Barrier Estuary Research Articles

Morphological and geological responses of barrier estuaries to dam-opening systems

River discharge depending on the dam gate operation method is an important factor underlying changes in estuarine sedimentation dynamics, but comprehensive studies are limited by data collection constraints. The hydrological operation of Nakdong Dam, designed to prevent seawater intrusion by opening during ebb tides and heavy rains and closing during flood tides, presents an ideal setting for such investigations. This study focused on the Nakdong River Estuary (NRE), utilizing aerial photography, bathymetry data, and sedimentation rate data to determine alterations in sediment transport and deposition patterns within the two tributary estuaries. Spatial discrepancies in sedimentation patterns across the eastern and western regions of the NRE were particularly evident in the spatial variations of textural characteristics and sedimentation rates. An enhanced ebb tidal prism resulting from dam gate opening in the eastern region caused the development of erosional topography and a rough depositional environment, forming a landward migrating (transgressive) barrier island system. Conversely, the predominantly closed dam gates in the western region maintained a consistent deposition pattern due to a regular tidal prism, facilitating the formation of sedimentary topography and fine sediment deposition. Consequently, barrier islands followed a seaward building (prograding) system. Therefore, the results suggested a role of modern dam gate operation as a major sedimentation mechanism, providing valuable insights for the sustainable management of estuarine sediment environments.

Rapid barrier estuary infill in a geologically-constrained setting: Aireys Inlet/Painkalac Creek, Victoria, Australia

The Holocene infill of an intermittently open-closed barrier estuary (Painkalac Creek/Aireys Inlet) was examined on the southern margins of Australia, through aerial LiDAR, coring, sedimentology, radiocarbon dating and pollen analysis. It is found that almost all the accommodation space within the estuary has been occupied, with a thick sequence (>9 m) of sediments being deposited rapidly soon after flooding by the sea. The rate of sedimentation was 4.8 mm/year which closely matches the rate of sea level rise (4.5 mm/year) during the 1200 year period that the estuary infilled, from around 8000–6800 years ago. The evidence of estuarine deposition above the elevation of the modern intertidal zone provides significant further evidence for the presence of the mid-Holocene highstand sea level in this relatively unstudied part of the Australian coast. The pattern of infill departs from the traditional barrier evolution models. The first phase of infill was likely tide dominated with subtidal sand shoals being found landward of a subtidal bedrock sill. Once a beach-barrier sequence formed, the estuary infilled in a more traditional manner, with lateral progradation of a flood tide delta and central mud basin infill. The barrier estuary of Painkalac Creek therefore has undergone a complex evolutionary history, characterised by a change in process dominance from classic tidal to wave-dominated form. Infill occurred in a keep-up (or fast catch-up) mode, with geological control occurring in regard to the presence of an intertidal bedrock sill at the mouth of the estuary.

Large structural estuaries: Their global distribution and morphology

Estuaries are marine-flooded topographic depressions that receive sediment from both marine and fluvial sources. The morphology of the underlying drainage basin acts as a boundary condition for the spatial distribution of hydrodynamic energy within an estuary, and by extension the resultant depositional environment. In some large estuaries, bedrock structures at the mouth restrict oceanographic forcing, which has impacts on sedimentary infill. Such estuaries are termed here as Large Structural Estuaries (LSEs). For LSEs, the shape of the basin and associated bedrock morphology is fixed on the millennial scale having been formed by geological processes operating over timescales greater than that of the Holocene. However, the distribution and abundance of these LSEs are poorly understood. In this study, a global survey of large estuaries was conducted to identify and characterize LSEs from the worldwide population to understand regional drivers of coastal geomorphology. A total of 271 large (>50 km2) estuaries were identified, and these were clustered into 5 classes using Gaussian Mixture Modelling. These clusters were interpreted as Barrier Estuaries, Rocky Bays, Sandy Bays, Funnelled Estuaries, and LSEs. LSEs comprised 28.04 % of all large estuaries identified and were spatially heterogenous, associated with collision coastlines, and predominantly located at higher latitudes except for the Malay Archipelago. The global distribution of LSEs suggests that regional distribution of tectonic processes strongly influences their formation. LSEs are important global features comprising many of the major ports in the world and this work indicates they are unique landforms and traditional approaches to understanding sedimentation in these settings require adjustment to their unique geomorphology.

The race for space: Modelling the landward migration of coastal wetlands under sea level rise at regional scale

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.

