Only science may rescue city’s south
The future of low-lying South Dunedin is in the hands of science, not those who misinterpret or politicise its flooding problems.
That was the warning from Dunedin-based GNS Science geologist Dr Simon Cox at a large gathering of scientists in Christchurch.
Often when South Dunedin flooded, it was held up as the exemplar of what a climate change induced sea-level rise might look like in New Zealand. However, there were other factors behind the suburb’s vulnerability that were being ignored, including run-off from Dunedin’s hills.
That urgently needed monitoring and modelling to inform decisions about South Dunedin’s future, Cox told the joint Meteorological Society-Hydrological Society conference.
‘‘South Dunedin has been a bit of a hot bed in terms of political problems associated with flooding and the hazard of flooding in association with groundwater rise.
‘‘It’s got a whole lot of run-off from the hills – with a topographic flow off the hills possibly, into the groundwater, and then run-off from the surface as well.
‘‘As well as that, we’ve got this issue where there’s a piping network that is at least 100 years old in places and it is leaking and it seems to be affecting the groundwater . . . The extent to which we’ve got this shallow groundwater is partly a function of that leakiness and that whole infrastructure. It then becomes a problem between hydrology and engineering to try and understand to what extent that is being influenced.’’
Results from Lidar surveying – in which light pulses are used to measure land elevation – had been misinterpreted into South Dunedin maps with ‘‘blue bits’’ showing where climate change flooding would supposedly be worst, he said.
‘‘If you look at the Lidar level and if you look at the Parliamentary Commissioner for the Environment reports, they highlight South Dunedin as being of concern because of the height of South Dunedin relative to sea-level. And that is really useful from the perspective of highlighting areas to do some work. But what then happens is people take the Lidar information and try to get clever and they say, ‘right, plus 1m [sea-level] and this is what it will look like with 1m of sea-level rise’.
‘‘And they couldn’t be further wrong. They couldn’t be further annoying for the people who have got their house sitting under that blue bit. The reason it is really annoying is the current problems in South Dunedin are where the run-off comes from the hills. It is a flooding issue from surface run-off . . . not a groundwater-rise issue.
‘‘When you do this sort of thing, you run the risk of basically getting all this knee-jerk reaction from the society against that model, because they haven’t had wet feet yet. But it is starting to affect their property values and they can’t get insurance because the insurance company actually believe it.’’
It was difficult to quantify how much water came from hill-slope run-off. The area urgently needed shallow and deep monitoring networks. Until that happened, accurate modelling of future conditions and understanding of options was much more difficult, Cox said.