A site plan shows contour lines labeled 102, 104, 106, and 108. If these represent elevations in feet, what is the slope percentage between elevations 102 and 108 over a horizontal distance of 120 feet?
Correct Answer
B) 5%
Slope percentage = (rise ÷ run) × 100. The rise is 108 - 102 = 6 feet, and the run is 120 feet. Therefore: (6 ÷ 120) × 100 = 5%.
Why This Is the Correct Answer
Option B is correct because slope percentage is calculated using the formula (rise ÷ run) × 100. The rise is the vertical change in elevation from 102 feet to 108 feet, which equals 6 feet. The run is the given horizontal distance of 120 feet. Applying the formula: (6 ÷ 120) × 100 = 0.05 × 100 = 5%.
Why the Other Options Are Wrong
Option A: 2%
Option A calculates to 2%, which would result from incorrectly using a rise of only 2.4 feet instead of the actual 6-foot elevation change, or from miscalculating the division.
Option C: 8%
Option C calculates to 8%, which might result from incorrectly calculating the rise as 9.6 feet or making an error in the division process.
Option D: 12%
Option D calculates to 12%, which would result from incorrectly using a rise of 14.4 feet or significantly miscalculating the slope formula.
Memory Technique
Remember 'RRH' - Rise over Run times Hundred. Rise is always vertical (elevation change), Run is always horizontal distance, and multiply by 100 to get percentage.
Reference Hint
Look up site work, grading, and surveying sections in your construction reference manual, typically found in earthwork or site preparation chapters.
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