Using Plants to Improve Flood Management - Floodplain Management

Roughness of Floodplain Plants 

Using Plants to Improve Flood Management

What is Roughness 

Texture 

Friction 

Blockage 

Resistance to 

flow

Plants have roughness

So do the 

Things we 

Put in channels

We measure roughness using Mannings n – 

a number used in a whole bunch of hydraulic 

analyses 

R= hydraulic radius 

S f = friction slope 

V = water velocity 

n=(1/v)R 2/3 S 1/2 

f

Manning's n Coefficients for Open Channel Flow 

The fluid mechanics calculations website 

Manning n has no units. 

Material Manning n Material Manning n 

Natural Streams 

Excavated Earth Channels 

Clean and Straight 0.030 Clean 0.022 

Major Rivers 0.035 Gravelly 0.025 

Sluggish with Deep Pools 0.040 Weedy 0.030 

Stony, Cobbles 0.035 

Metals 

Floodplains 

Brass 0.011 Pasture, Farmland 0.035 

Cast Iron 0.013 Light Brush 0.050 

Smooth Steel 0.012 Heavy Brush 0.075 

Corrugated Metal 0.022 Trees 0.15 

Non-Metals 

Glass 0.010 Finished Concrete 0.012 

Clay Tile 0.014 Unfinished Concrete 0.014 

Brickwork 0.015 Gravel 0.029 

Asphalt 0.016 Earth 0.025 

Masonry 0.025 Planed Wood 0.012 

Unplaned Wood 0.013

Type of Channel and Description 

Minimum Normal Maximum 

Natural streams - minor streams (top width at floodstage < 100 ft) 

1. Main Channels 

a. clean, straight, full stage, 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 

0.025 0.030 0.033 

0.030 0.035 0.040 

0.033 0.040 0.045 

0.035 0.045 0.050 

e. same as above, lower stages, more ineffective 

slopes and sections 

0.040 0.048 0.055 

f. same as "d" with more stones 0.045 0.050 0.060 

g. sluggish reaches, weedy, deep pools 0.050 0.070 0.080 

h. very weedy reaches, deep pools, or floodways 

with heavy stand of timber and underbrush 

0.075 0.100 0.150 

2. Mountain streams, no vegetation in channel, banks usually steep, trees and brush 

along banks submerged at high stages 

a. bottom: gravels, cobbles, and few boulders 0.030 0.040 0.050 

b bottom: cobbles with large boulders 

0.040 0.050 0.070

Mannings n – average of all textures, 

but used as a model fitting parameter. 

Inaccuracy in other model parameters: 

Shape, channel form, grade, slope, 

sinuosity, velocity, water density, … 

get assigned to roughness

O’Connor Lakes, Star Bend 

Lots of variation: 

∆ velocity = ∆ roughness 

Pre‐project velocities

The need for improving safety and 

ecosystem management prompted us 

to look at the role some common 

plants play in creating roughness. 

Plants selected because they are adapted to 

flooding, are common in our floodways, and 

provide good habitat value

Mulefat 

Wild Rose 

Sandbar willow 

California Blackberry

x= 47' 

x=33' 

x=19' 

Flume 

tail tank 

H 

V 

6’ 

Depth 

Control 

Weir 

x =60' 

C 

B 

A 

2’ 

x =0' 

Pumps 

Point gage locations 

and 

velocity measurement sections

Velocity measurement locations in a cross-section 

Flume width 5 ft 

Flume width 5 ft 

8 ft 

6 ft 

8 ft 

6 ft 

2 ft 

2 ft 

4 ft bin Width 

4 ft bin Width 

Water depth 

higher than 2 ft 

Water depth 

lower than 2 ft

Velocity and Water Surface Profiles 

V Scale X 19.5 33.5 47 

2 Z 2.65 2.54 2.56 

2.17 4.39 4.69 4.86 

1.67 4.18 4.56 4.64 

V (ft/s) 1.17 4.11 4.42 4.44 

0.67 4.00 4.20 4.03 

0.25 3.62 3.47 3.36 

8.00 

7.00 

6.00 

Flow Regime S15 

Elevation [ft] 

5.00 

4.00 

3.00 

2.00 

1.00 

V= 2 ft/s 

0.00 

0 10 20 30 40 50 60 70 

X [ft] 

Water Surface Flume Top Elevation Bottom Elevation 

Velocity at P#2 Velocity at P#3 Velocity at P#4

STREAMBED BARE 

SOIL SAMPLE 

PREPARATION

Sandbar Willow 

1 st replicate


3 rd replicate

Velocity Profile Sandbar Willow 

5 

4 

V=1.5 ft/s H=1.5 ft 

5 

V|x=16ft 

V|x=34ft 

V|x=47ft 4 

V=3.0 ft/s H=1.5 ft 

V|x=16ft 

V|x=34ft 

V|x=47ft 

3 

3 

Z(ft) 

Z(ft) 

2 

2 

1 

1 

0 

0 2 4 6 

Velocity (ft/s) 

0 

0 2 4 6 

Velocity (ft/s)


5 

4 

V=1.5 ft/s H=5 ft 

5 

V|x=16ft 

V|x=34ft 

V|x=47ft 4 

V=3.0 ft/s H=5 ft 

V|x=16ft 

V|x=34ft 

V|x=47ft 

3 

3 

Z(ft) 

