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5. Potential Shoreline Impacts & Management Requirements

As outlined previously, the sediment budget that was prepared for this investigation was based on available data. This included aerial photography, shoreline movement plans and some beach profile surveys. Given the type of data available, the sediment budget should be considered to be indicative of the general trends of sediment transport along the coastline. It should not be relied upon to provide an exact estimate of these transport rates. Similarly, whilst the sediment transport modelling was able to replicate the estimated sediment budget to a reasonable degree, it is the relative changes to the transport rates that are most important, rather than the actual numerical predictions. Additionally, the fact that there will always be variability in the natural environment, in particular seasonal and interannual changes in weather conditions, means that the impact of the marina construction and the subsequent management requirements could change from year to year.

The sediment budget clearly shows the historical accretion at Mullaloo Beach. As shown in the sediment budget, the source of this sediment is largely from the beaches to the south, around Pinnaroo Point. The numerical modelling of waves and sediment dynamics suggests that the construction of the proposed Ocean Reef Marina would not change this accretion trend.

The sediment transport modelling suggests that the construction of the Ocean Reef Marina could reduce the total volume of material that is transported to the north past the existing Boat Harbour. The modelling has shown that this would subsequently reduce the volume of sediment transport along the shoreline to the north of the marina. Nevertheless, the predictions within the sediment budget and from the modelling suggest that this could be in the order of a few thousand cubic metres of material per year on average.

The other point to note is that excavation of accumulated sediment from within the Ocean Reef Boat Harbour has removed an average of around 4,200 m3 of material per year in the period between 1998 and 2013. This means that the current beach system to the north of the Boat Harbour has already been running at a deficit of around 4,200 m3 per year since 1998. The decrease in the total volume of sediment that is transported in a northerly direction past the marina may therefore have little effect on the total volumes of material that move further north along the shoreline. Regardless, for the planning of the future coastal management, it is prudent to consider and allow for the case where management of the sediment is required to maintain the status quo.

5.1 Proposed Coastal Management Regime When considering the proposed management requirements for the construction of the Ocean Reef Marina it is important to remember that the total fluxes of sediment along the coastline adjacent to the marina are expected to be quite small. This is evidenced by the sediment budget and the fact that the adjacent shorelines are predominately rocky with transient embayed beaches. Given that these sediment fluxes are likely to be quite small, the management response to the marina construction should be based on monitoring and subsequent management of the sediment transport to maintain the status quo. This approach is slightly different to what would be advised if the marina was proposed to be constructed on an active sandy coastline with large sediment fluxes, which would typically require a more structured and pre-emptive management response.

The most important aspect of the future coastal management regime will be to ensure that adequate coastal monitoring is completed. This should consist of regular surveys every few years, at a minimum, in order to determine any changes that occur to the shoreline as a result

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of the marina construction. These surveys will allow the total volume of sediment that is either trapped on the southern side of the marina or lost from the northern side of the marina to be estimated. Further management responses, such as sand bypassing or beach nourishment using terrestrial sources could then be triggered if and when they are required.

The other important feature of the survey monitoring will be to assess whether the marina is trapping sediment in the wave shadow and consequently limiting the transport of that material further along the shoreline. In this instance back-passing or forward-passing of sand may be required. This is more likely to be an issue on the northern side of the marina on the proposed beach, as the spur groyne required to hold the beach may prevent sediment from being transported along the beach to the north. To combat this potential, the development of the marina should allow for access both to the proposed beach area, as well as to an area further north (outside of the wave shadow) in order to ensure that sand can be placed either on the beach or in an area north of the wave shadow where it can feed the littoral system to the north. A small sand trapping structure may be warranted to the north of the marina to limit the trapping in the wave shadow.

Where sand bypassing is required to maintain the flow of sediment along the coastline it would obviously be preferable to extract the sediment from the area where it has been trapped on the southern side of the marina. However, there is not currently any area where sediment deposits on the southern side of the Ocean Reef Boat Harbour that would allow it to be excavated with ease. This is due to the fact that the sediment transport pathway in this area is predominately along the edges of the nearshore reefs and platforms (as discussed in Section 2). As a result, it may be that any additional sediment trapped to the south after the construction of the marina is also in an area where it cannot be easily accessed, at least in its entirety, for excavation. In this instance terrestrial sources of sediment could be considered as an alternative to renourish the beaches and feed the littoral system to the north. Given the total volumes of sediment required to do so would be small, nourishing or supplementing any bypassing with these artificial sources is considered to be a reasonable outcome for the project.

The other item to be considered regarding the potential management requirements for the marina would be the potential requirement for dredging of the entrance channel from time to time. This could be required due to the entrance partially capturing some of the sediment transport that may occur around the marina. Given the volume of material involved, it is not expected that this would be a frequent requirement, however monitoring of the area should still be completed in order to highlight if management measures are required. Based on a potential accumulation rate of up to 2,000 m3 per year, such dredging may be warranted every decade to remove between 10,000 – 20,000 m3.

The following are recommended for inclusion in the monitoring and management strategy for the proposed Ocean Reef Marina.

Beach and entrance monitoring surveys every second year.

Coastal engineering analyses of the monitoring data and identification of any beach nourishment requirements.

