DrainageCalculators

Outlet Protection Calculator

Design riprap aprons and outlet protection structures for culverts using FHWA HEC-14 methodology. Calculate D50 stone size, blanket thickness, and apron dimensions.

What This Solves

Designs riprap aprons and outlet protection pads at culvert and pipe outlets using FHWA HEC-14 methodology to prevent scour from high-velocity discharge.

Best Used When

  • You need to design a riprap apron at a culvert outlet to prevent downstream erosion
  • You want to determine the required stone size (D50), blanket thickness, and apron dimensions
  • You are evaluating whether existing outlet protection is adequate for the design flow

Do NOT Use When

Key Assumptions

  • Riprap sizing follows FHWA HEC-14 relationships for pipe and culvert outlets
  • The apron is placed on a stable subgrade with appropriate filter fabric or bedding
  • Tailwater depth affects the required stone size and apron length
  • Riprap is angular, well-graded stone meeting standard gradation specifications
  • No significant debris or ice loading on the riprap apron

Input Quality Notes

Outlet velocity and tailwater depth are the key inputs. Calculate outlet velocity from pipe flow analysis and tailwater from downstream channel conditions. Use conservative (low) tailwater for riprap sizing.

HEC-14 Design Coefficients

d50 Sizing Coefficient 0.2
Minimum Blanket Thickness 1.5 x d50
Apron Length (low TW) 4.0 x De
Apron Length (high TW) 3.0 x De
Width Expansion Factor 3.0 x De
TW Ratio Threshold 0.5

Tailwater Effects on Design

Low Tailwater (TW/De < 0.5)

  • Full d50 sizing equation applies
  • Extended apron length (4 x De)
  • More energy dissipation needed

High Tailwater (TW/De ≥ 0.5)

  • 14% reduction in d50 sizing
  • Standard apron length (3 x De)
  • Tailwater provides some energy dissipation

Input Parameters

Outlet Configuration

ft

Flow Parameters

cfs
ft/s
ft

Depth downstream of outlet

Calculated if not provided

Riprap Options

Ready to Design

Enter outlet configuration and flow parameters to design riprap protection.

For educational purposes only. Not a substitute for professional engineering judgment.

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

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Last verified: February 2026

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

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PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.