Gravity pipe flow capacity (Manning)
Full-pipe flow and velocity for a gravity storm or sewer pipe, and how full your design flow runs it.
Expert check pending. This tool shows its formula, cited sources and assumptions with every result, and it's included in its role plan. A qualified expert in this trade hasn't signed the method off yet, so check the result against your own code, engineer or supplier before you rely on it.
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- The full grade sheet you can print or save to My work
- 3 cited sources, linked, with the checks to make
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Example result · fictional numbers
What you get: grade sheet
Worked out by this tool from its default inputs. Its role plan lets you run your own numbers, print the sheet and save it.
18 in at 0.50%: 7.43 cfs full; design flow runs 52% deep at 4.28 ft/s
- Full-pipe capacity
- 7.43 cfs (3,334 gpm, 4.80 MGD)
- Full-pipe velocity
- 4.20 ft/s (self-cleansing)
- Design flow as % of capacity
- 54% (fits)
Design flow in this pipe
- Design flow
- 4.00 cfs (1,795 gpm)
- Depth of flow
- 9.4 in, 52% of the diameter
- Velocity at design flow
- 4.28 ft/s
- Capacity at your 75% fill limit
- 6.77 cfs
- Most the pipe carries by gravity
- 7.99 cfs at about 94% deep (a little more than full)
- Verdict
- Works: 52% deep at design flow, 4.20 ft/s full
Also on the sheet
- Velocity and slope check · 3 lines, starting with “Velocity flowing full”
- Sizes at 0.50% and n = 0.013 · 6 lines, starting with “12 in”
- Grade sheet · 7 lines, starting with “Fall over the run”
The plain-words read of this example
This 18 in pipe at 0.50% with n = 0.013 carries 7.43 cfs full at 4.20 ft/s. The 4.00 cfs design flow runs about 52% deep at 4.28 ft/s, against your 75% limit. Full-pipe velocity meets the 2.00 ft/s minimum; 0.113% slope would give exactly that. The smallest size that works at this slope is 15 in. Over 300 ft the pipe falls 1.50 ft to invert 98.50.
3 cited sources
- Recommended Standards for Wastewater Facilities (Ten States Standards), 2014 edition (Great Lakes - Upper Mississippi River Board (copy hosted by the Minnesota Department of Health))
- WSDOT Hydraulics Manual M 23-03, Chapter 6: Storm sewers (Washington State Department of Transportation)
- N.J.A.C. 5:21-7.3, Design of runoff collection system (Cornell Law School, Legal Information Institute)
Plus 5 step-by-step checks, a print layout and a link that reopens it with your numbers.