Quick Facts

Project Proponent(s): City of Holdfast Bay
Project Manager(s): City of Holdfast Bay
Principal Contractor(s): Lucas Total Contract Solutions
Total Cost: $2 million
SMA Contribution: $1 million
Other Funding Contributors:
• City of Holdfast Bay

The City of Holdfast Bay is a local government area in the south-western coastal suburbs of Adelaide, South Australia.

Prior to European settlement, stormwater from the area would have descended to low points behind shallow, undulating coastal dunes, where it would have infiltrated into the sandy soils. Urban development has occurred over the top of these natural dune systems. In the 1960’s and 1970’s a number of underground drains were constructed to pipe stormwater underneath the dunes and out to sea.

Increasing urban development in the catchment has meant that in some cases these drains no longer have the capacity to protect the area from significant ponding and flooding. The area around Tarlton Street in the suburb of Somerton Park is one such area. During a storm event in October 2016 which was estimated to be a 2% annual exceedance probability (AEP) storm (or ‘1 in 50 chance’), major ponding of stormwater occurred within Tarlton Street and Bickford Terrace. Flood plain mapping undertaken for the Glenelg to Marino Stormwater Management Plan (2014) indicated that in a 1% AEP storm (or ‘1 in 100 chance’), some of these properties would be inundated up to 800 millimetres above ground level.

Modelling showed that a major outfall was required to collect flows in Tarlton Street to substantially reduce the extent of flooding in the area and prevent overflow of floodwaters further north. The design of the outfall included approximately 220 metres of 400 millimetre diameter pipe running beneath Bickford Terrace between Tarlton Street and the coast, and a large 1.8 metre high x 3.6 metre wide by 83 metre long culvert beneath the intersection of Tarlton Street and Bickford Terrace. This large underground culvert provides detention storage (530 000 litres) in heavy rainfall events, collecting stormwater and then releasing it slowly to the sea.

The new outfall was constructed during the autumn of 2017, and eliminates or reduces the flood risk to over 80 properties in the area. The contractor, Lucas Total Contract Solutions, employed an innovative ‘trenchless’ Guided Boring System to install the 220 metres of pipework beneath Bickford Terrace, in some cases up to 13 metres below ground level. This construction technique eliminated the requirement for large open cut excavations, which not only reduced the total cost of construction, but also resulted in fewer traffic disruptions, less risk to existing services (power, gas, water), less inconvenience to residents and lower Greenhouse gas emissions when compared to conventional open cut excavations and trenching. The innovative approach of the project was recognised by the Institute of Public Works Engineering Australia (IPWEA) at IPWEA Excellence Awards for South Australia, where the project won an award for Design and/or Construction of a Public Works Project in the ‘Water, over $1 million’ category.