A plumbing drawing shows a 4-inch diameter storm drain with a 2% slope running 200 feet. What is the total elevation change from inlet to outlet?
Correct Answer
A) 48 inches
A 2% slope means 2 feet of fall per 100 feet of run. For 200 feet: (200 × 2) ÷ 100 = 4 feet = 48 inches of total elevation change. Proper slope ensures adequate drainage flow.
Why This Is the Correct Answer
A 2% slope means 2 feet of vertical drop per 100 feet of horizontal run. For a 200-foot run: (200 feet × 2%) = (200 × 0.02) = 4 feet of elevation change. Converting to inches: 4 feet × 12 inches/foot = 48 inches. This calculation ensures proper drainage flow velocity and prevents standing water in storm drain systems.
Why the Other Options Are Wrong
Option B: 96 inches
96 inches represents 8 feet of elevation change, which would be a 4% slope (8 feet ÷ 200 feet = 0.04 = 4%). This doubles the actual slope requirement and would create excessive velocity that could cause erosion and system damage in storm drainage applications.
Option C: 4.0 inches
4.0 inches represents only 1/3 foot of elevation change, which would be a 0.2% slope (4 inches ÷ 2400 inches = 0.0017 = 0.17%). This slope is too flat and would not provide adequate drainage flow, leading to standing water and potential system failure.
Option D: 2.4 inches
2.4 inches represents 0.2 feet of elevation change, which would be a 0.1% slope (0.2 feet ÷ 200 feet = 0.001 = 0.1%). This extremely flat slope would cause water to pool and create drainage problems, failing to meet minimum slope requirements for storm systems.
Memory Technique
Remember 'Percent Slope = Rise over Run × 100'. For 2% slope: 2 feet drop per 100 feet run. Think '2-4-8': 2% slope, 4 feet total drop, 48 inches final answer for 200-foot run.
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