The sediment budget as a management tool: the Shoalhaven Coastal Compartment, southeastern NSW, Australia

Sediment budgets are fundamental approaches in coastal studies for allowing estimates of volumes of sediments entering and exiting a selected area of the coast, resulting in net erosion or accretion of that compartment under consideration. This assessment is crucial for understanding current processes and predicting future effects of sedimentimpact activities, promoting the sustainability of coastal environments over the next centuries. In this paper we present a series of preliminary spatial, sedimentological and geophysical analyses undertaken in order to understand the sources, sinks, transport and pathways for the sediment budget of the Shoalhaven coast, a compartment whose sediment provision is supplied primarily by the Shoalhaven River (draining a catchment of 7,151 km2) and that stretches ~32 km from the rocky headland of Black Head at Gerroa (north) to the Beecroft Peninsula near Currarong (south). Analysis included the use of sub-bottom profiler, ground penetrating radar, RTK-GPS, aerial photographs, satellite images, LiDAR, echosounding, computer modelling, as well as grain size parameters from ~200 sediment (and mineralogy for selected) samples from the estuary, beach and shoreface

Terrestrial laser scanning to quantify above-ground biomass of structurally complex coastal wetland vegetation

Above-ground biomass represents a small yet significant contributor to carbon storage in coastal wetlands. Despite this, above-ground biomass is often poorly quantified, particularly in areas where vegetation structure is complex. Traditional methods for providing accurate estimates involve harvesting vegetation to develop mangrove allometric equations and quantify saltmarsh biomass in quadrats. However broad scale application of these methods may not capture structural variability in vegetation resulting in a loss of detail and estimates with considerable uncertainty. Terrestrial laser scanning (TLS) collects high resolution three-dimensional point clouds capable of providing detailed structural morphology of vegetation. This study demonstrates that TLS is a suitable non-destructive method for estimating biomass of structurally complex coastal wetland vegetation. We compare volumetric models, 3-D surface reconstruction and rasterised volume, and point cloud elevation histogram modelling techniques to estimate biomass. Our results show that current volumetric modelling approaches for estimating TLS-derived biomass are comparable to traditional mangrove allometrics and saltmarsh harvesting. However, volumetric modelling approaches oversimplify vegetation structure by under-utilising the large amount of structural information provided by the point cloud. The point cloud elevation histogram model presented in this study, as an alternative to volumetric modelling, utilises all of the information within the point cloud, as opposed to sub-sampling based on specific criteria. This method is simple but highly effective for both mangrove (r 2 = 0.95) and saltmarsh (r 2 \u3e 0.92) vegetation. Our results provide evidence that application of TLS in coastal wetlands is an effective non-destructive method to accurately quantify biomass for structurally complex vegetation

Probabilistic coastal hazard lines for risk based coastal assessment

As part of a recent NCCARF funded project Approaches to Risk Assessment on Australian Coasts , a modelling framework was developed which integrated geological, engineering and economic approaches for assessing the risk of climate change along the Australian Coast. This paper aims to demonstrate the working of the framework in deriving probabilistic coastal hazard lines. Within the framework, means for combining results from models that focus on the decadal to century time scale (geomorphic), and those that focus on the short term and seasonal time scales (storm bite and recovery) have been developed. This combination is necessary for the derivation of probabilistic hazard lines. The Narrabeen - Collaroy embayment on the northern beaches of Sydney was chosen as an appropriate study site due to its data rich nature, with directional wave records extending back 20 years, and ongoing repeated beach survey available since the mid 1970\u27s. The site has been subject to extensive study over recent decades. To demonstrate operation of the framework two models with stochastic capabilities were adapted for use in the study. These are the Shoreface Translation Model (STM), for century scale geomorphic evolution, and the Joint Probability Method - Probabilistic Coastline Recession (JPM-PCR) for shorter term beach erosion and recovery. Both models are introduced and discussed. When projecting forward to future scenarios involving sea level rise, the framework also enables sea level rise over time to be input as a probabilistic variable. Recent research has also provided some guidance as to how this can be achieved using outputs from the most recent IPCC estimates. Overall, the research efforts have aimed to point a way forward that enables the quantitative assessment of coastal hazard likelihood for use in robust coastal risk assessment. This contrasts with present practice which typically adopts a more qualitative approach to risk assessment

Reply to: Punctuated transgression (?): Comment on Oliver, T.S.N., Donaldson, P., Sharples, C., Roach, M., and Woodroffe, C.D. Punctuated progradation of the Seven Mile Beach Holocene barrier system, southeastern Tasmania

Our interpretation of the depositional history of the prograded barrier at Seven Mile Beach in Tasmania, described in Oliver et al. (2017a), was based on the morphology of ridges apparent in the LiDAR-based digital elevation data and a sample of 14 optically-stimulated luminescence (OSL) ages. Dougherty (2018) has identified gaps in the chronology and speculated that progradation may have occurred as sea level fell from a mid-Holocene highstand inappropriately applying sea-level curves from mainland Australia. Despite a highstand being inferred by early research in Tasmania, glacio-isostatic modelling and recent sea-level studies adopted a prevailing view that excluded a highstand. Our observations led us to question this prevailing view and to suggest that it might be appropriate to reopen the debate on Holocene sea-level change in Tasmania. We welcome the renewed interest in the chronology and sea-level history of this prograded barrier, and look forward to further clarification based on new evidence. The site may have the potential to become one of the more continuous and better-constrained sea-level records in southern Australia

Postglacial fringing-reef to barrier-reef conversion on Tahiti links Darwin's reef types.

