HY-8 — The Complete Guide: Free Hydraulic Design of Road Culverts
HY-8 is Windows software from the US Federal Highway Administration (FHWA) for the hydraulic design of culverts under roads and embankments. It is in the public domain and completely free, and its hydraulic calculation does not depend on US data — you enter local flows, geometry and water levels and get a result that is valid anywhere.
HY-8
US Federal Highway Administration (FHWA)
Hydraulic design of road culverts: headwater and tailwater, outlet velocity, roadway overtopping and energy dissipators.
Get it from the vendorWhat you get
- Barrel shapes — circular pipes, boxes and other common shapes, in various materials.
- Inlet and outlet control — computing both conditions and selecting the governing one for each flow.
- Roadway overtopping — splitting the flow between the culvert and water passing over the road.
- Multiple barrels — a culvert with several barrels in parallel, and combinations of several culverts.
- Energy dissipation — designing an outlet structure when the exit velocity is too high.
How engineers use it
A road or drainage engineer takes the design flow from the hydrologic analysis, enters the road section and the downstream channel, and tests several options of size and number of barrels until the headwater meets the requirement. It focuses on the culvert itself; for the whole channel, a broader model is used.
Related tools
For a full channel analysis including culverts and bridges — HEC-RAS. For urban drainage networks — SWMM. For computing the design flow from the catchment — HEC-HMS.
HEC-RAS
US Army Corps of Engineers
1D/2D hydraulic model for rivers, bridges and culverts, with floodplain mapping. The world standard.
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SWMM
US Environmental Protection Agency
Urban drainage model — runoff, sewers, detention and infiltration, including runoff quality.
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HEC-HMS
US Army Corps of Engineers
Rainfall-runoff hydrological model — infiltration, unit hydrograph and routing. Pairs with HEC-RAS.
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HEC-SSP
US Army Corps of Engineers (HEC)
Statistical analysis of hydrologic data: flood-flow frequency, volume-duration and coincident frequency, for deriving design floods from gauge records.
Get it from the vendorThe bottom line
HY-8 is the focused, well-known tool for culvert design. It is public domain and completely free, including for commercial use.