Development of a Multi-purpose Reef at Orewa Beach, New Zealand

Orewa's long sandy beach is considered its major attraction and the most significant natural resource in the area. However, since early last century a number of contributing factors have dramatically reduced the width of 'dry' beach along much of its length. In recent years sand has been placed on the beach to increase the dry beach area and protect the back beach from further erosion that can occur during storm events, but the nourish material does not stay in place in the long-term. This project's aim was to investigate the feasibility of using an offshore submerged reef to retain sand placed on the beach, to provide coastal protection and enhanced amenity, (a wide sandy beach and surfing break) and environmental values (incorporation of marine habitat) - i.e. a multi-purpose reef. An independent investigation supported claims that enhancing the amenity and environmental value of Orewa Beach would have positive impacts on the social and economic sectors of the area. Here, we first describe the investigations undertaken to determine the existing coastal processes, which were critical since without a thorough understanding of the existing physical environment, the performance/functions and impacts of any new structures would be difficult to predict in a complex environment such as Orewa Beach. The complexities of the site include a low wave climate in relation to the tidal range, a very gently sloping beach gradient, the close proximity of an estuary and existing beach protection structures. The initial investigations included field measurements, analysis of long-term wave, wind and beach monitoring data and numerical modelling (wave transformation and sediment transport). The detailed design process, which incorporated coupled refraction/diffraction, Boussinesq and sediment transport modelling, is then described. The results demonstrate that sediment can be retained on the beach through the combination of wave energy dissipation and re-direction with an appropriately positioned and designed submerged reef.