Sediment dynamics at different timescales on an embayed coast in southeastern Australia

The concept of coastal sediment compartments has recently been adopted at a national scale in Australia to better understand sediment and shoreline dynamics and to underpin management of future shoreline behaviour in response to impacts of climate change. Geomorphological studies in southern NSW have provided a foundation for development of conceptual models of estuary and sandy barrier evolution. Geochronological reconstructions using radiocarbon, optically-stimulated luminescence, and other dating techniques, reviewed in this paper, demonstrate that adjacent compartments are at successive stages. Three compartments, Illawarra, Shoalhaven and Moruya, are compared, each with different catchment characteristics and different levels of human intervention. Landform change and sediment accumulation at millennial timescales enable estimates of past sediment accretion (vertical accumulation) and horizontal displacement of shorelines (particularly progradation), as a first step towards quantifying volumetric changes of morphology. Lake Illawarra is a barrier estuary at an early stage of infill, but land-use change, urbanisation, and engineering structures at the entrance have accelerated rates of sediment accumulation. The Shoalhaven River has infilled its estuary and delivers sand to the coast. It has been subject to several conspicuous anthropogenic interventions. At Moruya, ongoing supply of sand, primarily from offshore rather than from the catchment, has resulted in beach-ridge plains (strandplains) with changes in their alongshore inter-connectivity driven by differential embayment infilling. Millennial-scale geomorphology indicates landform change providing a means to determine natural trajectories of sediment transfer. However, variability is apparent at century and decadal timescales, compounded by various anthropogenic interventions. Disentangling natural and anthropogenic influences will be necessary to provide greater confidence in estimating past and present sediment budgets. Assessing sand sources and transport rates is important in relation to engineering interventions at entrances, and long-term resilience of coastal habitats. Such issues are the focus of coastal management programs, and this synthesis emphasises the relevance of a sediment budget approach to understand contemporary sediment pathways and provide an indication of future response to engineering interventions and sensitivity to climate change.

An Eco-Morphodynamic Modelling Approach to Estuarine Hydrodynamics & Wetlands in Response to Sea-Level Rise

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.

Holocene evolution and depositional model of a bayhead delta, Lake Illawarra, Australia

AbstractExtensive research into the evolution of wave‐dominated estuaries has been documented since the 1960s. However, there has only been limited research on the prograding bay‐head deltas that are the primary drivers for the rate and stage of estuary evolution. This paper presents the findings of a high‐resolution spatial study into the evolution of the Macquarie Rivulet bayhead delta in the Lake Illawarra barrier estuary. The delta’s evolution has been established based on sedimentological analysis of 74 cores, two14C ages and 45 amino acid racemization ages. This study intersected two Pleistocene and 10 Holocene sedimentary facies associations representing both the retrogradational and progradational phases of delta development. The distribution of bayhead delta facies associations in shallow barrier estuaries is initially controlled by the antecedent morphology, but, in contrast to deeper estuaries, this influence decreases as sedimentation proceeds. Changing relative sea level also has a major control on facies distribution with transgressive facies deposited as relative sea‐level rises being replaced by prograding deltaic facies during the highstand and subsequent minor relative sea‐level fall. Fluvial sediment supply and river flood events affect the rate of delta progradation, and produce low and high flow sediment deposits within the overall deltaic sequence. The low tidal flux in barrier estuaries, the muddy cohesive nature of the sediments and the prevalence of bioturbation means that primary sedimentary structures are rarely preserved in these bayhead delta facies. The depositional model of bayhead delta evolution shown by the Macquarie Rivulet delta would be widely applicable to other similar barrier estuary settings.

Intertidal wetland geomorphology influences main channel hydrodynamics in a mature barrier estuary

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.

