Griffith Asia Insights
When the flood comes from below: Groundwater, sea-level rise and a hidden coastal threat
SIMON COX, MARC ETTEMA, LEE CHAMBERS, SCOTT STEPHENS, GREGORY BODEKER, QUYEN NGUYEN, IVAN DIAZ-RAINEY AND ANTONI MOORE |
As climate change accelerates, rising sea levels are visibly redrawing coastlines, triggering coastal erosion, storm surges, and inundation. Yet, another quieter and less visible threat is creeping beneath the surface—shallow groundwater rise.
In low-lying coastal cities like Dunedin, New Zealand, this rising groundwater isn’t just a future concern. It’s already affecting infrastructure, compounding flood risks, and presenting complex challenges for urban planning and climate adaptation. A recent study led by Simon Cox and colleagues outlines a compelling approach for understanding and forecasting this underappreciated hazard.
The hidden role of groundwater in coastal flooding
While most attention focuses on water coming overland—from the sea or sky—flooding from below is a growing risk in many coastal cities. This occurs as sea-level rise (SLR) causes groundwater to “shoal” (move closer to the surface), reducing the ground’s ability to absorb rainfall. Eventually, this leads to emergent groundwater flooding, where water appears at the surface, sometimes far inland from the coast.
Because groundwater is invisible and poorly mapped, its dynamics—and dangers—are often misunderstood. But as the study shows, it plays a critical role in multi-hazard coastal flood forecasting.
A case study from Dunedin
In Dunedin, researchers installed a network of shallow groundwater monitoring piezometers to observe how tides, rainfall, and sea levels influence groundwater levels. By interpolating this site-specific data, they were able to map:
- Current groundwater elevations
- Depth to groundwater (DTW)
- Temporal variations and trends
- Spatial responses to tides and rainfall
This empirical information was then used to model future scenarios under different increments of sea-level rise. Importantly, the findings revealed that some inland areas—not just those on the shoreline—are most vulnerable due to the shape of the land and subsurface conditions.
Why groundwater matters more than you think
Rising groundwater presents a compound hazard. It can:
- Increase the frequency of pluvial (rainfall-driven) flooding
- Compromise building foundations and roads
- Overwhelm ageing stormwater and wastewater systems
- Redistribute subsurface contaminants
- Lead to mould and dampness in homes, with negative public health outcomes
- Facilitate saltwater intrusion, damaging freshwater aquifers
The insidious nature of groundwater flooding—often occurring in advance of visible coastal inundation—makes it harder to detect, predict, and defend against.
A new approach to forecasting multi-hazard flood risk
The empirical models used in this study offer a cost-effective and spatially refined method to understand and plan for future groundwater-related hazards. While not as detailed as computationally intensive numerical models, these risk-averse projections are valuable for their spatial and temporal insight, particularly when data and resources are limited.
Crucially, these models can highlight how the relative contribution of different flood sources—rainfall, sea-level rise, and groundwater—will evolve over time and space. This allows for adaptive planning that reflects the unique topography, geology, and infrastructure of each coastal area.
What this means for coastal communities
The findings from Dunedin underscore a broader truth for cities across the globe: groundwater hazards are already here, and in many places, they’ll worsen before visible coastal inundation occurs.
This “flooding from below” threatens to outpace our capacity to respond unless integrated into multi-hazard planning. Monitoring networks and groundwater-focused geospatial models can give communities a head start—showing where and when impacts are likely, and helping to prioritise adaptation efforts.
As the researchers conclude, the response must be holistic. Understanding and mapping shallow groundwater is a vital step in preparing for the creeping, complex, and often invisible challenges of climate change.
Simon Cox and Lee Chambers (GNS Science), Marc Ettema (Otago Regional Council), Scott Stephens (NIWA, Wellington), Gregory Bodeker (Bodeker Scientific), Quyen Nguyen and Antoni Moore (University of Otago) and Ivan Diaz-Rainey (Griffith Asia Institute).
This blog is a synopsis of the research presented in “Empirical Models of Shallow Groundwater and Multi-Hazard Flood Forecasts as Sea-Levels Rise” by Simon C. Cox, Marc H. J. Ettema, Lee A. Chambers, Scott A. Stephens, Gregory E. Bodeker, Quyen Nguyen, Ivan Diaz-Rainey, and Antoni B. Moore. It draws on insights from the authors’ case study of Dunedin, New Zealand, to explore how rising groundwater contributes to evolving coastal flood risks.
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