Forecasting Extreme Sea Level Events And Coastal Inundation From Tides, Surge And Wave Setup

A new marine forecasting system for the New Zealand region has been developed, including global and regional wave models, and regional tide and storm surge models. These form part of an integrated weather-related hazards forecasting capability, which also includes an accurate, data-assimilating high-resolution weather forecasting system and a national-scale flood forecasting system. Predicted variations in sea surface height and depth-averaged current due to tides and storm surge are provided in twice-daily 48-hour regional forecasts. Storm surge is predicted for the New Zealand region using the RiCOM hydrodynamic model on an unstructured grid in depth-averaged mode, using semi-implicit integration in time and a semi-langrangian scheme for advection. The model reproduced the storm surge event resulting from the passage of cyclone "Funa" across New Zealand during January 2008. Root-mean-square (RMS) errors in predicted storm surge height were 4-13 cm. For a second simulated storm, which occurred in Marlborough in late July/early August 2008, the model predicted the storm surge height with RMS errors at Kaikoura and Lyttelton of 7 and 4 cm, respectively. Wave-breaking also contributes to sea levels close to the coast through radiation stresses. The forecast system runs Wavewatch III on nested global and regional structured grids, providing twice-daily 48-hour forecasts. Model developments incorporating wave radiation stresses into the hydrodynamic model forcing are described, and some investigations into the effects of wave forcing on surface setup and coastal inundation are presented. A case study of inundation around the fringes of Hawkes Bay in response to extreme sea levels exacerbated by wave-induced stresses is presented. The case study used the SWAN wave model to provide the near-shore wave characteristics and radiations stresses, and demonstrates the ability of the coupled tide-surge-wave model to simulate and predict coastal flooding on a spatial resolution of a few metres.