Developing effective vegetation bioshield for tsunami protection

To elucidate the effectiveness and limitations of coastal vegetation for tsunami protection, the impact of vegetation structure on drag forces was analysed using the observed characteristics of reference tree species. The drag coefficient, including the vertical stand structures of trees, Cd-all, and the vegetation thickness per unit area, dNu (d, reference diameter of trees; Nu, number of trees per unit area), varies greatly with different species. Based on data analyses, dense Casuarina equisetifolia and Pandanus odoratissimus grown in beach sand were found to be especially effective in providing protection from tsunami damage due to their density and complex aerial root structure. The breaking moment of trees was investigated as a function of tree diameter. The breaking moment equation of P. odoratissimus explains well the damage caused to trees by the 2004 Indian Ocean tsunami and the 2006 Java tsunami. Numerical simulation indicates that inertia is the dominant force (99.1%) in comparison with the drag force when a wave front collides with a vegetation face, but while the water depth is very shallow and the total force is 6% of the maximum. Drag resistance is the dominant force in reducing both water depth and current velocity, but inertia resistance is active in reducing current velocity only in front of the vegetation. The breaking condition of trees can be discussed by the drag-force moment alone because when the total moment reaches the maximum, the contribution of the inertia moment is in the range of 0.1-0.3%. Considering the limitations of P. odoratissimus in reducing tsunami water depth and the other roles that coastal vegetation can play in mitigating tsunami-related damage, a forest with two layers in the vertical direction of P. odoratissimus and dense C. equisetifolia was found to be effective for increasing drag and trapping floating debris.