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Purpose of the Report 

To update the hydraulic model of the Otaki River and floodplain using modern data and techniques to support floodplain management planning. 

Geographic Area 

The report covers the Otaki River and its floodplain in the Kapiti Coast District, including the Mangaone, Waitohu, and Mangapouri Streams. 

Summary of the Report 

This report, prepared by River Edge Consulting for the Greater Wellington Regional Council, presents an updated hydraulic model of the Otaki River and its floodplain. The model supports the review of the Otaki Floodplain Management Plan and incorporates modern topographic data (LiDAR) and advanced 1D–2D hydraulic modelling using MIKE FLOOD software. The model includes the Otaki River and key tributaries such as the Mangaone, Waitohu, and Mangapouri Streams, as well as the Chrystalls Extended Stopbank. Flood scenarios ranging from 5-year to Probable Maximum Flood (PMF) events were simulated, with climate change impacts considered through increased flows and sea level rise. The modelling showed that the Mangaone Stream experiences flooding even in minor events, while the Otaki River begins to spill in 50-year and 100-year events. Climate change significantly increases flood extents, particularly in the Katihiku area. Breach scenarios at Chrystalls Bend and downstream of SH1 were also modelled, showing potential flooding across SH1 and the railway. Flood maps were generated for each scenario, and freeboard analysis was conducted to account for uncertainties. A 600 mm freeboard is recommended for the Otaki River and 400 mm for the Mangaone Stream. Two methods of applying freeboard were tested, with the preferred method adding freeboard to channel levels only. The report recommends future work including improved hydrological modelling using a 'rain-on-grid' approach, integration of stormwater and floodplain models, enhanced flood data collection, updated stream surveys, and refined freeboard analysis. Overall, the updated model provides a more accurate and comprehensive understanding of flood risks in the Otaki area, supporting better floodplain management and planning.