Holocene sea-level change and estuary infill in North West Nelson, central New Zealand

The sedimentary sequences found within estuaries in the north west Nelson region of central New Zealand are investigated in order to quantify the timing of the end of the Post Glacial Marine Transgression. This region has been identified as being relatively stable in terms of vertical tectonic movement during the Holocene, but is yet to yield any reconstructions of eustatic sea level. In this study, we investigate the Holocene infill of a barrier estuary (Parapara Inlet) through sedimentological analysis and radiocarbon dating of 18 vibracores up to 4.2 m in length. It is found that the estuary infilled through a combination of lateral flood tide and fluvial delta progradation as well as vertical central basin infill. The central basin infilled at a consistent rate of 0.4 mm/year in both the mid (7.0–6.0 ka) and late-Holocene (2.5–1.5 ka). By the time of early human (Maori) settlement (c. 1 ka), the estuary surface was at low intertidal elevations with sediment being transported from the fluvial to tidal delta. A discernible change in sedimentation rates could not be associated with Maori settlement; however, infill rates increased to at least 12.5 mm/year in the past 150 years due hydraulic sluicing associated with mining. The sedimentary history of Parapara Inlet is compared to nearby Whanganui Inlet, d’Urville Island and Nelson to establish the character of regional Holocene sea level movement. It is found that relative sea level reached modern elevations between 8 and 7 ka in the region. The similarity between sea level curves for the end of the post glacial marine transgression (PMT) to other tectonically stable sites in northern New Zealand suggests that this curve can now be considered a true eustatic signal for the New Zealand archipelago.

Sediment budget of a river-fed wave-dominated coastal compartment

A sediment compartment approach is an appropriate framework within which to undertake planning and management of coastal environments, and a hierarchical scheme has been adopted by several state governments in Australia. This study applies a geospatial comparison of terrain modelling to estimate decadal-scale sediment transfer in a closed wave-dominated coastal compartment in southeastern Australia. The Shoalhaven River, one of the larger rivers in southern New South Wales, drains from a temperate catchment of 7151 km2 into Shoalhaven Bight, a secondary compartment. The river has infilled the barrier estuary at its mouth in recent millennia, and delivers sediment that nourishes a well-developed strandplain to its north. We estimate sediment yield from the heterogeneous catchment based on deposition between 2003 and 2014 bathymetric surveys in a reservoir formed as a result of dam construction. Delivery of ~86,000 m3/y to the estuarine channel was calculated, adopting a known trap efficiency at the dam, augmented by sediment from a further unmodified tributary. Volumetric change of the estuary and adjacent nearshore was determined by comparison of surveys in 1981 and 2006. A complex pattern of estuarine surficial sediments reflects modification of the river's natural course, which is now artificially diverted to exit at Crookhaven Heads. During major flood events, the river intermittently discharges material to the shoreface at Shoalhaven Heads, its former mouth, which is impounded again by deposition of a sandy berm in the months following such storms. Between 1981 and 2006, at least 1,020,000 m3 was added to the estuary, and 1,065,000 m3 of sand was discharged to the shoreface. Sand sourced from the Shoalhaven River is retained within the northernmost of three tertiary compartments, Seven Mile Beach, as cliffed headlands and rocky reefs restrict along-shore sediment contributions from the south and inhibit leakage to the north. Balance of the budget based on shoreline accretion during 41 years is consistent with cross-shore transport and an ongoing shoreface supply rate of 1–2 m3/m/y. Although a number of uncertainties constrain the final sediment budget, with complex patterns of erosion or accretion in response to natural events or engineering interventions, this preliminary study indicates the potential for deriving decadal-scale estimates of sediment pathways and transfer rates using geospatial analysis of terrain changes, which can provide a basis for planning and management actions.

Heterogeneous tidal marsh soil organic carbon accumulation among and within temperate estuaries in Australia

The scarcity of data on tidal marsh soil accumulation rates (SAR) and soil organic carbon accumulation rates (CAR) globally precludes a comprehensive assessment of the role of tidal marshes in climate change mitigation and adaptation. Particularly few data exist from the southern hemisphere and for Australia in particular, which contains ~24% of globally recognised tidal marsh extent. Here we estimate SAR and CAR over the last 70 years using 210Pb-based geochronologies in temperate estuarine tidal marsh ecosystems in southern Western Australia (WA). Specifically, we assessed tidal marsh ecosystems situated in two geomorphic settings (marine vs. fluvial deltas) within 10 wave-dominated, barrier estuaries. Overall, average SAR (1.1 ± 0.3 mm yr−1) and CAR (32 ± 9 g m−2 yr−1) estimates were 5-fold lower than global mean estimates. Furthermore, we showed that hotspots of soil organic carbon stocks are not indicative of current hotspots for CAR. The lack of significant differences (P > 0.05) in SAR, CAR, and excess 210Pb inventories between marine and fluvial settings can be explained by the high heterogeneity among and within estuaries throughout the region. The relative stability of recent and Holocene relative sea-levels in WA likely explains the limited CAR potential in tidal marshes under relatively stable sea-level conditions. However, further research exploring interactions among biotic and abiotic factors within estuaries is required to shed more light on the small spatial-scale variability in SAR and CAR across tidal marsh ecosystems in WA and elsewhere. This study provides baseline estimates for the inclusion of tidal marshes in national carbon inventories, identifies hotspots for the development of blue carbon projects, and supports the use of site-specific assessments opposed to regional means for estimating blue carbon resources.

