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Levee Vegetation Research Symposium 2012: Scientific progress informing a path forward

The Levee Vegetation Research Symposium 2012 was held on August 28-30, 2012, in Sacramento, California. Researchers, levee experts, and policy makers came from across the United States and abroad to participate, give presentations, be part of panel discussions, and to learn from each other. Approximately 350 people attended the 2012 symposium , with participants travelling from all corners of the US, as well as the United Kingdom, France, Austria, and Germany.

The 2012 symposium program was densely packed with presentations related to root architecture, new tree windthrow research, sophisticated seepage and stability modeling, and mammal burrow excavations. Some of the notable findings from the many excellent presentations included:

  1. Forensic analysis of past California levee failures/incident reports indicate that levee managers rarely, if ever, attributed significant performance incidents to the presence of woody vegetation (Dr. Sujan Punyamurthula, URS).
  2. Levee vegetation may have beneficial effects to levee seepage and stability in some cases (Dr. Andrew Simon and Dr. Natasha Bankhead, Cardno ENTRIX).
  3. Tree root pull-out resistance increases with root diameter but varies less among tree species than among levee sites (Dr. Johannes Wibowo and Dr. Maureen Corcoran, ERDC).
  4. Ground squirrels can burrow extensively into earthen levees within California’s Central Valley, even spanning the width of the levee (Dr. Diego Cobos-Roa, UC Berkeley).
  5. Burrowing mammal holes generally controlled the water flow patterns and early seepage locations in levees, and stratigraphy was very important in determining seepage patterns. No evidence of flow through decomposing roots was observed in seepage test (Dr. Michelle Shriro, UC Berkeley).
  6. Cottonwood tree roots can grow very long distances (100 to 150 feet) from their trunks and, where they are in close proximity to a levee, can penetrate into the interior of a levee by exploiting cracks in slurry walls (Dr. Leslie Harder, Jr., HDR). Cottonwoods had the longest roots of any species tested for pull-out resistance (Dr. Johannes Wibowo and Dr. Maureen Corcoran, ERDC).
  7. Computer models of hypothetical cases show that levee seepage is insensitive to effects of trees on the waterside slope or growing on the landside slope above the phreatic surface (water table). Trees growing at or near the landside toe have the greatest influence on seepage, but this effect is local to the tree root zone and depends on the assumed effect of the tree roots on soil permeability. If the tree has only moderate effects (increasing or decreasing soil permeability less than a factor of 10 to 100), then seepage effects are also very small. Even with significant local effects, isolated trees have almost no impact on large-scale seepage patterns such as global flow field, location of the seepage face, or pore pressure gradients (Dr. Fred Tracy, Dr. Chris Kees et al., ERDC).

For 2012 symposium proceedings and details, visit the 2012 Symposium Overview Website.

The Vegetation Challenge: A scientific and engineering examination of managing vegetation along California’s Central Valley levees that protect urban and rural areas from devastating floods

In April 2007, the U.S. Army Corps of Engineers (USACE) released a draft white paper regarding maintenance standards for vegetation on levees. The draft white paper requirements called for the removal of trees and shrubs along the waterside and landside of USACE program levees nationwide in order to meet compliance with existing USACE standards.

In response to the draft white paper, the Vegetation Challenge symposium was held August 28-29, 2007, to explore and assess the existing science, experiences, challenges, and policy solutions related to levee vegetation. The symposium brought together over 500 people from across the U.S. representing over 150 agencies from federal, State and local levels, as well as academic institutions and consulting and environmental firms.

While the 2007 symposium covered many subjects, there was not enough science available on which to inform or base policy. In order to fill this need, the CLVRP was formed and the first pilot study was conducted in 2008 to begin researching and filling the data gaps.

For 2007 symposium proceedings and details, visit the 2007 Symposium Overview Website.