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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 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