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.

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........

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