Holocene infill of the Anglesea Estuary, Victoria: a keep-up estuary in a geologically constrained environment

Holocene coastal infill was examined in Anglesea Estuary, an intermittently open barrier-type estuary in Victoria. Based on coring and radiocarbon dating, it was found that accommodation space in the estuary was almost entirely occupied in the early–mid Holocene corresponding to periods of higher sea-level. The pattern of infill is in stark contrast to established models of estuarine sedimentation developed on the east Australian coast. The contemporary estuary is composed of two distinct sedimentary basins. The upper basin is infilled by autochthonous peaty-marsh sediments, whereas the lower basin is infilled by saltmarsh material. The traditional tripartite flood-tide delta–central mud basin–fluvial delta sequence is absent. The contemporary estuary appears to have evolved as two distinct basins, which are connected through hydrodynamic rather than sedimentological processes. This pattern of connected basins occurs from the top of the catchment to the wave base, with the contemporary estuary infilling the two basins coincident with modern sea-level. Low sediment supply from both terrestrial and marine sources and bedrock-limited accommodation space means that Anglesea can be considered to be geologically constrained barrier estuary. KEY POINTS Geological-control limits mud basin development. Low allochthonous sediment supply significantly affected estuarine facies in Anglesea. Anglesea Estuary infilled to higher mean sea-level elevations in the mid Holocene. Anglesea Estuary is composed of two distinct sedimentary basins.

SALINITY RESPONSE TO ENVIRONMENTAL FLOW RELEASE IN ESTUARIES

The Snowy River in southern Australia has been impacted by flow diversion since the construction of a dam in the upper catchment, constructed between 1955 and 1967. As part of a monitoring program the effects of two flow releases were studied in 2010 and 2011. The estuarine component of the monitoring and the estuarine modelling phase of the Snowy River Increased Flows Program has been presented. The impact on the estuarine salinity distribution for the selected flow releases is reported and a subsequent modelling exercise outlined. A simple numerical model has been used to simulate about 100 events in a mature barrier estuary, from which a sequence of response types has been identified. The occurrence of each response type has been related to the duration, inflow volume and peak flow rate of the inflow event and to relevant parameters of the estuary. It has been found that the salinity changes may be classified in terms of a dimensionless "estuary flushing parameter" E, which represents the ratio of the direct flushing by the river inflow to the tidal exchange.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/76fefltUCro

STABILITY OF SMALL BARRIER ESTUARIES: FIRST PASS ASSESSMENT USING PUBLIC DATA AND ATTRACTOR MODELLING

Small barrier estuaries are common in temperate latitudes where catchment runoff is small. The entrance state controls the tidal exchange and hence the salinity regime and flushing. To aid the study and management of these estuaries, predictions of the stability and future evolution of the estuary entrance must be made. This paper demonstrates the application of the attractor method to determine the stability of a wide sample of estuaries on the south-eastern coast of Australia, using only data available in broad-based public data bases. The method uses a simple hydrodynamic-sediment balance model, run for thousands of scenarios and thousands of tide cycles to identify the long-term dynamic equilibria - the attractors. The model predictions are shown to match stability data in the data bases and to provide realistic predictions of the entrance evolution. The results have direct applicability to high-level assessment of coastal assets and to optimal selection of model scenarios for more detailed modelling of any selected estuary.

