Abstract
The Great East Japan Earthquake in March 11, 2011 caused economic and ecological damages along with the coastal area of Pacific Ocean in east Japan. The huge tsunami was generated due to the earthquake registered 9.0 on the Richter scale. Most things such as buildings, trees, cars etc. were swept up by the tsunami. After the tsunami, wind environment in open areas near the coastal line which had been rice field and cultivation changed for the worse because almost of all windbreak trees planted along with the coastal line over 10km swept away by the tsunami. Actually, reconstructed PVC greenhouses for strawberry were broken down by strong wind. Such situations also have the potential for increasing an occurrence frequency of blown sand from open areas.
This study aims to assess current wind environment in Watari area located around 20 km south from Sendai city as an example and to investigate the effects of new windbreak trees and coastal breakwaters which will be put into the place on flow field by using OpenFOAM. First, we analyzed observed frequency of occurrence of mean wind speed during 10 minutes in each wind direction at an Automated Meteorological Data Acquisition System (AMeDAS) site in Watari as a reference point. Wind speed was measured at 10m high in the Watari AMeDAS site. The Weibull parameters for each direction were calculated from the observed wind speed. Second, CFD analyses for each wind direction in current situation were conducted to obtain the horizontal distribution of wind speed at ground level. Then, in order to calculate the exceedance frequency of wind speed at ground level, the ratios of wind speed in each point to the wind speed at the reference AMeDAS site were calculated in each wind direction. Next, CFD analyses were carried out with the condition where the new windbreak trees and the coastal breakwaters were reproduced in the computational domain to investigate their effects on the flow field. Finally, the simulation results with the new windbreak trees and the coastal breakwaters were compared with the result in current situation without the windbreak trees. In addition, the horizontal distribution of the occurrence frequency of blown sand was estimated by combining the occurrence frequency of wind speed with the threshold wind speed for blown sand. We showed that the windbreak trees reduced the occurrence frequency of blown sand to -25% in the area of 1km from the coast.
