When was the seawall constructed?
The seawall at Belmont Street, Swansea was constructed in in the 1990s. A timber wharf was also constructed in the 90s, along with landscaping of the area. The timber wharf closed to the public in 2019, and was replaced with a new wharf, aluminium gangway and floating pontoon. During the replacement works, movement of the seawall was discovered prompting further investigations and ultimately the closure of the foreshore area.
What options for the seawall were considered?
In 2021, we engaged a consultant to investigate options to inform its decision making on the future of the seawall. Some of the options considered were:
- Conventional armouring of underwater slope
- Rock bag armouring of underwater slope with tie backs
- Lateral spur dikes and armouring beneath wharf and pontoon
- Anchored, full depth sheet pile wall
It was determined that Council would proceed with designing and constructing the rock bag armouring of the underwater slope.
What is the cost of the project?
We are still going through the detailed design process so exact costs are not yet known; however, we expect the project cost will be in the millions.
How was the design developed?
Consultants identified four possible repair options. A multi-criteria analysis that considered the effectiveness, constructability, impact on channel hydraulics and morphodyanmics, ecology, monitoring and maintenance, public safety and aesthetics was then undertaken. Following this analysis, the rock bag armouring option was identified as the preferred option.
What construction method is proposed to repair the seawall?
Rock bag armouring has been selected as the preferred construction method to repair the steepening of the underwater slope. This will involve placing bags of rock along the slope to stabilise the area. The rock bags will be anchored to stabilise and remediate the seawall.
We are still working through the detailed design process; however, below is a diagram demonstrating the rock bag placement.
Are there any environmental impacts and how will they be managed?
In April 2023, the environmental assessment identified sightings of threatened species in the project location, the threatened species being White’s Seahorse and Black Rockcod. The proposed design using rock bags will likely impact the White’s Seahorse population and habitat, and potentially have similar impacts to Black Rockcod.
As part of the detailed design process, we will work with relevant approval authorities to determine an appropriate construction methodology that will minimise impacts on the threatened species. The project design has capacity to install a variety of habitat options for White’s Seahorse and Black Rockcod in the design, such as seahorse hotels and pipe installation into ballast rock. We will also be looking to relocate the White's Seahorse habitat to protect this threatened species into the future. The project also provides an opportunity to involve a University student to undertake research on soft coral establishment on rock bags.
Why weren't the threated species a concern when undertaking the jetty works?
An environmental assessment including an aquatic habitat survey was completed for the design of the Belmont Street Jetty works. The project involved removing existing piles, and the installation of new piles for the floating pontoon.
The aquatic survey identified brown algae and soft corals established on the existing piles; however only common fauna and flora species were found colonising this habitat, and they were likely to relocate to nearby habitat during construction. Removing the piles was considered to have a short-term and minor impact to the area and it was expected that the new piles would provide an increase in the available area for colonisation of soft corals and algae long-term. The assessment concluded there was no significant impact to threatened species.
Is the steepening of the underwater slope caused by dredging of the channel?
The steepening of the underwater slope is due to the natural movement of sand through Swansea channel. Sand movement has become more active since the breakwalls were constructed at Swansea Heads and Blacksmiths, and monitoring has shown that the channel is deepening and widening.
Some of the sand deposits upstream of the Swansea Bridge forming sand islands. This is dredged to maintain access through the channel. Based on recent studies, dredging is considered to have a minor impact to the overall sand movement and steepening of underwater slopes in the channel.