Vegetation effects on river hydraulics

Johannes J. (Joe) DeVriesDavid Ford Consulting Engineers, Inc.

Sacramento, CAjjdevries@ford-consulting.com

SAC05 D2P31 – RM 99.0L VIEW UPSTREAM AT UPSTREAM END DWR Photo

SAC05 D3P49 – RM 26.4R VIEW OF RIGHT BANK AT DOWNSTREAM END DWR Photo

SAC05 D3P28 – RM 44.7R VIEW OF BANK AT UPSTREAM END DWR Photo

SAC05 D5P6 – FEATHER RIVER RM 17.8L VIEW OF LEFT BANK DWR Photo

SAC05 D5P5 – FEATHER RIVER RM 7.0L VIEW UPSTREAM DWR Photo

Sacramento River below Hood

Sacramento River below Sacramento

Vegetation effects on river hydraulics

• Floodway conveyance, velocity response, & near-bank erosion

• Effects of vegetation-free areas next to levees

• Erosion around trees• Wind wave/boat wake effects

River hydraulics

• Water flows downhill - gravity

• Resistance to flow characterized by “boundary resistance”:• Manning's n-values express boundary

surface roughness• Manning’s equation quantifies

boundary resistance

Manning’s equation

21

32486.1 ASR

nQ =

Where Q = discharge (cubic feet per second) n = Manning's roughness coefficient R = hydraulic radius (ft) = P/A (wetted

perimeter/area) A = flow area (ft2) S = energy slope (energy loss per ft of

channel)Manning's n-values are used to characterize boundary surface roughness

What do typical Central Valley river sections look like?

• Very wide with respect to depth• River bed and overbanks form most of

wetted perimeter (“boundary”)• Banks (including levees) comprise small

fraction of wetted perimeter• Vegetation on levees typically a small

contribution to overall flow resistance

DWR Photo

Sacramento River below Hood

Representation of river cross sections

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Sac River section Plan: With vegetation 8/23/2007 River = Sacramento River Reach = Abv Sacramento RS = 78.75

Station (ft)

Ele

vatio

n (ft

)

Legend

WS PF 1

Ground

Levee

Bank Sta

.08 .033 .08

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Sac River section Plan: With vegetation 8/23/2007 River = Sacramento River Reach = Abv Sacramento RS = 78.75

Station (ft)

Ele

vatio

n (ft

)

Legend

WS PF 1

Ground

Levee

Bank Sta

.08 .033 .08

Undistorted plot of river section

How we typically view sections

Bear and Yuba River cross sections

Yuba River RS 0.83

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0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

Distance (ft)

Elev

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n (f

t)

B e a r R i v e r R S 3 . 4 4

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D is t a n c e ( f t )

Elev

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4H:1V distortion

4H:1V distortion

Feather and Sacramento River cross sections

F e a th e r R iv e r R S 3 3 .0

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0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0

D is t a n c e ( f t )

Elev

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n (ft

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S a c r a m e n t o R iv e r - R M 7 8 .7 5

- 1 00

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0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0

D is t a n c e ( f t )

Elev

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20H:1V distortion

4H:1V distortion

Tree & shrub corridors near levees

Wallace, Clifford, Dwyer and Hershey (1994)

Reduced conveyance due to levee vegetation

• Hydrologic Engineering Center’s River Analysis System (HEC-RAS) computer program used for analysis• Runs for typical cross sections with and

without levee vegetation• Comparisons based on change in

conveyance capacity for typical river section• HEC-RAS provides data on velocity

distributions in river cross section

Computed reduction in conveyance capacity

RiverReduction in conveyance, %

Feather River 1.6Bear River 5.6Yuba River 3.3Sacramento River 2.4

Change in conveyance of typical river sections on four Sacramento Valley rivers (vegetation on levees vs. no vegetation)

Effects of vegetation-free areas next to levees

• Increase in velocity near banks• Potential for increase in scour on levee

slopes• In general, higher roughness equates to

lower near-bank velocity and lower shear stresses

Effect of vegetation on velocity

Velocity profiles with submerged and unsubmerged vegetation (Fischenich, 2000)

Typical near-bank velocities

0 1000 2000 3000 4000 5000 6000 700040

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Feather River Section Plan: without veg 8/23/2007 River = Feather Reach = Typ section RS = 2

Station (ft)

Elev

atio

n (ft

)

