Wave hindcast for the New Zealand region: Nearshore validation and coastal wave climate

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Abstract
Historically, wave data coverage of New Zealand's coast has been poor, particularly for directional records. With very few data sets available of more than 1 year's duration, it has been difficult to establish accurate wave climatologies. To help fill in the gaps in our wave records, the wave generation model WAM (WAve Model) has been implemented over a domain covering the south‐west Pacific and Southern Oceans. The model has been used to hindcast the generation and propagation of deep‐water waves incident on the New Zealand coast over a 20‐year period (1979–98), using winds from the European Centre for Medium‐Range Weather Forecasts (ECMWF). The resulting synthetic climatology is expected to provide a valuable tool for researchers and coastal planners. The hindcasts were compared with data from wave buoy deployments at eight representative sites around the New Zealand coast. With appropriate interpolation and correction for the effects of limited fetch and sheltering by land, the hindcast was found to provide a satisfactory simulation of wave conditions at sites on exposed coasts. Regression between measured and hindcast significant heights at the four deep‐water sites (100–120 m) achieved scatter indices (ratio of root mean square error to mean) averaging 0.28. At the four shallower sites (30–45 m), the corresponding scatter index averaged 0.49, indicating that for regions of complex coastal topography, deep‐water spectra do not represent inshore conditions well. Wave spectra can be considerably modified by the processes of refraction and shoaling. To address these effects, nearshore wave transformations in the outer Hauraki Gulf were investigated using the shallow water model SWAN (Simulating WAves Nearshore), which was used to derive wave statistics at nearshore locations from deep‐water wave spectra obtained from the hindcast. The simulations were validated using data from an inshore site in 30 m water depth at Mangawhai on the north‐east coast of the North Island. Use of the nested model improved the agreement between model and measured significant wave height, decreasing the scatter index from 0.50 to 0.26. The suite of tools provided by the hindcast and localised, shallow water models can provide accurate new wave information for most of New Zealand's coastline.

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