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Purpose of the report: To identify flood risks and assess hazards in the Pinehaven Stream catchment using hydraulic modelling and related analyses.

Geographic Area: The Pinehaven Stream catchment, located on the eastern side of the Hutt Valley in Upper Hutt, New Zealand, covering approximately 4.5 km². 

Summary:

This report presents the findings of a flood hazard assessment for Pinehaven Stream, conducted by Sinclair Knight Merz for the Greater Wellington Regional Council and Upper Hutt City Council. The study area includes a steep upper catchment and a flatter lower catchment, with land use ranging from pine forest to residential and commercial zones. 

Historical flood events, notably in December 1976, 2004, 2005, and July 2009, have caused significant damage, especially in areas such as Birch Grove, Blue Mountains Road, Sunbrae Drive, and Silverstream Village. The 1976 flood was exacerbated by logging debris and led to the construction of bypass culverts and a detention dam. 

Hydraulic modelling using MIKE FLOOD software revealed that much of the stream has less than a 5-year flow capacity, with culverts and bridges acting as major constraints. Blockages at these structures significantly increase flood extents. The model was calibrated using historical data and community input, and it closely matched observed flood extents. 

Design scenarios included 5 to 100-year storm events and a Probable Maximum Flood (PMF), with climate change considerations increasing rainfall intensity by 16%. Future development, including up to 1,665 new dwellings, was assessed and found to have a minor impact on flood extents. 

Flood hazard maps were developed for various scenarios, including erosion setback recommendations. A flood damage assessment estimated potential damages up to $2.47 million for the base 100-year scenario and up to $7.15 million with blockages. Lower reaches of the stream are highly susceptible to erosion, with 13 erosion-prone locations identified. 

The planning framework review found that Pinehaven Stream is not currently included in flood overlays or specific hazard controls. However, future plan changes aim to incorporate risk-based flood management. Community consultation involved over 150 residents and provided valuable insights for model calibration. 

The report concludes that Pinehaven Stream poses a significant flood risk due to limited capacity and structural constraints. It lays the groundwork for Phase 2, which will explore mitigation options such as stream upgrades, planning controls, and emergency response strategies.