Riprap Sizing Calculator (D50 & FHWA HEC-23)

Screen FHWA HEC-23 D50, nominal class, thickness, and application geometry for channels, bridges, overtopping, bottomless culverts, and wave attack, with bounded granular and geotextile filter checks.

FHWA Hydraulic Toolbox parity

Application-Specific Riprap Design

HEC-23 uses a different design equation for each application. Circular culvert aprons are handled by the HEC-14 Outlet Protection Calculator.

FHWA Application

Application

Channel Hydraulics

Local Flow Depth

Average Velocity

ft/s

Longitudinal Channel Slope

decimal

HEC-23 Equation 4.1 is limited to 2% or flatter

Riprap Placement

Channel Type

Rock Shape

Bank Angle

deg

Safety Factor

Protection Location

Bend Centerline Radius

Upstream Water Width

Rock Specific Gravity

FHWA Results

Select an application, enter its hydraulic inputs, and run the FHWA design.

Legacy / Comparative Sizing Methods

The methods below are retained for comparison. Use the application-specific FHWA section above when matching Hydraulic Toolbox results.

Legacy Isbash coefficients

These coefficients apply only to the comparative Isbash method below. Use the HEC-23 Table 4.1 classes for an FHWA class specification.

Condition	C
Loose, minimum stability	0.86
Loose, average	1
Loose, stable	1.2
Placed, stable	1.4
Keyed (embedded)	1.6

Input Parameters

Sizing Method

Calculation Method

Comprehensive method with Froude number consideration

Flow Parameters

Design Velocity

fps

Average channel velocity

Flow Depth

Stone Properties

Specific Gravity

Typically 2.65 for granite/limestone

Rock Shape

Conditions

Installation Method

Flow Condition

Stability Factor

Safety factor (1.0-1.5 typical)

Side Slope Angle

degrees

For side slope protection (optional)

Ready to Calculate

Select a sizing method and enter flow parameters to determine riprap requirements.

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