A Serious Problem for South Dunedin AND Thames

Following serious floods in South Dunedin in 2015 due to sea level/groundwater rising the Dunedin City Council (DCC) is actively finding out the cause of the problem and what can be done to reduce future flooding. 

In 2017 DCC commissioned a report from Golders Associates to review international case studies on rising groundwater in urban areas.  This follows an earlier report from BECA which looked at possible engineering options, and the Otago Regional Council placing a series of groundwater monitoring wells in flood prone areas.

Meanwhile, Thames township also has a serious groundwater flooding problem but no research whatsoever has been undertaken into groundwater flooding issues. The groundwater level along much of the Thames foreshore is less than 1 m below ground level and has a clear “tidal footprint” which means the groundwater level rises and falls with the tide.  Sea flood maps produced by Waikato Regional Council show that extensive groundwater surface flooding (Green areas) will occur from Kopu to Moanataiari even with a present-day storm surge or with low/moderate sea level rise. 

It is also well documented that much of the Thames foreshore is sinking which means that the rate of sealevel rise is far greater because the local rate is a combination of the rate of sinking land and the rising sea.  

We, therefore, have much to learn locally from the groundwater research undertaken in Dunedin, and WRC and TCDC must catch up fast. 

Here are some extracts from the Golders Report.   

“In recent times before European settlement the area comprised salt marshes, lagoons, dunes and intertidal mudflats. South Dunedin was developed into a predominantly residential and commercial / retail area from the 1800’s onward, following land reclamation and land filling. Land filling was often poorly compacted and some residual land settlement is still occurring (ORC 2016).”  (Precisely the situation on the Thames foreshore)

“Groundwater levels in South Dunedin are expected to rise as a consequence of sea level rise, exacerbating existing water ponding and drainage issues. Groundwater levels are strongly influenced by the sea in South Dunedin and sea level rise over the past century has increased groundwater levels there (ORC 2012 and ORC 2016). With sea levels expected to further increase in response to climate change, groundwater levels in South Dunedin are projected to continue to rise. This rise will increase South Dunedin’s susceptibility to water ponding and drainage issues.”

“In urban areas where groundwater levels are relatively close to the surface several issues may arise, such as surface water ponding, damage to infrastructure and buildings, and increased risks of liquefaction during earthquakes. These issues can be severe and widespread unless appropriate adaptive or management measures have been put in place. Factors such as sea level rise, increased weather extremes and land subsidence all influence the potential change in susceptibility.”

“To control rising groundwater levels in South Dunedin there are options to combine multiple protection measures. Protection options can be introduced slowly in a progressive manner rather than as a one-off operation and can be focussed initially to protect sensitive areas and buildings. There is potential to develop new recreational surface water bodies and features. South Dunedin’s stormwater and wastewater networks are up for renewal in the near future. This may provide opportunity for implementing some measures to manage groundwater levels.”

“Mitigation options include horizontal and vertical drainage, pumped open canals, ground improvement, seepage canals and cut-off walls. 

I will look into these mitigation options in a future blog.