A site plan shows a sanitary sewer line marked 'S' with an invert elevation of 42.3 feet at manhole MH-1 and 41.8 feet at manhole MH-2, which is 250 feet downstream. What is the pipe slope?
Correct Answer
A) 0.2%
Slope = (elevation change ÷ horizontal distance) × 100. (42.3 - 41.8) ÷ 250 × 100 = 0.5 ÷ 250 × 100 = 0.2%.
Why This Is the Correct Answer
The correct answer is A (0.2%) because pipe slope is calculated by dividing the elevation change by the horizontal distance and converting to percentage. The upstream invert elevation (42.3 ft) minus the downstream invert elevation (41.8 ft) equals 0.5 ft of drop over 250 ft of horizontal distance. When calculated as (0.5 ÷ 250) × 100, this equals 0.2%. This represents the proper slope calculation for gravity-fed sewer systems.
Why the Other Options Are Wrong
Option B: 1.0%
This would result from major calculation errors, such as reversing the distance and elevation values or miscalculating the elevation difference by a factor of 4
Option C: 0.5%
This would be the result if you forgot to multiply by 100 to convert to percentage, giving you 0.005 instead of 0.2%
Option D: 2.0%
This would result from incorrectly calculating the elevation change as 2.5 feet instead of 0.5 feet, or making an error in the division
Memory Technique
Remember 'Rise over Run times 100' - but for sewers it's 'Fall over Run times 100' since water flows downhill. Think 'FRH' - Fall, Run, Hundred (multiply by 100 for percentage)
Reference Hint
Florida Building Code Chapter 7 - Plumbing Systems, or reference materials covering site utilities and drainage calculations
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