of the windbreak. As the height and length of the windbreak increase, so does
the depth of the sheltered area. The sheltered area also increases with
windbreak depth, up to a depth of two windbreak heights (2H). If the
windbreak depth is increased beyond 2H then the flow "reattaches" to the top
of the windbreak and the length of the sheltered area decreases (see Figure
10). An area of slightly lowered velocity also exists for 10H in front of the
shelterbelt or windbreak (see Figure 11).
Effect of Porosity of the Windbreak on Sheltered Area. The extent
of the sheltered area produced also varies with the porosity of the barrier.
Porous barriers cause less turbulence and can create a greater area of total
shelter (reduced speeds) than solid barriers. The more solid the barrier, the
shorter the distance to the point of minimum wind velocity and the greater the
reduction in velocity at that point. The velocity, however increases more
rapidly downwind of the minimum point providing less sheltered area than
behind a more porous barrier (see Figure 11).
Wind Incidence. The incidence angle of the wind also affects the
length of the sheltered area. Tree and hedge windbreaks are most effective
when the wind is normal to the windbreak. If the wind approaches a windbreak
at an oblique angle, the sheltered area is reduced (see Figure 12).
Type of Vegetation. Hedges provide a more pronounced sheltering
effect than trees because they have foliage extending to the ground level. In
fact, the flow beneath the branches (around the trunks) of trees can actually
be accelerated above the free wind speed upwind of the tree (see Figure 13).
Recommendations for Windbreaks. If a sheltered area is desired for
a zoned or seasonally adjustable building, it is recommended that the
landscaping be designed to allow for reduced velocities without large scale
turbulence. To achieve this, windbreaks should be at least 35 percent porous.
The windbreak is most effective when the building it is to protect is located
within 1-1/2 to 5 heights of the windbreak.