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Culvert Design: Inlet Control vs Outlet Control

Capacity is the lesser of what the entrance admits and what the barrel plus tailwater can pass. HW/D limits, Ke, Manning n, and a screening example.

Updated 16 August 202610 min read

Key takeaways

  • Governing Q is min(inlet control, outlet control).
  • Inlet control is an entrance problem; the barrel could take more.
  • Outlet control is friction plus tailwater through a full (or nearly full) barrel.
  • Keep HW/D modest so you do not flood the roadway.

A culvert is a short gravity barrel under an embankment. Unlike a long sewer, either end can throttle the flow. Design compares inlet control and outlet control and takes the smaller discharge at the allowable headwater.

Two controls

  • Inlet control: the barrel can take more than the entrance admits. HW and entrance type set Q. Slope and n hardly matter.
  • Outlet control: barrel friction and tailwater back water up to the inlet. Energy from HW to TW through Ke + Manning.
Open solver: Culvert inlet vs outlet control

Outlet-control energy

HW = TW + (1 + K_e + n² L / R^{4/3}) V² / (2g)
SI screening form for a full circular barrel. R = D/4.

Allowable HW is often 1.2–1.5 D so the roadway stays dry and the inlet does not go heavily submerged. Outlet velocity should be checked against downstream scour.

Open solver: Barrel Manning checkOpen solver: Upstream rational-method peak

Frequently asked questions

HY-8 and the FHWA nomographs are more precise (inlet charts by entrance type). The Solver360 culvert tool is a first-cut energy/orifice model for circular barrels.

Keywords

culvert design calculatorinlet control culvertoutlet control culvertheadwater depth HW/Dculvert entrance loss Kehighway drainage culvertcircular barrel capacity

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Inlet vs outlet control capacity for a circular barrel
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Gravity sewers and open-channel flow, including part-full circular pipes
Storm Sewer / Rational Method
Peak runoff Q = CiA and Manning full-pipe check

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