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)). 

HEC15
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 onetenth 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 openweave 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 depthdependent for roadside channel applications. More indepth liningspecific information on roughness is provided in Chapter 6. Roughness guidance for vegetative and gabion mattress linings is in Chapters 4 and 7, respectively.
Manning’s n^{1}  

Lining Category^{2}  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 
Rock Cut (smooth, uniform)  0.045  0.035  0.025  
RECP  Openweave 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  
^{1}Based on data from Kouwen, et al. (1980), Cox, et al. (1970), McWhorter, et al. (1968) and Thibodeaux (1968).
^{2}Minimum value accounts for grain roughness. Typical and maximum values incorporate varying degrees of form roughness.

Manning’s n for Selected Flow Depths^{1}  

Lining Category  Lining Type  0.15 m (0.5 ft)  0.50 m (1.6 ft)  1.0 m (3.3 ft) 
Gravel Mulch  D_{50} = 25 mm (1 in.)  0.040  0.033  0.031 
D_{50} = 50 mm (2 in.)  0.056  0.042  0.038  
Cobbles  D_{50} = 0.1 m (0.33 ft)  –^{2}  0.055  0.047 
Rock Riprap  D_{50} = 0.15 m (0.5 ft)  –^{2}  0.069  0.056 
D_{50} = 0.1 m (0.33 ft)  –^{2}  –^{2}  0.080  
^{1}Based 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.
^{2}Shallow relative depth (average depth to D_{50} ratio less than 1.5) requires use of Equation 6.2 (Bathurst, et al., 1981) and is slopedependent. See Section 6.1.

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