SUPPLIED

Properties in Elmslie Rd bared the brunt of the flooding of the Pinehaven Stream in 1976. A dispute over an updated flood risk map is now headed to the Environment Court.

A dispute over flood management in Upper Hutt has spilled into the Environment Court.

Community activists have appealed an Upper Hutt City Council district plan change that will have a flood risk map built into council decision making. 

Flood management is a point of contention for the Upper Hutt suburb of Pinehaven, where a 1976 flood had 22 families evacuated and many homes destroyed.

Save Our Hills (SOH) campaigner Stephen Pattinson says the group's appeal was bolstered by a Greater Wellington Regional Council (GWRC) decision to remodel a Wairarapa flood hazard map after community outcry.

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But the council says the court action will delay structural work and continue to expose the community to flooding. 

After a lengthy consultation process, the council decided on a plan change in March that would include a new flood hazard map for both Pinehaven Stream and Mangaroa River catchments. 

The plan change was supported by independent commissioner David McMahon.

Pattinson said SOH supported flood mapping and structural work to mitigate flood risk, but the current modelling was "grossly inaccurate" and inflated.

The increased risk of runoff from a potential housing development in the hills above Pinehaven is central to the dispute.

"The new flood map for Greytown is a greatly reduced flood extent, just showing the thing we need over here is better modelling and mapping for a once in 100-year flood."

GWRC conceded its flood hazard map for the Waiohine Gorge was flawed in August 2017, releasing a new interim map in March.

The new map reduced the perceived impact of flooding for Greytown. 

While the original Waiohine modelling was deemed fatally flawed in an independent report by engineering firm Beca, many aspects were described as having major issues.

The same Beca hydrologist has audited and supported the Pinehaven model, satisfied any future development has been properly accommodated. 

Pattinson said the assessment was superficial, and required further investigation. 

"If that development does go ahead on the hill, unless these flood maps get fixed  … we will see much worse flooding in the future than we see now."

SOH wants the plan change to be declined or delayed, to allow for an independent expert to review the modelling.

Upper Hutt City Council city planning director Richard Harbord said the plan change provided a framework for future housing developments, and guided incoming structural work on the Pinehaven Stream.  

"If that's delayed, then that means that the community is exposed to flood risk."

Harbord said the council was confident in the flood risk modelling, and review by the independent planning commissioner. 

It is expected Environment Court mediation will begin in late August. 

 - Stuff