HY-8 alternative: culvert analysis online

What HY-8 is and who it suits

HY-8 is the Federal Highway Administration’s culvert hydraulic analysis program. Its methods follow FHWA Hydraulic Design Series No. 5 (HDS-5) for culvert hydraulics and Hydraulic Engineering Circular No. 14 (HEC-14) for energy dissipators. It is public-domain software that anyone may download. The current release on the FHWA download page is version 8.0.1.2, built on 5 March 2025. FHWA describes it as a 64-bit program “only compatible with 64-bit Windows-based operating systems”, and installing it needs administrator rights.

HY-8 suits engineers on US highway work, reviewers who check submissions against HY-8 output, and anyone who needs its whole module set in one program: several culverts at one crossing, roadway overtopping, energy dissipators and the aquatic organism passage (AOP) calculator. FHWA does not provide user assistance or support for it.

Where HY-8 is the better choice

Check this list before you switch. Each item is a real reason to stay with HY-8.

• Cost. HY-8 is free. Studio’s free plan keeps five saved designs per tool and watermarks its reports; unlimited saves and clean reports need a paid plan.
• Unlike culverts at one crossing. An HY-8 crossing holds one to six culverts, each with its own barrels, and one project can hold several crossings. Studio’s Culvert Designer analyses one culvert size and shape per crossing, with any number of identical barrels. It does not split flow between, say, a pipe and a box sharing an embankment.
• Barrel shapes. HY-8 covers circular, concrete box, elliptical, pipe-arch, open-bottom, low-profile and high-profile arches, metal box and user-defined sections. Studio analyses circular and box barrels today; pipe-arch, elliptical and arch are marked as coming soon.
• Energy dissipators and AOP. HY-8 sizes USBR Type VI stilling basins, Saint Anthony Falls (SAF) basins and riprap basins, and has an AOP module. Studio’s Culvert Designer stops at the outlet velocity, which you send to the separate Riprap Design tool to size a HEC-14 outlet apron.
• US customary units. HY-8 works in US customary or SI units. Studio’s Culvert Designer takes SI input (metres, millimetres, m³/s). US-customary pipe and box sizes appear only in its sizing catalogue, for US jobs.
• Reviewer expectation. If the approving agency asks for HY-8 files, send HY-8 files.

Where Delta Hydro Studio differs

It runs in a browser. There is nothing to install and no Windows requirement. A saved design opens on any machine you sign in on, and colleagues on the Delta team plan open the same project.

Same method, same vocabulary. The default inlet-control solver uses HY-8’s fifth-degree polynomial fits to the HDS-5 nomographs, falls back to the HDS-5 Appendix A regressions, and applies the −0.5·S₀ slope correction. Outlet control is an energy balance from tailwater to headwater: entrance, friction and exit losses, with a direct-step backwater profile through part-full barrels. Every result row carries the HY-8 flow-type code (for example 5-S2n or 4-FFf) beside the USGS flow type, so a reviewer used to HY-8 reads the regime at a glance. HY-8 treats the inlet as submerged at HW ≥ D while the USGS classification uses HW/D ≥ 1.5, so the two codes can legitimately differ between 1.0 and 1.5.

A second national method. A toggle switches inlet control to the SANRAL Drainage Manual (6th edition) Table 7.2 relations for South African road-authority submissions. The report names the solver either way.

A sizing assistant. Set the limits you design to: maximum HW/D, maximum headwater elevation, minimum freeboard to the road, an outlet velocity range, minimum cover, and whether overtopping is allowed. The assistant analyses every commercial section in its catalogue that matches your shapes, materials, inlets and barrel counts, using the same analysis as the main tool. Sections that pass are ranked smallest area first; sections that fail stay visible with the limit they broke and by how much. Adopting one writes it into the ordinary design form, and the study goes into the report.

Batch import. A road with a dozen crossings goes in as one CSV or Excel file built on a 33-column template. Every row is analysed and previewed before anything is saved, rows with problems are flagged with the field at fault, and each row saves as its own design.

Reports and projects. Designs save to projects. Word, Excel and PDF reports carry the inputs, the solver, the summary table by flow, the performance curve, the side view of the crossing and the longitudinal profile with hydraulic and energy grade lines.

The rest of the job is in the same workspace. A design flow arrives from Flood Frequency Analysis, TR-55, the Rational Method or the USGS flood estimate with Send to. The outlet velocity goes to Riprap Design. A saved culvert can be imported as an outlet in Flood Routing, where its HDS-5 rating drives the pond routing. Tailwater can be a fixed depth or normal depth computed in the downstream channel.

HY-8 and Studio side by side

How to move a job across

1. Convert units first. Diameters, spans and rises go in millimetres, stations and inverts in metres, flows in m³/s (1 ft³/s = 0.0283 m³/s).
2. Site data. Enter the inlet and outlet station and invert elevation. Barrel length and slope follow from them.
3. Discharge. Give the minimum, design and maximum flow. The performance curve runs from zero to 1.5 times the maximum, so you see behaviour beyond the design event.
4. Tailwater. A constant HY-8 tailwater becomes a direct depth. A rectangular, trapezoidal or triangular downstream channel becomes Compute from Channel, with bottom width, side slope, slope and Manning’s n. A rating-curve tailwater cannot come in through batch import.
5. Roadway. Enter crest elevation, crest length, top width and a paved or gravel surface.
6. Inlet. Match the inlet configuration to what will be built: square edge, groove end, bevelled ring, mitred, projecting, or wingwall flare band. In a batch file, an entrance loss coefficient that differs from the catalogue value for that inlet is used as an override and named in the report.
7. Check one crossing in both programs at the design flow, comparing headwater, control and flow-type code, before moving a whole project. Crossings with unlike culverts side by side stay in HY-8.

Start with the culvert tools

Open the culvert calculator to size a barrel against your headwater limit. For a channel tailwater, the Manning’s equation calculator gives the normal depth, and the riprap sizing calculator sizes the outlet apron from the culvert’s outlet velocity. If the design flow is still open, the flood frequency analysis and TR-55 calculator pages show where Studio’s flows come from.