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Appendixes A, B, C, D, and E

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Research Questions

  1. How might the Pittsburgh metropolitan region's vulnerability to future stormwater runoff and sewer overflow change with a changing climate and population patterns?
  2. To what extent could combined sewer overflows (CSOs) and sanitary sewer overflows (SSOs) be reduced using innovative approaches, either in current conditions or with future change?
  3. How do green stormwater infrastructure (GSI) and other "source reduction" solutions compare with traditional "gray" infrastructure solutions in terms of overflow reduction benefits, costs, and cost-effectiveness?
  4. What trade-offs must be resolved to implement improved stormwater management across the region?

Cities and larger metropolitan regions are at the forefront of efforts to reduce greenhouse gas emissions, understand and respond to current and future effects of climate change, and develop resilience and adaptation capacity in response to climate change.

This study focused on the ongoing challenge of stormwater management in the Pittsburgh metropolitan region. The city of Pittsburgh and other municipalities in Allegheny County, Pennsylvania, face significant challenges in meeting water-quality requirements and upgrading their aging and inadequately sized regional combined sewer system, a problem that could grow with future climate, population, or land-use changes. The research in this report provides an independent study of the growing stormwater problem and discusses potential long-term solutions using new analytical approaches developed by RAND. Specifically, the study applied a participatory decision support framework and method for improving decisions under deep uncertainty called Robust Decision Making (RDM). RDM is an iterative, quantitative decision analytic framework that brings together experts and decisionmakers to help identify the full extent of a challenge, as well as potentially robust strategies to address it. The framework enables the participants to characterize the vulnerabilities of proposed strategies and evaluate the trade-offs among them.

This research is intended to support improved stormwater, wastewater, and climate resilience planning in the Pittsburgh region. The intended audience includes local government agencies and regional authorities addressing this challenge, local stakeholders engaged in stormwater and wastewater planning, state and federal regulators, and planners in other cities facing similar challenges with aging infrastructure and climate uncertainty.

Key Findings

Analysis of Future Vulnerability

  • Simulations of the recent past suggest that sewer overflow volumes are up to 15 percent higher than previously estimated.
  • Future rainfall, population, and land-use changes could further increase overflow volumes.

Strategies to Reduce Future Overflow

  • Expanding wastewater treatment plant capacity or cleaning existing deep interceptors could represent low-regret, near-term options.
  • Large-scale investments in source reduction, or combining source reduction with treatment expansion and/or interceptor cleaning, could help reduce sewer overflow, but with a wide range of uncertainty regarding cost-effectiveness and relative strategy performance.
  • None of these combined strategies, even including substantial source reduction investments, fully eliminates sewer overflows in current or plausible future conditions.

GSI Cost-Effectiveness

  • GSI strategies, evaluated in isolation, yield poor cost-effectiveness for overflow reduction under commonly used rainfall, capital cost, and GSI performance assumptions.
  • GSI and other source reduction strategies are more cost-effective in higher rainfall scenarios, and could provide hedging value against future climate change.

Recommendations

  • Plausible future change should inform near-term planning and design for stormwater and wastewater infrastructure investments.
  • Evaluations of GSI should consider co-benefits beyond sewer overflow reduction, such as such as flood risk reduction, ecosystem services, and economic development.
  • Source reduction could help reliably reduce overflows, but additional research is needed to fully define a long-term, adaptive stormwater and wastewater management strategy.

Table of Contents

  • Chapter One

    Introduction

  • Chapter Two

    Regional Stormwater Planning in Allegheny County

  • Chapter Three

    Future Sewer Overflow Vulnerability

  • Chapter Four

    Comparing Source Reduction and Wastewater Policy Levers

  • Chapter Five

    Robust Decision Making Strategy Comparison

  • Chapter Six

    Key Findings and Next Steps

  • Appendix A

    Study Participants

  • Appendix B

    Stormwater and Wastewater Modeling

  • Appendix C

    Technical Inputs for Scenario Development

  • Appendix D

    Stormwater Management Strategy Development

  • Appendix E

    Final Design of Experiments

The research reported here was conducted in the Infrastructure Resilience and Environmental Policy Program within RAND Justice, Infrastructure, and Environment.

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