Trapezoidal Channel: Manning’s n

Caltrans Highway Design Manual

Commonly accepted values for Manning’s roughness coefficient are provided in Table 866.3A. The tabulated values take into account deterioration of the channel lining surface, distortion of the grade line due to unequal settlement, construction joints and normal surface irregularities. These average values should be modified to satisfy any foreseeable abnormal conditions (Reference: Caltrans Highway Design Manual Index 866.3(3)).

**Table 866.3A Average Values for Manning’s n**
Type of Channel		n value
Unlined Channels:
	Clay Loam	0.023
	Sand	0.020
	Gravel	0.030
	Rock	0.040
Lined Channels:
	Portland Cement Concrete	0.014
	Sand	0.020
	Gravel	0.030
	Rock	0.040
Lined Channels:
	Portland Cement Concrete	0.014
	Air Blown Mortar (troweled)	0.012
	Air Blown Mortar (untroweled)	0.016
	Air Blown Mortar (roughened)	0.025
	Asphalt Concrete	0.016 – 0.018
	Sacked Concrete	0.025
Pavement and Gutters:
	Portland Cement Concrete	0.013 – 0.015
	Hot Mix Asphalt Concrete	0.016 – 0.018
Depressed Medians:
	Earth (without growth)	0.016 – 0.025
	Earth (with growth)	0.05
	Gravel (d₅₀ = 1 in. flow depth < 6 in.)	0.040
	Gravel (d₅₀ = 2 in. flow depth < 6 in.)	0.056
NOTES:
	For additional values of n, see HEC No. 15, Tables 2.1 and 2.2, and “Introduction to Highway Hydraulics”, Hydraulic Design Series No. 4, FHWA Table 14. (No such table. Table B-2 provides n values.)

HEC-15

Section 2.1.3 Resistance to Flow

For rigid channel lining types, Manning’s roughness coefficient, n, is approximately constant. However, for very shallow flows the roughness coefficient will increase slightly. (Very shallow is defined where the height of the roughness is about one-tenth of the flow depth or more.)

For a riprap lining, the flow depth in small channels may be only a few times greater than the diameter of the mean riprap size. In this case, use of a constant n value is not acceptable and consideration of the shallow flow depth should be made by using a higher n value.

Tables 2.1 and 2.2 provide typical examples of n values of various lining materials. Table 2.1 summarizes linings for which the n value is dependent on flow depth as well as the specific properties of the material. Values for rolled erosion control products (RECPs) are presented to give a rough estimate of roughness for the three different classes of products. Although there is a wide range of RECPs available, jute net, curled wood mat, and synthetic mat are examples of open-weave textiles, erosion control blankets, and turf reinforcement mats, respectively. Chapter 5 contains more detail on roughness for RECPs.

Table 2.2 presents typical values for the stone linings: riprap, cobbles, and gravels. These are highly depth-dependent for roadside channel applications. More in-depth lining-specific information on roughness is provided in Chapter 6. Roughness guidance for vegetative and gabion mattress linings is in Chapters 4 and 7, respectively.

**Table 2.1. Typical Roughness Coefficients for Selected Linings**
		Manning’s n¹
Lining Category²	Lining Type	Maximum	Typical	Minimum
Rigid	Concrete	0.015	0.013	0.011
	Grouted Riprap	0.040	0.030	0.028
	Stone Masonry	0.042	0.032	0.030
	Soil Cement	0.025	0.022	0.020
	Asphalt	0.018	0.016	0.016
Unlined	Bare Soil	0.025	0.020	0.016
Unlined	Rock Cut (smooth, uniform)	0.045	0.035	0.025
RECP	Open-weave textile	0.028	0.025	0.022
	Erosion control blankets	0.045	0.035	0.028
	Turf reinforement mat	0.036	0.030	0.024
¹Based on data from Kouwen, et al. (1980), Cox, et al. (1970), McWhorter, et al. (1968) and Thibodeaux (1968). ²Minimum value accounts for grain roughness. Typical and maximum values incorporate varying degrees of form roughness.

**Table 2.2. Typical Roughness Coefficients for Riprap, Cobble, and Gravel Linings**
		Manning’s n for Selected Flow Depths¹
Lining Category	Lining Type	0.15 m (0.5 ft)	0.50 m (1.6 ft)	1.0 m (3.3 ft)
Gravel Mulch	D₅₀ = 25 mm (1 in.)	0.040	0.033	0.031
Gravel Mulch	D₅₀ = 50 mm (2 in.)	0.056	0.042	0.038
Cobbles	D₅₀ = 0.1 m (0.33 ft)	–²	0.055	0.047
Rock Riprap	D₅₀ = 0.15 m (0.5 ft)	–²	0.069	0.056
Rock Riprap	D₅₀ = 0.1 m (0.33 ft)	–²	–²	0.080
¹Based on Equation 6.1 (Blodgett and McConaughy, 1985). Manning’s n estimated assuming a trapezoidal channel with 1:3 side slopes and 0.6 m (2 ft) bottom width. ²Shallow relative depth (average depth to D₅₀ ratio less than 1.5) requires use of Equation 6.2 (Bathurst, et al., 1981) and is slope-dependent. See Section 6.1.

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