Whangamata's Stormwater System​

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1. How Our Stormwater System Works

When it rains, water has three possible pathways:

• It soaks into the ground

• It flows into pipes

• It travels overland into rivers or the ocean

 

2. Soakage – Only Works in Sandy Soils

Soakage works well in sandy soil, but not in clay. Stormwater soakage systems must handle 100mm of rain in a 24-hour period—either by holding the water (storage) or letting it seep into the ground (infiltration), or both.

Cyclones Hale and Gabrielle filled our water table. Then heavier-than-normal winter rain caused persistent flooding because the ground was saturated and couldn't absorb more water.

 

3. Pipes – Limited but Fast

Some roads in Whangamatā have stormwater pipes—but homes are not connected. Pipes are expensive, so councils are only required (under the Building Act) to cater for a 2% AEP (Annual Exceedance Probability) event.

In Whangamatā, that’s equivalent to 137mm/hour for 10 minutes.
Designing for rarer events (like 1-in-100-year storms) would require much larger and more expensive infrastructure.

 

4. Overland Flow – Nature’s Drainage

Anything the pipes and soakage can’t handle flows overland. Naturally, this water would follow valleys, rivers, creeks, and streams to the ocean.

But we’ve disrupted those natural pathways by building roads and subdivisions.
We've replaced sandy dunes and grass with hard surfaces like concrete and roofing, which increases runoff—by up to four times per square metre, according to Waikato Regional Council (WRC).

Our old pipe systems didn’t anticipate climate change, and now they’re undersized.

 

5. Detention Basins – Holding Extra Water

These are low-lying areas (like swales) that collect water when everything else is full. If your home is on one, you’ve likely experienced flooding.

Council rules require:

• Floor heights above flood level

• High ground can't drain onto lower properties

These rules are meant to reduce flood risk—but enforcement has been inconsistent.

 

6. How It All Works Together

• Primary flow paths = Pipes

• Secondary flow paths = Overland flow

• Supporting systems = Soakage + Detention basins

Council can use all four in a proper Stormwater Master Plan to protect homes and people.

 

7. Climate Change Is Raising the Bar

Global warming means councils must now plan for:

• A 1% AEP event (more intense rainfall)

• Sea level rise through to 2090
  This means:

• Pipes need more capacity and must discharge at higher levels

• Detention areas and overland paths must be expanded

Whangamatā has two master plans already developed from flood modelling—but these are still being withheld from the public.

 

8. Development Has Outpaced Infrastructure

Subdivisions, large homes, and wider roads have made our streets act like rivers when pipes and soakage overflow.

Without good planning, runoff collects in low areas, flooding homes built there.
The Building Code says houses must be 150mm above the road crown—but that hasn’t always been enforced.

 

9. Council’s Responsibility

Development should only happen after a stormwater master plan and infrastructure are in place.

This infrastructure is supposed to be funded by:

• Reserve contributions

• Resource consent approvals

But that hasn’t happened. We're now facing an infrastructure deficit—and growing risk to property owners with low floor levels.

 

10. Legal Waivers and Flood Risk

The Building Act allows homeowners to build below the recommended floor level—but only if they sign a waiver stating council can’t be held liable for flood damage.

These waivers should appear on property titles.
As far as WRSAG can determine, Council has not consistently applied these waivers.

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