Intermittently closed estuaries provide important ecosystem services but are often overlooked in coastal and catchment research and management. These estuaries are highly vulnerable to human and climate disturbances due to their episodic closure to the ocean, yet remain understudied. This study maps 2245 intermittent estuaries globally, whose catchments currently support 55 million people, with projections of up to 101 million by 2100. Analysis of three decades of scholarly literature revealed that only 7% of these estuaries have been studied. Research on intermittent estuaries comprises 0.5% of all estuarine literature, despite representing 4-5% of estuaries globally. Major research gaps exist in Asia, South America, and Africa-regions with large, vulnerable populations. Over 90% of research on intermittent estuaries is conducted in (southern) Africa, Oceania, and North America, with most studies focusing on local physico-chemical and eco-hydro-geomorphological processes. This assessment underscores the need to expand research priorities to include ecosystem services, climate and human disturbances, and management, with greater international collaboration and leadership from intergovernmental organisations.

Supratidal forests are defined by their position relative to the tidal frame where inundation and salinity patterns are potentially influenced by both tidal and nontidal regimes. Despite their recent inclusion in national blue carbon initiatives, knowledge of the processes that influence their carbon storage in supratidal forests remains limited. In this study, we report on new datasets of vegetation structure, carbon cycling parameters, inundation, and salinity patterns across 18 sites spanning more than 4000 km of Australia's temperate coastlines. We report site-specific ecosystem carbon stocks ranging from 169 to 635 Mg Corg ha-1, with mean aboveground biomass (134 ± 63 Mg DM ha-1) and belowground carbon stocks to 1 m soil depth (193 ± 98 Mg Corg ha-1), which are within the range of national estimates for mangrove and saltmarsh ecosystems. While there are variations in vegetation structure between sites dominated by the genera Melaleuca and Casuarina, this does not lead to discernible differences in above- or belowground carbon stocks. Organic matter decomposition trends within supratidal forest substrates were similar to those of adjacent mangrove and saltmarsh, though there were differences among study sites and between labile and recalcitrant tea litters. Soil-atmospheric flux measurements conducted at one site were also within the range of adjacent blue carbon ecosystems. We hypothesize that the high degree of preservation of belowground carbon and low soil-atmosphere flux of greenhouse gases is driven by a combination of infrequent surface inundation, high water tables, and typically saline groundwater in supratidal forests, as measured across multiple settings. Supratidal forests are carbon-rich ecosystems influenced by coastal processes associated with tidal inundation. While further research is required to understand the full distribution, carbon cycling, and abiotic drivers of supratidal forests, our findings strongly support their inclusion in blue carbon and other management initiatives that support the response and recovery of these endangered ecological communities in a time of change.

Estuarine intertidal wetlands, positioned at the interface of land and sea, are highly vulnerable to sea-level rise and other climate change threats such as flooding and storm-driven erosion. Sustaining these ecosystems and their services requires evidence-based projections of wetland resilience under multiple climate futures while explicitly accounting for uncertainty. Here, we integrated the revised universal soil loss equation with updated rainfall projections from the New South Wales and Australian Regional Climate Modelling (NARCliM2.0) to estimate potential sediment delivery to New South Wales (NSW) estuarine wetlands under both optimistic (SSP1-2.6) and pessimistic (SSP3-7.0) climate change scenarios. We compared modelled catchment sediment yields with both projected rates of sea-level rise and empirically derived vertical accretion rates for mangrove and saltmarsh intertidal wetlands. We found that potential future sediment delivery in NSW coastal catchments is unlikely to assist intertidal wetland accretion to keep pace or exceed sea-level rise. Across near (2020-2039), mid (2050-2069), and far (2080-2099) future epochs, most projections indicated neutral change or modest declines in catchment-derived (allochthonous) sediment supply. Our results provide the first process-based, region-wide assessment of vertical resilience for NSW estuarine wetlands. They highlight the urgent need for adaptation strategies that facilitate landward migration of intertidal wetlands to maintain their representation and services.

