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Culvert Headwater Depth Calculator

Highway culverts operating under inlet control restrict stormwater discharge at the pipe entrance, causing upstream water to pond into headwater.

Internal circular barrel diameter.

Peak storm runoff flow through the culvert.

Entrance headwall and bevel edge geometry per FHWA HDS-5.

Invert slope (m/m, e.g., 0.015 = 1.5% drop).

Calculated Result
1.18 m

Required Headwater Depth (HW)

Headwater Depth (HW)

1.18 m (3.87 ft)

Headwater Ratio (HW/D)

0.98

Inlet Hydraulics Regime

Submerged Inlet (Orifice full pipe entrance, HW/D > 1.5)

Barrel Velocity (v)

3.09 m/s

Culvert Diameter (D)

1.2 m

Calculation Breakdown

  1. Flow Parameter CalculationBarrel Area A = π × (1.2)² / 4 = 1.131 m² → Q / (A × √D) = 3.09 / √1.2 = 2.83
  2. FHWA HDS-5 Inlet Control EquationHW/D = 0.98 (submerged)
  3. Total Required Headwater ElevationHW = 0.98 × 1.2 m = 1.18 m

Hydraulic Properties

Interactive visualization based on your current inputs

Value
0.07.7152331HW (m × 10)HW/D Ratio (× 10)Diameter (m × 10)Velocity (m/s × 10)ParameterValue

What Is the Culvert Headwater Depth Calculator?

The Culvert Headwater Depth Calculator determines upstream ponding depth (headwater HW) for highway culverts operating under inlet control.

How Does the Culvert Headwater Depth Calculator Work?

Evaluates dimensionless flow parameters against Federal Highway Administration (FHWA HDS-5) empirical coefficients for square-edged, mitered, and beveled entrance collars.

Culvert Headwater Depth Calculator Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

\frac{HW}{D} = \frac{H_c}{D} + K \left( \frac{Q}{A \sqrt{D}} \right)^M - 0.5 S \quad \text{or} \quad c \left( \frac{Q}{A \sqrt{D}} \right)^2 + Y - 0.5 S

FHWA HDS-5 semi-empirical hydraulic inlet control design equations.

How to Use the Culvert Headwater Depth Calculator

  1. Input pipe internal diameter and design storm peak discharge rate.
  2. Select entrance edge configuration and invert slope.
  3. Review upstream headwater depth and HW/D ratio.

Step-by-Step Example Calculation

Highway Stormwater Culvert Inlet Sizing

Input Values:

pipeDiameterM:1.2
flowRateM3s:3.5
inletEdgeType:square-edge-headwall
barrelSlopeMPerM:0.015
Worked Steps: Requires 2.38 m headwater depth (HW/D = 1.98, submerged orifice control).

Understanding Your Result

Headwater Depth (HW): Upstream water elevation above the culvert inlet invert.

HW/D Ratio: Design criterion (typically kept ≤ 1.2 to 1.5 to prevent road embankment overtopping).

Inlet Regime: Classifies flow as weir, transition, or submerged orifice flow.

Factors That Affect the Result

  • Adding entrance bevels can reduce upstream ponding depth by up to 25% for the same discharge.

When Should You Use This Calculator?

  • Highway bridge replacements, railway embankment drainage, and subdivision road crossings.

Assumptions & Limitations

  • Assumes inlet control governs; outlet control must be verified separately if high downstream tailwater exists.

Frequently Asked Questions

Calculation Accuracy & Reference Note

Standard FHWA HDS-5 (Hydraulic Design of Highway Culverts) formulation.

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