Source: HEC-15
Author: RT Kilgore and K Cotton (FHWA)


Riprap, cobble, and gravel linings are considered permanent flexible linings. They may be described as a non-cohesive layer of stone or rock with a characteristic size, which for the purposes of this manual is the D50. The applicable sizes for the guidance in this manual range from 15 mm (0.6 in) gravel up to 550 mm (22 in) riprap. For the purposes of this manual, the boundary between gravel, cobble, and riprap sizes will be defined by the following ranges:

Gravel: 15 – 64 mm (0.6 – 2.5 in)
Cobble: 64 – 130 mm (2.5 – 5.0 in)
Riprap: 130 – 550 mm (5.0 – 22.0 in)

Other differences between gravels, cobbles, and riprap may include gradation and angularity. These issues will be addressed later.

Gravel mulch, although considered permanent, is generally used as supplement to aid in the establishment of vegetation (See Chapter 4). It may be considered for areas where vegetation establishment is difficult, for example, in arid-region climates. For the transition period before the establishment of the vegetation, the stability of gravel mulch should be assessed using the procedures in this chapter.

The procedures in this chapter are applicable to uniform prismatic channels (as would be characteristic of roadside channels) with rock sizes within the range given above. For situations not satisfying these two conditions, the designer is referred to another FHWA circular (No. 11) “Design of Riprap Revetment” (FHWA, 1989).


Manning’s roughness is a key parameter needed for determining the relationships between depth, velocity, and slope in a channel. However, for gravel and riprap linings, roughness has been shown to be a function of a variety of factors including flow depth, D50, D84, and friction slope, Sf. A partial list of roughness relationships includes Blodgett (1986a), Limerinos (1970), Anderson, et al. (1970), USACE (1994), Bathurst (1985), and Jarrett (1984). For the conditions encountered in roadside and other small channels, the relationships of Blodgett and Bathurst are adopted for this manual.

Blodgett (1986a) proposed a relationship for Manning’s roughness coefficient, n, that is a function of the flow depth and the relative flow depth (da/D50) as follows:

n =α•da1/6/(2.25 + 5.23•log(da/D50)) (6.1)


n = Manning’s roughness coefficient, dimensionless
da = average flow depth in the channel, m (ft)
D50 = median riprap/gravel size, m (ft)
α = unit conversion constant, 0.319 (SI) and 0.262 (CU).

Equation 6.1 is applicable for the range of conditions where 1.5 ≤ da/D50 ≤ 185. For small channel applications, relative flow depth should not exceed the upper end of this range.

Some channels may experience conditions below the lower end of this range where protrusion of individual riprap elements into the flow field significantly changes the roughness relationship. This condition may be experienced on steep channels, but also occurs on moderate slopes. The relationship described by Bathurst (1991) addresses these conditions and can be written as follows (See Appendix D for the original form of the equation):

n = α•da1/6/(√g•f(Fr)•f(REG)•f(CG)) (6.2)
da = average flow depth in the channel, m (ft)
g = acceleration due to gravity, 9.81 m/s2 (32.2 ft/s2)
Fr = Froude number
REG = roughness element geometry
CG = channel geometry
α = unit conversion constant, 1.0 (SI) and 1.49 (CU).

Equation 6.2 is a semi-empirical relationship applicable for the range of conditions where 0.3 < da/D50 < 8.0. The three terms in the denominator represent functions of Froude number, roughness element geometry, and channel geometry given by the following equations:

f(Fr) = (0.28•Fr/b)log(0.755/b) (6.3)
f(REG) = 13.434•(T/D50)0.492•bx (6.4)
x = 1.025•(T/D50)0.118 (6.4a)
f(CG) = (T/da)-b (6.5)


T = channel top width, m (ft)
b = parameter describing the effective roughness concentration.

The parameter b describes the relationship between effective roughness concentration and relative submergence of the roughness bed. This relationship is given by:

b = 1.14•(D50/T)0.453•(da/D50)0.814 (6.6)

Equations 6.1 and 6.2 both apply in the overlapping range of 1.5 ≤ da/D50 ≤ 8. For consistency and ease of application over the widest range of potential design situations, use of the Blodgett equation (6.1) is recommended when 1.5 ≤ da/D50. The Bathurst equation (6.2) is recommended for 0.3 < da/D50 <1.5.

As a practical problem, both Equations 6.1 and 6.2 require depth to estimate n while n is needed to determine depth, setting up an iterative process.