The large tidal lake systems along the Southeast Australian coast are amongst the most vulnerable estuaries in Australia to the effects of sea level rise. In these lakes, reduced tide ranges compared with the ocean, in combination with modest flood extremes, have allowed development to occur in close vertical proximity to the current mean sea level. In this study, we examine water levels within Lake Macquarie, Australia’s most exposed estuary to sea level rise. We analyse water level data from the entrance channel and the lake to investigate recent changes to the frequency and duration of inundation or flooding of low-lying streets and examine the potential impacts of future rises in sea level. Our analysis shows that the numbers of days each year when water levels exceed those of low-lying streets, while subject to some variability, have increased significantly over recent decades. The increasing frequency of inundation is attributed to both mean sea level rise and an increase in tide range over the period of available data, which is thought to be associated with scour processes related to ongoing morphological adjustment to entrance training works undertaken over a century ago. Comparison of the projected behaviour of lake and open coast water levels under sea level rise shows the lake has significantly greater sensitivity to sea level rise. Projected inundation frequency for a given amount of sea level rise within the lake is double that of open coast sites, exposing infrastructure in the estuary to increasing risk of damage.

Globally, sea-level rise (SLR) is a major environmental challenge for coastal ecosystems. Of particular concern are the impacts on intertidal wetlands, the loss of which would have detrimental consequences for both human and ecological communities. On the south-east Australian coast, case studies suggest that the future of intertidal wetlands will greatly depend on landward migration as surface accretion may not keep up with the predicted SLR in many estuaries. However, due to differences in geomorphological settings and land-use, estuaries vary in their capacity to accommodate lateral migration. Regional scale assessment of the lateral accommodation space is therefore critical for pre-emptive planning to conserve these valuable coastal ecosystems. In this study, we analysed wetland lateral accommodation space distribution within 110 estuaries under three SLR scenarios and three land management options on the New South Wales coast, south-east Australia. From the wetland distribution predictions, we calculated and mapped the lateral accommodation space in each estuary associated with each sea level and land use scenario. We further investigated the relationships between wetland migration capacity, intertidal hypsometry represented by elevation skewness, and estuary type within a Bayesian analysis framework. Our results showed that while a few large riverine estuaries dominated the state's total accommodation space, saline wetlands were at risk of disappearing from most intermittently closed-open estuaries if they cannot vertically accrete at the pace of SLR. These distinct responses to SLR are due to different elevation distributions. Furthermore, our assessment of land use adaptation options suggested that the promotion of landward migration without impairing other important ecosystems could be achieved by making low intensity land uses available within several riverine estuaries and barrier (open entrance) estuaries. Through identifying migration opportunities and barriers, the findings of the study could support regional scale adaptation strategies to ensure the sustainability of wetland-associated ecosystem goods and services.

Tidal inundation is the primary driver of intertidal wetland functioning and will be affected by sea- level rise (SLR). The morphology of estuaries and friction across intertidal surfaces influences tidal propagation; accordingly, sea-level rise not only increases inundation frequency, but will also alter other tidal parameters, such as tidal range. To investigate responses of estuarine intertidal vegetation, primarily mangrove and saltmarsh, to SLR an eco-morphodynamic modelling approach was developed that accounted for some of the feedbacks between tidal inundation and changes to wetland substrate elevations. This model partially accounts for adjustment in estuarine hydrodynamics, and was used to examine the potential effect of SLR on mangrove and saltmarsh distribution in a micro-tidal channelised infilled barrier estuary in southeast Australia. The modelling approach combines a depth-averaged hydrodynamic model (Telemac2D) and an empirical wetland elevation model (WEM) that were coupled dynamically to allow for eco-geomorphological feedbacks. The integrated model was parameterised to consider two SLR scenarios, and two accretion scenarios within the WEM. Time series of observed water levels, tidal inundation and flow velocity were used to validate the hydrodynamic model for present-day sea level, whereas wetland mapping was used to verify predictions of mangrove and saltmarsh distribution. Tidal range varied along the estuary, increasing in response to low and high SLR scenarios (by up to 8%), and responded non-linearly under high SLR. Simulations of low and high SLR scenarios indicated that wetlands mostly withstand modest SLR rates (+ 5mm yr(-1)) through sedimentation, but submerge and convert to subtidal areas under fast SLR rates (> 10mm yr(-1)). Projected changes in tidal range are linked to eco-geomorphological feedbacks caused by changing wetland extents and adjustments of intertidal wetland geomorphology through sedimentation. Potential changes arising from morphological change at the entrance and in the tidal channels is not obtained from the model. The results of this study demonstrate interconnections between hydrodynamics and intertidal wetlands, which need to be accounted for when estimating wetland response to SLR in channelised estuaries. Integrated models of estuarine-wetland systems are more precise as they account for the dynamic feedbacks between hydrodynamics and wetlands. For example, they also consider alterations to tidal range resulting from SLR and the effects of these on wetland inundation and sedimentation.