Evolution from estuary to delta: Alluvial plain morphology and sedimentary characteristics of the Shoalhaven River mouth, southeastern Australia

The Shoalhaven River in southern New South Wales (NSW) drains a catchment of ~7000 km2, forming a barrier estuary at its mouth with a prograded strandplain to its north. Previous coring and dating across the floodplain and adjacent strandplain indicate progressive infill of the estuarine basin over the past 7000 years. Over this time the system transitioned from a wave-dominated estuary to a wave-dominated delta. The Shoalhaven River appears to have initially discharged to the sea at Crookhaven Heads, until it adopted a more direct route through an intermittently open mouth at Shoalhaven Heads; construction of Berrys Canal in 1822 now permanently links the Shoalhaven River to the adjacent smaller Crookhaven River. Airborne LiDAR reveals details of topography of prominent levees and meander scroll bars that flank the lower reaches of the river and beach/foredune ridges to the north. Sedimentological and mineralogical characteristics of modern sediment samples collected from the river channel, the beach, and the barrier reveal that the Shoalhaven River is dominated by quartz sand along its predominantly straight course through the infilled estuarine plains. This is in contrast to the tripartite classification of sedimentary depositional environments, comprising sandy fluvial delta, central mud basin, and sandy marine barrier, that characterised the initial wave-dominated estuary 7000 years ago, and is typical of most barrier estuaries in NSW. Grain size decreases downstream and sands have a higher feldspar content, and angularity, than is typical of marine sand that has been reworked in the nearshore. This fluvially-derived component is delivered to the coast intermittently by floods that re-open Shoalhaven Heads, as shown by retrospective analysis of aerial photography and satellite imagery. Grain size decreases with distance alongshore from the river mouth indicating that these sands are transported north by longshore drift to augment a gradual onshore delivery of sand within this coastal compartment. Scanning electron microscopy of selected grains implies that this fluvial sand has been contributing to incremental formation of beach ridges for much of the past 3000 years since the estuarine basin has been largely infilled, with a slight detectable acceleration in the rate of progradation.

Using macrofaunal communities to inform estuarine classification

Worldwide, geomorphological classifications of estuaries are often used to guide the design of monitoring programs and management strategies. However, if classifications do not reflect biotic patterns, the effectiveness of monitoring and management is potentially reduced. In this study, we consider the effectiveness of one classification scheme in describing biotic patterns by examining and comparing spatial variation of macrofaunal assemblages and their relationship with the environment in 12 estuaries of 2 geomorphological types (mesotidal river dominated and permanently open barrier estuaries). Estuaries were sampled at three locations (upper, mid and lower) for macroinvertebrates and environmental characteristics. Differences in macrofaunal assemblages occurred between the estuary types at the lower and mid locations, but not the upper. Similarities in the upper locations were related to sediment, whereas at the mid and lower locations differences were linked to salinity, dissolved oxygen concentrations and seagrass area. Within estuaries, location effects were definitive and unique within each estuary type, correlating to changes in sediment particle size, nitrogen concentration, microphytobenthos and percentage organic carbon. These results suggest that estuarine classification based on physical attributes alone does have the potential to capture important biological attributes if the biological scales of variability within these systems are well understood.

Response of a barrier estuary to climate change and river regulation using attractor analysis: Snowy River estuary, Australia

AbstractThe evolution of the entrance channel of the Snowy River estuary in response to river regulation and climate change is predicted. The predictions are made in terms of the physical attractors that define possible long‐term states of the estuary entrance condition. The classification of these attractors shows the dependence of the entrance stability on the catchment inflows and the present entrance depth. The Snowy River estuary in south‐eastern Australia is a barrier estuary with an unstable entrance that tends to closure. The classification from the attractor map shows that the estuary entrance has changed from predominantly stable to a predominantly unstable state attributable to diversion of water from the upper catchment. The introduction of a series of environmental flow regimes, commencing in 2002, has returned 8% rising to 21% of the mean annual natural flow, but this study shows that the releases provide limited improvement in entrance stability. Additionally, the predicted effects of climate change for this region include increased mean sea level (MSL), decreased annual rainfall, and increased incidence of storms. These changes will decrease stability, primarily through the rise in MSL. The rise in sea level will increase the plan area of the tidal basin, increasing the tidal prism, and hence drawing in more marine sand. The application to the Snowy River estuary provides a proof of concept of the attractor classification to support estuary management.

