Why “1 in 100” Isn’t Enough: We Need Running Stormwater Totals for Development and Pipe Headroom in NZ
- ajraea
- Sep 2
- 5 min read
Updated: Sep 25
When the Hindenburg disaster happened, the shock wasn’t just the tragedy itself, it was that people believed rigid airships were almost unimpeachably safe. In the same way, flood risk is still being communicated to the public in misleadingly “safe” terms.
We say an event is “1 in 100”implying a remote 1% chance. But the climate is shifting. A “1-in-100” storm yesterday may now happen as often as 1 in 50 or less today. For a homeowner with a 30 year mortgage, that’s not once in a century; it’s close to a coinflip that they’ll face such a storm, possibly more than once or more. (In the Tasman region we’ve even seen this year severe events arrive twice in two consecutive weeks.)
Why “1 in 100” in NZ Creates Confusion
Engineers mean “1 in 100” = a 1% chance this year. But as rainfall intensifies, the threshold for what qualifies as “1 in 100” moves.
Today: 20 mm of rain in one hour might be labelled 1 in 100.
Tomorrow: with a wetter climate, that exact same 20 mm/h could be reassessed as 1-in-10.
To the public, the storm didn’t change but the label did. Stop banks 1 in 200 when built could be 1 in 50. Communities cling to the old measure without realising it’s changed. That erodes trust, leaves people unsure how to act, exposed and unprepared. Clearer signals are needed.
Better message is to talk in capacity terms the public can understand.
“Our pipes safely move X mm per hour and Y mm per day. Above that, we must store, slow, or safely bypass water upstream.”
The Problem: Using Terms and Numbers that Confuse Instead of Clarify for NZ Communities
The Annual Exceedance Probability (AEP) behind “1 in 100” makes sense to engineers and insurers, but it doesn’t help communities.
What people really want to know is: can our system cope with the storm overhead today?
The truth is simpler:
Pipes have a physical limit. They can carry X mm of rain per hour and Y mm in a day.
Catchments have a storage budget. Once that headroom is gone, the water has nowhere to go but into homes, roads, and businesses.
Why Running Totals Matter
Every development adds hard surface roofs, driveways, roads. Each change adds stormwater runoff.
Yet assessments are often done in isolation, treating one subdivision as if it exists in a vacuum.
But the NZ pipe network feels the combined effect. What matters is the running total:
How many mm/hour are already committed to the network?
How many buildings can the catchment still support before upgrades are needed?
How much detention or slowing is already in place upstream and how much more is required?
The Shift: From Rare Event Thinking to Capacity Thinking
We need to move from “1 in 100” language to pipe headroom and catchment in NZ budgets. That means:
Publish pipe capacity numbers (e.g., “This catchment can safely convey 40 mm/h and 90 mm/day”).
Track development as a running total every consent adds to the cumulative load.
Require additional mitigation per property watertanks, raingardens, or upstream storage that keep peak flows below the catchment’s budget.
Update headroom publicly so councils, insurers, and residents know limits and risk denoted by 1 in 100 risk measures per year in their catchment. Communities know when to prepare when they look at weather forecasts compare to catchment limits without being told.
Such measures will drive innovation in stormwater management
Example in Practice
A pipe system can safely move 40 mm/hour.
Current subdivision contributes 35 mm/hour.
Another 100 houses in the upper catchment each add 0.1 mm/hour (= 10 mm/hourmore).
Total = 45 mm/hour → over capacity.
With a running total approach of all developments , this consent would be flagged. storage or pipe upgrades must come first before we develop more buildings
The NZ Overland Flood Path Problem
Too often, our response to overcapacity is simply to designate an overland flood path to take the excess water.
On paper, this “solves” the hydraulic calculation. In reality, it pushes risk onto communities downstream. Floodplains widen, water velocity increases, and people in the lower catchment are left bearing the cost.
When those areas flood, the refrain is predictable:
“They shouldn’t have built there. They should have known.” How? Just because it is a flood plain today, doesn’t mean it was yesterday.
That isn’t planning, it’s deferring responsibility. It ignores the cumulative effect of upstream decisions and leaves downstream residents unprotected, uninsured, and too often blamed for circumstances outside their control.
Why This Matters
Confusion around “1 in 100” gives NZ communities false comfort just as zeppelins were once assumed safe until disaster proved otherwise.
By switching to running totals and capacity reporting combined with risk, we give NZ residents, councils, insurers, and developers clear visibility on when the system is reaching its limits. And by ending the default reliance on NZ overland flood paths, we stop shifting risk downstream and start sharing responsibility catchment wide.
Beyond NZ Stopbanks: Bundles of Measures
With climate change, stopbanks designed for a 1 in 200 events are being resized downward in likelyhood as rainfall increases. This shows we cannot rely on single measures or outdated benchmarks so communities aren’t complacent.
What’s needed is a bundle of measures that considers both:
Likelihood of occurrence annually (probability based thresholds), and
Level of water in millimetres (physical volumes catchment systems can handle).
By pairing these together, we provide clarity in terms non technical people can understand. If NZ communities can see both the risk of a certain event, scale of the storm risk (mm) and compare to pipe/catchment capacity, stakeholders can plan their response with confidence as risk profile increases.
Solutions exist!!! Preparing Beyond the Pipes NZ
As climate changes and more rain falls, knowing these numbers lets communities prepare for tomorrow’s storms. If a catchment is already over or near capacity, we can deploy:
Early warning systems buying time to act
Fixed, Modular and Self Activating NZ flood barriers, protecting key infrastructure businesses and homes, jobs and GDP
Catchment storage leaky dams, wetlands, detention ponds to reduce NZ peaks
Community wide planning ensuring downstream neighbours aren’t left with the consequences
Talk to us
Reach out to Dutch Water Prevention NZ to see how we help councils, businesses and communities, prepare with practical solutions we have developed to help communities mitigate risk through warnings and water management :
(09) 440 9820
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