Many coastal wetlands are under pressure due to climate change and the associated sea level rise (SLR). Many previous studies suggest that upslope lateral migration is the key adaptive mechanism for saline wetlands, such as mangroves and saltmarshes. However, few studies have explored the long-term fate of other wetland types, such as brackish swamps and freshwater forests. Using the current wetland map of a micro-tidal estuary, the Manning River in New South Wales, Australia, this study built a machine learning model based on the hydro-geomorphological settings of four broad wetland types. The model was then used to predict the future wetland distribution under three sea level rise scenarios. The predictions were compared to compute the persistence, net, swap, and total changes in the wetlands to investigate the loss and gain potential of different wetland classes. Our results for the study area show extensive gains by mangroves under low (0.5 m), moderate (1.0 m), and high (1.5 m) sea level rise scenarios, whereas the other wetland classes could suffer substantial losses. Our findings suggest that the accommodation spaces might only be beneficial to mangroves, and their availability to saltmarshes might be limited by coastal squeeze at saline–freshwater ecotones. Furthermore, the accommodation spaces for freshwater wetlands were also restrained by coastal squeeze at the wetland-upland ecotones. As sea level rises, coastal wetlands other than mangroves could be lost due to barriers at the transitional ecotones. In our study, these are largely manifested by slope impacts on hydrology at a higher sea level. Our approach provides a framework to systematically assess the vulnerability of all coastal wetland types.

A modelling framework for using regional climate projections to assess flooding hazard has been developed and applied to the Gwydir River (catchment 26 600 km2 and floodplain 8100 km2), NSW, Australia. The model framework uses NSW and ACT Regional Climate Modelling version 1.5 projections combined with computationally efficient hydrologic and hydraulic models. Although it required model management and high-performance computing resources, the modelling framework successfully processed 18 regional climate projections into flood projections. Specifically, a six-member set of climate model combinations simulating a historical period (1951–2005) and a future period (2006–2100) under two global emission pathways (RCP4.5 and RP8.5) were used to predict flood depth and speed. In total, 1470 continuous years were simulated at hourly time steps. These flood (depth and speed) projections were analysed to assess the flood hazard changes under future climate scenarios by estimating changes in the annual probability of occurrence of a range of flood hazard classes. The six-member ensemble indicates that the flood hazard in the Gwydir Valley will decrease in the short, medium and long term. There are also cases within the ensemble, which includes increases in all non-safe flood hazard classifications while decreasing the safe flood hazard classification.

