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Current Projects and Research
Vegetation Assessment Working Group

A subgroup of the CLVRP, the Vegetation Assessment Working Group is utilizing the most recent research to recommend updated policies and procedures for ongoing monitoring and management of vegetation. The working group is composed of experts in geotechnical engineering, root architecture, tree windthrow, arboriculture, and ecology. Representatives from DWR’s inspections, maintenance, permitting, and policy groups are also members of the working group. Recommendations are being developed for the following:

  1. Levee Tree Assessment. Using the best available knowledge, the working group will develop and implement a field procedure for identifying and appropriately remediating trees that pose an unacceptable risk to levee integrity.
  2. Levee Vegetation Data Collection Procedures. These procedures are  composed of two parts:
    • Development of Standardized Field Data Collection Procedures. These procedures will serve as the foundation for future statistical studies by helping to standardize and improve levee maintaining agencies’ data collection.
    • Tree-fall Data Rapid Response. This procedure will guide the development of a mobile research team that will deploy soon after trees fall on levees to collect real-time data, including root pit measurements, soil compaction, soil saturation, tree species, and condition.  Collected data will be used in future analyses and research. 
Evaluation of the Incremental Probability of Levee Failure Due to the Effects of Woody Vegetation

UC Berkeley is conducting a study for the CLVRP that will use a peer-informed risk assessment methodology to evaluate the probability of failure of selected representative levee reaches in California’s Central Valley. The study will quantify the incremental probability of levee failure due to the effects of vegetation on the selected case study levees. Updated analytical tools will be used, and the most recent scientific knowledge from CLVRP studies and others will be incorporated.

This study follows up on the December 13, 2011, paper titled CLVRP California Levee Vegetation Research Needs/Priorities, (PDF: 319 KB) which identified the following aspect of woody vegetation impact on the levee performance and integrity that needs to be examined further:

What is the relative  [or conditional probability] of failure due to retention of existing vegetation, as compared to general failure modes such as seepage, underseepage, slope instability, erosion, or overtopping, and as compared to known risk factors such as encroachments, penetrations, sub-standard levee geometry, and animal burrows? How does vegetation change the probability of failure posed by these failure modes as influenced by such risk factors?

Using the best available science and collaboration with CLVRP managers and researchers,  the UC Berkeley team will incorporate woody vegetation factors into the probability aspect of the risk assessment methodology by rationally dividing each general failure mode (e.g., seepage, underseepage, slope stability, erosion, and overtopping) into risk factor subcategories based on important characteristics and overall effect on levee stability. Thus, the proposed research will only evaluate one component of the risk equation in terms of failure probability. It will not explicitly address the other component of risk: consequences of failure.

The study will quantify the change in probability of failure correlated with vegetation as a risk factor.  It will also identify the relative impact of vegetation on failure probability in comparison to risk factors (e.g., sub-standard levee geometry or animal burrows). A description and discussion of analytical methods and tools will support the findings.  In addition, a layperson-friendly report will be developed that highlights and summarizes significant findings.

Good science is often produced by highly specialized workers who focus intensely on a narrow class of problems. In order to apply their findings to levee policy or management, it is useful to synthesize their results so that the “forest” is visible - not just individual “trees.” Following the 2007 levee vegetation symposium, considerable research on levee vegetation was conducted by the CLVRP, the U.S Army Engineer Research Development Center (ERDC), and European scientists. Much of this work was reported at the 2012 symposium. In consultation with scientists studying various aspects of levee vegetation effects, the CLVRP has completed a synthesis of key findings of selected research (PDF: 8 MB) with emphasis on recent findings from 2007 to 2013. The synthesis report compiles the findings of the principal investigators who are separated by discipline and geography, as well as identify questions for which a consensus answer has emerged, data gaps, and issues that remain controversial or unresolved.

Journal Publication: Influence of Vegetation on Levee Performance in California

In 2011, URS examined more than 10,000 levee records and prepared a technical memorandum that summarized their findings, which were also presented at the 2012 symposium. These findings are now being formalized into a manuscript that will be submitted to the International Journal of Geoengineering Case Histories.