Culvert Hydraulic Design Calculator – FHWA Inlet/Outlet Control, 2D Profile, Batch, PDF

Culvert Hydraulic Design Calculator

FHWA HDS‑5 method · Inlet/Outlet control · Circular, box & arch culverts · 2D profile · PDF report · Batch · Trend chart

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📐 1. Culvert Hydraulic Design – Why It Matters

Culverts convey water under roads, railways, or embankments. Proper hydraulic design prevents flooding and road overtopping. The FHWA HDS‑5 method considers both inlet control (governed by inlet geometry) and outlet control (affected by friction, tailwater). This calculator supports circular (pipe), box (rectangular), and arch culverts, computes the governing control, headwater depth, and required culvert size.

Inlet control (circular): HW = (Q / (C·D²·√g))^(1/1.5) · D
Outlet control (energy equation): HW = (H_L + TW) – (inlet loss + friction + exit loss). The tool uses simplified empirical curves for preliminary design.

🚀 2. Live Culvert Design Calculator

Culvert shapeCircular (pipe)Box (rectangular)Arch (elliptical)

Diameter / height (m)

Box width (m)

Design flow Q (m³/s)

Tailwater depth TW (m)

Culvert length L (m)

Inlet typeSquare edge with headwallGroove end with headwallProjecting

Manning's n (concrete = 0.013, corrugated = 0.024)

Batch CSV Upload

Get PDF Report via Email

CSV Template

📊 3. Culvert Profile & Headwater Visualisation

📈 4. Inlet vs Outlet Control & History

📘 5. How to Use the Calculator

1. Select culvert shape, enter dimensions (diameter/height, box width if applicable).
2. Input design flow (Q), tailwater depth, culvert length, Manning's n.
3. Choose inlet type (affects inlet control coefficient).
4. Click "Compute Hydraulics" – tool calculates inlet control headwater, outlet control headwater, and governing condition.
5. The 2D profile shows culvert, water surface, and headwater depth.
6. Bar chart compares required headwater for both control types; history stores last 20 runs.
7. Batch CSV: upload multiple designs; output table with governing headwater.
8. PDF report includes profile image and summary.

❓ Frequently Asked Questions

❓ What is the difference between inlet and outlet control?

Inlet control occurs when the culvert barrel can carry more flow than the inlet can admit; outlet control is limited by friction and tailwater. The governing control is the one that requires the higher headwater.

❓ How accurate is this calculator?

It uses simplified FHWA equations for preliminary design. For final design, use HEC‑RAS or HY‑8.

❓ What Manning's n should I use?

Concrete pipe: 0.013; corrugated metal: 0.024; smooth plastic: 0.010.

🌐 Industry Standards & References

FHWA HDS‑5 – Hydraulic Design of Highway Culverts | USACE Hydraulic Design

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© ERH Culvert Design Calculator – Based on FHWA HDS‑5. All data processed locally.