Possible beach nourishment using terrestrial sources of sand. The quantity is expected to be small and only required once or twice a decade. For financial planning, it is recommended to allow for about 15,000 m3 of beach nourishment every 5 years.

Possible dredging of the marina entrance. The quantity is expected to be very small and probably only required each decade or so. For financial planning it is recommended to

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allow for dredging about 10,000 – 20,000 m3 every decade. This dredged sand may be able to be placed on nearby beaches, reducing the need for beach nourishment listed above (provided the sediment passes a contamination analysis).

Based on the proposed management regime and the results of this study, it is expected that any changes to the coastal processes within the region will be limited to those areas shown in Figure 5.1. Changes in these areas are expected to be short term changes due to the timing of the coastal management not perfectly matching the regional sediment dynamics. For example, the placement of sand bypassing (or nourishment if from terrestrial sources) would deposit a large volume of sand in an area in a very short time. This is different to how the sand would typically move within the system. Such discrepancies are a common occurrence where bypassing is completed, however in this location the presence of the rocky shoreline limits the potential impact of these short term effects.

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Figure 5.1 Potential Extent of Coastal Process Changes

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6. Summary & Conclusions This study has investigated the sediment dynamics associated with the construction of the proposed Ocean Reef Marina. The key findings of this investigation are outlined below.

A sediment budget for the coastline between Hillarys Boat Harbour and Mindarie Marina was estimated based on available shoreline movement information since 1995/96. Despite the impact of the construction of the marina on the local wave climate being limited to a much smaller area, the extent of this sediment budget was chosen to ensure that any resultant changes to the sediment transport pathways between Hillarys Boat Harbour and Mindarie Marina could be captured.

The sediment budget clearly shows that Mullaloo Beach has been accreting over at least the last couple of decades.

It is estimated that around 7,000 m3 (net) of sediment per year could be transported past the existing Ocean Reef Boat Harbour in a northerly direction. It is believed that the majority of this sediment is transported along the edges of nearshore reefs and rock platforms.

The wave model was calibrated to the 2011 inshore measurement at Ocean Reef Boat Harbour.

The calibrated wave model was used to generate the nearshore wave climate for the sediment modelling. The wave modelling highlighted that the construction of the marina would result in only relatively minor wave shadows on the shoreline to the north of the marina during summer sea breeze conditions and to the south during winter storms.

The sediment model was calibrated to the sediment budget.

The calibrated sediment model, coupled to the calibrated wave model, was used to examine the sediment dynamics along the shoreline and the potential impacts of the marina development. The results of the modelling showed that for both a typical and atypical year, the net northerly transport of sediment around the marina could be reduced by a few thousand cubic metres per year compared to the transport around the existing Boat Harbour.

A management solution is proposed to overcome the issues associated with the changes to the sediment dynamics as a result of the marina construction. This management solution would involve monitoring of the shoreline surrounding the marina. The results of this monitoring would then be used to prompt management actions to ensure that the status quo of sediment movement along the coastline is maintained.

Overall, the key finding of this investigation is that the construction of the Ocean Reef Marina is unlikely to cause any significant changes to the shoreline compared to the presence of the Ocean Reef Boat Harbour. Changes that do occur are likely to be on a relatively small scale and should be able to be managed using the approach recommended within this report.

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7. References

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Damara WA, 2012. Coastal Sediment Cells between Cape Naturaliste and the Moore River, Western Australia. Report prepared by Damara WA Pty Ltd and Geological Survey of Western Australia for the Western Australian Department of Transport, Fremantle.

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Department for Planning & Infrastructure, 2008. Perth Recreational Boating Facilities Study. Government of Western Australia.

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Madsen, O., Y.-K. Poon and H. Graber, 1988. “Spectral wave attenuation by bottom friction: Theory." In Proceedings 21th International Conference Coastal Engineering, ASCE, pages 492-504. 48, 133, 138

Masselink, G. and Pattiaratchi, C.B. 2001. Seasonal changes in beach morphology along the sheltered coastline of Perth, Western Australia. Marine Geology 2001.

MRA 2005. Northern Perth Metropolitan Coast Coastal Setback Study. R157 Rev 0. Prepared for Department of Planning and Infrastructure.

MRA, 2012a. Marmion – Sorrento Coastal Protection Study. Report R284 Rev 2 prepared for City of Joondalup.

MRA, 2012b. Hillarys to Ocean Reef Coastal Vulnerability. Report R316 Rev 1 prepared for City of Joondalup.

MRA, 2014. Iluka to Burns Beach – Coastal Vulnerability Assessment. Report R487 Rev 1 prepared for City of Joondalup.

Sanderson, P. G. & Eliot, I. 1999, Compartmentalization of beachface sediments along the southwestern coast of Australia, Marine Geology, vol. 162, no. 1, pp. 145-164.

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Searle, D. J. & Semeniuk, V. 1985. “The natural sectors of the inner Rottnest Shelf coast adjoining the Swan Coastal Plain,” Journal of the Royal Society of Western Australia, vol 67; p 116:136.

van Rijn, L.C., 1989. Handbook of Sediment Transport by Currents and Waves. Delft Hydraulics, The Netherlands.

WorleyParsons, 2009. Ocean Reef Marina – Coastal Sediment Transport Assessment. Report 00745A17 Rev 1 prepared for Coffey Projects.