The latest update of nationwide flood maps, to be released next year, will give communities across New Zealand the clearest picture yet of the regional risk of flooding.
Authors
- Amandine Bosserelle
Lecturer in Hydrogeology, The University of Western Australia
- Leanne K Morgan
Associate Professor in Hydrology, University of Canterbury
But as it stands, this state-of-the-art tool leaves out a critical piece of the puzzle: the water table in shallow coastal aquifers, which fluctuates with the seasons and, over the long term, rises with sea levels.
As our recent research shows, coastal groundwater responds to sea level rise in different ways. Where the water table is free to rise with the sea, it does exactly that, rising toward the surface and increasing the risk of groundwater flooding.
This maintains the flow of fresh groundwater to the coast and keeps saltwater intrusion at bay.
However, where the water table is instead held down by low, flat topography, the flow to the coast weakens, increasing the risk of saltwater intrusion .
Either way, these changes happen out of sight underground and rarely make it into adaptation planning tools, even though nearly one in five New Zealanders live in low-lying coastal areas.
Waters rising from beneath
We usually think of floods as storm surges, crashing waves and ocean water topping sea walls. But rising seas are not only pushing against the shore from the outside – water is also rising from beneath.
Beneath coastal landscapes, there is a vast and mainly uncharted resource of freshwater. When rain falls on the land, it slowly drains down into the subsurface where layers of sand, gravel and rocks form a saturated groundwater zone.
Driven by gravity, this shallow freshwater flows slowly through the ground toward the coast, eventually reaching estuaries, tidal creeks and the ocean. Where these aquifer systems meet the coast, the water table often sits just below the ground most people live and work on.
New Zealand’s low-lying coastal areas span around 9,000 square kilometres nationwide. This thin coastal margin also reaches inland along river valleys and basins such as the Hauraki Plains and the vast Canterbury Plains, making up just over 3% of the total land area.
A twin hazard to infrastructure and freshwater
It helps to imagine coastal landscapes as a bathtub being filled both from the tap at the top and a source of water percolating from below.
Most coastal adaptation planning focuses on surface and atmospheric waters: rivers, heavy rainfall and ocean swells. Yet along much of New Zealand’s coastline, the water table lies just meters, sometimes centimetres, below the surface.
In many low-lying coastal areas, rising seas will fill aquifers high enough to spill over the rim, with groundwater rising above the land surface long before the ocean’s saltwater ever breaches the coastline.
Groundwater flooding damages natural resources, agricultural land and built environments. Saltwater intrusion threatens drinking water supplies and coastal ecosystems.
When groundwater levels rise, they diminish the capacity of the soil to absorb heavy rainfall, making surface flooding far worse. Slow moving and persistently high water tables compromise soils and critical underground infrastructure such as wastewater and stormwater pipes long before land is permanently submerged.
Road sub-bases weaken, drainage lacks, wastewater pipes crack or float, septic systems fail and basements experience persistent dampness and structural damage.
Attention to coastal aquifers is growing, both in New Zealand and globally . But planning regulations and infrastructure design still treat groundwater as a static variable rather than a dynamic and emerging hazard.
Closing the gap between sky, surface and subsurface
To build genuinely resilient communities, we must bridge the gap between atmospheric weather, surface water flows and subsurface hydrology.
Excluding shallow water table dynamics from national flood modelling risks giving local councils, homeowners and insurers a false sense of security. A flood map that only looks at surface runoff tells only half the story.
Historically, groundwater monitoring has focused on deep aquifers to secure drinking and agricultural water supplies. This has left shallow coastal water tables largely unmapped.
Gathering data on these shallow systems is frequently viewed as too costly and technically complex for individual regional councils to manage alone. However, patching together fragmented regional data is no longer sufficient. We urgently need a coordinated effort to track water tables.
Planning for climate adaptation requires observing, measuring and modelling these systems together. Integrating long-term water table responses and short-term seasonal variations into New Zealand’s national flood maps is entirely achievable and essential.
Most people don’t think about the water table sitting just below their feet, let alone plan for it. Until models account for what happens below the surface as well as what falls from the sky, we will keep being caught out by floods rising quietly from below.
