How to use it
- Enter the internal diameter of the pipe.
- Enter the slope as a percentage. A fall of 1 m over 100 m is 1%.
- Choose the pipe material, which sets Manning's roughness n.
- Enter the design flow to see the flow depth and velocity, or 0 for the capacity only.
Worked example
A 200 mm concrete pipe (n = 0.013) laid at 1% carries 32.8 L/s when running full, at 1.04 m/s. At a design flow of 10 L/s, the water is 38% of the diameter deep and moves at 0.92 m/s, which is above the 0.6 m/s commonly required for self-cleansing.
Why part-full flow matters
Gravity sewers are designed to run part full. The space above the water lets air move and absorbs peaks. Velocity matters as much as capacity: if the flow is too slow at normal flows, solids settle and the pipe blocks. That is why small sewers often need steeper slopes than their capacity alone would suggest.
Frequently asked questions
What is the minimum velocity in a sewer?
A self-cleansing velocity of about 0.6 to 0.75 m/s (2 to 2.5 ft/s) is common, reached at least once a day. Your local sewer authority sets the exact figure.
Why can a pipe carry more than its full-bore capacity when not quite full?
Near the top of the pipe, the wetted perimeter grows faster than the flow area, which adds friction. A circular pipe carries its maximum, about 7.6% more than full bore, at about 94% of its depth.
What Manning n should I use?
Around 0.011 for smooth plastic, 0.013 for concrete and vitrified clay, and higher for old or rough pipes. Many authorities require 0.013 for all materials to allow for ageing and deposits.
What is the difference between slope in percent and 1 in X?
A slope of 1% means 1 unit of fall per 100 units of length, the same as 1 in 100. A slope of 0.5% is 1 in 200.