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Purpose of the Report 

To update flood models and hazard maps for the Waikanae River to support emergency management and planning. 

Geographic Area Covered 

The report covers the Waikanae River and its adjacent floodplain on the Kāpiti Coast, New Zealand. 

Summary of the Report 

This report was commissioned by the Greater Wellington Regional Council to update hydraulic flood modelling and mapping for the Waikanae River and surrounding floodplain. The modelling supports civil defence, emergency management, floodplain planning, and updates to the Kapiti Coast District Plan.

The modelling used MIKE FLOOD software, which integrates one-dimensional (MIKE 11) and two-dimensional (MIKE 21) simulations. MIKE 11 was used for river channels and MIKE 21 for floodplain areas. Topographic data was sourced from a 2010 LiDAR survey with a 4m grid resolution. The model included major structures such as road and rail bridges but excluded minor footbridges.

Calibration was based on two historical flood events: the 2005 flood (estimated 80-year return period) and the 2008 flood (10-year return period). Conservative channel resistance values were used, especially for smaller events.

Flood scenarios were modelled for return periods of 5, 10, 20, 50, and 100 years, as well as the Probable Maximum Flood (PMF). Climate change scenarios assumed a 20% increase in flows and an 800 mm sea level rise. Sea level scenarios were based on spring tide plus storm surge.

Five breach scenarios were modelled at key locations: Jim Cooke Memorial Park, Woodleigh Stud, Chillingworth, Kauri-Puriri, and Otaihanga. Each scenario included assumptions about breach timing, width, and crest level reductions.

Flood mapping outputs included flood depth, hazard (depth × velocity), duration, and time to inundation maps. These maps were produced for each scenario, including breach and climate change variants.

Freeboard analysis added 600 mm for the Waikanae River and 300 mm for smaller streams. This increased the inundation area by 30% in the 100-year climate change scenario, with some areas showing increases greater than 600 mm.

Recommendations for future work include detailed modelling of the Muaupoko Stream, rain-on-grid hydrological modelling, integration with the Otaki floodplain model, three-way coupling of stormwater, open channel, and floodplain systems, improved flood data collection, review of freeboard methodology, and development of emergency response planning using model outputs.