ASR - A Global Partner for the Sustainable Management of Coastal and Marine Resources

Taranaki

Location: Taranaki, New Zealand

Type of Project: Wave Modelling

ASR Projects- Taranaki

Left: An ariel view of Port Taranaki. Right: Modelled Hrms wave heights in the harbour for the one of the dredged scenarios under the mid-range incident IG waves (50-70 s periods) at MHWS.

Project Overview:

ASR was commissioned to create a numerical model of Port Taranaki and use it to compare the effects of long period waves on the current harbour configuration with the harbour under two dredging scenarios.

Project Outcomes:

The long wave energy band of concern is the infragravity(IG) ‘surfbeat’ range (T 25-120s), which is responsible for occasions of excessive motion of vessels at various berths in the harbour basin. The model was developed using MODEL 3DD with a spectral wave boundary. The model results were analysed to define the energy distribution in the existing harbour configuration, and to define and quantify the changes under the two dredging scenarios.

Model results showed that the capital dredging proposed for the harbour does not result in large changes to the overall pattern and distribution of IG energy. However, for some frequencies and in some regions of the harbour (e.g. the harbour basin), there are likely to be significant increases in the IG wave heights and orbital velocities.

This will occur because deepening certain regions of the harbour by capital dredging has the effect of increasing the local resonant frequency (i.e. lowering the resonance period). For example, the model simulations indicate that in the harbour basin, incident IG waves from the 40-60 s period range band will have wave height and wave velocity increases as a result of dredging. As this range includes over half of all the incident IG wave periods, it is clear that increased long wave activity at berths is a likely overall result.

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