Shoreface response to sea level change and the evolution of barrier coasts

This thesis investigates shoreface response to sea level change, and the evolution of wave-dominated coastal barrier systems, in response to late-Quaternary and future environmental change. A numerical stratigraphic model (BARSIM) was calibrated using chronostratigraphic evidence from barrier coasts of southeastern Australia, and was used to explore shoreface response and coastal evolution during the last-glacial cycle and Holocene highstand. The modelling supported previous suggestions that Holocene strandplain progradation was primarily sourced from disequilibrium-stress-induced onshore sand supply, due to the erosion of lower-shoreface sand bodies (e.g. coastal barriers that were overstepped during late transgression). Simulated mid- to late-Holocene sea-level fall, within the bounds of existing evidence, was insufficient to supply observed strandplain progradation. Furthermore, external sand supply resulted in progradation well beyond the depth of present-day barrier complexes. Highstand barrier stacking was found to vary with prior transgressive-barrier behaviour: sea level change and substrate physiography controlled barrier rollover and overstepping behaviours, which resulted in alternative stacking relationships. Hypothetical forward simulations were also carried out to assess the sensitivity of shoreface response (i.e. active shoreface extent, depth-dependent shoreface response rates) and coastal evolution to sea level change. For increasing rates of sea level change typical of the late Quaternary and projected sea-level rise (0.05-10 mm/yr), time-invariant active-shoreface behaviour contracted from the lower shoreface toe to the surf zone. Depth-dependent lower-shoreface erosion was most significant for mesoscale coastal evolution (i.e. 102-105 yrs), which is characterised by partial shoreface activity. The findings suggest that depth- and timescale-dependent shoreface response limit the reliability of equilibrium-profile models for mesoscale problems

Shoreface response to sea level change and the evolution of barrier coasts

This thesis investigates shoreface response to sea level change, and the evolution of wave-dominated coastal barrier systems, in response to late-Quaternary and future environmental change. A numerical stratigraphic model (BARSIM) was calibrated using chronostratigraphic evidence from barrier coasts of southeastern Australia, and was used to explore shoreface response and coastal evolution during the last-glacial cycle and Holocene highstand. The modelling supported previous suggestions that Holocene strandplain progradation was primarily sourced from disequilibrium-stress-induced onshore sand supply, due to the erosion of lower-shoreface sand bodies (e.g. coastal barriers that were overstepped during late transgression). Simulated mid- to late-Holocene sea-level fall, within the bounds of existing evidence, was insufficient to supply observed strandplain progradation. Furthermore, external sand supply resulted in progradation well beyond the depth of present-day barrier complexes. Highstand barrier stacking was found to vary with prior transgressive-barrier behaviour: sea level change and substrate physiography controlled barrier rollover and overstepping behaviours, which resulted in alternative stacking relationships. Hypothetical forward simulations were also carried out to assess the sensitivity of shoreface response (i.e. active shoreface extent, depth-dependent shoreface response rates) and coastal evolution to sea level change. For increasing rates of sea level change typical of the late Quaternary and projected sea-level rise (0.05-10 mm/yr), time-invariant active-shoreface behaviour contracted from the lower shoreface toe to the surf zone. Depth-dependent lower-shoreface erosion was most significant for mesoscale coastal evolution (i.e. 102-105 yrs), which is characterised by partial shoreface activity. The findings suggest that depth- and timescale-dependent shoreface response limit the reliability of equilibrium-profile models for mesoscale problems

