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Few Dunedin people lie awake at night tormented by this question, but geography PhD student Duc Nguyen is working hard to answer it.
The security, wellbeing and regeneration of erosion-threatened sand dunes at St Kilda Beach, and possibly elsewhere, could be improved by insights from Mr Nguyen’s studies.
"I feel very proud that I can contribute something," Mr Nguyen said yesterday.
Three notches were initially cut into St Kilda sand dunes in a trial, aimed at helping to regenerate the heavily eroding sandhills, in 2016.
Added in 2018 were 30 more notches 1.5m to 2m wide and 1m-2m deep at 10m-15m intervals, between the St Kilda Surf Life Saving Club and Lawyers Head, to encourage growth of the dunes and reduce the impact of erosion.
Dunedin City Council coastal specialist Tom Simons-Smith said in 2019 that the extra notches had since helped shift several hundred cubic metres of sand into the dunes.
They were designed so that sand was blown to the back of the dunes in strong west to southwest winds, enabling the dunes to grow, and become more resilient to erosion, Mr Simons-Smith said.
Mr Nguyen said good progress was being made in understanding how the notches worked.
He was studying how their various angles to the coast and other characteristics, including their slope angle, shape and size, had contributed to moving sand.
University of Otago coastal geographer Associate Prof Mike Hilton said Mr Nguyen was undertaking the first doctoral research in the world to investigate how air flows linked to notches acted as "sand pumps" to help regenerate sand dunes.
His work was of significant community benefit, and he was also passing on his knowledge effectively to younger students.