Delta Levee Investment Strategy: Risk Analysis Methodology

Levees in California’s Sacramento-San Joaquin Delta (Delta) reduce flood risk to people, property, natural resources, and infrastructure systems of statewide importance. Levee failure (such as a levee breach) could cause devastating flooding, potentially resulting in injury or loss of life, damage to property and infrastructure, disruption of water supply, and harm to environmental resources of statewide significance.

Currently, the State of California (State) does not have a long-term strategy to guide future investments of its limited funding for Delta levees. The 2009 Delta Reform Act directed the Delta Stewardship Council (Council) to lead the effort to develop and recommend priorities for State investments in Delta levees to reduce flood risks and to advance the coequal goals of water supply reliability and restoring the Delta ecosystem. Working closely with other agencies and the public, the Council is developing a Delta Levees Investment Strategy (DLIS) to establish priorities for State investments in the Delta levee system.

Risk, the potential for an unwanted outcome, is a principal factor in establishing priorities for State investments in levees. Specifically, risk is the probability that an adverse event (such as a flood or earthquake) will occur multiplied by the consequences of that event: Risk = Probability x Consequences

As presented in this report, the DLIS is based on a comprehensive methodology that quantifies risk by considering the threats to Delta levees and the assets protected by Delta levees. The DLIS also considers the multiple beneficiaries of Delta levee investments. Building on the results of previous Delta levee planning efforts, the DLIS collects and uses the best available existing data and information from numerous sources to evaluate risks to State interests in the Delta as defined by risk to public safety; property and infrastructure; water supply reliability; the Delta ecosystem; and the unique attributes of the Delta as an evolving place. The DLIS evaluates those risks under current and potential future conditions.

In early 2015, the DLIS team prepared an Interim Progress Report summarizing the work conducted during the first 9 months of the DLIS project. The report focused primarily on the methodologies planned for implementing the DLIS project to facilitate a “mid-course” review by an independent science panel, which completed its work in July 2015. Panel members offered comments for improvement and concluded that the “overall methodology of using existing data, identifying important metrics, aggregating results, and making comparisons across islands is conceptually sound” (Mitchell et al. 2015).

The DLIS project includes outreach to State and local agencies, concerned stakeholders in the Delta, knowledgeable technical resources, owners and operators of Delta infrastructure, and Delta leaders and residents. The final methodologies employed in the DLIS project are reported herein and incorporate the input from the independent science panel as well as feedback and comments resulting from the outreach efforts.