National sediment compartment framework for Australian coastal management

Abstract

The concept of coastal sediment compartments was first used in the 1960s in the United States. It has since been recognised as appropriate for defining sections of the Australian coast, but had not been uniformly adopted around the nation in the way that has underpinned management, as in other countries. In 2012, the Australian Government supported a project to better understand coastal sediment dynamics using the sediment compartment approach as a framework within which to consider future shoreline behaviour and the impacts of climate change, including rising sea level, changing wave climates and sediment budgets. This paper outlines the sediment compartment project and uses case studies to demonstrate its application. The project consisted of three steps. The first step involved delineation of a hierarchy of coastal sediment compartments following a nationally agreed set of criteria, integrating the onshore/offshore geologic framework with known patterns of sediment movement and those inferred from surface landforms. This identified more than 100 primary compartments bounded by major structural features such as headlands or changes of shoreline orientation. At a finer scale, approximately 350 secondary compartments were identified, many of which encompass smaller scale structural features that define tertiary scale compartments or cells. For verification of this sediment compartments approach to coastal planning and management, the second step of the study comprised case studies of contrasting compartments with different patterns of sediment supply, transport and deposition. The third step, involved embedding all secondary compartments around the continental coast into the Shoreline Explorer, within the CoastAdapt toolbox (National Climate Change Adaption Research Facility). Information regarding the sensitivity of shorelines to change was compiled at the compartment scale, based upon evidence such as substrate, sediment transport attributes and oceanographic forcing, including waves, tides and storm processes. Presentation of information through CoastAdapt within the compartments framework provides a resource to facilitate improved coastal planning and management over different implementation levels, from national strategy scale down to local policy scale. Case studies from several contrasting settings around the Australian coast demonstrated the potential and feasible application of the sediment compartment approach at different spatial and temporal scales.

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

“…The hierarchy is used to conceptualise and map the cross-shore and alongshore extents of sediment sharing between sources and sinks, for timescales relevant to coastal management and planning (years to centuries) [34]. The framework reflects the natural hierarchy of coastal depositional systems [21], and allows for the model parameterisation to reflect the spatial and temporal scales of the forecast, and the resolution of available geomorphology and process data.…”

“…Within the primary sediment compartments, 359 secondary compartments have been identified, representing sediment sharing at intermediate (decadal to centennial) timescales [33,34]. The NSW coast includes 9 primary and 47 secondary sediment compartments ( Figure 1A).…”

“…While the loss of sand offshore is usually temporary, full beach recovery to the pre-storm state may take several months to several years, depending on the post-storm wave climate and frequency of storms [2][3][4]. Future change in modal and storm wave climates, due to climate change, may drive changes in shoreline orientation [33,34]; and, the seven NSW coastal regions considered in the exposure assessment. The small red dots indicate identified erosion hot spots [44].…”

“…Development is often concentrated on the coastal plains due to their flat and regular morphology. Unlike low-gradient passive margins (e.g., US Atlantic and Gulf coasts), which feature narrow, low-lying barrier islands that are dominated by barrier-bypassing processes (e.g., washover and tidal inlet migration), the bay barriers of NSW are typically higher, wider, and more stable in comparison, and mostly feature well-developed dune morphology formed over hundreds [33,34]; and, the seven NSW coastal regions considered in the exposure assessment. The small red dots indicate identified erosion hot spots [44].…”

“…The small red dots indicate identified erosion hot spots [44]. Representative secondary sediment compartments [33,34] Sediment cover across the inner shelf is relatively thin, except where relict coastal barriers and shelf sand bodies accumulated during lower sea levels, prior to erosional reworking during the late Holocene post-glacial marine transgression and subsequent sea-level highstand [45,46]. A marine abrasion surface extends along 300 km of the central to southern coast, frequently outcropping across the submerged inner-shelf seabed [46].…”