The percent of slope (rise in feet per horizontal distance) shall be established by measurement of distance perpendicular to the contour of the slope. The percent of slope shall be calculated for each two-foot contour interval. For example, any location on the site where there is a one-foot rise over a ten-foot horizontal run constitutes a 10% slope; a 1.5 foot rise over a ten-foot horizontal run constitutes a 15% slope; a two-foot rise over a ten-foot horizontal run constitutes a 20% slope.