Legend

WS PF 1

1 ft/s

2 ft/s

3 ft/s

4 ft/s

5 ft/s

Ground

Bank Sta

.035

.08 .035 .08 .08

n = 0.035 V = 1.5 ft/sec

n = 0.080 V = 0.6 ft/sec

Channel V = 4.1 ft/sec

SAC05 D2P21 – RM 116.5L VIEW DOWNSTREAM FROM UPSTREAM END DWR Photo

Bank n = 0.035 V = 1.5 ft/sec

Channel V = 4.1 ft/sec

SAC05 D1P17 – RM 163.0L VIEW DOWNSTREAM DWR Photo

Channel V = 4.1 ft/sec

Bank n = 0.080 V = 0.6 ft/sec

Wave runup on levees

• Estimates depend on wind wave characteristics, levee geometry, and levee surface conditions

• Wave runup elevations represent expected height of a breaking wave up the levee slope

Relative wave runup for different surfaces

• Grassed levees act as smooth surface• Shrub and tree vegetation will attenuate

waves to some degree• Vegetation also provides additional

erosion protection• Rock and riprap provide the greatest

degree of protection

Rock and riprap

• Based on Dutch experience, reduction factor in wave height for rock and riprap structures varies between 0.5 and 0.6

• Rock and riprap structures impede runup not only by slope roughness, but also by permeability of riprap and under layers placed over an impermeable slope

Estimating Irregular Wave Runup on Rough, Impermeable Slopes,Steven A. Hughes, USACE

Grass mat on levee surface

Grass mat provides high erosion resistance:

• Due to structure of root layer and not, as often thought, the thickness of layer of grass leaves and stems above ground

• Plant roots also important in keeping particles of soil together

Effects of rooting of grass mat

• Well-rooted grass mats can resist 1-2 days of sustained exposure to flood velocities • Typically very little sod erosion; even parts

above ground remain intact• Poorly rooted grass mats may fail within a few

hours • What is required is an actively growing,

perennial grass (sod forming)

Effectiveness of plant roots

• Plant roots keep particles in soil together• Very fine root hairs and fungal threads in soil

keep fine soil particles together, anchored within the substrate

• Coarser plant roots keep large and small particles together in a network

• Network of fine and coarse roots makes sod a strong, springy and flexible layer that can deform without tearing

Erosion resistance of grassland as dike covering, Technical Advisory Committee on Flood Defence, The Netherlands

Erosion around trees

• Hydraulic model study by USACE tested use of tree stands as a method for protecting levee embankments from waves caused by wind and boat-generated wakes

• 12 plans covered a range of thickness of tree stands, using 4-in.- & 12-in.-diameter tree stems, with and without defoliated branch systems

Levee Wave Wash Protection by Trees; Hydraulic Model Investigation. D. G. Markle, WES, USACE 1979

Erosion around trees (continued)

• For tree stems without branches, wave reduction was small (9 to 15% of wave height)

• For tree stems with branch systems, maximum wave attenuation was 45 percent, although the average was less

Levee Wave Wash Protection by Trees; Hydraulic Model Investigation. D. G. Markle, WES, USACE 1979

Case Study - Williams River in Australia

• Riparian vegetation was cleared with only small pockets of remnant vegetation

• Significant portion of river has either no trees or limited stands 1-2 trees wide, and little or no understory vegetation

• Later recommended a riparian reforestation program to restore a vegetative buffer and reduce bank erosion

Williams River Bank Erosion Study, Final Report, GHD Pty. Ltd., NSW

Recommendations – Williams River

• Provide bank resilience and roughness with mixture of deep rooted and shallow rooted plants to stabilize and bind soil

• Use shallow-rooted plants to provide ground cover to protect top of bank from potential erosion by flood flows

• Use trees and shrubs to decrease velocity of flood flows to reduce scour potential

• Implement a boating management plan to reduce impacts of boating practices and minimize wake erosion

Concerns – Williams River

• Restoring vegetation along the river would increase hydraulic resistance during flood events

• Could potentially increase flood heights and reduce flood protection

Pros and cons

• Levee armoring with properly designed woody material and vegetation will: • Slow floodwater velocity near banks• Dissipate wave energy• Reduce scouring potential• Increase soil shear strength

• Minor decreases in river conveyance capacity on wide, perennial channels

Benefits of vegetation near levees

Vegetation can be desirable and can positively affect the maintenance and stability of levees.• Levee designs should incorporate woody

materials as corridor plantings between the levee and river channel

• Protective vegetated cover on levee structure is desirable if inspection & flood fighting capability is maintained

Risks of removing vegetation

• Banks and levee slopes may be exposed to greater near-bank velocity and wind and boat wake wave scour

• Protection from wave runup may be reduced, especially on slopes without rock