In 1842 Charles Darwin claimed that vertical growth on a subsiding foundation caused fringing reefs to transform into barrier reefs then atolls. Yet historically no transition between reef types has been discovered and they are widely considered to develop independently from antecedent foundations during glacio-eustatic sea-level rise. Here we reconstruct reef development from cores recovered by IODP Expedition 310 to Tahiti, and show that a fringing reef retreated upslope during postglacial sea-level rise and transformed into a barrier reef when it encountered a Pleistocene reef-flat platform. The reef became stranded on the platform edge, creating a lagoon that isolated it from coastal sediment and facilitated a switch to a faster-growing coral assemblage dominated by acroporids. The switch increased the reef's accretion rate, allowing it to keep pace with rising sea level, and transform into a barrier reef. This retreat mechanism not only links Darwin's reef types, but explains the re-occupation of reefs during Pleistocene glacio-eustacy

Shifting perspectives on coastal impacts and adaptation

The Intergovernmental Panel on Climate Change reports reflect evolving attitudes in adapting to sea-level rise by taking a systems approach and recognizing that multiple responses exist to achieve a less hazardous coast.info:eu-repo/semantics/publishedVersio

Winners and losers as mangrove, coral and seagrass ecosystems respond to sea-level rise in Solomon Islands

A 2007 earthquake in the western Solomon Islands resulted in a localised subsidence event in which sea level (relative to the previous coastal settings) rose approximately 30-70 cm, providing insight into impacts of future rapid changes to sea level on coastal ecosystems. Here, we show that increasing sea level by 30-70 cm can have contrasting impacts on mangrove, seagrass and coral reef ecosystems. Coral reef habitats were the clear winners with a steady lateral growth from 2006-2014, yielding a 157% increase in areal coverage over seven years. Mangrove ecosystems, on the other hand, suffered the largest impact through a rapid dieback of 35% (130 ha) of mangrove forest in the study area after subsidence. These forests, however, had partially recovered seven years after the earthquake albeit with a different community structure. The shallow seagrass ecosystems demonstrated the most dynamic response to relative shifts in sea level with both losses and gains in areal extent at small scales of 10-100 m. The results of this study emphasize the importance of considering the impacts of sea-level rise within a complex landscape in which winners and losers may vary over time and space

A global biophysical typology of mangroves and its relevance for ecosystem structure and deforestation

Mangrove forests provide many ecosystem services but are among the world's most threatened ecosystems. Mangroves vary substantially according to their geomorphic and sedimentary setting; while several conceptual frameworks describe these settings, their spatial distribution has not been quantified. Here, we present a new global mangrove biophysical typology and show that, based on their 2016 extent, 40.5% (54,972 km2) of mangrove systems were deltaic, 27.5% (37,411 km2) were estuarine and 21.0% (28,493 km2) were open coast, with lagoonal mangroves the least abundant (11.0%, 14,993 km2). Mangroves were also classified based on their sedimentary setting, with carbonate mangroves being less abundant than terrigenous, representing just 9.6% of global coverage. Our typology provides a basis for future research to incorporate geomorphic and sedimentary setting in analyses. We present two examples of such applications. Firstly, based on change in extent between 1996 and 2016, we show while all types exhibited considerable declines in area, losses of lagoonal mangroves (- 6.9%) were nearly twice that of other types. Secondly, we quantify differences in aboveground biomass between mangroves of different types, with it being significantly lower in lagoonal mangroves. Overall, our biophysical typology provides a baseline for assessing restoration potential and for quantifying mangrove ecosystem service provision

Widespread retreat of coastal habitat is likely at warming levels above 1.5 °C

Several coastal ecosystems—most notably mangroves and tidal marshes—exhibit biogenic feedbacks that are facilitating adjustment to relative sea-level rise (RSLR), including the sequestration of carbon and the trapping of mineral sediment. The stability of reef-top habitats under RSLR is similarly linked to reef-derived sediment accumulation and the vertical accretion of protective coral reefs. The persistence of these ecosystems under high rates of RSLR is contested. Here we show that the probability of vertical adjustment to RSLR inferred from palaeo-stratigraphic observations aligns with contemporary in situ survey measurements. A defcit between tidal marsh and mangrove adjustment and RSLR is likely at 4 mm yr−1 and highly likely at 7 mm yr−1 of RSLR. As rates of RSLR exceed 7 mm yr−1, the probability that reef islands destabilize through increased shoreline erosion and wave over-topping increases. Increased global warming from 1.5 °C to 2.0 °C would double the area of mapped tidal marsh exposed to 4 mm yr−1 of RSLR by between 2080 and 2100. With 3 °C of warming, nearly all the world’s mangrove forests and coral reef islands and almost 40% of mapped tidal marshes are estimated to be exposed to RSLR of at least 7 mm yr−1. Meeting the Paris agreement targets would minimize disruption to coastal ecosystems