In a previous post, I highlighted the Golder Associates comparative international study obtained by Otago Regional Council on high groundwater and flooding issues caused by rising sea levels. This post will show some of the options used to reduce groundwater flooding – particularly in the Netherlands.
Some options involving wholesale ground improvement and raising ground levels that are sometimes applied in new developments are often not feasible in existing urban areas. The focus in the selected case studies is on retrofitting systems in existing urban areas, which is most relevant to South Dunedin’s (and Thames’) circumstances.
Thames already suffers from rapid land subsidence due to soils/marine deposits that are prone to consolidation. Lowered groundwater levels can accelerate land subsidence so Thames will have to be careful to ensure any measures to reduce groundwater don’t make sea level rise hazard worse
In existing urban areas the establishment of open land drains may require purchase of property and the removal of dwellings.
The potential for clogging is an important consideration when designing these systems. Iron-rich groundwater can lead to iron oxides precipitating, which clog the systems over time. Thames’ groundwater is iron-rich from discharges from old mines – (red/orange flows – eg. at Kuranui Bay)
Combined Stormwater and Groundwater Systems
In some cases communities may choose to retreat from a certain area, or a part of it. Managed retreat entails the rezoning of affected areas to a non-urban function, which means building consents will no longer be issued and services no longer maintained. Buildings are vacated and this is usually followed by demolition and site clearing (Figure 13). In the context of coastal protection, managed retreat may also include the removal of coastal protection and allowing an area to become flooded.
The red zoning of large parts of Christchurch, including buying out property owners following the Canterbury Earthquake Sequence (CES), is an example of managed retreat.
Some of the main causes of high and rising groundwater levels are:
- Tidal fluctuations and sea level rise raising groundwater levels.
- High water levels in rivers and streams raising nearby groundwater levels.
- Cessation of pumping from large municipal or industrial groundwater abstractions.
- Land subsidence.
- Upgrading leaking stormwater and sewage pipes and connections.
- Urban development effects on groundwater recharge and drainage
At least 3 of these apply to Thames.
“Groundwater levels in areas close to the sea can rise at high tide as a result of seepage beneath coastal protection measures such as sea walls, stop banks or dikes. A similar situation may occur near a river at high flows. Whilst sea walls or stop banks may protect against flooding, the seepage can cause high groundwater levels issues.” P.18 of Golder report
The Tonkin and Taylor report on the proposed aquatic centre for Thames failed to address these seepage issues.
“The case studies highlight that other matters to consider when protection options are considered are ownership of land, financial means for installation, and clearly defined responsibilities for operation and maintenance. High groundwater levels are often only one of the issues that low-lying urban areas face, as these areas typically also have stormwater management challenges and can be prone to inundation from the sea.”
We have much to learn from overseas case studies. But first, as a community and at District and Regional Council level, we must stop pretending that we don’t have a groundwater crisis and immediately get the essential research done. Then we must tackle groundwater flooding with funding and mitigation options – as Dunedin has done.