# Effects of Wind Barrier Porosity and Inclination on Wind Speed Reduction
## 4 Discussion  
Previously, the porosity of the wind barrier was regarded as the key factor influencing its effectiveness in protecting against wind (reviewed by [8,18]). To further advance the understanding of the wind flow characteristics and determine the effectiveness of wind barriers, this study examined the effects of inclinations and porosity on the effectiveness of the following wind barriers based on a wind tunnel experiment. The reduction in wind speed varied among the three cases. For Case A, although all inclinations showed a six-fold decrease in wind velocity at the last measuring distance (Table 1), wind flow could reattach and recirculate at the leeward side of the wind barrier. [PERSON] [33] reported that a reattachment point occurred at a distance of approximately 10-15 times the height of a wind barrier. The reattachment point and recirculation of wind flow may damage tree saplings as sea breezes with salinity, sand, and dust in coastal areas are particularly harmful to vegetation [2,9,16,17], resulting in lower growth rates [15]. Thus, when establishing a windbreak forest within a distance equal to or less than 10 times the height of the wind barriers, it is advisable to use a non-porous wind barrier in an embanked area rather than on flat land. For Case B, the reduction ratio of wind velocity for each inclination increased with increasing distance (Table 2, Figure 5), whereas the opposite trend was observed for Case A (Table 1, Figures 4 and 7). This could be explained by the increase in porosity from 0% to 25% [18,32]. Although the change in the reduction ratio of wind velocity was similar between 90\({}^{\circ}\) and 80\({}^{\circ}\), there was an abrupt decrease between 50 cm and 100 cm at 70\({}^{\circ}\), probably (Table 2, Figure 7) due to the recirculation of wind flow [26]. Overall, the effect of wind protection was weak within 200 cm. Thus, to attain protection of over 60% with wind  
Figure 7: The relationship between average wind velocity (m/s) and distance (cm) on the windward and leeward sides of the wind barriers. The percentages (%) and degrees (\({}^{\circ}\)) in the legend represent the porosity and inclination of the wind barriers, respectively. The bars represent the standard deviation. barriers of 25% porosity, it is recommended to utilize vertical 90\({}^{\circ}\) wind fences. Further saplings should not be planted immediately behind the wind barriers. Following a previous review that reported porosity as one of the influential factors on wind speed reduction alongside other factors such as the type, shape, and height of wind barriers [18], numerous earlier studies primarily focused on porosity [8; 9; 24; 26; 28]. Previous studies generally concluded that windbreaks with lower porosity had a higher reducing effect on wind velocity. However, the construction of wind barriers with lower porosities is expensive in practical applications [28]; therefore, the optimal porosity range is from 30% to 50% [24; 25]. [PERSON] et al. [8] reviewed recent studies that examined wind barrier porosity for wind protection effectiveness and reported that the highest sheltering efficiency of wind fences was achieved at a porosity within the range of 20% to 40%. In contrast, in this study, the wind barrier of Case C, with the highest porosity of 50%, substantially decreased the wind velocity and showed relatively stable reduction ratios for each inclination at varying distances relative to Cases A and B (Table 3, Figures 6 and 7). In addition, the effectiveness of Case C was higher than that of Case B in this study. This implies that a lower porosity does not always result in a greater wind velocity reduction. The different results of this study from the previous studies reviewed by [PERSON] et al. [8] may be attributed to the inclinations that were not examined in the previous studies. Among the different inclinations, the decreasing effect was the greatest at 90\({}^{\circ}\) within 300 cm; however, the counterpart was largest at 70\({}^{\circ}\) from 400 to 600 cm (Table 3, Figures 6 and 7).