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Why 10 Percent? <br /> Aids Planning <br /> ' The 10 percent average figure provides a framework for sustainable design and can be very helpful when con- <br /> ceptualizing a trail.You'll be able to calculate the approximate length of trail needed to reach the top of a <br /> given hill at a sustainable grade, and you'll be able to plot possible trail corridors with sustainable grades on <br /> a topo map. <br /> Applies to Most Soil Types <br /> There are many types of soil and each has different qualities of cohesion and drainage. Some soils support <br /> steeper trail grades than others. By employing a 10-percent average,you won't need to rely on your soil- <br /> identification skills.A 10-percent average grade is a trustworthy guideline for sustainable trails in all but the <br /> most unique soil conditions. (See page 84 for a more thorough discussion on soil.) <br /> Minimizes User-Caused Erosion <br /> Average grades of 10 percent or less help minimize erosion caused by users. Sustained grades of more than <br /> 10 percent can increase the amount of soil loosened by visitors who must work harder to travel up or down <br /> the slope. This loosened material is more easily carried off by water and gravity, resulting in a damaged trail. <br /> Allows Design Flexibility <br /> A trail that climbs at conservative grades allows flexibility in case there is an obstacle in the path. By staying <br /> at or below a 10 percent average,you can adjust the route without necessarily starting at the beginning or <br /> routing the trail too steeply to reach your targeted destination. <br /> Helps Future Reroutes <br /> Future reroutes are much easier if the average grade is roughly 10 percent. For example, if a trail with an <br /> average grade of 20 percent develops an erosion problem,a reroute around the problem area may require <br /> very steep grades or a switchback to reach the destination.When average grades are closer to 10 percent, , <br /> there is greater flexibility for the trail's future. <br /> Accommodates Undulations <br /> Average grades of 10 percent allow the trail to rise and fall without resulting in overly steep sections. <br /> Visualize a trail climbing to a targeted destination at an average grade of 20 percent. The trail dips slightly to <br /> cross a drainage and then resumes climbing. Following the dip, the trail must now climb at 25 percent—an <br /> unsustainable grade—in order to reach the destination.A better design would have the trail climb at an aver- <br /> age of 8 to 10 percent,with short sections of 15 percent when needed (as long as the sideslope grade is <br /> greater than 30 percent, to ensure the trail meets the half rule). <br /> i <br /> Ilt Bright fde <br /> 4 Begin flagging the trail with conservative grades below 8 or 10 per- <br /> __ cent. This allows flexibility in case you run into something like a water <br /> { <br /> shallow grades as you move along, you can steepen the route periodically without <br /> seep or major tree and need to route the trail around the object. By using <br /> F , <br /> necessarily reconfiguring the whole trail. <br /> 65 <br /> Part Three: Principals of Sustainable Trails <br />