FREE SURFACE FLOW MODELING
P3
Roughness parameters
for open surface flow
7 July 2005
Prof S J van Vuuren
Department of Civil & Biosystems
Engineering
Roughness parameters
for open surface flow
Layout of the presentation
• Introduction
• Variation of Manning’s n-value versus
Hydraulic radius, R
• Typical Manning n-values
• Examples of roughness in rivers
Roughness parameters
for open surface flow
Introduction
• Most hydraulic computations related to indirect
estimates of discharge, require an evaluation of the
roughness characteristics of the channel.
• In the absence of a satisfactory quantitative
procedure the estimation remains “an art”.
• The ability to evaluate roughness coefficients must
be developed through experience.
• One means of gaining this experience is by
examining and becoming acquainted with the
hydraulic characteristics of some typical channels
whose roughness coefficients are known.
1
Roughness parameters
for open surface flow
Variation of Manning nvalue versus Hydraulic
radius
1
C=
R6
n
Chezy
 12 R 

C = 18 log
 ks 
yn
V = C RS
Manning
5
Q=
1 A 3 12
So
n 23
P
yn
Roughness parameters
for open surface flow
Practical exercise
Calculate the associated
Manning n-value and
absolute roughness
parameter (ks), for this
stone, if it represents the
roughness parameter of
an extremely large and
wide river such as the
Zambezi River, when
the water depth is 5 m.
Roughness parameters
for open surface flow
Answer to practical exercise
Stone diameter =
50 mm
Hydraulic radius, R =
5,0 m
Ks =
50 mm
C=
55,425
n=
0,0236 s/m 0.333
2
Roughness parameters
for open surface flow
Manning n-values
Natural channels
Type of Channel and Description
A. Natural Streams
1. Main Channels
a. Clean, straight, full, no rifts or deep pools
b. Same as above, but more stones and weeds
c. Clean, winding, some pools and shoals
d. Same as above, but some weeds and stones
e. Same as above, lower stages, more ineffective
slopes and sections
f. Same as "d" but more stones
g. Sluggish reaches, weedy. deep pools
h. Very weedy reaches, deep pools, or floodways
with heavy stands of timber and brush
2. Flood Plains
a. Pasture no brush
1. Short grass
2. High grass
b. Cultivated areas
1. No crop
2. Mature row crops
3. Mature field crops
c. Brush
1. Scattered brush, heavy weeds
2. Light brush and trees, in winter
3. Light brush and trees, in summer
4. Medium to dense brush, in winter
5. Medium to dense brush, in summer
d. Trees
1. Cleared land with tree stumps, no sprouts
2. Same as above, but heavy sprouts
3. Heavy stand of timber, few down trees, little
undergrowth, flow below branches
4. Same as above, but with flow into branches
5. Dense willows, summer, straight
3. Mountain Streams, no vegetation in channel, banks
usually steep, with trees and brush on banks submer
a. Bottom: gravels, cobbles, and few boulders
b. Bottom: cobbles with large boulders
Minimum
Normal
Maximum
0,025
0,030
0,033
0,035
0,040
0,030
0,035
0,040
0,045
0,048
0,033
0,040
0,045
0,050
0,055
0,045
0,050
0,070
0,050
0,070
0,100
0,060
0,080
0,150
0,025
0,030
0,030
0,035
0,035
0,050
0,020
0,025
0,030
0,030
0,035
0,040
0,040
0,045
0,050
0,035
0,035
0,040
0,045
0,070
0,050
0,050
0,060
0,070
0,100
0,070
0,060
0,080
0,110
0,160
0,030
0,050
0,080
0,040
0,060
0,100
0,050
0,080
0,120
0,100
0,110
0,120
0,150
0,160
0,200
0,030
0,040
0,040
0,050
0,050
0,070
Roughness parameters
for open surface flow
Manning n-values
Lined channels
Type of Channel and Description
B. Lined or Built-Up Channels
1. Concrete
a. Trowel finish
b. Float Finish
c. Finished, with gravel bottom
d. Unfinished
e. Gunite, good section
f. Gunite, wavy section
g. On good excavated rock
h. On irregular excavated rock
Minimum
Normal
Maximum
0,011
0,013
0,015
0,014
0,016
0,018
0,017
0,022
0,013
0,015
0,017
0,017
0,019
0,022
0,020
0,027
0,015
0,016
0,020
0,020
0,023
0,025
2. Concrete bottom float finished with sides of:
a. Dressed stone in mortar
b. Random stone in mortar
