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

BUILDING STRONG ® 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

BUILDING STRONG ® 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

BUILDING STRONG ® 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

BUILDING STRONG ® Research Tasks  Task 1 – Conduct an Extensive Literature review  Task 2 – Select Study Sites  Task 3 – Field Data Collection  Task 4 – Numerical Model Simulation

BUILDING STRONG ® 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

BUILDING STRONG ® Lake Providence, LA

BUILDING STRONG ® 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

BUILDING STRONG ® Field Data Collection Task 3a- Tree Properties and Identification Common NameScientific NameLocation Height (ft) Root Depth (in) Oregon ashFraxinus latifoliaPortland, OR3524 CedarThuja plicataBurlington, WA4030 Valley oakQuercus lobataSacramento, CA3542 AlderAlnus rubraBurlington, WA5025 Purpose – Trees and their properties were recorded both from the field and existing literature for slope stability and seepage model input

BUILDING STRONG ® Task 3. Field Data Collection b. Root Architecture – Geophysical  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 effect on the levee profile Pocket Levee, Sacramento, CA 3D Resistivity field results ERDC Veg Field Team, 2009 Pocket Levee, Sacramento, CA 3D Resistivity field results ERDC Veg Field Team, 2009 Pocket Levee, Sacramento, CA Seepage analysis ERDC Veg Model Team, 2010 Pocket Levee, Sacramento, CA Seepage analysis ERDC Veg Model Team, 2010 Cohesive Root Ball

BUILDING STRONG ® 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 effect on the levee profile (A,D): Photographs of tree roots (B): Ground penetrating radar (C): Electromagnetic digitization (E): Interpreted roots (F): Calibrated photographic modeling

BUILDING STRONG ® 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 effect on the levee profile

BUILDING STRONG ® Root Length = 30 ft Force = 3000 lbs Force/Root Length = 100 lbs/ft Root Length = 30 ft Force = 3000 lbs Force/Root Length = 100 lbs/ft 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

BUILDING STRONG ® Task 3. Field Data Collection d. Soil properties for slope stability and seepage Distance (ft) Elevation (ft) Sacramento, CA - Site B Station Seepage Analysis Geologic Model Burlington, WA Soil collection ERDC Veg Field Team 2010 Burlington, WA Soil collection ERDC Veg Field Team 2010  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

BUILDING STRONG ® 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

BUILDING STRONG ® Task 4. Numerical Model Simulation a. Modeling for Sensitivity Analysis K vegetation zone = 10 x K surrounding soil Purpose – explore the sensitivity of levee performance to changes in levee and vegetation parameters 2-D Seepage Root modeled as a defect

BUILDING STRONG ® Cohesive Root Ball Tree Weight Wind Load Ground Surface Root Reinforcement Failure Plane Task 4. Numerical Model Simulation a. Modeling for Sensitivity Analysis Phreatic Surface

BUILDING STRONG ® Task 4. Numerical Model Simulation b. Deformation Analysis Purpose – Improve understanding of tree root effects on levee performance Velocity Pressure head gradient

BUILDING STRONG ® 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

BUILDING STRONG ® Vidalia, LA 12 May 2011 Mississippi River Levee Mississippi River Flood stage 48 ft River ft Expected 64 ft