Culvert Hydraulic Design Calculator – FHWA Inlet/Outlet Control Method for Box, Pipe & Arch Culverts

Culvert Hydraulic Design Calculator – FHWA Inlet/Outlet Control

Size culverts using FHWA method · Box, circular & arch culverts · Headwater depth · 2D profile · PDF · Batch

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📐 1. What is Culvert Hydraulic Design?

Culverts are closed conduits that convey water under roads, railways, or embankments. Proper hydraulic design ensures that the culvert can pass the design flood without overtopping the road. The FHWA (Federal Highway Administration) method considers both inlet control and outlet control to determine the required size and headwater depth.

Inlet control: Flow is controlled by the inlet geometry (shape, area, headwater).
Outlet control: Flow is controlled by friction, tailwater, and exit conditions.
This calculator uses simplified empirical equations for quick preliminary design.

🚀 2. Live Culvert Calculator (FHWA Method)

Culvert ShapeCircular (Pipe)Box (Rectangular)Arch (Elliptical)

Diameter / Height m

Box Width (B) m

Design Flow Rate (Q) m³/s

Headwater Depth (HW) m

Tailwater Depth (TW) m

Culvert Length (L) m

Inlet TypeSquare edge with headwallGroove end with headwallProjecting

Batch CSV Upload

✅ CSV: shape, dim, flow, headwater, tailwater, length, inletType

Get PDF Report via Email

CSV Template

📊 2D Culvert Profile (Inlet/Outlet Control)

📊 Inlet vs Outlet Control Capacity

📈 Design Flow & Headwater Trend

📂 3. How to Use the Calculator

1. Select culvert shape: Circular (pipe), Box (rectangular), or Arch (elliptical).
2. Enter dimensions: For circular: diameter; for box: height and width; for arch: equivalent height.
3. Enter hydraulic parameters: Design flow rate (Q), headwater depth (HW), tailwater depth (TW), culvert length.
4. Select inlet type: Square edge, groove end, or projecting (affects inlet control coefficient).
5. Compute: Click "Compute Culvert" – the tool calculates inlet and outlet control capacities, determines the governing control, and recommends required headwater.
6. Batch CSV: Upload multiple designs for rapid analysis.
7. PDF Report: Download a detailed report with the 2D profile and charts.
8. History: All calculations saved locally; export or clear anytime.

📈 4. Real‑World Example

Box culvert under a rural road, Bangladesh
Shape: Box (1.5 m width, 1.2 m height), Q = 2.5 m³/s, HW = 1.8 m, TW = 0.5 m, L = 30 m, inlet: square edge with headwall.
Inlet control capacity: about 2.8 m³/s (passing). Outlet control capacity: about 2.6 m³/s (governs).
The calculator identifies that outlet control governs and recommends a slightly larger culvert if Q exceeds capacity.

❓ 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 accept; outlet control is limited by friction and tailwater. The governing control is the one with the lower capacity.

❓ How accurate is this calculator?

It uses simplified FHWA equations for preliminary design. For final design, use HEC‑RAS or consult a hydraulic engineer.

❓ Is the tool free?

Yes, completely free. Batch processing supports up to 100 rows per CSV.

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© ERH Culvert Design Calculator – Based on FHWA HDS‑5 methodology. For preliminary design only.