Roughness of Floodplain PlantsUsing Plants to Improve Flood Management

What is Roughness?

TextureFrictionBlockage

Resistance toflow

Plants have roughness

So do theThings we Put in channels

We measure roughness using Mannings n –a number used in a whole bunch of hydraulic

analyses

(1/ )R2/3S 1/2n=(1/v)R2/3Sf1/2

R= hydraulic radiusSf= friction slopeV = water velocity

Manning n has no units.

Manning's n Coefficients for Open Channel Flow

Material Manning n Material Manning nNatural Streams Excavated Earth ChannelsClean and Straight 0.030 Clean 0.022

The fluid mechanics calculations website

Major Rivers 0.035 Gravelly 0.025Sluggish with Deep Pools 0.040 Weedy 0.030

Stony, Cobbles 0.035Metals FloodplainsBrass 0.011 Pasture, Farmland 0.035Cast Iron 0.013 Light Brush 0.050Smooth Steel 0.012 Heavy Brush 0.075Corrugated Metal 0.022 Trees 0.15Non-MetalsGlass 0.010 Finished Concrete 0.012Clay Tile 0.014 Unfinished Concrete 0.014Brickwork 0.015 Gravel 0.029Asphalt 0.016 Earth 0.025Masonry 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 0.025 0.030 0.033

b. same as above, but more stones and weeds 0.030 0.035 0.040

c. clean, winding, some pools and shoals 0.033 0.040 0.045

d. same as above, but some weeds and stones 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 densitysinuosity, velocity, water density, …

get assigned to roughness

O’Connor Lakes, Star Bend

Lots of variation:

Pre‐project velocities

Lots of variation:∆ velocity = ∆ roughness

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

H

Depth Control Weir

x= 47'

x =60'

V

2’

Flume tail tank

x =0'

x=33'

Pumps

6’

AC

x=19'

B

Point gage locations and

velocity measurement sections

Pumps

Velocity measurement locations in a cross-section

Flume width 5 ft

6 ft

Flume width 5 ft

6 ft

4 ft bin Width

8 ft

2 ft

4 ft bin Width

8 ft

2 ft

Water depth higher than 2 ft

Water depth lower than 2 ft

Velocity and Water Surface Profiles

V Scale X 19.5 33.5 472 Z 2.65 2.54 2.56

2.17 4.39 4.69 4.861.67 4.18 4.56 4.641.17 4.11 4.42 4.440.67 4.00 4.20 4.030.25 3.62 3.47 3.36

V (ft/s)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

0 10 20 30 40 50 60 70

X [ft]

Elev

atio

n [f

t]

Water Surface Flume Top Elevation Bottom Elevation

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

V= 2 ft/s

Flow Regime S15

STREAMBED BARE SOIL SAMPLE PREPARATION

Sandbar Willow1st replicate

Sandbar Willow3rd replicate

3

4

5V|x=16ftV|x=34ftV|x=47ft

3

4

5V|x=16ftV|x=34ftV|x=47ft

V=1.5 ft/s H=1.5 ft V=3.0 ft/s H=1.5 ft

Velocity Profile Sandbar Willow

0

1

2

0 2 4 6Velocity (ft/s)

Z(ft)

0

1

2

0 2 4 6Velocity (ft/s)

Z(ft)

3

4

5V|x=16ftV|x=34ftV|x=47ft

3

4

5V|x=16ftV|x=34ftV|x=47ft

V=1.5 ft/s H=5 ft V=3.0 ft/s H=5 ftVelocity Profile Sandbar Willow

0

1

2

0 2 4 6Velocity (ft/s)

Z(ft)

0

1

2

0 2 4 6Velocity (ft/s)

Z(ft)

3

4

5V|x=16ftV|x=34ftV|x=47ft

3

4

5V|x=16ftV|x=34ftV|x=47ft

V=4.5 ft/s H=3 ft V=6.0 ft/s H=3 ft

Velocity Profile Sandbar Willow

0

1

2

0 2 4 6Velocity (ft/s)

