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.

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.

Powered by DrainageCalculators.com — free drainage & stormwater calculators