Variability of depth-limited waves in coral reef surf zones

Wave breaking and transformation on coral reef flats is an important process protecting tropical coastlines and regulating the energy regimes of coral reefs. However, the high hydraulic roughness, shallow water, and steep bathymetries of coral reefs may confound common surf zone assumptions, such as a depth-limited and saturated surf zone with a constant wave height to water depth ratio (γ). Here, we examine wave transformation across a coral reef flat, during three separate swell events, on both a time-averaged and a wave-by-wave basis. We use the relationship between significant wave height and water depth (γ) to examine the change in surf saturation across the reef flat and compare the measured wave height decay to results of modelled wave energy dissipation in the surf zone. Our results show that γ was not cross-reef constant and varied according to location on the reef flat and local water depth. On average, γ was greatest at the outer reef flat, near the reef crest, and progressively reduced towards the inner reef flat, near the reef lagoon. This was most pronounced in shallow water with large γ values (γ > 0.85) at the outer reef flat and small γ values (γ < 0.1) at the inner reef flat. This indicates that there is an increase in wave energy dissipation in shallow water, most likely due to increased breaker and bed frictional dissipation. The measured wave energy dissipation across the entire reef flat could, on average, be modelled accurately; however, this required location specific calibration of the free parameters, the wave friction factor (f) and γ, and further suggests that there is no value for either parameter that is universally applicable to coral reef flats. Despite model calibration inaccuracies were still observed, primarily at the outer reef flat. These inaccuracies reflected the observed cross-reef variation of γ on the reef flat and potentially the limitations of random wave breaker dissipation models in complex surf zones. Our results have implications for the use of wave energy dissipation models in predicting breaker dissipation and subsequent benthic community change on coral reef flats, and suggest that careful consideration of the free parameters in such models (such as f and γ) is required

Rising tides: Tidal inundation in South east Australian estuaries

Global sea levels are rising and are likely to have an increasing impact on coastal communities over the coming decades and centuries. Within Australia, recent studies have identified that considerable development and infrastructure is at risk from sea level rise with low-lying areas adjacent to estuaries in New South Wales being subject to the greatest exposure. Increasing extent and frequency of tidal inundation is likely to be one of the more obvious impacts of sea level rise. In this study we examine the occurrence of tidal inundation at selected estuary sites in NSW. We identify sites experiencing tidal inundation now and examine recent changes to the frequency of inundation at two case study sites. We combine observations of local inundation and high-resolution digital elevation data with local tide gauge data to identify thresholds for tidal inundation. When these thresholds are exceeded, tidal water inundates streets and footpaths for a short period of time - so called \u27nuisance inundation\u27 or \u27sunny day flooding\u27. We identify numerous sites along the NSW coast which are subject to tidal inundation. At two case study sites, Woy Woy and Botany, we show increasing frequency of inundation with nuisance events (5 year average) at Woy Woy more than doubling since the late 1980\u27s. By examining current inundation exceedance, we show that even modest near-future increases in sea level are likely to lead to a significant increase in the number of days when inundation occurs in the lowest lying streets

Second-Pass Assessment of Potential Exposure to Shoreline Change in New South Wales, Australia, Using a Sediment Compartments Framework

The impacts of coastal erosion are expected to increase through the present century, and beyond, as accelerating global mean sea-level rise begins to enhance or dominate local shoreline dynamics. In many cases, beach (and shoreline) response to sea-level rise will not be limited to passive inundation, but may be amplified or moderated by sediment redistribution between the beach and the broader coastal sedimentary system. We describe a simple and scalable approach for estimating the potential for beach erosion and shoreline change on wave-dominated sandy beaches, using a coastal sediment compartments framework to parameterise the geomorphology and connectivity of sediment-sharing coastal systems. We apply the approach at regional and local scales in order to demonstrate the sensitivity of forecasts to the available data. The regional-scale application estimates potential present and future asset exposure to coastal erosion in New South Wales, Australia. The assessment suggests that shoreline recession due to sea-level rise could drive a steep increase in the number and distribution of asset exposure in the present century. The local-scale example demonstrates the potential sensitivity of erosion impacts to the distinctive coastal geomorphology of individual compartments. Our findings highlight that the benefits of applying a coastal sediment compartments framework increase with the coverage and detail of geomorphic data that is available to parameterise sediment-sharing systems and sediment budget principles. Such data is crucial to reducing uncertainty in forecasts by understanding the potential response of key sediment sources and sinks (e.g., the shoreface, estuaries) to sea-level rise in different settings

