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Purpose of the Report 

To assess flood hazards and inform floodplain management for the Mangaone Stream catchment. 

Geographic Area Covered 

The report covers the Mangaone Stream catchment, which originates in the western Tararua Ranges and flows northwest to Te Horo Beach, south of Ōtaki, encompassing a total area of approximately 54.5 km². 

Summary of Key Findings 

This report presents a comprehensive flood hazard assessment of the Mangaone Stream, commissioned by the Wellington Regional Council and prepared by Montgomery Watson Harza. The catchment spans from the Tararua foothills to Te Horo Beach, with varying terrain and land use including native bush, pasture, horticulture, and residential areas. Hydrological modelling was conducted using the Hydrol tool, calibrated with local rainfall and flow data, to simulate flood hydrographs for return periods ranging from 2 to 200 years, including extreme and probable maximum flood (PMF) scenarios.

A detailed hydraulic model was developed using Mike11 software, incorporating 121 surveyed cross-sections and calibrated with Manning’s roughness coefficients. The model simulated flood depths, extents, and velocities across the catchment, identifying key flow paths and ponding areas. Flood hazard maps were produced based on the 100-year average recurrence interval (ARI) event, with recommended freeboard levels of 300 mm near the coast and 400 mm further upstream. 

Damage assessments estimated potential costs for various flood scenarios, ranging from $16,000 for a 10-year event to over $1.1 million for an extreme flood. These costs include impacts on residential and commercial properties, notably a poultry factory, while farming damages were noted but not quantified. Stream morphology analysis identified erosion zones, deposition areas, and historical alterations, recommending a 40 m erosion hazard corridor and potential weir construction to stabilise gradients and reduce upstream erosion. 

An environmental register was compiled, documenting water quality monitoring, ecological observations, riparian vegetation, planning documents, and cultural sites. The report concludes with hazard mapping and management recommendations to support sustainable floodplain planning and risk mitigation for the Mangaone Stream catchment.