No abstract

This study explored the effects of coastal vegetation on tsunami damage based on field observations carried out after the Indian Ocean tsunami on 26 December 2004. Study locations covered about 250 km (19 locations) on the southern coast of Sri Lanka and about 200 km (29 locations) on the Andaman coast of Thailand. The representative vegetation was classified into six types according to their habitat and the stand structures of the trees. The impact of vegetation structure on drag forces was analyzed using the observed characteristics of the tree species. The drag coefficient, including the vertical stand structures of trees, C d-all , and the vegetation thickness (cumulative trunk diameter of vegetation in the tsunami direction) per unit area, dN u (d: reference diameter of trees, N u : number of trees per unit area), varied greatly with the species classification. Based on the field survey and data analysis, Rhizophora apiculata and Rhizophora mucronata (hereafter R. apiculata-type), kinds of mangroves, and Pandanus odoratissimus, a representative tree that grows in beach sand, were found to be especially effective in providing protection from tsunami damage due to their complex aerial root structure. Two layers of vegetation in the vertical direction with P. odoratissimus and Casuarina equisetifolia and a horizontal vegetation structure of small and large diameter trees were also important for increasing drag and trapping floating objects, broken branches, houses, and people. The vertical structure also provided an effective soft landing for people washed up by the tsunami or for escaping when the tsunami waves hit, although its dN u is not large compared with R. apiculata-type and P. odoratissimus. In addition, the creeks inside mangroves and the gaps inside C. equisetifolia vegetation are assumed to be effective for retarding tsunami waves. This information should be considered in future coastal landscape planning, rehabilitation, and coastal resource management.

Given the molecular phylogenies, and considering chromosome number and morphology, three species and one species complex comprising six lineages were discerned. A putative allotriploid, an allotetraploid, and a lineage of hybrid origin were identified within the species complex, and a hybrid was found outside the species complex, and their respective putative parental taxa were inferred. With respect to biogeography, a remarkably discontinuous distribution was identified in two cases, for which bird-mediated seed dispersal may be a reasonable explanation.

The characteristics of flow structures around a colony-type emergent roughness model, hereafter called 'colony model', mounted on a flat plate in uniform flow and the drag coefficient C dc for the colony model are investigated by flow visualization, spectral analysis, velocity measurement and drag force measurement. Two types of colony models, each comprising seven equally spaced cylinders with grid or staggered arrangement are mounted on a water flume bed. The flow structure around the colony model changes depending on L/D and G/D (L: space between neighboring cylinders, D: diameter of a cylinder, G: space between cylinders in the cross-stream direction). Two types of flow structures, a large-scale Kármán vortex street (LKV) behind the colony models and a primitive Kármán vortex street (PKV) behind the individual cylinders, are generated. LKV is formed along the shear layer for G/D < 0.4. When G/D > 1.8, PKV behind each cylinder is stably formed because of the decrease in difference between the velocity through the colony model and that of the detour flow. The velocity through the colony model, which plays a key role for the phenomenon change, strongly increases with G/D at first and then gradually. The tendency of the velocity curve changes around G/D = 1.8 for each arrangement. It almost coincides with the initiation of the formation of PKV. On the other hand, C dc for the colony model increases in the range of 0.08 < G/D < 0.75 (0.25 < L/D < 1) and 0 < G/D < 1 (1 < L/D < 3) for the grid and staggered arrangements, respectively. The changing point of the tendency in the C dc curve is located in the middle of the transition zone for the two vortex structures