A site plan shows contour lines with elevations of 100', 102', 104', and 106'. If the horizontal distance between the 100' and 106' contours is 120 feet, what is the slope percentage?
Correct Answer
D) 5%
Slope percentage = (rise/run) x 100. The rise is 6 feet (106-100) and the run is 120 feet. (6/120) x 100 = 5%.
Why This Is the Correct Answer
Option B (5%) is correct because slope percentage is calculated using the formula (rise/run) × 100. The rise is the vertical elevation change from 100' to 106', which equals 6 feet. The run is the horizontal distance of 120 feet. Therefore: (6/120) × 100 = 0.05 × 100 = 5%.
Why the Other Options Are Wrong
Option A: 2%
Option C (8%) is incorrect because it overestimates the slope. This error might occur from using an incorrect rise value of approximately 9.6 feet or from mathematical errors in the calculation.
Option B: 8%
Option D (12%) is incorrect and significantly overestimates the slope. This could result from confusing the calculation method or using an incorrect rise value of approximately 14.4 feet.
Memory Technique
Remember 'RRH': Rise over Run times Hundred. Rise is always vertical (elevation change), Run is always horizontal (map distance), and multiply by 100 to get percentage.
Reference Hint
Look up site work and grading sections in your construction reference manual, typically found in earthwork or site preparation chapters that cover topographic maps and slope calculations.
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