Cover: Managing Heavy Rainfall with Green Infrastructure

Managing Heavy Rainfall with Green Infrastructure

An Evaluation in Pittsburgh's Negley Run Watershed

Published Oct 26, 2020

by Jordan R. Fischbach, Michael T. Wilson, Craig A. Bond, Ajay K. Kochhar, David Catt, Devin Tierney

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Research Questions

  1. What are the current risks posed by heavy rainfall to the Negley Run watershed, including both sewer overflows and flooding?
  2. How might these risks change in futures where heavy rainfall grows more frequent or intense?
  3. How might a new daylighted system of GSI help to reduce sewer overflow and flood risk, and how does this vary at different levels of investment or build-out?
  4. What other cobenefits would proposed GSI strategies provide?
  5. How do the benefits and cobenefits from proposed GSI strategies compare with their costs across a range of plausible assumptions?

Urban stormwater management is a growing challenge in many U.S. cities. Continued population growth, urbanization, and inadequate investment in storm- and wastewater infrastructure have left many cities exposed to sewer overflows, stormwater flooding, and reduced water quality. Climate change is expected to add to this challenge by increasing the intensity or volume of rainfall from storms in many regions. There is also a growing acknowledgment that these vulnerabilities are environmental justice and equity challenges, as flooding and other negative outcomes disproportionately affect low-income or majority-minority neighborhoods.

Pittsburgh's Negley Run watershed is a prime example of these stormwater management challenges, draining a diverse area of Pittsburgh's East End, including neighborhoods that have suffered heavily from underinvestment. It also represents an urgent flood-risk challenge in the city, as heavy rainfall in the area leads to regular flooding of a key road corridor. In this project, RAND researchers use simulation modeling to evaluate present and future risks in Negley Run from sewer overflows and flooding given future rainfall uncertainty. The authors then evaluate proposals for a phased series of green stormwater infrastructure (GSI) investments.

In addition to estimating stormwater benefits and implementation costs, the authors provide economic estimates of recreational, amenity, and other cobenefits to local residents; compare total benefits to costs; and explore potential trade-offs. Results show that a centralized system of stormwater management in Negley Run could yield cost-effective sewer-overflow reduction, reduce street flooding, and provide positive net economic benefits across a range of assumptions about future rainfall and implementation costs.

Key Findings

Additional investment is needed to mitigate present and future risks

  • Sewer overflow from Negley Run is higher in a range of recent historical rainfall patterns compared with a single typical year.
  • Sewer overflow increases further in plausible future scenarios.
  • Negley Run already faces notable flood risk from heavy rainfall events.

GSI strategies could reduce sewer overflow and flood risk

  • Flood risk increases with heavier rainfall from plausible future climate change.
  • A new centralized daylighted system and subsequent upstream investments could substantially reduce, but not eliminate, sewer overflows in Negley Run.
  • A centralized daylighted system reduces flood depths along Washington Boulevard but does not eliminate flooding even in more-frequent, lower-intensity events.

The benefits of proposed GSI strategies outweigh their costs

  • Strategies that build on a daylighted surface collection system could yield highly cost-effective sewer-overflow reduction across a range of assumptions.
  • Scenarios with higher average annual rainfall generally lead to greater sewer-overflow cost-effectiveness.
  • GSI cobenefits from the strategies evaluated could contribute substantial value to residents when accounting for uncertainty and using conservative assumptions.
  • The net economic value of the strategies considered — taking into account life-cycle strategy costs, water-quality benefits, and cobenefits — is nearly always positive across a wide range of assumptions.
  • A midranged strategy could provide positive economic benefit across a wide range of assumptions.

Recommendations

  • Invest in design and engineering efforts for the Central Daylighting strategy (Strategy 4) as a solution with few, if any, downsides.
  • Employ an adaptive planning approach to support assessment and decisionmaking for further increments of connected GSI.
  • Expand the range of options to address street and basement flooding through agency and stakeholder collaboration.
  • Continue to invest in distributed local GSI projects in Negley Run neighborhoods in order to take advantage of benefits for residents, irrespective of flood-risk reduction.
  • Further refine the detailed simulation model developed for this study and integrate it into regional system models to support additional sewer-overflow and flood-risk evaluation.
  • Gather additional data to inform flood-risk assessment for Negley Run, including combined system descriptions and flow monitoring.
  • Explore the use of two-dimensional surface-flow modeling to support flood-risk assessment and site-specific GSI design.
  • Consider adopting an integrated, watershed-based approach for other areas of focus across the region.

Research conducted by

This research was funded by the Henry L. Hillman Foundation, the Heinz Endowments, and 3 Rivers Wet Weather and conducted by the Community Health and Environmental Policy Program within RAND Social and Economic Well-Being.

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