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Purpose: To assess flood risks, model flood scenarios, and inform land use planning for Mangaone Stream and School Road Drain catchments. 

Geographic Area: The Mangaone Stream catchment, extending from the Western Tararua Ranges to Te Horo Beach on the Kāpiti Coast, New Zealand. The study focuses on the lower catchment between Mangaone North Road Bridge and the stream mouth, including the School Road Drain tributary. 

Summary:

This report presents the findings of a hydraulic modeling study conducted to assess flood hazards in the Mangaone Stream catchment. Using MIKE 11 software, the model was calibrated against historical flood events and validated for accuracy. The catchment spans 54.5 km², with the upper portion in native bush and the lower portion comprising alluvial floodplain used for pasture and horticulture. 

Field inspections and surveys identified overland flow paths and ponding areas. A total of 121 cross-sections were surveyed, with higher resolution near bridges and constrictions. Floodplain channels were mapped using aerial photos and contour data. Manning’s roughness coefficients were set at 0.04 for the main stream and 0.08 for floodplains. 

The model simulated 10, 50, 100-year and Extreme flood events, incorporating scenarios such as sedimentation, river mouth status, bridge blockage, bunding, and breaching. Sedimentation downstream of SH1 raised water levels by up to 0.141 m. Bridge blockage increased upstream levels and diverted flow to overflow channels. Bunding of flood channels altered flow distribution and raised water levels in the main stream. Breaching was modeled using link channels to simulate overflow. 

Flood hazard maps were produced using MapInfo, showing flood extents, depth contours, and recommended freeboard allowances—400 mm upstream and 300 mm downstream of Chainage 7800. Peak flows at key locations ranged from 32 m³/s at Ratanui to 57 m³/s at SH1 during Extreme events. The School Road Drain was modeled separately, with peak flows of 2.88 m³/s in the upper catchment. 

Recommendations include preserving overland flow paths, improving bridge hydraulics, investigating downstream channel capacity, maintaining culvert sizes, and recording future flood events. The model can be adapted for overflow scenarios from the Otaki River. The study provides a comprehensive basis for flood risk management and land use planning in the region.