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Purpose of the Report: 

To assess flood risk to Carterton from Mangatarere Stream during a 1% AEP event and under climate change scenarios. 

Geographic Area Covered: 

The report covers the Mangatarere Stream catchment and its floodplain, particularly the area surrounding Carterton in the Wairarapa region. 

Summary of the Report: 

This report presents a flood hazard assessment for Carterton, Wairarapa, focusing on the Mangatarere Stream. Commissioned by Greater Wellington Regional Council and prepared by Opus International Consultants Ltd, the study evaluates the potential flood extents from a 1% Annual Exceedance Probability (AEP) event and considers future impacts under climate change. Using hydrological data and LiDAR terrain information, a combined one-dimensional and two-dimensional hydraulic model was developed with TUFLOW software. The model simulates stream flow and floodplain inundation, incorporating bridge structures and land roughness. Design flows were estimated from recent hydrological analysis and compared with previous GWRC estimates. The results show that floodwaters from Mangatarere Stream can reach Carterton via two main flow paths, originating near Smith’s cow shed and travelling along Anderson’s Line and toward the northwest of the town. Simulations indicate that Carterton is currently at risk of flooding during a 1% AEP event, and this risk increases under climate change scenarios. Sensitivity testing with higher flow estimates further supports the potential for significant inundation. While the model uses simplified assumptions and unclassified LiDAR data, it provides a useful preliminary assessment of flood hazard. The report recommends further detailed modelling and community engagement to refine flood maps and inform planning.