Do you support the call for flood protection for Pinehaven, Upper Hutt, NZ?

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Upper Hutt City Council promised that all flooding in this Pinehaven flood map (above) would be removed by Council's $10million Pinehaven Stream improvements - BUT NONE OF IT WIIL BE! Now the stream works are costing about $30million (plus loan interest = approx. $60million) ... BUT WON'T MAKE ANY DIFFERENCE TO FLOODING IN THE ABOVE PINEHAVEN FLOOD MAP! This leaves Pinehaven at risk from current and future flooding. Council is pushing for very large-scale residential development (called 'Guildford') on the forested hills in the upper catchment. Accurate baseline flood modelling is essential for protecting Pinehaven from extra stormwater runoff from future development on the hills.  

This petition requests that Upper Hutt City Council provide proper flood protection for Pinehaven, as promised.


We want accurate flood modelling that will protect Pinehaven. Do you support us? Council held public meetings in Pinehaven, promising local residents that Council would upgrade Pinehaven Stream to remove all flooding in Pinehaven up to and including a 1-in-25 year storm event (shown in Council's Pinehaven flood map above).

The purpose of this petition is to find out how much public support there is for accurate flood modelling to protect Pinehaven from flooding. There are obvious reasons why flood protection is important - people's safety; property protection; insurance eligibility and cost of premiums; real estate values; development opportunities.

But in Pinehaven's case, we also want flood protection from proposed large-scale future residential development on the hills above us. And for that Pinehaven needs accurate flood modelling of the pre-development (existing) and post-development situations.

However, Council has created a grossly inaccurate existing pre-development (baseline) flood model. Rather than protecting Pinehaven, it will expose Pinehaven to worse flooding that will be caused by future development. Council refutes this, but we have sound, reliable expert evidence that shows Council's baseline flood model is grossly inflated. It cannot be relied on as an accurate baseline for assessing additional stormwater runoff from future development. A grossly inflated baseline model won't detect additional runoff from future development!  Additional runoff should be managed on site by the developer so that it will not make existing flooding worse. 

Council's inflated flood model has actually set Pinehaven up for WORSE flooding in the future from the proposed Guildford development! We've discovered this by engaging flood experts who have analysed Council's flood modelling for Pinehaven and found it fatally flawed - their flood model is grossly inflated. An audit by Council found their Pinehaven flood model "fit for purpose", but this is not correct. 

In simple terms, Council's flood model misrepresents the existing Pinehaven Stream catchment (about 80% of which is forested hills) by treating it as impervious, like a rock. And so nearly all rainfall in Council's model runs off the hills, resulting in flood maps that show widespread flooding. In reality, actual flood extents are much less than Council's flood model predicts because the forested hills soak up huge volumes of stormwater, like a sponge - so only a portion of rainfall runs off the hills into the valley. 

Proof that Council's Pinehaven flood model is grossly inflated is an actual flood event. Council's 1-in-25 year flood map shows flood extents that are way in excess of flooding that actually happened in the 1-in-25 year flood on 8 December 2019, proving Council's food model greatly exaggerates flood extents.

On site tests show that rainfall infiltration in the forest is massive (500mm to 900mm per hour). But Council's modelling uses only 2mm per hour. This grossly UNDER-estimates infiltration, and therefore grossly OVER-estimates stormwater runoff. Significant over-estimation of runoff results in grossly exaggerated flood extents in the flood maps, as proven by actual flood extents in the 8 December 2019 flood event.

Guildford's very large residential development will replace about 100 hectares of forest on the hills in the upper Pinehaven catchment. Large impervious areas of roads, roofs and driveways will replace existing highly absorbent forest. This will result in 300% to 500% increase in stormwater runoff that will greatly increase flooding in Pinehaven if it is not managed properly on site. 

Council's inflated baseline (pre-development) flood model will mask all this extra runoff (tens of thousands of tons) from the proposed Guildford development. Instead of being managed on site it will simply be dumped downhill, making flooding in Pinehaven much WORSE. Future floods in Pinehaven will be bigger, more frequent and widespread than current flooding because of uncontrolled additional stormwater from Guildford's roads, roofs and driveways.

The only way to protect Pinehaven community and the Pinehaven environment from much bigger future floods from the Guildford development is to INSIST that Council abandon their current grossly inflated baseline flood model and do a new baseline flood model that accurately represents the existing highly absorbent forests.

An honest and accurate baseline flood model will ensure that extra stormwater runoff from the Guildford development will be identified and managed properly so it doesn't make existing flooding in Pinehaven any worse. Given the better data we now have on the Pinehaven catchment, an accurate baseline flood model could be created quickly and easily. Fixing the flood model for proper flood protection is not a big ask of Council. So this petition requests that Council fix the Pinehaven flood model and provide proper flood protection for Pinehaven.

Although this is a local issue, the inflated Pinehaven flood modelling reflects inconsistent flood engineering standards in New Zealand. New Zealand has no national flood hazard management standard, and flood engineering practice is fairly ad hoc. Every region and district does it differently. A review of flood hazard assessment and management is well overdue and urgently needed to develop a uniform approach in New Zealand to rainfall-runoff flood modelling and mapping.