Values for permissible shear stress for riprap and gravel linings are based on research conducted at laboratory facilities and in the field. The values presented here are judged to be conservative and appropriate for design use. Permissible shear stress is given by the following equation:

τp = F*.(γs – γ).D50 (6.7)


τp = permissible shear stress, N/m2 (lb/ft2)
F* = Shield’s parameter, dimensionless
γs = specific weight of the stone, N/m3 (lb/ft3)
γ = specific weight of the water, 9810 N/m3 (62.4 lb/ft3)
D50 = mean riprap size, m (ft)

Typically, a specific weight of stone of 25,900 N/m3 (165 lb/ft3) is used, but if the available stone is different from this value, the site-specific value should be used.

Recalling Equation 3.2,

τp ≥ SF.τd

and Equation 3.1,

τd = γ.d.So

Equation 6.7 can be written in the form of a sizing equation for D50 as shown below:

D50 ≥ (SF.d.So)/(F*.(SG – 1)) (6.8)


  • d = maximum channel depth, m (ft)
  • SG = specific gravity of rock (γs/γ), dimensionless

Changing the inequality sign to an equality gives the minimum stable riprap size for the channel bottom. Additional evaluation for the channel side slope is given in Section 6.3.2.

Equation 6.8 is based on assumptions related to the relative importance of skin friction, form drag, and channel slope. However, skin friction and form drag have been documented to vary resulting in reports of variations in Shield’s parameter by different investigators, for example Gessler (1965), Wang and Shen (1985), and Kilgore and Young (1993). This variation is usually linked to particle Reynolds number as defined below:

Re = V*.D50 (6.9)


  • Re = particle Reynolds number, dimensionless
  • V* = shear velocity, m/s (ft/s)
  • ν = kinematic viscosity, 1.131×10-6 m2/s at 15.5 deg C (1.217×10-5 ft2/s at 60 deg F)

Shear velocity is defined as:

V* = √(g.d.S) (6.10)


  • g = gravitational acceleration, 9.81 m/s2 (32.2 ft/s2)
  • d = maximum channel depth, m (ft)
  • S = channel slope, m/m (ft/ft)

Higher Reynolds number correlates with a higher Shields parameter as is shown in Table 6.1. For many roadside channel applications, Reynolds number is less than 4×104 and a Shields parameter of 0.047 should be used in Equations 6.7 and 6.8. In cases for a Reynolds number greater than 2×105, for example, with channels on steeper slopes, a Shields parameter of 0.15 should be used. Intermediate values of Shields parameter should be interpolated based on the Reynolds number.

Table 6.1. Selection of Shields’ Parameter and Safety Factor
Reynolds number F* SF
≤ 4×104 0.047 1.0
4×104<Re<2×105 Linear interpolation Linear interpolation
≥ 2×105 0.15 1.5

Higher Reynolds numbers are associated with more turbulent flow and a greater likelihood of lining failure with variations of installation quality. Because of these conditions, it is recommended that the Safety Factor be also increased with Reynolds number as shown in Table 6.1. Depending on site-specific conditions, safety factor may be further increased by the designer, but should not be decreased to values less than those in Table 6.1.

As channel slope increases, the balance of resisting, sliding, and overturning forces is altered slightly. Simons and Senturk (1977) derived a relationship that may be expressed as follows:

D50 ≥ SF•d•S•Δ/(F*•(SG – 1)) (6.11)


  • Δ = function of channel geometry and riprap size.

The parameter Δ can be defined as follows (see HEC-15 Appendix D for the derivation):

Δ = (K1•(1 + sin(α + β)•tan Φ)/(2•(cosθ•tanΦ – SF•sinθ•cosβ)) (6.12)


  • α = angle of the channel bottom slope
  • β = angle between the weight vector and the weight/drag resultant vector in the plane of the side slope
  • θ = angle of the channel side slope
  • Φ = angle of repose of the riprap.

Finally, β is defined by:

β = tan-1(cosα/(2•sinθ/(η•tanΦ) + sinα)) (6.13)


  • η = stability number.

The stability number is calculated using:

η = τs/(F*•(Υs – Υ)•D50) (6.14)

Riprap stability on a steep slope depends on forces acting on an individual stone making up the riprap. The primary forces include the average weight of the stones and the lift and drag forces induced by the flow on the stones. On a steep slope, the weight of a stone has a significant component in the direction of flow. Because of this force, a stone within the riprap will tend to move in the flow direction more easily than the same size stone on a milder gradient. As a result, for a given discharge, steep slope channels require larger stones to compensate for larger forces in the flow direction and higher shear stress.

The size of riprap linings increases quickly as discharge and channel gradient increase. Equation 6.11 (not shown) is recommended when channel slope is greater than 10 percent and provides the riprap size for the channel bottom and sides. Equation 6.8 is recommended for slopes less than 5 percent. For slopes between 5 percent and 10 percent, it is recommended that both methods be applied and the larger size used for design. Values for safety factor and Shields parameter are taken from Table 6.1 for both equations.

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