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Purpose of the Report 

To assess flood hydrology in the Pinehaven Stream catchment and provide design flood estimates for hydraulic modelling. 

Geographic Area Covered 

The Pinehaven Stream catchment in Upper Hutt, Wellington Region, New Zealand, including tributaries upstream of Chatsworth Road. 

Summary of Key Findings

This report presents a flood hydrology assessment of the Pinehaven Stream catchment, commissioned by Greater Wellington Regional Council and prepared by MWH in 2009. The catchment covers approximately 4.7 km² and includes steep upper reaches and urbanised lower areas. The study aimed to estimate design flood flows for use in hydraulic modelling. 

Rainfall data was sourced from multiple gauges including Tasman Vaccine, Wallaceville, and Pinehaven. Rainfall frequency analysis was conducted for durations ranging from 30 minutes to 6 hours and return periods from 2 to 100 years. Probable Maximum Precipitation (PMP) was also calculated.

Due to limited flow data, regional flood frequency methods and the Rational Method were used to estimate peak flows. A rainfall-runoff model was developed using Hydstra software and calibrated using a flood event from July 2009. The model was divided into 15 sub-catchments and used design rainfall inputs to generate hydrographs. 

The modelled peak flows for the 100-year event were approximately 22 m³/s at Chatsworth Road. The PMP scenario produced a peak flow of 86 m³/s. The rainfall-runoff model results were consistent with regional methods and are recommended for use in hydraulic modelling. 

The report recommends continued data collection and future calibration of the model. It also suggests incorporating climate change scenarios into future assessments.