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The Five Essential Elements , <br /> ,., <br /> Trailsof Sustainable <br /> ., <br /> 0 The Half Rule <br /> r <br /> ® The Ten Percent Average Guideline ' ': <br /> 0 Maximum Sustainable Grade <br /> Grade Reversals <br /> © Outslope Half Ruleh'i <br /> Trail 15% Grade t <br /> ::� <br /> i` <br /> Sideslope <br /> 20% Grade i{ <br /> The Half Rule i <br /> i, va <br /> 1 <br /> A trail's grade shouldn't exceed half the grade of \ A, \\ , <br /> the hillside or sideslope that the trail traverses.If i, <br /> the grade does exceed half the sideslope, it's con- - IIT \\ <br /> sidered a fall-line trail.Water will flow down a <br /> fall line trail rather than run across it. '� This trail breaks the <br /> Measure the sideslope with a clinometer Water will flow <br /> Half Rule. <br /> (we'll discuss how to do this shortly),then be down trail. <br /> sure to keep the tread grade below half of that <br /> figure in order to ensure good drainage. For <br /> example, if you're building across a hillside with Trail 8% Grade <br /> a sideslope of 20 percent,the trail-tread grade Sideslope <br /> should not exceed 10 percent. 20% Grade <br /> The half rule is especially important in gen- Il <br /> a <br /> 000eV:0/.-- <br /> tly sloping areas.A common mistake occurs '+ . <br /> when trails are routed down gradual slopes, \ <br /> based on the assumption that erosion won't be a <br /> concern in nearly flat areas.Yet,water will fun- <br /> nel down trails and ruin them even on gentle \\ <br /> slopes.A trail passing through an area with a This trail meets the v <br /> mere 6 percent sideslope must have a trail tread Water will sheet Half Rule. , <br /> across trail. <br /> grade less than half of that figure—only 3 per- is <br /> cent—in order to escape the fall line. <br /> :) <br /> 63 <br /> Part Three: Principals of Sustainable Trails <br />