The City of Pittsburgh’s incorporation of climate-change projections into its stormwater-control regulations have been highlighted by the latest National Climate Assessment as an example of how a city can prepare itself for the bigger, more frequent rain storms produced by the changing climate.

The federal document cited the former steel capital, Pennsylvania’s second-largest city, population 300,000, for its work requiring developers of new properties covering about a quarter of an acre of land, or with impervious surfaces of about an eighth of an acre, to install various kinds of green infrastructure so that their projects don’t worsen runoff.

The city is an early adopter of stormwater rules based on the expectation of increased future rainfall, which threatens worsening floods unless new development enhances the ability of land to absorb and store water rather than just deflecting it as runoff.

The rules “require new developments to plan for projected increases in heavy rainfall under climate change rather than building to historical rainfall amounts,” according to the assessment, published on Nov. 14. It also noted that Pittsburgh committed in 2021 to achieve net-zero carbon emissions by 2050.

While bigger cities like New York, Boston and Philadelphia are making “extensive” climate preparations, Pittsburgh is an example of an “innovative” approach to climate planning taken by a geographically, economically and politically diverse sampling of mid-sized cities, the climate assessment said.

“Their efforts generally receive less visibility, but the need among similarly sized cities in the [Northeast] region—to learn about best practices and lessons learned in developing and implementing climate action plans to inform their own efforts—can be significant,” the assessment  said.

Pittsburgh’s existing stormwater rule, Title 13, was updated in April 2022 to include rainfall projections calculated two years earlier by Carnegie Mellon University and the Rand Corporation. Although the forecasts for more-likely but less damaging storms were similar to projections from the National Oceanic and Atmospheric Administration, the CMU/Rand forecast predicted even greater increases in rainfall than NOAA. 

For example, a so-called 100-year storm—that which is expected to occur only 1 percent of the time—is expected to dump 6.4 inches of rain on Pittsburgh in a 24-hour period, or about a sixth of what the city typically gets in a whole year, according to the CMU/Rand forecasts. That’s about two inches more than projected by NOAA, according to Kyla Prendergast, a senior environmental planner with the city.

By contrast, a more-likely “one-year storm” would mean 2.1 inches of rain fell on the city in 24 hours, similar to the federal projections.

“The rainfall depths are a bit higher than the NOAA ones, and that helps us to ensure that we’re holding developments to a higher standard, and whatever we’re building now is actually going to be able to manage the rainfall that we know we are going to be seeing in the next 10, 20, 50 years,” Prendergast said.

Myron Arnowitt, Pennsylvania director for the nonprofit Clean Water Action, welcomed the city’s inclusion of climate projections into its stormwater rules as a change that is much more likely to protect the city from flooding than an earlier version of the rule.

“What Pittsburgh is doing is making sure their regulations will actually work so that they reflect the reality of the climate crisis we’re in,” Arnowitt said. “If you’re writing stormwater rules, and you’re using rainfall amounts based on what happened in 1900 to 1950 before climate change really took off, you’re going to be controlling much less water than if you base it on what we expect rainfall to be like in the next 10, 20, 50 years. It makes a lot of sense.”

NOAA’s National Weather Service is working on an update of a regional estimate of precipitation frequency. The new report, called NOAA Atlas 15, will use climate-change information to “derive precipitation frequency estimates” when it is published in 2026, said NWS spokesman Michael Musher. 

NOAA said the new estimates will provide “critical information to support the design of state and local infrastructure nationwide under a changing climate.”

Climate scientists predict an increasingly warm, wet future and widespread disruption of historic weather patterns worldwide as a result of trapped greenhouse gases. The latest National Climate Assessment, the fifth in a series of the Congressionally mandated reports, said the United States has cut carbon emissions from their peak in 2007, and has done more to adapt to the effects of climate change in the last five years, but it urged much stronger action to lessen severe effects including flooding, wildfires, heat waves and sea-level rise.

In Pittsburgh, the latest rule blames more runoff for a range of ills including erosion and sedimentation, exceeding the carrying capacity of streams and sewers, increasing public costs to control stormwater, and threatening public health with the backup of raw sewage in basements.

“A comprehensive program of stormwater management, including regulation of development and activities causing accelerated runoff, is fundamental to the public health, safety and welfare,” the rule says.

To ensure that new developments lessen or at least do not increase runoff, developers can incorporate a variety of techniques such as rain gardens, green roofs and “construction wetlands”—areas that restore a landscape’s capacity to absorb rainfall, she said.

Projects that disturb at least 10,000 square feet of land or create 5,000 square feet or more of impervious surface are now required to submit their plans to the city’s stormwater permitting process, and to show that the project would not increase the amount of runoff during a rainstorm.

So far, the city has approved about 50 projects that include the new rainfall forecasts although none are yet under construction, so there’s limited evidence so far on the effectiveness of the new standard, Prendergast said.

But she argued that incorporation of the new climate projections into the development plans, and the city’s approval of them, already show that the standard is working.

The new rules will inevitably increase costs for developers, Prendergast said, but that would be less than the cost of reacting to future flooding based on outdated precipitation forecasts. There has been some pushback from developers since the rules were introduced but the pace of development hasn’t slowed, she said. The city has produced a “design manual” for developers, explaining the rule and the reasons for it.

Pittsburgh, located at the confluence of three rivers, and with many populated hillsides, is especially vulnerable to flooding, and has suffered loss of life during the worst storms. In 2011, a downpour left waters that overtopped cars and left drivers standing on their roofs or wading through chest-deep waters, Prendergast said. Four people died in that storm.

“We do have a lot of water in the city,” Prendergast said. “Historically, Pittsburgh has tried to fight against the water; we have a lot of streams that have been paved over, and turned into pipes. It worked for what we’ve seen in the past but as we move forward, we see that it’s not the most resilient approach to managing water. We’re hoping that with this code and other policies, we are going to be able to work with the water rather than against it.”