Pinehaven and Silverstream Homes Are At Risk If Flood Maps Are Not Fixed!

Why Council’s flood modelling is flawed & urgent action is needed to protect our homes

See those hills above Pinehaven, covered with pine forest?

Tests show those hills act like a giant sponge, soaking up lots of rainfall, reducing flooding in a storm. 

Greater Wellington Regional Council says if all those trees were replaced with hundreds of houses, driveways and roads – all that permeable soil replaced with concrete, bitumen and roofing – it wouldn’t make any noticeable difference to flooding in Pinehaven and Silverstream in a big storm.

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This is grossly incorrect. We know that stormwater increases enormously when housing is added to a greenfields site.

So how did Council conclude that there would be no change?

An auditor discovered that in their baseline model Council modelled the forested hills as if high density housing already exists on the hills (left).

So, naturally, the comparison model with a housing development added to the hills showed no increase in flooding for Pinehaven! Instead of disclosing this large error in the baseline flood model, the auditor dismissed it, wrongly stating the baseline flood model is ‘fit for purpose’. Council’s flood model is not ‘fit for purpose’! It grossly misrepresents the forested hills as if they are like bare rock. It is not a reliable baseline model for assessing the true impact of future development.

What do we know about stormwater runoff & housing developments?

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Auckland Council’s stormwater design guide (GD04:2015 p32) states that development on a greenfield site typically increases runoff 200% – 500%. The Pinehaven flood model shows only 1% increase in runoff from urban development on Pinehaven’s forested hills. This is clearly shown to be out by at least 200% - 500%, as shown by R J Hall & Associate’s peer-reviewed evidence.

The consequences of Greater Wellington’s flawed Pinehaven flood model could be more frequent and more widespread flooding in Pinehaven like below:

Other Consequences

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Flood expert R J Hall warns there is:

“the potential to destabilise slopes and result in an increase in the propensity for landslides ... increased sediment loading being deposited in the flatter downstream reaches of the stream ... [removing] ... any gains from the channel upgrade ... and blocking of culvert entries.  If they result in the formation and failure of debris dams during major storms then there can be very serious consequences ( viz. Blandswood Settlement incident, Peel Forest, South Canterbury January 1975, four lives lost ), including landslide debris dam formations and sequential failures.”

The Council’s flawed flood model has set Pinehaven up for future trouble like this …

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It is imperative for the safety and wellbeing of residents in Pinehaven and Silverstream, and for the protection of the environment, that the Council’s deeply flawed Pinehaven flood modelling is fixed. Pinehaven has no flood protection, and the Pinehaven Streamworks won’t save Pinehaven or Silverstream from the consequences of the flawed flood model.

 

Only a correct baseline flood model can ensure that stormwater runoff from future Guildford development on the Pinehaven and Silverstream hills will be properly managed so as to protect Pinehaven and Silverstream from worse future flooding.

 

The best thing you can do to protect Pinehaven and Silverstream from worse future floods is to support the urgent call to Council to fix the Pinehaven Stream flood model!