Previous research utilising water level observations and hypsometric data has suggested that intertidal areas exert some control on main channel flow dynamics in estuaries, lagoons and tidal creeks. This has been demonstrated in more detail for saltmarsh and mangrove creeks utilising measurements of tidal velocity. However, understanding of relationships between tidal hydrodynamics and intertidal wetlands is still lacking for mature barrier estuaries. Improved understanding of hydrodynamics in these systems, as well as potential interactions with tidal wetlands, may facilitate their effective management and modelling. This study investigates relationships between main channel hydrodynamics and vegetated intertidal wetlands at Minnamurra River estuary, southeast Australia, using observations of tidal dynamics and wetland inundation regime. Tidal data was collected over five spring-neap cycles utilising tidal gauges and drag-tilt flow meters at six locations in the estuary's main channel, and 14 pressure transducers along three wetland transects. Comparison of stage-velocity plots and hypsometric curves indicates that estuarine flow dynamics were spatially variable and strongly influenced by the geomorphology of intertidal wetlands in the upper estuary, where water surface gradients developing during flood and ebb phases created velocity pulses. Spatiotemporal analysis of tidal asymmetry showed variability along the estuary, as well as between spring and neap conditions. The estuary studied displayed an ebb-dominated deeper main channel (velocity asymmetry) surrounded by wide shallow flood-dominated margins, indicating that caution should be adopted when interpreting an estuary's tidal asymmetry using a single asymmetry characteristic. Overall, results presented here demonstrate a strong connection between estuarine hydrodynamics and vegetated intertidal wetlands, which implies that integrated approaches considering estuarine hydrodynamics and vegetated intertidal wetlands simultaneously are required to manage and model mature barrier estuaries.

Intertidal wetlands such as mangrove and saltmarsh are increasingly susceptible to areal losses related to sea level rise. This exposure is potentially offset by processes that might enable wetlands to accrete in situ or migrate landward under sea level rise, and planning policies that might open new opportunities for migration. We present and demonstrate a method to predict intertidal wetland distribution in the present-day landscape using random forest classification models, and use these models to predict the intertidal wetland distribution in future landscapes under specified sea level scenarios. The method is demonstrably robust in predicting present-day intertidal wetland distribution, with moderate correlation or better between predicted and mapped wetland distributions occurring in nearly all estuaries and strong correlation or better occurring in more than half of the estuaries. Given the accuracy in predicting present-day wetland distribution the method is assumed to be informative in predicting potential future wetland distribution when combined with best available models of future sea level. The classification method uses a variety of hydro-geomorphological surrogates that are derived from digital elevation models, Quaternary geology or soils mapping and land use mapping, which is then constrained by a representation of the future sea level inside estuaries. It is anticipated that the outputs from applying the method would inform assessments of intertidal wetland vulnerability to sea level rise and guide planning for potential wetland migration pathways.

Tidal inundation is a principal driver in intertidal wetland functioning. A combination of surface elevation and its relation to tidal range creates inundation regimes that influence a range of abiotic factors that affect wetland species distribution. Hydroperiod is a term frequently used to describe inundation regimes; however, the term has been vaguely defined in the past and typically quantified using tide gauge data rather than empirical field observations. This study explores relationships between various characteristics, such as frequency, total duration, average duration, average depth, non-tidal exposure, and mangrove/saltmarsh distribution in micro-tidal estuarine settings of southeast Australia. Inundation was measured directly using 35 pressure transducers embedded into mangrove and saltmarsh substrates over a period of two lunar tidal cycles along six transects located at Clyde and Minnamurra River estuaries, New South Wales, Australia. Results indicated differences in wetland inundation characteristics between mangrove and saltmarsh that are a function of wetland microtopography and site-specific tidal regime (e.g. local tidal range). For example, mangroves were inundated approximately 15–70% of the time, and 0.9–2 times a day, whereas saltmarshes were submerged only <15% of the time and less than 0.9 times per day. Comparison of direct field measurements and tide gauge-derived estimates of wetland inundation regime revealed that nearby gauge data may provide an acceptable estimate; however, distant tide gauge data provided poor estimates due to the effect of tidal dampening and amplification, respectively. Detailed analysis of wetland inundation characteristics as presented in this study demonstrates a method for describing inundation regimes at local scale. Improved understanding of contemporary tidal inundation dynamics in mangrove and saltmarsh can support modelling of wetland response to sea-level rise, as well as restoration practises by providing estimates of inundation tolerances.

