Riprap Rock Sizing Calculator

Compute stable rock size. River channel erosion control, scour prevention. Isbash equation


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Water velocity, V: 
Rock diameter, D: 
Isbash constant, C:    Enter 0.86 or 1.2. See below
Rock specific gravity, S:    Typically 2.65. See below
© 2017 LMNO Engineering,  Research, and Software, Ltd.  

Units in riprap stone sizing calculator: ft=foot, knots=nautical miles per hour, m=meter, mph=miles per hour.

Riprap in a River

riprap photo

Riprap Sizing
Riprap is used for erosion control, to prevent scour, and to minimize sediment transport in rivers and streams. A stable riprap rock size is desired. The Isbash equation computes the smallest diameter stone D having specific gravity S that if dropped in water flowing at velocity V will settle and remain stationary on the channel bed (Blevins, 2003, p. 218; NCHRP, 2006, p. 14).

riprap equation

Our calculation will also solve the equation in reverse. In that case, riprap rock size D is entered. The computed water velocity V is the maximum water velocity at which the riprap will remain in place.

The variables in the riprap sizing equation are (units shown in SI, but our calculation allows a variety of units):
C = Isbash constant. Per references below, C=0.86 for highly turbulent conditions or C=1.2 for low turbulence.
D = Median diameter of spherical stone or rock. Also known as D50 (m).
g = Acceleration due to gravity, 9.8066 m/s2.
S = Specific gravity of stone or rock. Typically varies from 2.56 to 2.92 depending on the rock material. A commonly used value is 2.65.
V = Water velocity approaching the riprap (m/s).

Blevins, Robert D. 2003. Applied Fluid Dynamics Handbook. Krieger Publishing Co.

National Cooperative Highway Research Program [NCHRP]. 2006. Riprap Design Criteria, Recommended Specifications, and Quality Control. Report 568. Transportation Research Board. Retrieved from


© 2017 LMNO Engineering, Research, and Software, Ltd.  All rights reserved.

Please contact us for consulting or other questions about riprap sizing.

LMNO Engineering, Research, and Software, Ltd.
7860 Angel Ridge Rd.   Athens, Ohio  45701  USA   Phone: (740) 592-1890


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