Common Misconceptions and What You Need To Know

You’ll notice several of the points below are detailed and, at times, quite technical. That’s because many of the problems with the Pinehaven flood model are technical in nature.

We’ve done everything possible to keep the explanations simple, but some technical detail is unavoidable. We have kept the language as clear as possible without oversimplifying and missing the point. All the information below is supported by evidence and expert reports.

If some parts feel hard to follow, that’s okay. The key message is simple: the Pinehaven flood maps contain serious errors, and these errors must be addressed and fixed by including Pinehaven catchment in the current city-wide flood-mapping review.

What matters most is this: the current Pinehaven flood maps have known, significant errors, and must be re-evaluated now.

The call to action is to make a submission here … Upper Hutt Flood Hazard Modelling | Have Your Say | Greater Wellington before the consultation period closes on Friday 12 December 2025 and to request in your submission that:

• the Pinehaven and Silverstream flood maps be included in the current consultation
• the consultation period be extended to provide better information and allow reasonable time for engagement and submissions

What You Need To Know: Completion does not equal accuracy. The maps were finalised before the 2019 flood, before the 1976 and 2009 events were correctly re-classified, and before the rainfall-loss issue was discovered in the “future development” error.

Updating the whole city while excluding the one catchment with known defects is indefensible.

What You Need To Know: The 2015 audit missed or omitted key errors:

• wrong rainfall-loss parameters,
• misclassified 1976 and 2009 events,
• failure to adequately scrutinize the wrong peak water level value for the 2009 event,
• misrepresentation of Save Our Hills’ ‘case study’ evidence,
• and altered flood data and diagrams on the key ‘case study’ property.

A flawed audit cannot justify excluding Pinehaven from a city-wide update.

What You Need To Know: Calibration was done to only one event (23 July 2009) which was wrongly assessed, using:

• no local rainfall data (SKM stated that the Pinehaven rain gauge “malfunctioned” on 23 July 2009, but a LGOIMA response revealed that the Council removed the Pinehaven rain gauge from the catchment for the full 2-year period of the flood study (2008 – 2010),
• the wrong peak water level of 1.2 m at the stream gauge site (the stream gauge recorded a peak water level of 1.6m, but SKM claimed the model produced a “close match” with flood extents observed in the field when the model used a peak water level of 1.2m),
• but Council staff and their consultants did not observe flood extents in the field; they didn’t visit the Pinehaven catchment until the day after the flood on 23 July 2009, and
• Council published or displayed nothing at the drop-in session on 12 September 2009 to demonstrate a “close match” between modelled and observed flooding on 23 July 2009,
• GWRC’s estimated peak flow of 8.8 m³/s for the flood on 23 July 2009, which they state was a 5 – 10-year flood, contradicts Council’s own modelling of a 5-year flood as 15m³/s and a 10-year flood as 17m³/s, not 8.8 m³/s, a serious anomaly discrediting the model,
• later independent analysis following the 25-year flood on 8 December 2019 found the 23 July 2009 flood to be a 38–40-year event, not a 5–10-year event; evidence clearly shows the 2009 flood was larger than the 2019 flood, and
• independent analysis found the 23 July 2009 flood to have a peak flow of 12 – 12.5m³/s, not 8.8m³/s as miscalculated by GWRC.

A model calibrated to just one mis-calculated flood event is not a reliable flood model, and it should therefore be included in the current flood map review.

What You Need To Know: Being operative does not make them correct. They became operative before real data (2019 flood) and before key errors were identified.

The purpose of consultation is to fix outdated maps. Pinehaven is among the most outdated.

What You Need To Know: There is extensive new information:

• the fully documented 25-year flood on 8 December 2019,
• revised Annual Return Intervals for 1976 (at least a 1,000-year) and 2009 (38–40-year),
• independent hydrologists’ findings,
• discovery of the rainfall-loss error,
• evidence of grossly overstated flood extents.

MWH and Beca themselves both said major new data should trigger re-modelling, therefore the new information must be considered in the current flood map review.

What You Need To Know: “Later” has meant never for more than a decade: no new stream gauge, no recalibration, no validation, no update after the 2019 flood.