Measurements of near‐bed shear stress were undertaken in the shallow subtidal zone at Durras Beach, NSW, Australia using a sideways‐looking acoustic velocity meter installed within the wave boundary layer. The wave climate was swell‐dominated and wave conditions comprised shoaling and breaking waves as well as surf bores. The sediment at the field site was medium‐grained sand, and observations of bedform geometry were conducted using a pencilbeam‐sonar system. Using frequency‐filtering techniques, the measured stresses were partitioned into terms representing turbulent (Reynolds) stress, stresses due to gravity and infragravity‐scale oscillatory motions, and wave‐turbulence‐mean current cross‐terms. Gravity wave‐orbital scale motions contributed the largest fraction of the stresses, comprising 24% on average, followed by long‐wave advection of vertical orbital motion (16%). The presence of wave orbital‐scale motions near or at the water/sediment interface was likely due to the porous nature of the seabed, facilitating interfacial flow. Shear stresses did not scale with bed roughness but exhibited a linear relationship with the relative wave height. This indicates that for the experimental conditions, surf zone processes overwhelmed bed roughness effects on shear stress and friction. Calculations of the wave friction factor, fw, showed that in a natural surf zone, this was a factor 3–4 larger than conventional predictions. © 2020 John Wiley & Sons, Ltd.

The existence of sandy beaches relies on the onshore transport of sand by waves during post-storm conditions. Most operational sediment transport models employ wave-averaged terms, and/or the instantaneous cross-shore velocity signal, but the models often fail in predictions of the onshore-directed transport rates. An important reason is that they rarely consider the phase relationships between wave orbital velocity and the suspended sediment concentration. This relationship depends on the intra-wave structure of the bed shear stress and hence on the timing and magnitude of turbulence production in the water column. This paper provides an up-to-date review of recent experimental advances on intra-wave turbulence characteristics, sediment mobilization, and suspended sediment transport in laboratory and natural surf zones. Experimental results generally show that peaks in the suspended sediment concentration are shifted forward on the wave phase with increasing turbulence levels and instantaneous near-bed sediment concentration scales with instantaneous turbulent kinetic energy. The magnitude and intra-wave phase of turbulence production and sediment concentration are shown to depend on wave (breaker) type, seabed configuration, and relative wave height, which opens up the possibility of more robust predictions of transport rates for different wave and beach conditions.

This chapter describes the morphological features and key processes related to the swash zone, situated at the landward edge of the inundated part of the beach system. It is where incoming surf zone waves force oscillatory motion of the shoreline at a variety of frequencies. Morphological features addressed include the beach face slope, beach berms, beach steps and beach scarps. Sediment sorting across the beach face profile and the formation of heavy mineral deposits are also discussed. In the recent literature, swash flow kinematics/dynamics and sediment transport mechanics have received more attention than the morphology. This research is discussed in the context of its relevance to an improved understanding of swash zone morphodynamics. Rather than presenting a detailed review of the available literature, the authors present their perspective on current research directions and outstanding issues still to be resolved.

Coastal wetlands are a critical component of the coastal landscape that are increasingly threatened by sea level rise and other human disturbance. Periodically mapping wetland distribution is crucial to coastal ecosystem management. Ensemble algorithms (EL), such as random forest (RF) and gradient boosting machine (GBM) algorithms, are now commonly applied in the field of remote sensing. However, the performance and potential of other EL methods, such as extreme gradient boosting (XGBoost) and bagged trees, are rarely compared and tested for coastal wetland mapping. In this study, we applied the three most widely used EL techniques (i.e., bagging, boosting and stacking) to map wetland distribution in a highly modified coastal catchment, the Manning River Estuary, Australia. Our results demonstrated the advantages of using ensemble classifiers to accurately map wetland types in a coastal landscape. Enhanced bagging decision trees, i.e., classifiers with additional methods to increasing ensemble diversity such as RF and weighted subspace random forest, had comparably high predictive power. For the stacking method evaluated in this study, our results are inconclusive, and further comprehensive quantitative study is encouraged. Our findings also suggested that the ensemble methods were less effective at discriminating minority classes in comparison with more common classes. Finally, the variable importance results indicated that hydro-geomorphic factors, such as tidal depth and distance to water edge, were among the most influential variables across the top classifiers. However, vegetation indices derived from longer time series of remote sensing data that arrest the full features of land phenology are likely to improve wetland type separation in coastal areas.

