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Sizing culverts and storm drains without silting them up

Choosing a barrel that passes the design flow, and the self-cleansing velocity that stops it silted up within a few seasons.

The engineering question

The pipe passes the flow calculation and then blocks with sediment. What was left out of the sizing that matters as much as capacity?

Worked example

The figures below were produced by running the Storm Drain Sizing Calculator with the same inputs — during the build, from its authoritative engine.

Results
Required pipe size 675 mm
Capacity at that size 0.5944 m³/s
Velocity when full 1.661 m/s
Self-cleansing velocity met 1
Assessment 675 mm pipe at the selected slope runs full at 0.59 m³/s (1.7 m/s). Self-cleansing velocity met.
  1. Manning gives the capacity of each standard size at the available gradient.
  2. Select the smallest barrel whose full-flow capacity exceeds the design flow.
  3. Then check the velocity when full — a pipe sized too large for its gradient runs slowly and silts.

Sanity check

Capacity and self-cleansing velocity pull in opposite directions. If the chosen pipe is slow, split the run rather than accepting an oversized flat pipe.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

References & standards

  • FHWA HDS-5 — culvert design, inlet and outlet control
  • BS EN 752-4 — underground drainage and piping systems
  • Chow — Open-Channel Hydraulics

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