Pohutukawa bio-shield on the coast of New Zealand as a tsunami mitigation strategy

New Zealand is vulnerable to both far-field and near-field tsunamis. The Pohutukawa tree (Metrosideros excelsa), a native tree to New Zealand, is commonly found in the New Zealand coastal environment. This tree has strong roots that grow deep into the ground, as well as have a wide spreading branching system. It is considered that these characteristics would enable the Pohutukawa tree to act effectively as a bio-shield against tsunamis in New Zealand. However, there is no research to show its capability in reducing the hydrodynamic force of a tsunami. Field investigation of the Pohutukawa tree in Maraetai Beach, and the use of a one-dimensional depth-integrated shallow water numerical model, was conducted to assess the capability of a Pohutukawa bio-shield against a tsunami wave. A Pohutukawa bio-shield of 35 m width against a tsunami wave of height of 3.5 m at shoreline with wave period of 20 min reduced the hydrodynamic force of a tsunami directly behind the bio-shield by approximately 21%. This study, therefore, elucidates the capability of a Pohutukawa bio-shield as a tsunami mitigation strategy.