Tips for Sizing Storm Pipes

June 5, 2017 by Siri Soth
  • Design downstream up; find your control and go from there.
  • If your site needs to drain to existing culverts or to a water quality and/or detention facility, determine the flowline elevation at the final destination and start designing upstream from there.
  • Use a program like Bentley’s StormCAD or AutoDesk’s Storm and Sanitary Analysis to help design your network. Both programs make it simple to model your network with inputs for elevations, pipe sizes, inlet details and more. You can also generate profiles of your network to see, and even export, your final design.

  • When transitioning from a small pipe to a larger pipe, the elevation of the top of both pipes should be the same. This is called matching crowns.
    • This helps flow and prevents instances of tailwater effects


  • At a manhole, place the invert of the outlet 0.1 feet lower than the invert of the inlet.
    • This is to accommodate the downward slope of the base of the manhole structure.
  • Using a wye connection? Need to find the flowline elevation of the smaller pipe? Use this formula:

Where Z is the flowline elevation in feet and D is the diameter in inches of the different sized connections.

BIG RED DOG is an industry leader in civil, MEP, structural and traffic engineering. Contact us today to see how we can work together to make your project a success.

Written by Siri Soth

Siri Soth

Siri Soth is a graduate engineer for BIG RED DOG. She works in our Austin office.