The raceway was for decades the home of the Forbury Park Trotting Club, but the venue became surplus to the racing industry’s requirements in 2021.
The Dunedin City Council announced the $13.2 million purchase of the former racecourse last week, saying it had several ideas for the site.
But semi-retired professional flood control engineer Neil Johnstone — who worked on the lower Clutha and lower Taieri flood protection schemes — said he was wary of the risks the purchase posed.
Some people had enthusiastically backed the purchase, seemingly believing that stormwater flooding akin to what happened in June 2015 can be reduced or eliminated by diverting stormwater to what might be in future called Lake Forbury, he said.
"It would appear that [they had not] considered how stormwater from across greater South Dunedin could ever find a pathway to the ‘lake’.
"There are no waterways across the floodplain, and gravity drainage is non-existent; hence the pumped drainage system that was installed in the early 1960s."
Mr Johnstone said it would presumably be possible to replicate such a system of pipes and pumps to divert stormwater to the "lake", but this would be less cost-efficient that upgrading the existing system that feeds directly into Otago Harbour, he said.
"The other key issue that seems to have been overlooked is the toxic nature of urban stormwater in general, and in South Dunedin in particular.
"Just ask anyone whose home was flooded in 2015.
"While industrial pollution would create something of an issue, the far greater concern would be the concentration of untreated sewage that inevitably pollutes South Dunedin floodwaters."
Residents of South Dunedin should not hold out much hope that any variant of "Lake Forbury" would reduce their flood risk to any meaningful extent, he said.
"Instead, they may question why ... no material improvements have been made to South Dunedin’s stormwater infrastructure following the disastrous flood event of June 2015."
The South Dunedin floods of June 2015 were estimated to have cost the city more than $138 million.
A council spokesman said all potential options for flood alleviation and climate adaptation for the wider South Dunedin area were being explored through the South Dunedin Future programme.
"This has and will continue to involve extensive conversations with the community, including on the future use of Forbury Park. Mr Johnstone is welcome to feed his views into this process."
The spokesman also took issue with Mr Johnstone’s claim that no material improvements have been made to South Dunedin’s stormwater infrastructure.
"The DCC’s approach has been to carry out the immediate works needed in South Dunedin post-2015 flood; maintaining, renewing and upgrading stormwater infrastructure across the city to reduce pressure on the system; and carefully planning and consulting over the much more extensive (and expensive) work to come.
"Ultimately, it’s about making South Dunedin both a safer and better place to live. Upgrading the stormwater system will assist in providing resilience, but it’s not necessarily going to be the best, cheapest or only approach to be adopted, or the most successful or affordable standalone approach."
