Assessing coastline recession for adaptation planning: sea level rise versus storm erosion.

The Sixth Assessment report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) states with high confidence that most sandy coasts around the world will experience an increase in coastal erosion over the twenty-first century. An increase in long term coastal erosion (coastline recession) along sandy coasts can translate into massive socio-economic impacts, unless appropriate adaptation measures are implemented in the next few decades. To adequately inform adaptation measures, it is necessary to have a good understanding of the relative importance of the physical processes driving coastline recession, as well as of linkages between consideration (or not) of certain processes and the level of risk tolerance; understandings that are hitherto lacking. Here, we apply the multi-scale Probabilistic Coastline Recession (PCR) model to two end-member sandy coastal types (swell dominated and storm dominated), to investigate where and when coastline recession projections are dominated by the differential contributions from Sea Level Rise (SLR) and storm erosion. Results show that SLR substantially increases the projected end-century recession at both types of coasts and that projected changes in the wave climate have only a marginal impact. An analysis of the Process Dominance Ratio (PDR), introduced here, shows that the dominance of storm erosion over SLR (and vice versa) on total recession by 2100 depends on both the type of the beach and the risk tolerance levels. For moderately risk-averse decisions (i.e. decisions accounting only for high exceedance probability recessions and hence do not account for very high amounts of potential recession-for example, the placement of temporary summer beach cabins), additional erosion due to SLR can be considered as the dominant driver of end-century recession at both types of beaches. However, for more risk-averse decisions that would typically account for higher potential recession (i.e. lower exceedance probability recessions), such as the placement of coastal infrastructure, multi-storey apartment buildings etc., storm erosion becomes the dominant process. The results of this study provide new insights on which physical processes need to be considered when and where in terms of numerical modelling efforts needed for supporting different management decisions, potentially enabling more streamlined and comprehensive assessments of the efficacy of coastal adaptation measures.

Probabilistic projections of the stability of small tidal inlets at century time scale using a reduced complexity approach.

Climate change is widely expected to affect the thousands of small tidal inlets (STIs) dotting the global coastline. To properly inform effective adaptation strategies for the coastal areas in the vicinity of these inlets, it is necessary to know the temporal evolution of inlet stability over climate change time scales (50-100 years). As available numerical models are unable to perform continuous morphodynamic simulations at such time scales, here we develop and pilot a fast, probabilistic, reduced complexity model (RAPSTA - RAPid assessment tool of inlet STAbility) that can also quantify forcing uncertainties. RAPSTA accounts for the key physical processes governing STI stability and for climate change driven variations in system forcing. The model is very fast, providing a 100 year projection in less than 3 seconds. RAPSTA is demonstrated here at 3 STIs, representing the 3 main Types of STIs; Permanently open, locationally stable inlet (Type 1); Permanently open, alongshore migrating inlet (Type 2); Seasonally/Intermittently open, locationally stable inlet (Type 3). Model applications under a high greenhouse gas emissions scenario (RCP 8.5), accounting for forcing uncertainties, show that while the Type 1 STI will not change type over the twenty-first century, the Type 2 inlet may change into a more unstable Type 3 system around mid-century, and the Type 3 STI may change into a less unstable Type 2 system in about 20 years from now, further changing into a stable Type 1 STI around mid-century. These projections underscore the need for future adaptation strategies to remain flexible.

Synthesising wave attenuation for seagrass: Drag coefficient as a unifying indicator.

An estimated 100 million people inhabit coastal areas at risk from flooding and erosion due to climate change. Seagrass meadows, like other coastal ecosystems, attenuate waves. Due to inconsistencies in how wave attenuation is measured results cannot be directly compared. We synthesised data from laboratory and field experiments of seagrass-wave attenuation by converting measurements to drag coefficients (CD). Drag coefficients varied from 0.02-5.12 with CD¯ = 0.74 for studies conducted in turbulent flow in non-storm conditions. A statistical model suggested that seagrass species affects CD although the exact mechanism remains unclear. A wave model using the estimated CD¯ as an input parameter demonstrated that wave attenuation increased with meadow length, shoot density, shoot width and canopy height. Findings can be used to estimate wave attenuation by seagrass, in any given set of conditions.

Resilience of branching and massive corals to wave loading under sea level rise--a coupled computational fluid dynamics-structural analysis.

Measurements of coral structural strength are coupled with a fluid dynamics-structural analysis to investigate the resilience of coral to wave loading under sea level rise and a typical Great Barrier Reef lagoon wave climate. The measured structural properties were used to determine the wave conditions and flow velocities that lead to structural failure. Hydrodynamic modelling was subsequently used to investigate the type of the bathymetry where coral is most vulnerable to breakage under cyclonic wave conditions, and how sea level rise (SLR) changes this vulnerability. Massive corals are determined not to be vulnerable to wave induced structural damage, whereas branching corals are susceptible at wave induced orbital velocities exceeding 0.5m/s. Model results from a large suite of idealised bathymetry suggest that SLR of 1m or a loss of skeleton strength of order 25% significantly increases the area of reef flat where branching corals are exposed to damaging wave induced flows.

Coastal retreat and improved water quality mitigate losses of seagrass from sea level rise.

The distribution and abundance of seagrass ecosystems could change significantly over the coming century due to sea level rise (SLR). Coastal managers require mechanistic understanding of the processes affecting seagrass response to SLR to maximize their conservation and associated provision of ecosystem services. In Moreton Bay, Queensland, Australia, vast seagrass meadows supporting populations of sea turtles and dugongs are juxtaposed with the multiple stressors associated with a large and rapidly expanding human population. Here, the interactive effects of predicted SLR, changes in water clarity, and land use on future distributions of seagrass in Moreton Bay were quantified. A habitat distribution model of present day seagrass in relation to benthic irradiance and wave height was developed which correctly classified habitats in 83% of cases. Spatial predictions of seagrass and presence derived from the model and bathymetric data were used to initiate a SLR inundation model. Bathymetry was iteratively modified based on SLR and sedimentary accretion in seagrass to simulate potential seagrass habitat at 10 year time steps until 2100. The area of seagrass habitat was predicted to decline by 17% by 2100 under a scenario of SLR of 1.1 m. A scenario including the removal of impervious surfaces, such as roads and houses, from newly inundated regions, demonstrated that managed retreat of the shoreline could potentially reduce the overall decline in seagrass habitat to just 5%. The predicted reduction in area of seagrass habitat could be offset by an improvement in water clarity of 30%. Greater improvements in water clarity would be necessary for larger magnitudes of SLR. Management to improve water quality will provide present and future benefits to seagrasses under climate change and should be a priority for managers seeking to compensate for the effects of global change on these valuable habitats.