Coastal communities with shallow groundwater need to plan for some of the lesser-known impacts of rising sea levels, according to findings from a new report by GNS Science.
The report said factors such as the loss of ground capacity to absorb rainfall and flooding from groundwater emerging from the surface are some of the challenges some regions will face.
As sea levels begin to rise, it is expected the upper surface of hidden water underground – known as the water table – will also rise in areas near the coast where the groundwater is shallow and permeable, said GNS Science.
“Impacts may include an inundation of stormwater and wastewater systems, building instability and increased liquefaction vulnerability. Eventually groundwater can emerge at the surface as springs and cause localised flooding.”
The study, produced in partnership with the Otago Regional Council, sheds light on the behaviour of groundwater beneath the suburbs of South Dunedin and Harbourside.
It forecasts when and where the groundwater-related hazards, such as increased flooding from rainfall and flooding below, are likely to occur.
Principal scientist at GNS Simon Cox said: “The flat-lying coastal land of Harbourside and South Dunedin is vulnerable to effects from the harbour and ocean.”
“Our study shows that in some areas, as the sea level rises, groundwater will contribute to flood problems in two ways – through loss of the ground’s capacity to store rain, and then flooding from below. This can be expected before any inundation directly from the sea.”
The research will help to inform the South Dunedin Future programme – an ORC and Dunedin City Council partnership that tackles climate change, sea-level rise and flooding problems in South Dunedin.
Otago Regional Council Manager Natural Hazards Jean-Luc Payan said: “This unique research is key to understanding the complexity of what’s happening under the ground in South Dunedin and Harbourside.”
“It builds on monitoring work ORC and GNS have been doing together since 2009, and provides a detailed picture of where and when issues will arise. This helps us to understand how hazards will evolve and enable planning for the future.”
“It’s an important tool in determining adaptation options for the future of South Dunedin.”
The project was based on analysis of four years of data from a groundwater monitoring network in South Dunedin and Habourside, and looked at what causes change to groundwater levels, such as tides, storm surge and rain.
It also used modelling to understand how sea-level rise will alter the future fluctuations and elevation of groundwater levels.
The forecasts of the groundwater-related impacts were mapped against 10 cm increments of sea-level rise, with estimated timeframes provided based on IPCC (Intergovernmental Panel on Climate Change) emissions scenarios.
“We know that sea-level rise is already happening but can’t be specific about the timeframe of exactly how quickly – this depends on global warming and international efforts to reduce emissions,” Cox said.
“The research shows that Dunedin needs to plan for water coming from above, below and sideways from the harbour and ocean. However, the monitoring shows the ground is less prone to tidal flow and entry of seawater than initially feared, which provides a little more time for the city to plan.”
The report forecast the following changes to South Dunedin:
- As groundwater rises, the ability for rainfall to be absorbed into the ground like a sponge decreases. This exacerbates existing rain-related flood issues. This is likely to remain the dominant issue for the next 30 to 40 years.
- Groundwater rising to the surface will move from periodic occurrences (at less than 40cm of sea-level rise expected) to permanent springs and/or flooding (after 40cm of sea-level rise has been reached as early as about 50 years from now).