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Box Culvert Barrel Discharge Capacity Calculator

Precast concrete box culverts convey stormwater and stream discharges under roadways and railways where headroom or cover is limited.

Inside clear horizontal width of each individual culvert barrel (typically 1.2 to 4.5 m).

Inside clear vertical height of each individual culvert barrel.

Total longitudinal length of the culvert barrel under the roadway embankment.

Longitudinal invert slope along the barrel alignment (typically 0.1% to 2.0%).

Roughness factor (typically 0.012 for smooth precast concrete, 0.015 for cast-in-place).

Number of identical side-by-side barrels sharing the flow (1, 2, 3, etc.).

Calculated Result
39.82 m³/s

Full-Flow Discharge Capacity

Total Full-Flow Discharge

39.82 m³/s (1,406.3 CFS)

Barrel Flow Velocity (V)

3.98 m/s

Hydraulic Radius (R)

0.556 m

Total Barrel Flow Area

10 m²

Barrel Arrangement

2 × (2.5m Span × 2m Rise)

Calculation Breakdown

  1. Box Barrel Cross-Section GeometryArea A = 2.5m × 2m = 5.00 m², Wetted P = 2 × (2.5 + 2) = 9.00 m → R = 0.556 m
  2. Manning's Open/Closed Conduit EquationQ_single = (1 / 0.012) × 5.00 × (0.556)^0.667 × √(0.005) = 19.91 m³/s
  3. Total Multi-Barrel CapacityQ_total = 2 barrels × 19.91 m³/s = 39.82 m³/s (1406.3 CFS)

Box Culvert Hydraulic Conveyance

Interactive visualization based on your current inputs

Value
0.09.7192939Span (m)Rise (m)Velocity (m/s)Hyd. Radius (m)Discharge (m³/s)ParameterValue

What Is the Box Culvert Barrel Discharge Capacity Calculator?

The Box Culvert Barrel Discharge Capacity Calculator evaluates the hydraulic carrying capacity of rectangular precast and cast-in-place culverts.

How Does the Box Culvert Barrel Discharge Capacity Calculator Work?

It computes conduit flow area, wetted perimeter, hydraulic radius, and discharge capacity using Manning equation under gravitational slope.

Box Culvert Barrel Discharge Capacity Calculator Formula & Variables

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

Q = N \cdot \frac{1}{n} A R_h^{2/3} S_0^{1/2}, \quad R_h = \frac{B \cdot D}{2(B + D)}

Manning conduit flow formula for pressurized or just-full rectangular barrel hydraulics.

How to Use the Box Culvert Barrel Discharge Capacity Calculator

  1. Enter barrel clear span (width) and clear rise (height).
  2. Input longitudinal bed slope and barrel length.
  3. Specify Manning roughness and number of parallel cells.

Step-by-Step Example Calculation

Twin 2.5m × 2.0m Precast Concrete Culvert

Input Values:

barrelWidthSpanM:2.5
barrelHeightRiseM:2
barrelLengthM:30
barrelSlopePercent:0.5
manningsRoughnessN:0.012
numberOfBarrels:2
Worked Steps: Delivers 38.64 m³/s (1,364 cfs) total discharge capacity at 3.86 m/s flow velocity.

Understanding Your Result

Design Discharge: Full pipe flow carrying rate in m³/s and cubic feet per second (cfs).

Full Flow Velocity: Mean water speed through the barrel.

Hydraulic Radius: Efficiency metric of the rectangular conduit geometry.

Factors That Affect the Result

  • Conduit slope increases discharge with the square root of slope.
  • Smooth precast forms (n = 0.012) carry ~25% more flow than rough cast-in-place concrete (n = 0.015).

When Should You Use This Calculator?

  • Highway bridge replacements, railway crossings, urban channel enclosure, and flood mitigation canals.

Assumptions & Limitations

  • Assumes uniform flow along barrel length without inlet control submerged contraction headloss.

Frequently Asked Questions

Calculation Accuracy & Reference Note

Conforms to FHWA HDS-5 Hydraulic Design of Highway Culverts and AASHTO guidelines.

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