Automated Sensing of Wave Inundation across a Rocky Shore Platform Using a Low-Cost Camera System

Rocky coastlines are frequently used for recreation, however, they are often highly exposed and hazardous environments resulting in high risk to visitors. Traditional approaches to managing human safety in coastal settings (such as the surf lifesaving clubs that have proven effective on beaches) are not necessarily transferable to rock platforms due to their often remote and fragmented distribution and the different recreational uses. As such, a different approach is required. To address this, we present a low-cost camera system to assess the wave hazard on a high-visitation rocky shore platform: the Figure Eight Pools Rock Platform, New South Wales, Australia. The camera system is shown to be highly effective and allows identification of both the distance and frequency of wave inundation on the platform using a novel pixel analysis technique. Nearshore wave height is shown to be the primary factor driving inundation frequencies along the cross-platform transect investigated with some influence from wave period. The remotely sensed camera data are used to develop a preliminary overwash hazard rating system, and analysis of the first month of data collected suggests that the platform is highly hazardous to visitors. Future work will expand this hazard rating system, developing a predictive tool that estimates the overwash hazard level based on forecast wave and tide conditions to improve visitor safety at the site

Sea level rise and the increasing frequency of inundation in Australia’s most exposed estuary

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

Identifying sediment compartment dynamics on the Illawarra Coast

This project aims to produce a framework for assessing compartment dynamics within two sediment compartments in the Illawarra region to assist in assessing coastal hazards. Sediment sources, pathways and sinks will be examined for the Wollongong and Illawarra Coast - South compartments, defined by Geoscience Australia and CoastAdapt. A compartment based approach allows for more holistic coastal planning and management which considers sediment transport at differing scales, and interconnectivity of beaches. This type of approach underpins national guidance on open coast risk assessment and has been incorporated within the NSW Coastal Reforms and the Draft Coastal Management Manual. The adjacent sectors of the Wollongong Coast and Illawarra Coast-South compartments extend for approximately 30 km from Bellambi Point to Bass Point. The rock platform of Red Point marks the shoreline division between these two contrasting compartments. The Wollongong Coast is an urbanized relatively little studied leaky compartment, whereas the Illawarra Coast-South is a well-defined and confined compartment whose main sedimentary characteristics are represented by the infilling of the Lake Illawarra barrier estuary and the erosionprone Warilla Beach. This detailed examination of sediment resources brings together the state-wide coastal seabed mapping program being undertaken by the NSW Office of Environment and Heritage (OEH), and coastal geomorphological investigations being undertaken along the southern NSW coast by the University of Wollongong (UOW). These initiatives involve collation of historical data, sediment sampling, and the use of recently available sophisticated remote sensing technologies, such as terrestrial airborne LiDAR, single and multibeam bathymetry, sidescan sonar imagery, and underwater video and still camera

Foredune erosion, overtopping and destruction in 2022 at Bengello Beach, southeastern Australia

The beach–foredune system at Bengello Beach has been monitored monthly to bimonthly at four profiles (P1–P4) since 1972 and documented the building of a foredune. This paper addresses the remarkable changes which occurred in 2022 as storm waves overtopped and trimmed this foredune at all profiles, then later removed this entire feature at two of the profiles (P3, P4) but not the others (P1, P2). Wave parameters for these storm events, measured by deepwater and nearshore wave buoys, enable a comparison of storm characteristics and resulting beach–foredune impact. During the storm event which destroyed the foredune, nearshore wave height exceeded deepwater wave height, in contrast with other storms that year. The beach–foredune lost 78 m3/m in 2022 and the notable 1974 storms that impacted this coastline resulted in 95 m3/m volume loss. During 2023, beach recovery has occurred, but not rebuilt the foredune. It had persisted for ~40 years enduring many other severe storm events, and the coastal protection afforded by the dune system has been compromised. This highlights the need to consider dune morphology in assessments of erosion hazard and inundation risk along similar coastlines