SEPA
January 2025
Communities with Combined Sewers
Adapting to a Changing Climate:
Pittsburgh, Pennsylvania
Background
Pittsburgh Water is responsible for managing stormwater and
wastewater conveyance in Pittsburgh, Pennsylvania and its
surrounding areas. Pittsburgh Water is the largest water, sewer,
and stormwater authority in Pennsylvania and serves
approximately 500,000 people across the City of Pittsburgh and
the surrounding areas. The city's sewer system is around 75%
combined The wastewater and stormwater collected in the sewer
system is conveyed to the Allegheny County Sanitary Authority
(ALCOSAN) for wastewater treatment.
Challenges
There are numerous stormwater and wastewater challenges in
Pittsburgh, including aging infrastructure, population growth,
changing climate conditions, combined sewer overflows
(CSOs), basement backups, flash flooding, landslides, and
impaired waterbodies. Pittsburgh Water is working to provide
solutions that balance affordability and resiliency to current and
future climate stressors.
Climate Impacts
Over the last century, Pittsburgh has experienced a steady
increase in annual precipitation and storm intensity—a trend
that is expected to continue. Based on an analysis conducted
by Pittsburgh Water of over 60 years of historical rainfall data
from the National Weather Service, the size of the 95th
percentile annual storm event has increased nearly 20% and
the annual frequency of storm events greater than one inch
has nearly doubled. Furthermore, the number of days with
severe storm events has increased from around 15 per year in
the 1950s to over 100 per today. Large, intense storms can
pose challenges for managing CSOs and flooding.
To account for the impacts from changing climate conditions,
Pittsburgh Water partnered with the RAND Corporation and
Carnegie Melon University (CMU) to generate localized climate
projections. The results suggest precipitation rates and
temperatures will continue to increase in the Pittsburgh area
over the coming decades. By 2100, the depth of the 95th
Key Information
• Location: Pittsburgh, PA
• Population served: 500,000
Permit Number: PA0217611
(Pittsburgh) and PA0025984
(ALCOSAN)
• Key hazards: increasing storm
intensity, increased precipitation
smw 1
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Top image: basement flooding following a large storm
event. Bottom image: road flooding and damage from
landslide.
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Communities with Combined Sewers Adapting to a Changing Climate: Pittsburgh, PA
percentile storm event is projected to increase from 1.3 to 1.65 inches, and the annual volume of stormwater is
projected to increase by 15-20%.
Solutions
Updating Stormwater Code
Using the localized climate projections, Pittsburgh
Water produced projected design storm rainfall
depths from 2020-2099, The updated design
storms enabled Pittsburgh Water and the City of
Pittsburgh to update their stormwater code to be
responsive to projected changes in precipitation
patterns. Instead of referencing precipitation
depths from the National Oceanic and
Atmospheric Administration's Atlas 14.
Pittsburgh's revised stormwater code requires
stormwater best management practices for new
development be designed using the projected
design storms developed by modeling for the
year 2100. To support permittees and engage
with the public, the city also developed a design
manual, which includes guidance on the updated
stormwater code requirements.
Revised Fee Structure
Gathering data on potential changes to climate
impacts also helped Pittsburgh Water assess their aging infrastructure, including the necessary changes to
minimize CSOs and water quality degradation and prepare for the future. After realizing that the majority of their
water quality and quantity issues, including CSOs, were due to their outdated sewer infrastructure, they began
developing a more integrated stormwater management plan, as described in the Climate Resilience Toolkit case
study. The plan is designed to adapt with the growth of Pittsburgh and provide a way to pay for necessary
changes. Pittsburgh Water created a fee structure based on impervious areas to raise the funds required to pay
for future sewer projects. The impervious area of each customer parcel was assessed using geographic
information system (GIS) mapping. Throughout the development of the stormwater plan, Pittsburgh Water
engaged community and stakeholder groups through a stormwater advisory group to ensure development of an
equitable stormwater rate plan.
Additional Information
For more information on Pittsburgh Water, contact James Stitt, Manager of Sustainability, at
itstitt@pqh2o.com. For more information on the City of Pittsburgh, contact Kyla Prendergast, Senior
Environmental Planner, at kyla.prendergast@pittsburqhpa.gov. Additional information on Pittsburgh's
CSOs and climate adaptation can be found here:
• ALCOSAN's 2019 NPDES Permit (PA0025984)
. ALCOSAN's. 2019 Ciesri Water Plan
• Robust Stormwater Management in the Pittsburgh Region Report
• City of Pittsburgh Stormwater Design Manual
Stormwater BMP installed to meet updated stormwater code requirements.
EPA Publication 830-F-25-007
January 2025
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