§ 6.3.1. Existing and Natural Channels

Because several primary factors affect the roughness coefficient, a procedure has been developed to estimate this value, n. By this procedure, the value of n may be computed by:

n = (n ₀ + n ₁ + n ₂ + n ₃ + n ₄ )m (Eq. 6-2)

Where n ₀ is a basic n value for a straight, uniform, smooth channel in the natural materials involved, n ₁ is a value added to n0 to correct for the effect of surface irregularities; n ₂ is a value for variations in shape and size of the channel cross section; n ₃ is a value for obstructions; n ₄ is a value for vegetation and flow conditions; and m is a correction factor for meandering of the channel. Proper values of n ₀ to n ₄ and m may be selected from Table 6-1 according to the given conditions.

In selecting the value of n1, the degree of irregularity is considered smooth for surfaces comparable to the best attainable for the materials involved; minor for good dredged channels, slightly eroded or scoured side slopes of canals or drainage channels; moderate for fair to poor dredged channels, moderately sloughed or eroded side slopes of canals or drainage channels; and severe for badly sloughed banks of natural streams, badly eroded or sloughed sides of canals or drainage channels, and unshaped, jagged and irregular surfaces of channels excavated in rock.

In selecting the value of n2, the character of variations in size and shape of cross section is considered gradual when the change in size or shape occurs gradually; alternating occasionally when large and small sections alternate occasionally or when shape changes cause occasional shifting of main flow from side to side; and alternating frequently when large and small sections alternate frequently or when shape changes cause frequent shifting of main flow from side to side.

The selection of the value of n3 is based on the presence and characteristics of obstructions such as debris deposits, stumps, exposed roots, boulders and fallen and lodged logs. One should recall that conditions considered in other steps must not be re-evaluated or double-counted in this selection. In judging the relative effect of obstructions, consider the following: the extent to which the obstructions occupy or reduce the average water area, the obstruction characteristics (sharp-edged or angular objects induce greater turbulence than curved, smooth-surfaced objects) and the position and spacing of obstructions transversely and longitudinally in the reach under consideration.

Table 6-1 Computation of Composite Roughness Coefficient For Excavated and Natural Channels n = (n 0 + n 1 + n 2 + n 3 + n 4 )m
Channel Conditions		Values
n 0 Material Involved	Earth Rock Cut Fine Gravel Coarse Gravel	0.020 0.025 0.024 0.028
n 1 Degree of Irregularity	Smooth Minor Moderate Severe	0.000 0.005 0.010 0.020
n 2 Relative Effect of Channel Cross Section	Gradual Alternating Occasionally Alternating Frequently	0.000 0.005 0.013
n 3 Relative Effect of Obstructions	Negligible Minor Appreciable Severe	0.000 0.013 0.025 0.050
n 4 Vegetation	Low Medium High Very High	0.008 0.018 0.038 0.075
m Degree of Meandering	Minor Appreciable Severe	1.000 1.150 1.300
Source: Chow, V.T. Open Channel Hydraulics , 1959.

 

In selecting the value of n4, the degree of effect of vegetation is considered in the following way:

A.

Low for conditions comparable to the following: (a) dense growths of flexible turf grasses or weeds, of which Bermuda and blue grasses are examples, where the average depth of flow is two (2) to three (3) times the height of vegetation and (b) supple seedling tree switches, such as willow, cottonwood or salt cedar where the average depth of flow is three (3) to four (4) times the height of the vegetation.

B.

Medium for conditions comparable to the following: (a) turf grasses where the average depth of flow is one (1) to two (2) times the height of vegetation, (b) stemmy grasses, weeds or tree seedlings with moderate cover where the average depth of flow is two (2) to three (3) times the height of vegetation and (K) brush growths, moderately dense, similar to willows one (1) to two (2) years old, dormant season, along side slopes of a channel with no significant vegetation along the channel bottom, where the hydraulic radius is greater than two (2) feet.

C.

High for conditions comparable to the following: (a) turf grasses where the average depth of flow is about equal to the height of vegetation, (b) dormant season — willow or cottonwood trees eight (8) to ten (10) years old, intergrown with some weeds and brush, where none of the vegetation is in foliage, where the hydraulic radius is greater than two (2) feet, and (c) growing season — bushy willows about one (1) year old intergrown with some weeds in full foliage along side slopes, no significant vegetation along channel bottom, where hydraulic radius is greater than two (2) feet.

D.

Very high for conditions comparable to the following: (a) turf grasses where the average depth of flow is less than ½ the height of vegetation, (b) growing season — bushy willows about one (1) year old, intergrown with weeds in full foliage along side slopes, or dense growth of cattails along channel bottom, with any value of hydraulic radius up to ten (10) or 15 feet and (c) growing season—trees intergrown with weeds and brush, all in full foliage, with any value of hydraulic radius up to ten (10) or 15 feet.

In selecting the value of m, the degree of meandering depends on the ratio of the meander length to the straight length of the channel reach. The meandering is considered minor for ratios of 1.0 to 1.2, appreciable for ratios of 1.2 to 1.5, and severe for ratios of 1.5 and greater.

In applying the above method for determining the n value, several things should be noted. The method does not consider the effect of suspended and bed loads. The values given in Table 6-2 were developed from a study of some 40 to 50 cases of small and moderate channels. Therefore, the method is questionable when applied to large channels whose hydraulic radii exceed 15 feet. The method applies only to unlined natural streams, floodways, and drainage channels and shows a minimum value of 0.02 for the n value of such channels. The minimum value of n in general, however, may be as low as 0.012 in lined channels and as low as 0.008 in artificial laboratory flumes.