Z(ft) 

2 

2 

1 

1 

0 

0 2 4 6 


0 

0 2 4 6 

Velocity (ft/s)


5 

4 

V=4.5 ft/s H=3 ft 

5 

V|x=16ft 

V|x=34ft 

V|x=47ft 4 

V=6.0 ft/s H=3 ft 

V|x=16ft 

V|x=34ft 

V|x=47ft 

3 

3 

Z(ft) 

Z(ft) 

2 

2 

1 

1 

0 

0 2 4 6 


0 

0 2 4 6 

Velocity (ft/s)

Roughness Coefficient vs Reynolds Number 


ng's n 

Manni 

0.09 

0.08 

0.07 

0.06 

0.05 

0.04 

0.03 

0.02 

0.01 

0.00 

0 200,000 400,000 600,000 

Bare Soil 

Willow1 

Willow2 

Willow3 

Linear (Bare Soil) 

Linear (Willow1) 



Re

Velocity Profile Mulefat 

V=4.5 ft/s H=3ft 

V=6 ft/s H=3 ft 

5 

4 

V|x=16ft 

V|x=34ft 

V|x=47ft 

5 

4 

V|x=16ft 

V|x=34ft 

V|x=47ft 

Depth 

(ft) 

3 

2 

Depth 

(ft) 

3 

2 

1 

1 

0 

0 2 4 6 


0 

0 5 10 

Velocity (ft/s)

Comparative Roughness: Mulefat & Sandbar Willow 

Man ning's n 

0.12 

0.10 

0.08 

0.06 

Bare Soil 

Willow (March) 

Willow (April) 

Willow (May) 

Mule Fat (Nov-Dec) 

Mule Fat (Oct-Nov) 

Linear (Bare Soil) 

Linear (Willow (March)) 

Linear (Willow (April)) 

Linear (Willow (May)) 

Linear (Mule Fat (Nov-Dec)) 

Linear (Mule Fat (Oct-Nov) ) 

0.04 

0.02 

0.00 

0 100,000 200,000 300,000 400,000 500,000 600,000 700,000 

Re

Velocity Profile California Blackberry 

V=3 ft/s Ht=5ft 

V=6 ft/s Ht=3.4ft 

6 

5 

V|x=16ft 

V|x=34ft 

V|x=47ft 

6 

5 

V|x=16ft 

V|x=34ft 

V|x=47ft 

4 

4 

Height 

(ft) 

3 

Heigh ht (ft) 

3 

2 

2 

1 

1 

0 

0.0 2.0 4.0 6.0 


0 

0.0 2.0 4.0 6.0 

Velocity (ft/s)

Roughness coefficients vs Reynolds Number 

California Blackberry 

Mannin ng's n 

0.16 

0.14 

0.12 

0.10 

008 0.08 

0.06 

Blackberry(Mar) 

Blackberry(Apr) 

Blackberry(May) 



Willow (May) 



Mule Fat (January) 

Mule Fat (February) 

Bare Soil 

Linear (Blackberry(Mar)) 

Linear (Blackberry(Apr)) 

Linear (Blackberry(May)) 

0.04 

0.02 

0.00 

0 100,000 200,000 300,000 400,000 500,000 600,000 700,000 

Re

Velocity profile Wildrose 

V=4.5ft/s H=3ft 

V=6ft/s H=3.4ft 

6 

5 

V|x=16ft 

V|x=34ft 

V|x=47ft 

6 

5 

V|x=16ft 

V|x=34ft 

V|x=47ft 

depth (ft) 

Water 

4 

3 

2 

Water 

depth (ft) 

4 

3 

2 

1 

1 

0 

0 2 4 6 8 


0 

0 2 4 6 8 

Velocity (ft/s)

Mannings Coefficient vs Reynolds Number 

Rose and Blackberry 

Manning's 

n 

0.16 

0.14 

0.12 

0.10 

0.08 

Blackberry(Mar) 

Blackberry(Apr) 

Blackberry(May) 



Willow (May) 



Mule Fat (January) 

Mule Fat (February) 

Bare Soil 

Wildrose#1(Jun) 

Wildrose#2(Jun) 

Wildrose#3(Jul) 

Linear (Blackberry(Mar)) 

Linear (Blackberry(Apr)) 

Linear (Blackberry(May)) 

Linear (Wildrose#1(Jun)) 

Linear (Wildrose#2(Jun)) 

Linear (Wildrose#3(Jul)) 

0.06 

0.04 

0.02 

0.00 

0 100,000 200,000 300,000 400,000 500,000 600,000 700,000 

Re

Erosion Summary 

Erosion (in) 

3 

Bare soil 

2 

mulefat 

1 

rose 

willow 

1 

3 5 

Velocity (ft/sec)

Swim Response to Sandbar Willow 

○ = TBF Swim Chamber 

● = MO 2 

■ =TBF Flume 

(1.5 ft/s) 

(3.0 ft/s) 

(0.5 ft/s)

A Mosaic of Plants 

As built – velocity

Using Plants to Improve Flood Management - Floodplain Management

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