US Army Corps of Engineers BUILDING STRONG ® Initial Research into the Effects of Woody Vegetation on Levees Maureen K. Corcoran U.S. Army Engineer Research.

US Army Corps of Engineers

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Initial Research into the Effects of Woody Vegetation on Levees

Maureen K. Corcoran

U.S. Army Engineer Research and Development Center (ERDC)

Vicksburg, MS

Presented to the Association of State Floodplain Managers, Louisville, KY

18 May 2011

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Background► Feb 2007 – Nationwide levee inspection identified

maintenance deficiencies on 122 levee systems, many due to woody vegetation closer than 15 ft from levee toe

► Aug 2007 – HQCorps engaged ERDC to perform extensive literature review of the effects of woody vegetation on the structural integrity of levees

► Dec 2007 – Literature review identified research gaps leading to present effort

► Sept 2009 – ERDC begins initial research

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Purpose

What is driving the research?The need for scientific support to quantify effects

of woody vegetation on levees

Quantify the impact of woody vegetation on levee performance using scientific and engineering methods

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Focus of Initial Research

Focus on two processes within failure mechanisms: ►Seepage analysis: Initiation of internal erosion ►Slope stability analysis: Simple, deep-seated

slope stability

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Research Tasks

Task 1 – Conduct an Extensive Literature review

Task 2 – Select Study Sites Task 3 – Field Data Collection Task 4 – Numerical Model Simulation

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Approach Task 1- Conduct an extensive literature review

► Compilation of documents, government reports, international guidance, and journal articles concerning woody vegetation on levees

Task 2- Select study sites► Considered levee geometry, soils, geology, geographic setting,

geotechnical reports, and tree species

Site Characterization Sacramento, CA Burlington, WA Portland, OR Albuquerque, NM

Site Characterization Sacramento, CA Burlington, WA Portland, OR Albuquerque, NM

Site Assessments Danville, PA Boca Raton, FL New Orleans, LA Lake Providence, LA Lewisville, TX Vicksburg, MS

Site Assessments Danville, PA Boca Raton, FL New Orleans, LA Lake Providence, LA Lewisville, TX Vicksburg, MS

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Lake Providence, LA

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Approach Task 3- Field Data Collection

• Tool selection based on published research and consultation with experts in academia and private industry

a. Tree properties and identification• Recorded tree species and their specific properties• Used existing literature to determine general root extent

b. Root architecture• Geophysical and in situ root mapping were used to define root system

c. Root reinforcement for slope stability• The strengthening effect of root systems were determined from a root

pull-out apparatus that was applied in the field to measure tensile strength of roots

d. Soil properties• Existing geotechnical reports contributed to representative cross sections

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Field Data Collection Task 3a- Tree Properties and Identification

Common Name Scientific Name LocationHeight

(ft)Root Depth

(in)

Oregon ash Fraxinus latifolia Portland, OR 35 24

Cedar Thuja plicata Burlington, WA 40 30

Valley oak Quercus lobata Sacramento, CA 35 42

Alder Alnus rubra Burlington, WA 50 25

Purpose – Trees and their properties were recorded both from the field and existing literature for slope stability and seepage model input

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Task 3. Field Data Collectionb. Root Architecture – Geophysical

Defines root ball for input into seepage and slope stability models

Defines root ball for input into seepage and slope stability models

Purpose – to better understand the interaction of roots within the soil regime and the subsequent effecton the levee profile

Depth = 1.25m

Pocket Levee, Sacramento, CA3D Resistivity field results ERDC Veg Field Team, 2009

Pocket Levee, Sacramento, CA3D Resistivity field results ERDC Veg Field Team, 2009

Pocket Levee, Sacramento, CASeepage analysisERDC Veg Model Team, 2010

Pocket Levee, Sacramento, CASeepage analysisERDC Veg Model Team, 2010

Cohesive Root Ball

Cohesive Root Ball

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Task 3. Field Data Collection b. Root Architecture – In Situ Root Mapping

Purpose – to better understand the interaction of roots within the soil regime and the subsequent effecton the levee profile

(A,D): Photographs of tree roots (B): Ground penetrating radar (C): Electromagnetic digitization (E): Interpreted roots (F): Calibrated photographic modeling

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Task 3. Field Data Collection b. Root Architecture – In Situ Root Mapping (LiDAR)

Purpose – to better understand the interaction of roots within the soil regime and the subsequent effecton the levee profile

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Root Length = 30 ftForce = 3000 lbsForce/Root Length = 100 lbs/ft

Root Length = 30 ftForce = 3000 lbsForce/Root Length = 100 lbs/ft

Slope Stability Model (UTEXAS4)Slope Stability Model (UTEXAS4)

Field Data Collection Task 3c - Root Reinforcement for Slope Stability

Purpose – The strengthening effect of root systems will be determined from a root pull-out apparatus that will be applied in the field to measure tensile strength of roots

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Task 3. Field Data Collection d. Soil properties for slope stability and seepage

Distance (ft)

Ele

vatio

n (f

t)

Sacramento, CA - Site B Station 292+60 Seepage Analysis Geologic Model

Burlington, WASoil collectionERDC Veg Field Team 2010

Burlington, WASoil collectionERDC Veg Field Team 2010

Soil properties used in levee profile for input into seepage and slope stability models

Soil properties used in levee profile for input into seepage and slope stability models

Purpose – to address the effects of roots on permeability and soil moisture

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Task 4. Numerical Model Simulation

Analyses used two-dimensional seepage and slope stability codes Representative levee cross sections and a relationship between

factor of safety and flood level were established► Task 4a Modeling for sensitivity analysis► Task 4b Deformation analysis

Purpose – used to address the issue of whether the presence of trees increase the stability of levees by reinforcing the slopes or decrease the stability by exacerbating the effects of seepage

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Task 4. Numerical Model Simulation a. Modeling for Sensitivity Analysis

Kvegetation zone = 10 x Ksurrounding soil

Purpose – explore the sensitivity of levee performance to changes in levee and vegetation parameters

2-D Seepage

Root modeled as a defect

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Cohesive Root Ball

Tree Weight

Wind LoadGround Surface

Root Reinforcement

Failure Plane

Task 4. Numerical Model Simulation a. Modeling for Sensitivity Analysis

Phreatic Surface

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Task 4. Numerical Model Simulation b. Deformation Analysis

Purpose – Improve understanding of tree root effects on levee performance

Velocity Pressure head gradient

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Products

Report documenting methods used in quantifying the effects of woody vegetation on levee integrity and includes:► Results quantified as a reduction in the factor of

safety from seepage and slope instability during high water events

► Approach developed and tested by ERDC from field data and geotechnical models at selected sites

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Vidalia, LA12 May 2011

Mississippi River LeveeMississippi River

Flood stage 48 ftRiver 59.31 ftExpected 64 ft