Wave Focusing in Response to a Dredged Sediment Mound

Dredged sediments from the navigation channel to Tauranga Harbour (New Zealand) have historically been deposited in approved dump grounds, located some 3-5 km off Mt Maunganui Beach. The refraction of approaching waves around the resultant spoil mounds focuses wave energy onto the adjacent beaches and, depending upon the angle of wave approach, the Entrance Channel through the ebb tidal delta. At the present time there is a proposal to significantly deepen and widen the shipping channels, with the dredged sediments likely to be deposited offshore. To simulate the influence of a deposited dredged sediment mound on local wave focusing patterns, a hypothetical spoil mound has been added to existing bathymetry (between ∼20-31 m water depth adjacent to the Mt Maunganui beaches) in two alternate scenarios. Modifications to local wave patterns, induced by the addition of either hypothetical spoil mound, are assessed through application of a wave refraction model and compared to the present conditions. Model predictions indicate that in each scenario inshore wave heights are likely to be altered by the addition of a disposal mound at these water depths (between 20-31 m). Wave regraction is enhanced along both sides of the mound, leading to varying levels of wave height amplification in the lee, depending on the specific mound design and wave event simulated. This coincides with wave height reduction on either side of this zone of convergent wave energy.