Located on the northern shores of Sydney Harbour, Mosman offers a rich cultural life amidst beautiful bush land surrounds. Mosman Council plans for building and development, maintains parks, beaches and sporting facilities, provides services for children, youth and seniors and runs cultural activities, a library and an art gallery.
Many of Mosman’s seawalls are nearly a century old and many are heritage listed. Some, including the walls at Mosman Bay, Quakers Hat Bay and the Spit are showing the effects of exposure to the elements, with collapsed and damaged sections, fallen sandstone blocks and subsidence behind the wall.
Council has developed a proposal for seawall works at Spit West, Spit East, Parriwi Point and Musgrave Street. Integrated with the seawall works in Spit East and Spit West is a proposed upgrade of access along the foreshore.
Seawall works will include rehabilitation, maintenance, upgrading walls and the establishment of new habitats for marine flora and fauna.
The proposal for improved access along the foreshore in Spit East will include improvements to the boat ramp, a new bicycle path, pedestrian path, bus bay and shared zone. In Spit West the proposed access improvements include a shared path for bicycles and pedestrians linking Pearl Bay Avenue to the Spit Bridge and mobility access from the car park to the foreshore.
I met with John and he took me to the Pearl Bay/Spit seawall which is about to the opened.
At Spit Bay, before there was a plain faced concrete seawall that covered the embankment. Besides some oysters that were growing along the bottom, there was no habitat to speak of. Also, the concrete was cracking in several spots and lifting out. Where they have implemented the new rock armory, an abundance of life has blossomed and provided a great comparison of an environment that provides habitat versus one that just has a concrete wall.
They used a rock armory to provide a barrier to tidal waves and structural stability at the base of the seawall where it is sliding out. This method of saving the seawall without having to rebuild it cost about 1/5 of the cost of rebuilding the seawall – this is quite significant across 800 meters of seawall. They implemented a rock armory to protect from tidal action and varied the placement and size of stone related to where the wave action comes from and its variance in strength. They scraped loose material at the bottom, placed geotextile and placed the stones – its slope is at one vertical to one and half horizontal. The rocks have been designed by coastal engineers. This provides stability at the bottom of the aging seawall and stops the seawall from sliding down.
They got the rocks in a quarry north of Sydney and they had to go through stringent testing to check the stability of the sandstone rocks. One of the major issues with seawall rocks is that they breakdown with the attack of sulfites and they had to test they could withstand the constant wetting and drying of being at a seawall.
The rocks provide shelter for smaller fish and it creates a breeding ground for habitat. There is quite a lot of sea life. The only minor problem they are getting is some litter issues that washes in from the harbour, which can be remedied by regular clean up maintenance.
On the ground, they implemented a bike trail and a pedestrian trail as well as planting that will eventually grow and become quite dense to act as a handrail for bicyclists and pedestrians. They also augmented a natural beach and put more sand in to create a high tide beach. At low tide you have a recreational beach area and it also creates habitat.
They worked with the Department of Energy and Climate Change as well as the University of Sydney – they have taken ideas from different sites to diversify the habitat and create a recreational area.
Another habitat they created was a salt marsh area where a section in inundated at high tide and allows the salt marsh to live within the correct habitat. They are providing it as an educational tool to show how seawalls should look like with signage and tours:
They also built a large tide pool that will flood at high tide and provide a shallow water habitat at low tide:
We also visited Quakers Hat Bay, where they were looking to create habitat for oysters and small fish. They’d like to study what actually has grown and lives there since it was built. They have worked with the University of Sydney about their research and gotten a lot of ideas of what can be done in each situation:
John Heptonstall is a Contract Engineer for the Mosman Council. He is the engineer principally involved in the project management of the design and construction of the Mosman seawalls.
Mosman’s walls have been designed and built in collaboration with Sydney University’s Centre for Environmental Impact in Coastal Cities (EICC). Also the state government’s Department of Environment and Climate Change has been involved with advice and funding as well as coordinating knowledge around the Sydney region for ecological engineering works.