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Research Questions

  1. Which Delta islands are most at risk with respect to lives, property, water supply, and other considerations due to island flooding from hydrologic and seismic events?
  2. How would investments designed to improve levee performance reduce risks and which investments would be most cost effective?
  3. What priorities for levee investments are arrived at using a decision support tool to support deliberations over analysis related to flood risks and investment priorities?

The Sacramento–San Joaquin Delta comprises over 100 islands and tracts northeast of the San Francisco Bay. Most are below sea level and are protected by levees, whereas some are tidal or above sea level. The Delta supports a unique ecosystem, communities, and agricultural land, and plays a key role in the California water distribution system. The flood risks facing the Delta are complex and varied. Some islands are at high risk from flood damage to human life or structures and property, whereas others are at risk of impacting the State's water supply, flooding important habitat, or compromising the Delta's historic towns, prime agricultural land, or public roadways. The possible investments to mitigate these risks are numerous, and they will affect Delta risks differently.

The Delta Stewardship Council commissioned the development of a risk modeling framework and decision support tool (DST) to aid in the formulation of a Delta Levees Investment Strategy. This document first reviews the risk analysis methodology used to evaluate the probability of flooding and risks to life, property, water supply, habitat, and Delta as Place under several future time periods and scenarios. It next describes in detail the functions of the interactive visualization function of the DST, including Assessing Risk, Ranking Islands by Risk, and Developing Levee Investment Portfolios.

Key Findings

The decision support tool aided the Delta Stewardship Council in developing a Delta Levees Investment Strategy through four key steps.

  • It compiled and displayed information about vulnerable assets throughout the Delta.
  • It estimated the probability of flooding and the associated risks to lives, property, water supply, habitat, and other Delta assets.
  • It provided interactive visualizations to support deliberations over how to weigh different types of risks to define high-risk islands.
  • It assimilated and displayed results of analyses of how different levee investments would reduce risk.

The Delta Levees Investment Strategy analysis seeks to prioritize investments based on how they meet important goals for the state — to reduce flood risks to lives and property, to water supply reliability, and to Delta ecosystem function — and to do so efficiently.

  • The foundation for this analysis is estimates of flooding probability due to seismic and hydrologic events for each leveed island and tract in the Delta.
  • These flooding calculations are then combined with estimates of different consequences of flooding (e.g., to lives, property, water supply, habitat, and Delta as Place) using a classical "probability x consequence" framing to calculate risk.

Recommendations

  • The decision support tool and supporting risk models were developed to provide continuing support of the Council for setting levee investment priorities. In the future, this could be done by refining the risk estimates through improved data and reconsidering how the islands compare in terms of the different types of risks.
  • This type of investment portfolio analysis could be conducted with a more comprehensive set of alternative levee investments and other risk reduction options to develop a balanced portfolio of cost-effectiveness actions to reduce risk.
  • The decision support tool can also evaluate investments that would improve the performance of the levees, and include other desirable features such as ecosystem enhancements.

Table of Contents

  • Chapter One

    Introduction

  • Chapter Two

    Summary of Risk Evaluation Methodology

  • Chapter Three

    Overview and Use of DST

  • Chapter Four

    Conclusion

Research conducted by

The research described in this report was sponsored by the Delta Stewardship Council and conducted by the Community Health and Environmental Policy Program within RAND Social and Economic Well-Being.

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