Field surveys were conducted on the eastern coastline of Sri Lanka to investigate which vegetation species are effective against a tsunami and to evaluate the effectiveness of existing Casuarina equisetifolia forests in tsunami mitigation. Open gaps in C. equisetifolia forests were identified as a disadvantage, and introduction of a new vegetation belt in front of the existing C. equisetifolia forest is proposed to reduce the disadvantages of the open gap. A numerical model based on two-dimensional nonlinear long-wave equations was applied to explain the present situation of open gaps in C. equisetifolia forests, and to evaluate the effectiveness of combined vegetation system. The results of the numerical simulation for existing conditions of C. equisetifolia forests revealed that the tsunami force ratio (R = tsunami force with vegetation/tsunami force without vegetation) was 1.4 at the gap exit. The species selected for the front vegetation layers were Pandanus odoratissimus. A numerical simulation of the modified system revealed that R was reduced to 0.7 in the combined P. odoratissimus and C. equisetifolia system. The optimal width of P. odoratissimus (W1) calculated from the numerical simulation was W1=10 m. Establishment of a new front vegetation layer except for open gaps that are essential, such as access roads to the beach, is proposed