c. Cement rubble masonry, plastered
d. Cement rubble masonry
e. Dry rubble on riprap
0,015
0,017
0,016
0,020
0,020
0,017
0,020
0,020
0,025
0,030
0,020
0,024
0,024
0,030
0,035
3. Gravel bottom with sides of:
a. Formed concrete
b. Random stone in mortar
c. Dry rubble or riprap
0,017
0,020
0,023
0,020
0,023
0,033
0,025
0,026
0,036
4. Brick
a. Glazed
b. In cement mortar
0,011
0,012
0,013
0,015
0,015
0,018
5. Metal
a. Smooth steel surfaces
b. Corrugated metal
0,011
0,021
0,012
0,025
0,014
0,030
6. Asphalt
a. Smooth
b. Rough
0,013
0,016
0,013
0,016
7. Vegetal lining
0,030
0,500
Roughness parameters
for open surface flow
Manning n-value = 0,016
Site Description
•Station name: Orange-Riet
canal, Gariep to Douglas
•Roughness coefficient:
Manning n = 0,016
•Description of channel:
Concrete lined channel
3
Roughness parameters
for open surface flow
Manning n-value = 0,024
Site Description
•Station name:Columbia River at
Vernita, Washington
•Computed roughness coefficient:
Manning n = 0,024
•Description of channel: Bed
consists of slime-covered cobbles
and gravel. The straight and
steep left bank is composed of
cemented cobbles and gravel. The
gently sloping right bank consists
of cobbles set in gravel and is free
of vegetation.
Roughness parameters
for open surface flow
Manning n-value = 0,025 – 0,030
Site Description
•Station name: Canalized
river (gabion mattresses)
•Roughness coefficient:
Manning n = 0,025 (for
handpacked) to 0,030
(unselected material filled
without care)
Roughness parameters
for open surface flow
Manning n-value = 0,024
Site Description
•Station name:Columbia River at
Vernita, Washington
•Computed roughness coefficient:
Manning n = 0,024
•Description of channel: Bed
consists of slime-covered cobbles
and gravel. The straight and
steep left bank is composed of
cemented cobbles and gravel. The
gently sloping right bank consists
of cobbles set in gravel and is free
of vegetation.
4
Roughness parameters
for open surface flow
Manning n-value = 0,032
Site Description
•Station name:Salt River below
Stewart Mountain Dam, Arizona
•Computed roughness
coefficient: Manning n = 0,024
•Description of channel: Bed and
banks consist of smooth cobbles
100 to 250 mm in diameter,
average diameter about 150 mm.
A few boulders are as much as
450 mm in diameter.
Roughness parameters
for open surface flow
Manning n-value = 0,050
Site Description
•Station name:Clear Creek
near Golden, Colorado
•Computed roughness
coefficient:
Manning n = 0,050
•Description of channel: Bed
and banks are composed of
angular boulders
Roughness parameters
for open surface flow
Manning n-value = 0,075
Site Description
•Station name:Rock Creek near
Darby, Montana
•Computed roughness coefficient:
Manning n = 0,075
•Description of channel:
Bed consists of boulders;
d50 = 220 mm, d84 = 415 mm. Banks
are composed of boulders and have
trees and brush.
5
Roughness in South African Rivers
Absolute roughness – 150 mm
Site Description – Komati River
Roughness in South African Rivers
Absolute roughness – 10 mm
Site Description – Klip River (Vaal River)
Roughness in South African Rivers
Absolute roughness – 50 to 80 mm
Site Description – Vaal River
6
Roughness in South African Rivers
Absolute roughness – 160 mm
Site Description – Berg River upstream from Skuifraam Dam
Roughness in South African Rivers
Absolute roughness – 180 mm
Site Description – Berg River
Roughness in South African Rivers
Absolute roughness – 25 mm
Site Description – Matlabas River
7
Roughness in South African Rivers
Absolute roughness – 220 mm
Site Description – Limpopo River
Roughness in South African Rivers
Absolute roughness – 70 mm
Site Description – Nlaleti River
Roughness in South African Rivers
Absolute roughness – 10 mm
Site Description – Steenkool River
8
Roughness in South African Rivers
Absolute roughness – 60 mm ?
Site Description – KZN Schotborough
Roughness parameters for open
surface flow
Thank you
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