Wind tunnel study on the effect of a wind break fence constructed on an embankment

Abstract

A planted Japanese black pine (Pinus Thunbergii) shelterbelt, which had grown upward satisfactorily in the back of an artificial embankment, ceased growing after reaching a certain height. The apparent reason for this was that the tree tops of the shelterbelt had come out above the area where the wind speed had been mitigated by the embankment and were exposed directly to strong sea winds. A wind break fence was thus constructed on the embankment in order to protect the trees from the wind, and the trees have begun growing again. The embankment is 2.66 m in height, 2.0 m in width at the top and the slope is 1: 1.5. The fence is 2.35 m in height. The position where tree height had stagnated in the past was 1.1-1.9 m above the ground. The present tree height range is 4.2-7.5 m, and the trees have grown 1.6-1.9 m during the last five years (in the five years since the fence was constructed). The wind tunnel experiment was conducted in order to consider how effectively the combination of the embankment and the fence has mitigated the wind speed, both in absolute terms and in comparison with a large embankment which has the same height as the embankment plus the fence. Three models were made, i.e., of the embankment, of the combination of embankment and fence, and of the large embankment. The model scale was 1/25. Wind speed in the wind tunnel was 4.8 m/s and 6.8 m/s, assuming local wind speed of 14 m/s and 20 m/s. The addition of the fence on the embankment remarkably expanded the area where the wind decreased, and there was even an area where the wind speed ratio was under 0.2, which had not been seen before the fence was constructed. The combination of the smaller embankment and the fence was much more effective than the large embankment and it appears to have enabled the trees to begin growing upward again.