Z(ft)

0

1

2

0 2 4 6Velocity (ft/s)

Z(ft)

0.050.060.070.080.09

ng's

n

Bare Soil

Willow1

Willow2

Willow3

Roughness Coefficient vs Reynolds Number

Sandbar Willow

0.000.010.020.030.04

0 200,000 400,000 600,000Re

Man

ni Linear (Bare Soil)

Linear (Willow1)

Linear (Willow2)

Linear (Willow3)

3

4

5

h (ft

)

V|x=16ftV|x=34ftV|x=47ft

Velocity Profile Mulefat

3

4

5

h (ft

)

V|x=16ftV|x=34ftV|x=47ft

V=4.5 ft/s H=3ft V=6 ft/s H=3 ft

0

1

2

0 2 4 6

Velocity (ft/s)

Dep

th

0

1

2

0 5 10

Velocity (ft/s)

Dep

th

0.06

0.08

0.10

0.12

ning

's n

Bare SoilWillow (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) )

Comparative Roughness: Mulefat & Sandbar Willow

0.00

0.02

0.04

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

Re

Man

3

4

5

6

ht (f

t)

V|x=16ftV|x=34ftV|x=47ft

V=6 ft/s Ht=3.4ft

4

5

6

t (ft

)

V|x=16ftV|x=34ftV|x=47ft

V=3 ft/s Ht=5ft

Velocity Profile California Blackberry

0

1

2

3

0.0 2.0 4.0 6.0

Velocity (ft/s)

Hei

gh

0

1

2

3

0.0 2.0 4.0 6.0

Velocity (ft/s)

Hei

ght

0 08

0.10

0.12

0.14

0.16

ng's

n

Blackberry(Mar)Blackberry(Apr)Blackberry(May)Willow (March)Willow (April)Willow (May)Mule Fat (Oct-Nov) Mule Fat (Nov-Dec)Mule Fat (January)Mule Fat (February)Bare SoilLinear (Blackberry(Mar))Linear (Blackberry(Apr))Linear (Blackberry(May))

Roughness coefficients vs Reynolds NumberCalifornia Blackberry

0.00

0.02

0.04

0.06

0.08

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

Man

nin Linear (Blackberry(May))

Velocity profile Wildrose

3

4

5

6

dept

h (f

t)

V|x=16ftV|x=34ftV|x=47ft

3

4

5

6

r de

pth

(ft)

V|x=16ftV|x=34ftV|x=47ft

V=4.5ft/s H=3ft V=6ft/s H=3.4ft

0

1

2

0 2 4 6 8

Velocity (ft/s)

Wat

er

0

1

2

0 2 4 6 8

Velocity (ft/s)

Wat

er

0.10

0.12

0.14

0.16s

nBlackberry(Mar)Blackberry(Apr)Blackberry(May)Willow (March)Willow (April)Willow (May)Mule Fat (Oct-Nov) Mule Fat (Nov-Dec)Mule Fat (January)Mule Fat (February)Bare SoilWildrose#1(Jun)Wildrose#2(Jun)Wildrose#3(Jul)Linear (Blackberry(Mar))Linear (Blackberry(Apr))Linear (Blackberry(May))Linear (Wildrose#1(Jun))Li (Wild #2(J ))

Mannings Coefficient vs Reynolds NumberRose and Blackberry

0.00

0.02

0.04

0.06

0.08

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

Man

ning

's Linear (Wildrose#2(Jun))Linear (Wildrose#3(Jul))

Erosion Summary

2

3Erosion (in)

mulefat

Bare soil

3 51

1

Velocity (ft/sec)

willow

rose

Swim Response to Sandbar Willow

○ = TBF Swim Chamber● = MO2■ = TBF Flume

(1 5 ft/s)

(3.0 ft/s)

(0.5 ft/s)

(1.5 ft/s)

As built – velocity A Mosaic of Plants