The only fair approach is to include Pinehaven now.

What You Need To Know: Correcting inaccurate maps is the responsible choice.
Excluding the catchment with the largest known errors is not.

What You Need To Know: Past consultations were based on incomplete and incorrect information.

Key assumptions, errors, and later evidence (1976, 2009, 2019 floods; rainfall losses; calibration issues) were never disclosed.

A consultation can’t be “fair” if the public was never shown correct and complete information.

What You Need To Know: Re-modelling does not require new gauge data and does not take years. Hydrologists routinely recalibrate small catchments from historical events. Pinehaven has at least three major usable calibration/validation floods (1976, 2009, 2019).

The real risk comes from continuing $60M streamworks based on a faulty flood model.

What You Need To Know: The key point is the flooding that actually happened on the ground.

The real 25-year flood in 2019 was far smaller than GWRC’s modelled 25-year flood.
This shows the Pinehaven flood maps are wrong and must be included in the current review for transparent reassessment.

What You Need To Know: Engagement was never properly informed. The public was not told about incorrect rainfall losses, misclassified floods, flawed calibration, altered diagrams, or the mismatch with real events. PC42 even ruled the accuracy of the underlying flood model “out of scope.”

Unfair and incomplete past processes cannot substitute for a transparent review today.

What You Need To Know: The streamworks are oversized because the baseline was inflated using future-development hydrology. The streamworks are for unmanaged future-development runoff.

Also, the streamworks provide no protection for properties upstream of Pinehaven Reserve.

Downstream of Pinehaven Reserve, the streamworks only partially prepare the catchment for unmanaged future-development runoff, at ratepayers’ cost.

This is not “extra protection”; it is subsidising the proposed future development on the hills.

Pinehaven must be included in the review to restore transparency.

What You Need To Know: The ‘fast-track’ panel cannot rebuild the flood model. It must rely on the existing flawed baseline model and evidence provided by the Councils and GTC which already masks the effects of GTC’s proposed development.

No panel can guarantee stormwater neutrality when the underlying baseline model is wrong.

What You Need To Know: Climate-change and blockage allowances are small; they are add-on adjustments applied after a correct baseline is achieved by matching the model with current flood events. Climate change does not explain the large over-predictions in Pinehaven.

A model must first match the real 25-year flood in 2019. This one doesn’t.

What You Need To Know: This is false. Recalibration can be done now using:

• comprehensive data from the 25-year flood on 8 December 2019 (over 500 photos, 32 detailed eyewitness accounts, surveyed debris marks and expert reports),
• the peak water level of 1.6m for the 23 July 2009 flood,
• 27 eyewitness accounts of the 20 December 1976 flood, the Wellington Regional Water Board report by R. G. Bishop, January 1977, and G. Horrell’s review of the 1976 flood,
• and NIWA rainfall data.

Waiting for new stream gauge data is a delay tactic, not a technical requirement.

What You Need To Know: The Environment Court did not approve the Pinehaven flood maps or order Upper Hutt City Council to adopt them. The Court has confirmed that claim is incorrect.

The UHCC Plan Change 42 (PC42) Consent Order issued by the Environment Court was for a separate appeal relating to the Mangaroa River. It had nothing to do with the Pinehaven flood maps. It resolved a complaint concerning the dumping of concrete in the Mangaroa river bed.

At the 14 August 2019 UHCC Council meeting, Mayor Wayne Guppy told councillors they were required by the Environment Court to adopt the PC42 Pinehaven flood maps and if Council did not adopt the Pinehaven flood maps then Council would be in breach of an Environment Court Consent Order. But the Pinehaven flood maps were outside the scope of the Consent Order.

Councillors adopted the Pinehaven flood maps under the false belief that the Environment Court required it. That was a misunderstanding of the Court’s Consent Order resolving the Mangaroa river bed issue.

The Environment Court did not approve or endorse the Pinehaven Stream flood hazard maps.

The Pinehaven flood maps grossly exaggerate flood extents because they are based on a seriously flawed and unreliable flood model. They must be included in the current flood map consultation to ensure transparency in reassessing the flood model to acceptable standards along with the rest of the Upper Hutt flood maps.