A grading plan shows a swale with a 2% minimum slope. If the swale is 150 feet long, what is the minimum elevation difference between the high and low points?
Correct Answer
A) 3.0 feet
A 2% slope means a 2-foot rise per 100 feet of horizontal distance. For 150 feet: 150 × 0.02 = 3.0 feet minimum elevation difference is required.
Why This Is the Correct Answer
Option B is correct because a 2% slope means 2 feet of elevation change per 100 feet of horizontal distance. To calculate the elevation difference for 150 feet, we multiply the length by the slope percentage in decimal form: 150 × 0.02 = 3.0 feet. This represents the minimum elevation difference required between the high and low points of the swale to maintain proper drainage.
Why the Other Options Are Wrong
Option B: 1.5 feet
Option A (1.5 feet) represents only a 1% slope over 150 feet, which would be insufficient for proper drainage and does not meet the 2% minimum requirement specified in the grading plan.
Option C: 7.5 feet
Option D (7.5 feet) represents a 5% slope over 150 feet, which is significantly steeper than required and would result from incorrectly calculating 150 × 0.05 instead of 150 × 0.02.
Memory Technique
Remember 'Rule of 100': For every 100 feet, the percentage equals the feet of rise. So 2% = 2 feet per 100 feet. For 150 feet: 150/100 × 2 = 3 feet.
Reference Hint
Florida Building Code Chapter 18 - Soils and Foundations, or grading and drainage sections in site development standards
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