Detailed vegetation maps are needed for wetland conservation and restoration as different vegetation communities have distinct water requirements. It is a continuous challenge to map the distribution of different wetland types on a regional scale, and a trade-off between the categorical details and availability of resources to ensure broad applications is often necessary for operational mapping. Here, we evaluated the capacity and performance of statistical learning in discriminating wetland types using Landsat time series and geomorphological variables computed from Light Detection and Ranging (LiDAR) and Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM). Our study showed that there was a discrimination limit of statistical learning in wetland mapping. The approach was clearly inadequate in distinguishing certain wetland types. In semiarid Australia, our results suggested that the appropriate level for floodplain wetland mapping included four classes: tree-dominated woodlands, shrublands, vegetated swamps, and non-flood-dependent terrestrial communities. Our results also demonstrated that the geomorphological metrics significantly improved the accuracy of wetland classification. Furthermore, geomorphological metrics derived from the freely available coarser resolution SRTM DEM were as beneficial for wetland mapping as those extracted from finer scale commercially-based LiDAR DEM. The finding enables the widespread applications of our approach, as both data sources are freely available globally.

Beach recovery is key to the continued existence of sandy beaches and is typically driven by the onshore‐directed transport of sediment by short waves during low‐moderate energy conditions. The physical processes governing beach recovery are not well understood, but the theoretically developed dimensionless fall velocity, Ω = H/wsT, was suggested to be important for separating onshore/offshore sediment motion (Dean, 1973). In this paper, the effect of wave period and sediment grain size on short‐wave suspended sediment flux was investigated based on field measurements obtained beneath shoaling and breaking waves at Durras and Vejers beaches. The efficiency of the breaking waves in transporting suspended sediment onshore was roughly the same for the two beaches, despite the wave periods being larger and the mean sediment grain size coarser at Durras beach. The flux efficiencies were, however, shown to be degraded by wave‐current interactions and long/short‐wave interactions at Durras beach, especially. Excluding time series of strong undertow velocities (< −0.1 m/s) and infragravity wave‐energy (>5 m2/s3) resulted in significantly larger flux efficiencies beneath breaking waves at Durras beach compared to Vejers beach. These results indicate that wave‐current interactions and long/short‐wave interactions should be taken into consideration along with the wave period and mean grain size when estimating short‐wave suspended sediment fluxes. The results also showed that plunging breakers were more efficient in suspending sediment and transporting it onshore compared to spilling breakers/surf bores. This finding suggests that wave breaker type also is an important parameter to incorporate when modeling beach recovery.

The microtidal coast of New South Wales (NSW), Australia contains a broad range of estuary types that can be classified according to their internal geomorphology and entrance hydraulic conditions. We present an analysis of intertidal wetland area and the proportion of mangrove and saltmarsh, and investigate the variety of tidal inundation regimes in NSW estuaries. Our work hypothesizes that regional patterns in wetland extent and tidal inundation regimes are related to estuary type, through controls on intra-estuary geomorphology and modification of the open-coast tide. Several styles of tidal inundation regime are identified, which display markedly different combinations of the average frequency, duration and depth of inundation events for a specified elevation in the tidal frame. The regional-scale patterns in saline wetland extent and the tidal inundation regimes identified herein are potentially representative of microtidal estuaries more broadly, including similar estuary types in South Africa and Brazil.