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Summary of the Decision Support Tool for the San Francisco Bay-Delta Levees Investment Strategy

Core Content

The Sacramento-San Joaquin Delta is a critical region in California, comprising over 100 islands and tracts northeast of San Francisco Bay. Most of these islands are below sea level and protected by levees, but some are tidal or above sea level. The Delta supports a unique ecosystem, communities, and agricultural land, and is central to the state's water distribution system.

Flood risks in the Delta are complex and vary across islands. These risks include potential harm to human life, property, water supply, habitat, and the Delta as Place (DAP), which encompasses legacy towns, prime agricultural land, and public roadways. The Delta Levees Investment Strategy (DLIS) was initiated by the Delta Stewardship Council (Council) to develop a transparent strategy for prioritizing state-funded levee investments. This aligns with California Water Code requirements, which emphasize reducing risks to people, property, and state interests through effective emergency preparedness, land use planning, and strategic investments.

To support this process, the Decision Support Tool (DST) was developed. It provides a framework for evaluating flood risk and assessing the impact of levee investments across different timeframes and scenarios. The DST enables the Council and stakeholders to explore trade-offs and make informed decisions based on quantitative analysis.

Main Points

Key Objectives of the DLIS Project

• Develop a transparent strategy for prioritizing levee investments.
• Reduce flood risks to human life, property, water supply, habitat, and the Delta as Place.
• Ensure efficient use of state resources to achieve maximum risk reduction.

Components of the DST

1. Instructions and Guide – Explains how to navigate and use the DST.
2. Islands and Vulnerable Assets – Identifies and maps vulnerable assets such as population, property, and infrastructure.
3. Assessing Risk (Risk Maps) – Provides visualizations of flood risk across different metrics.
4. Identify High Risk Islands – Highlights islands with the greatest risk based on individual metrics.
5. Ranking Islands by Risk – Offers a composite risk score and tiered rankings based on multiple risk factors.
6. Levee Investments and Change in Risk – Shows how different investments affect flood risk.
7. Draft Delta Levees Investment Priorities – Presents a preliminary prioritized list of islands for investment.
8. Developing a Portfolio of Levee Investments – Enables the selection of investment combinations that best align with state interests and constraints.

Risk Evaluation Methodology

• Flood Risk Metrics: The DST uses a set of metrics to evaluate risk across four key interests:
  • Life and Property: Measured by Expected Annual Fatalities (EAF) and Expected Annual Damage (EAD).
  • Water Supply: Measured by the probability of flooding of islands important to water conveyance and infrastructure.
  • Ecosystem Function: Measured by the expected flooding of high-value non-tidal habitat.
  • Delta as Place (DAP): Measured by the probability of flooding of islands with legacy towns, agricultural land, and public roadways.
• Risk Calculation Timeframes:
  • Current Conditions (2012 sea levels, 2010 population, current asset inventories).
  • Near-Term Future (2030, using two sea level rise estimates and one population and land use projection).
  • Long-Term Future (2050, using two sea level rise estimates and one population and land use projection).
• Uncertainty Handling: The DST accounts for uncertainty in future risk drivers and allows users to define assumptions and weights for different metrics to explore trade-offs.

Levee Investment Strategy

• The DST was used to develop a three-tier prioritization list of islands for levee investment.
• It also supports the creation of investment portfolios based on cost-effectiveness and risk metrics.
• The DST enables the Council to consider different investment scenarios, such as bringing levees up to the PL84-99 guidelines, which define minimum levee height and slope stability but not flood recurrence intervals.

Key Information

• The DST is an interactive tool available on a website, allowing users to explore flood risk data and investment options.
• The PL84-99 standard is a minimum requirement for levee rehabilitation and is used to assess eligibility for federal funding.
• The DST uses a multicriteria decision analysis approach to balance competing interests and constraints.
• The tool is flexible and can incorporate new data as it becomes available, such as updated levee stability, sea level rise projections, and project costs.

Conclusion

The DST is a critical component of the DLIS project, offering a structured and transparent method for evaluating flood risks and prioritizing levee investments. It supports the Council in making informed, evidence-based decisions that align with state interests and policy goals. The DST's use of risk metrics, visualizations, and portfolio analysis provides a comprehensive framework for addressing the complex and uncertain flood risks in the Delta.