Nepřímé odvození průtoku pomocí Manningovy rovnice
Stanoveni drsnostniho soucinitele n
(Cow a114956)
nb=2i base value of n for a straight, uniform, smooth channel in natural materials, ,
nl =a correction factor for the effect of surface irregularities,
n2— a value for variations in shape and size of the
channel cross section, n3 =a value for obstructions, n4~a value for vegetation and flow conditions, and m — a correction factor for meandering of the channel.
Manningova rovnice
n
Původní tvar rovnice pro imperiálni jednotky:
n e
Hodnota nb
Base n value
Median size of
Smooth
matenal (in millimeters)     ™«<™ channel'
channel
Stable channels and flood plains
Concrete...............                           — 0.012-0.018 0.011
Rock cut...............                           — — .025
Firm soil...............                         - 0.025-0.032 .020
Coarse sand............                     1-2 0.026-0.035
Fine gravel.............                        — — .024
Gravel................. 2-64 0.O2&-O.O35 —
Coarse gravel...........                     — — .026
Cobble................. 64-256 0.030-0.050 —
Boulder................ >256 0.040-0.070 —
Hodnota n1
Channel conditions
n value adjustment1
Example
Degree of irregularity
Smooth Minor
Moderate Severe
0.000 0.001-0.005
0.006-0.010
0.011-0.020
Compares to the smoothest channel attainable in a given bed material. Compares to carefully dredged channels in good condition but having slightly
eroded or scoured side slopes. Compares to dredged channels having moderate to considerable bed roughness
and moderately sloughed or eroded side slopes. Badly sloughed or scalloped banks of natural streams; badly eroded or sloughed
sides of canals or drainage channels; unshaped, jagged, and irregular surfaces
of channels in rock.
Hodnota n2
Channel conditions
n value adjustment1
Example
Variation in channel cross section
(«2)
Gradual Alternating occasionally
Alternating frequently
0.000 0.001-0.005
0.010-0.015
Size and shape of channel cross sections change gradually.
Large and small cross sections alternate occasionally, or the main flow
occasionally shifts from side to side owing to changes in cross-sectional
shape.
Large and small cross sections alternate frequently, or the main flow frequently shifts from side to side owing to changes in cross-sectional shape.
Hodnota n3
Channel conditions
n value adjustment1
Example
Negligible
Minor
Effect of obstruction
Appreciable
Severe
0.000-0.004
0.005-0.015
0.020-0.030
0.040-0.050
A few scattered obstructions, which include debris deposits, stumps, exposed roots, logs, piers, or isolated boulders, that occupy less than 5 percent of the cross-sectional area.
Obstructions occupy less than 15 percent of the cross-sectional area, and the spacing between obstructions is such that the sphere of influence around one obstruction does not extend to the sphere of influence around another obstruction. Smaller adjustments are used for curved smooth-surfaced objects than are used for sharp-edged angular objects.
Obstructions occupy from 15 to 50 percent of the cross-sectional area, or the space between obstructions is small enough to cause the effects of several obstructions to be additive, thereby blocking an equivalent part of a cross section.
Obstructions occupy more than 50 percent of the cross-sectional area, or the space between obstructions is small enough to cause turbulence across most of the cross section.
Hodnota n4
Channel conditions
n value adjustment1
Example
Small
0.002-0.010
Medium
0.010-0.025
Amount of vegetation
Large
0.025-0.050
Very large
0.050-0.100
Dense growths of flexible turf grass, such as Bermuda, or weeds growing where the average depth of flow is at least two times the height of the vegetation; supple tree seedlings such as willow, cotton wood, arrow weed, or saltcedar growing where the average depth of flow is at least three times the height of the vegetation.
Turf grass growing where the average depth of flow is from one to two times the height of the vegetation; moderately dense stemmy grass, weeds, or tree seedlings growing where the average depth of flow is from two to three times the height of the vegetation; brushy, moderately dense vegetation, similar to 1- to 2-year-old willow trees in the dormant season, growing along the banks, and no significant vegetation is evident along the channel bottoms where the hydraulic radius exceeds 2 ft.
Turf grass growing where the average depth of flow is about equal to the height of the vegetation; 8- to 10-year-old willow or cottonwood trees intergrown with some weeds and brush (none of the vegetation in foliage) where the hydraulic radius exceeds 2 ft; bushy willows about 1 year old intergrown with some weeds along side slopes (all vegetation in full foliage), and no significant vegetation exists along channel bottoms where the hydraulic radius is greater than 2 ft.
Turf grass growing where the average depth of flow is less than half the height of the vegetation; bushy willow trees about 1 year old intergrown with weeds along side slopes (all vegetation in full foliage), or dense cattails growing along channel bottom; trees intergrown with weeds and brush (all vegetation in full foliage).
Hodnota m
Channel conditions
n value adjustment1
Example
Degree of meandering2 (m)
Minor
Appreciable
Severe
1.00 1.15 1.30
Ratio of the channel length to valley length is 1.0 to 1.2. Ratio of the channel length to valley length is 1.2 to 1.5. Ratio of the channel length to valley length is greater than 1.5.