Current FHWA edition
HEC-22, Fourth Edition (2024)
FHWA publication FHWA-HIF-24-006 covers hydrology, pavement drainage, gutter flow, channels, inlet design, storm-drain structures and conduits, detention and retention, stormwater quality, and pump stations for transportation facilities.
The Third Edition is archived and superseded. Use the Fourth Edition for new work, together with the owning agency's criteria and standard details.
Where to find the design procedure
| Design task | Fourth Edition location | Use it for |
|---|---|---|
| Rainfall and runoff | Chapter 4 | Design rainfall, time of concentration, Rational Method, and hydrographs |
| Pavement and gutter flow | Chapter 5 | Spread criteria, cross slope, and gutter conveyance |
| Inlet interception | Chapter 7 | Grate, curb-opening, slotted, and combination inlets on grade and in sags |
| Storm-drain conduits | Chapter 9 | Capacity, energy losses, hydraulic grade line, and preliminary design |
| Detention and retention | Chapter 10 | Storage, outlet performance, water budgets, and storage routing |
Full-flow circular-pipe Manning equations
HEC-22 Section 9.1.3 gives these forms for a circular storm drain flowing full:
Mean velocity
V = (KV/n) D0.67 So0.5
Discharge
Q = (KQ/n) D2.67 So0.5
- V is mean velocity; Q is discharge; D is inside diameter.
- n is Manning's roughness coefficient and So is the energy-grade-line slope.
- For U.S. customary units, KV = 0.59 and KQ = 0.46; for SI, KV = 0.397 and KQ = 0.312.
What FHWA says changed in the Fourth Edition
FHWA's publication page describes three specific update themes: clearer rationale behind computations and a streamlined design process; stormwater-quality information updated for current practices and performance data; and a new curb-opening-inlet interception equation. Because that inlet equation changed, do not carry a Third Edition curb-opening formula into a Fourth Edition design without checking Chapter 7.
Primary Source
- Federal Highway Administration (2024). Urban Drainage Design Manual, Fourth Edition. Hydraulic Engineering Circular No. 22, FHWA-HIF-24-006. See Sections 9.1.2-9.1.4 for full-flow pipe hydraulics.
- FHWA HEC-22 publication page — official edition status, scope, publication number, and summary of Fourth Edition changes.