Artificial structures, such as seawalls, are increasingly disrupting the transition zones between terrestrial and marine systems. They can impede the transport of resources across habitat boundaries and impact adjacent sedimentary ecosystems by modifying hydrodynamics which, in turn, influence sedimentology and erosion. We assessed how structural elements of Avicennia marina mangrove forests along the Parramatta River estuary, Sydney, Australia, differ in the presence or absence of a seawall on the landward side of the forest. These forests are of importance to resident and transient fauna. Sampling of paired mangrove forests, with and without seawalls, supported our hypotheses of structural differences between them. Mangrove forests with seawalls were in some instances less than a third of the width of unconstrained mangrove forests, and had up to twice the pneumatophore density. They often contained less leaf litter and had fewer saplings than forests without seawalls. These results suggest that as shoreline armouring continues, urban mangrove forests and their important ecosystem functions may be negatively impacted. Studies are now needed to ascertain the mechanisms by which seawalls modify these systems.
We present findings from a geotechnical survey for a gravity-based Wave Energy Converter (WEC) to be installed in King Island, Tasmania. The goal of this work was to assess the deployment location for a 200 kW Oscillating Water Column (OWC) and to identify possible challenges for the foundation of the structure to make it Australia’s first operational offshore OWC for a remote offshore island. The proposed location for this OWC is the southeast coast of King Island, Tasmania, approximately in a depth of ~5.5 m LAT. The survey included sub-bottom profiling, sediment cores, surficial sediment strength by penetrometer drops, seabed imagery, as well as long-term deployment (>6 months) of pressure sondes and an acoustic wave current profiler (AWAC). Our findings demonstrate that the WEC can be installed in the proposed location with significant wave height Hs ~1–1.5 m and peak period Tp of 12–14 s, and that the site exhibits sufficient sand coverage and quasisteady bearing capacity. The period between the survey and prospective deployment is only one year, demonstrating the efficiency of the survey methods (in particular, the use of the penetrometer) and OWC design but also the suitability of the candidate site for this device design.
Marine renewable energy is still in its infancy and poses serious challenges due to the harsh marine conditions encountered for wave or tidal installations and the survivability of devices. Geophysical and hydrodynamic initial site surveys need to be able to provide repeatable, reliable, and economical solutions. An oscillating water column wave energy converter is to be installed on the west coast of King Island, Tasmania. The location is in a high-energy nearshore environment to take advantage of sustained shoaling non-breaking waves of the Southern Ocean and required site-specific information for the deployment. We provide insight into scalable geophysical site surveys capable of capturing large amounts of data within a short time frame. This data was incorporated into a site suitability model, utilising seabed slope, sediment depth, and water depth to provide the terrain analysis needed to match deployment-specific characteristics. In addition, short-term hydrology and geotechnical work found a highly energetic seabed (near seafloor water velocities <1 m/s) with sufficient bearing capacity (6 MPa). In a highly energetic environment, care was taken to collect the relevant data needed for an assessment of critical information to an emerging technology companies primary project. This is in addition to the malleable methodology for a site suitability model that can incorporate various weighted parameters to prioritise the location for shallow wave energy sites in general.
This study provides the first ever published measurements of scour and morphological change around an Oscillating Water Column (OWC) Wave Energy Converter (WEC) device at a real-world site, with the intention of informing future designs to reduce costs of the technology. A 200-kW prototype OWC WEC was deployed at King Island, Tasmania, Australia in January 2021, providing a unique opportunity to monitor the device using a combination of dive footage, multi-beam surveys and bedrock surveys. Settlement of the device was observed and monitored before ceasing once the foundation made contact with the underlying bedrock at the site. It is hypothesized that the settlement is caused by scour undermining the gravity structure’s foundations. The processes causing this scour are explored and possible future design modifications are suggested to reduce the risk of scour and settlement.
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