Document library

Geographic Coverage: Coastal area from Ōtaki Beach to Peka Peka, including Te Horo Beach and surrounding low-lying areas. 

This report, prepared by NIWA for Greater Wellington Regional Council, updates the 2012 coastal storm surge hazard assessment for the Kāpiti Coast. It focuses on the stretch from Ōtaki Beach to Peka Peka, an area previously excluded due to lack of topographic data. Using 2013 LiDAR data and the advanced XBGPU model, the study simulates coastal inundation from storm-tide and wave setup under four historical storm scenarios. Each storm is modelled at present mean sea level and with sea-level rises of 0.5 m, 1.0 m, and 1.5 m. 
 
The most severe inundation was observed during the 13 September 1976 storm, which had the largest wave height and longest period. Inundation areas increase significantly with sea-level rise, from 2.69 km² at present sea level to 7.97 km² at 1.5 m rise. Key entry points for inundation include the Ōtaki River mouth, Waitohu Stream, Mangaone Stream, and Te Kowhai Estuary. 
 
At Ōtaki Beach, inundation affects streets such as Moana Street, Mahoe Street, Marine Parade, and Atkinson Avenue. Te Horo Beach experiences flooding near Sea Road, Rodney Avenue, Dixie Street, and Sims Road. In Peka Peka, inundation is concentrated near Te Kowhai Estuary. The updated model provides higher resolution (10 m vs. 20 m in 2012) and better representation of stop-banks and wave dynamics, resulting in more accurate predictions. 
 
The report recommends using the 13 September 1976 storm scenario for district planning due to its worst-case nature. The updated results supersede the 2012 study for Ōtaki and Te Horo. This modelling does not account for river flooding or future morphological changes, and assumes current stop-bank heights remain unchanged. Despite limitations, the study offers a robust basis for coastal hazard planning.