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)).
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.
|Lining Category2||Lining Type||Maximum||Typical||Minimum|
|Rock Cut (smooth, uniform)||0.045||0.035||0.025|
|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.
|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|
|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|
|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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