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Purpose of the Report 

To provide updated flood estimates for the Mangaroa River catchment to support flood hazard assessment and planning. 

Geographic Area 

The report covers the Mangaroa River catchment, a tributary of the Hutt River in the Wellington Region of New Zealand. The catchment spans 104 km² and includes rural and semi-rural land with scrub, pasture, and forested areas. 

Summary

This report presents a comprehensive analysis of flood hydrology for the Mangaroa River catchment. It includes rainfall analysis, development and calibration of a rainfall-runoff model, and flood frequency estimation. Rainfall data was sourced from Greater Wellington and NIWA databases, with key stations including Tasman Vaccine Limited, Centre Ridge, and Te Marua. Rainfall depth-duration-frequency analysis was conducted using the EV1 distribution, and Probable Maximum Precipitation (PMP) was estimated using the Thompson & Tomlinson method. 

A rainfall-runoff model was developed using TimeStudio software and calibrated with six flood events, then validated with five additional events. The model showed good performance, with an average error in peak flow of approximately 9.3% and validation error around 5.4%. Design storms ranging from 2-year to 200-year return periods, as well as PMP, were simulated to estimate peak flows at various points in the catchment. 

At the Te Marua site, the critical storm duration was found to be 6 hours. Final flood frequency estimates were derived by pooling results from at-site analysis and rainfall-runoff modelling, which were closely aligned (within 5%). These estimates were on average 13% higher than previous estimates from 1990. Regional estimates were significantly lower and not used in the final recommendations. 

The report concludes that the updated flood frequency estimates provide a more reliable basis for flood hazard assessment due to the longer data record and catchment-specific modelling. The Probable Maximum Flood (PMF) at Te Marua was estimated at 1864 m³/s. The findings support improved planning and risk management for the Mangaroa River catchment.