Timeline to act on erosion

EROSION north of Two Rocks Marina was caused by the marina and needs to be addressed within 10 years, a Wanneroo council report says.

The report outlined a coastal management study held by MP Rogers and Associates for the City.

‘The construction of the Two Rocks Marina in 1973 interrupted the longshore drift in the area,’ it said.

The report said that resulted in sediment building up south of the marina and erosion on the shoreline north of it.

Since the consultant was appointed in November, 2012, it has reviewed previous studies for the coastline between the north Yanchep headland and Mallee Reef. It found the maximum erosion rate would be 70cm a year, with a possible 20m shoreline recession during a severe storm event.

‘Recession due to sea level rise was also investigated, resulting in an additional 2m and 5m of recession over a 10-year and 25-year planning timeframe respectively,’ the report said.

It said the consultant investigated various coastal management solutions and found two potential options. One was a managed retreat option that would involve relocating the Sceptre Court stairway, viewing platform and access path as well a navigation beacon on Sovereign Drive.

That option would also involve an 80m extension to the marina’s northern seawall and would cost about $1 million, ensuring the road reserve was not vulnerable to erosion for about 45 years.

The second option would be to build two groynes north of the marina and extend the northern seawall by 50m, as well as relocating the same infrastructure.

It was estimated to cost about $4.7 million and likely to be suitable for 80 years before erosion threatened the road reserve.

‘The managed retreat option was ranked first due to the lower capital costs and the planning timeframe of 25 years,’ the report said.

It said the Department of Transport commissioned Aurecon to do a geotechnical survey in May and June this year, which found limestone bedrock in the dunes north of the marina was above the mean sea level.

‘The limestone underlying the sand dune formation may provide some erosion protection,’ it said.

‘There is no immediate coastal erosion threat to the adjoining assets other than the navigational marker and the Sceptre Court stairway.

‘However, if no coastal management works are undertaken in the short to medium term (five to 10 years), the erosion trend suggests the loss of the foreshore dunes.’

Councillors will consider the report and authorising a public meeting in Two Rocks at their meeting tonight.