Blue carbon in coastal landscapes: a spatial framework for assessment of stocks and additionality

Efforts to incorporate blue carbon, the carbon associated with marine ecosystems, into carbon accounting and carbon markets are increasing. To fully leverage the capacity of marine ecosystems to sequester carbon requires information to guide prioritisation of coastal landscapes for conservation and regeneration. A comprehensive landscape-scale assessment of mangrove and saltmarsh blue carbon requires information regarding vegetation cover, sedimentological and geomorphological factors. This information should also be integrated with socio-economic factors that alter natural processes of blue carbon accumulation and storage. The purpose of this study was to provide a framework for undertaking a first-pass assessment of blue carbon storage, preservation, generation and permanency in coastal landscapes, and to incorporate socio-economic factors that will influence blue carbon storage in coastal landscapes. This was achieved using readily available datasets that were analysed using a raster-based approach to develop a proxy indication of biophysical and socio-economic factors relevant for mangrove and saltmarsh blue carbon. The approach demonstrated that large catchments were associated with areas highly suitable for blue carbon storage, preservation, generation and permanency. Small catchments associated with mature barrier estuaries had the highest proportional area that provides potential blue carbon ecosystem services and climate mitigation benefits. The qualitative approach used does not replace high-resolution quantitative assessments of blue carbon storage, flux and detailed site-specific assessment of socio-economic factors that may influence blue carbon services; however, it can be used to guide prioritisation of blue carbon landscapes for further assessment prior to conservation and/or regeneration.

Tidal inlets and estuaries: Comparison of Bruun, Escoffier, O'Brien and attractors

The authors have shown previously that over very long time scales a barrier estuary or tidal inlet will tend towards one of two states, called attractors. In this paper it is shown how that analysis represents an extension and generalisation of three earlier procedures. There are three widely recognised quantitative semi-empirical procedures describing the long term equilibrium dimensions of the entrance channel of barrier estuaries and tidal inlets. The best known of these laws is the tidal prism-entrance area relation, often referred to as the O'Brien equation. The second procedure is based on the Escoffier or closure diagram, comprising a simple hydrodynamic relationship between the entrance area and the entrance velocity together with an empirical "equilibrium velocity". The third is the set of rules developed by Bruun that relate the entrance channel stability to the longshore sediment supply and the entrance channel sediment transport capacity. Each of these is based on major simplifications that restrict its utility and range of validity more than is usually recognised.The attractor analysis, while still based on a lumped model, adds sediment transport and deposition/scour equations and enables the use of more realistic entrance hydrodynamics. It predicts the rates of change and presents the results on a practical “attractor map”. The predictions of the three empirical laws are compared with the attractor map and their practical application is critically compared. The empirical laws are shown to provide broadly equivalent predictions to the attractor map, but over limited ranges of estuary conditions. In particular, none of the empirical laws identifies the entrance closure state or describes conditions near closure.

Assessing climate change impacts on the stability of small tidal inlets: Part 2 - Data rich environments

Climate change (CC) is likely to affect the thousands of bar-built or barrier estuaries (here referred to as Small tidal inlets - STIs) around the world. Any such CC impacts on the stability of STIs, which governs the dynamics of STIs as well as that of the inlet-adjacent coastline, can result in significant socio-economic consequences due to the heavy human utilisation of these systems and their surrounds. This article demonstrates the application of a process based snap-shot modelling approach, using the coastal morphodynamic model Delft3D, to 3 case study sites representing the 3 main STI types; Permanently open, locationally stable inlets (Type 1), Permanently open, alongshore migrating inlets (Type 2) and Seasonally/Intermittently open, locationally stable inlets (Type 3). The 3 case study sites (Negombo lagoon – Type 1, Kalutara lagoon – Type 2, and Maha Oya river – Type 3) are all located along the southwest coast of Sri Lanka.After successful hydrodynamic and morphodynamic model validation at the 3 case study sites, CC impact assessment are undertaken for a high end greenhouse gas emission scenario. Future CC modified wave and riverflow conditions are derived from a regional scale application of spectral wave models (WaveWatch III and SWAN) and catchment scale applications of a hydrologic model (CLSM) respectively, both of which are forced with IPCC Global Climate Model output dynamically downscaled to ~50km resolution over the study area with the stretched grid Conformal Cubic Atmospheric Model CCAM. Results show that while all 3 case study STIs will experience significant CC driven variations in their level of stability, none of them will change Type by the year 2100. Specifically, the level of stability of the Type 1 inlet will decrease from ‘Good’ to ‘Fair to poor’ by 2100, while the level of (locational) stability of the Type 2 inlet will also decrease with a doubling of the annual migration distance. Conversely, the stability of the Type 3 inlet will increase, with the time till inlet closure increasing by ~75%. The main contributor to the overall CC effect on the stability of all 3 STIs is CC driven variations in wave conditions and resulting changes in longshore sediment transport; not Sea level rise as commonly believed.