On 1 February ODT readers were advised that DCC had purchased most of the former Forbury Park. No definitive purpose for the land was disclosed. I consider the purchase to be provisionally worthwhile, given the land’s potential to be used for housing, perhaps with a water feature, or as a reconfigured wetland. My personal preference is irrelevant.

Mayor Radich, Taieri MP Leary and the writers of this paper’s editorial of 3 february have 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. It would appear that neither Mr Radich nor Ms Leary has 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. It would presumably be possible to replicate such a system of pipes and pumps to divert stormwater to the “lake”, but this would surely be massively less cost-efficient that upgrading the existing system that feeds directly into Otago Harbour.

The writers of ODT’s editorial have unfortunately circumvented these difficulties by erroneously reporting a typical scenario for flooding across South Dunedin that has simply never occurred – and could not. The editorial refers to flood waves caused by heavy rain running down the hills and cutting across South Dunedin to the harbour. These, it is stated, can be intercepted by Lake Forbury et al. The reality is that the vast majority of stormwater flooding across South Dunedin is caused by rainfall intensities falling directly across the suburb that exceed the system’s capacity (around 7mm/hr, from memory and according to my analyses) over a reasonably pronged period. Flooding simply increases in depth gradually until the rain eases sufficiently. Any input from the hills merely adds to the pond. There are no flood waves to be intercepted.

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. The last thing that an expensively constructed water feature would need would be to be polluted thus risking the health of all its users, human or otherwise.

In my view, residents of South Dunedin should not therefore hold out much hope that any variant of “Lake Forbury” will reduce their flood risk to any meaningful extent. Instead, they may question why – at the time of writing, exactly 8 years and 8 months to the day –  no material improvements have been made to South Dunedin’s stormwater infrastructure following the disastrous flood event of 3-4 June 2015.

Neil Johnstone (semi-retired professional flood control engineer)

10 Tidewater Drive

Otago Peninsula

021 08728776