difference at the upstream boundary of the two seaward channels, when they are considered as single channel estuaries. This salinity difference occurs because conditions in the channels are not identical, which results in different net water transports (causing export of salt), exchange flows, and horizontal diffusion (causing import of salt). An analytical expression is derived, which explains the dependency of salt overspill to the factors mentioned above. Plain Language Summary: Estuarine deltas are areas where a river splits into multiple channels, such that it has several outflows to the sea. These regions are susceptible to salt intrusion: salty sea water intruding into the estuarine delta, which may for example hamper intake of fresh water. Specifically, we investigate the process of salt transport from one channel to another channel through a single junction in an estuarine delta. This salt transport is called salt overspill. Two estuarine deltas are taken as examples: part of the Yangtze Estuary and the Modaomen Estuary. We study why overspill happens in these systems and assess the sensitivity of overspill to river discharge, tidal strength, and changes of the geometry of the channels. We find that, because salt intrusion in the separate channels responds differently to these factors, the salt overspill also strongly depends on them. More specifically, we show that in the Yangtze Estuary salt is transported from the South Passage into the North Passage, because the South Passage is much wider than the North Passage. In the Modaomen Estuary, salt flows from the Hongwan Channel into the Modaomen Channel, because the Hongwan Channel only receives little fresh water from the Modaomen Waterway. Key Points: Dependence of salt overspill is quantified in case of a simple network for different river discharges, tidal strengths, and geometriesValues of overspill calculated with an idealized model are well approximated by an analytical expression derived from basic physicsSalt overspill strongly depends on the net water transport in the network and on the differential salt import in the channels [ABSTRACT FROM AUTHOR]

Freshwater pulses (during which river discharge is much higher than average) occur in many estuaries and strongly impact estuarine functioning. To gain insight into the estuarine salinity response to freshwater pulses, an idealized model is presented. With respect to earlier models on the spatiotemporal behavior of salinity in estuaries, it includes additional processes that provide a more detailed vertical structure of salinity. Simulation of an observed salinity response to a freshwater pulse in the Guadalquivir Estuary (Spain) shows that this is important to adequately simulate the salinity structure. The model is used to determine the dependency of the estuarine salinity response to freshwater pulses for different background discharge, tides, and different intensities and durations of the pulses. Results indicate that the change in salt intrusion length due to a freshwater pulse is proportional to the ratio between peak and background river discharge and depends linearly on the duration of the pulse if there is no equilibration during the pulse. The adjustment time, which is the time it takes for the estuary to reach equilibrium after an increase in river discharge, scales with the ratio of the change in salt intrusion length and the peak river discharge. The recovery time, that is, the time it takes for the estuary to reach equilibrium after a decrease in river discharge, does not depend on the amount of decrease in salt intrusion length caused by the pulse. The strength of the tides is of minor importance to the salt dynamics during and after the pulse. Plain Language Summary: The salinity distribution in an estuary, the transition area between river and sea, strongly depends on the river discharge. During periods of low river discharge, salt will move upstream, but when river discharge becomes high, salt is pushed downstream. This study focuses on the effect of freshwater pulses (short periods with sudden high river discharge) on estuarine salt intrusion. When applying an existing model to observed freshwater pulses in the Guadalquivir Estuary (Spain), it turned out that this model was not able to simulate the effect of strong pulses. A new model has been developed that performs well when being applied to the same situations. With this new model, it is shown that the intensity and duration of the pulse control the decrease in salt intrusion. The strength of the tides is found to be of minor importance. The time it takes before the salt intrusion has recovered to its initial location is determined by the river discharge after the pulse and does not depend on how much the salt intrusion moved downstream. Key Points: Modeling of salt dynamics during strong freshwater pulses requires a detailed description of the vertical structure of salinityDependence of salt intrusion length is quantified from the pulse duration and ratio of river discharge before and during a pulseThe recovery time after a freshwater pulse does not depend on the change in salt intrusion length induced by the pulse [ABSTRACT FROM AUTHOR]

Net water transport (NWT) in estuaries is important for, for example, salt intrusion and sediment dynamics. While NWT is only determined by river runoff in single channels, in estuarine networks, it results from a complex interplay between tides and residual flows. This study aims to disentangle the various contributions of these physical drivers to NWT in estuarine networks and investigate the sensitivities of NWT to variable forcing conditions, interventions, and sea level rise (SLR). To this end, a processes‐based perturbative network model is developed, which accounts for the vertical flow structure to resolve density‐driven flow driven by a vertically uniform along‐channel salinity gradient. Other identified drivers are river discharge and three tidal rectification processes: Stokes transport and its return flow, momentum advection, and velocity‐depth asymmetry. The model is applied to the Yangtze Estuary. NWT due to tidal rectifications and density‐driven flow can be comparable to river discharge. Specifically in the North Branch, the direction of NWT may differ from the direction of river discharge. Varying river discharge mainly affects NWT as tide‐river interaction is weak and density‐driven flow is shown to be insensitive to salt intrusion. Conversely, variations in tidal amplitude strongly affect NWT related to tidal rectification and density‐driven flow. The deepening (narrowing) of one channel (Deep Waterway Project), affected the NWT mostly through the density‐driven flow (advection). Furthermore, NWT distribution in the Yangtze is insensitive to SLR up to 2 m because the effects of SLR on transport due to different drivers compensate each other. Plain Language Summary: To complement numerical models, a partly analytical model is used to decompose net water transport in estuarine networks into different components according to their driving physical processes related to river discharge, differences in water density, and tidal rectification. Applying the model to the Yangtze Estuary (China), this study demonstrates the dependence of each component of net water transport on external forcing conditions (river and tide), as well as their sensitivity to local human intervention and sea level rise. Previous understanding of the tidal influence on net water transport is generalized to explain the creation of net water transport by arbitrary physical processes in estuarine networks. Key Points: Using a 2DV exploratory model, net water transport is disentangled into different components and attributed to their driving mechanismsThe theory of differential water level setup is extended to explain net water transport due to these various driving mechanismsIn the Yangtze Estuary, net water transport due to tidal rectifications and density gradients can be comparable to river water transport [ABSTRACT FROM AUTHOR]