Effective stormwater management is a challenging environmental problem, particularly for southwestern Pennsylvania where abundant water resources, frequent rainfall, hilly terrain, clay soils, antiquated sewer infrastructure and stormwater pollution all play an important role.
An average rain event in the Pittsburgh region is a quarter-inch; over a 12-month period, rainfall accumulates to a total of 37.5 inches on average. While southwestern Pennsylvania is not the wettest region in the country, it does face unique challenges due to its abundant water resources, hilly terrain, poor soils and land use development. One inch of rain falling, for example, over our rolling geography travels with a higher velocity and tends to pool at the bottom of valleys where flooding typically occurs. Less absorbent soils and increasing land development exacerbate the problem; clay soils absorb very little rainfall, and where once there were meadows and wetlands that soaked up stormwater, now there are hundreds of square miles of impenetrable surfaces, such as parking lots, sidewalks and roofs.
So stormwater runs off at higher speed, and if the storm is intense enough, it can cause flash flooding in low-lying areas. That same one-inch rain event over the plains of the midwest represents quite a different picture as the stormwater falls on flat terrain and soaks into the ground with little or no flooding.
How often do intense storms occur? You may have heard weather reporters refer to a storm as a 50-year storm or a 100-year storm, but what exactly does that mean? A 100-year storm refers to rainfall totals that have a one percent probability of occurring at that location in any one year expressed as a one-hour or 24-hour period. However, the occurrence of a 100-year storm on one day does not decrease the chance of a second 100-year storm occurring in that same year. In other words, there is a 1 in 100 or 1% chance that a storm will reach this intensity in any given year. Similarly, a 50-year rainfall event has a 1 in 50 or 2% chance of occurring in a year.
In addition, each region has its own criteria for how much rain must fall within one hour or 24 hours to classify as a particular rain event. According to the National Oceanic and Atmospheric Association (NOAA), in our region, 2.56 inches of rain needs to fall in one hour or 4.89 inches in a 24-hour period to be a 100-year storm. For a 50-year storm, 2.3 inches must accumulate in one hour or 4.38 inches over 24 hours. To find accurate rainfall amounts for a particular Allegheny County location, on a specific date and time, check out our Calibrated Radar Rainfall System.
Nationally, stormwater runoff is our most common cause of water pollution. Locally, it also contributes to our wet weather sewage overflow problem because in combined sewer systems, stormwater can exceed the capacity of the pipes, causing untreated stormwater to overflow before reaching the sewage treatment plant. In many cases, municipal sewer and stormwater systems have become deteriorated allowing extra stormwater and groundwater to flow into and out of the system at various points. Since 2004, municipalities, under municipal consent orders, have been working to assess, repair and rehabilitate their collection systems to help reduce the amount of inflow and infiltration into the system and thereby reduce the number of wet weather overflows that pollute our waterways.
Unlike pollution from industry or sewage treatment facilities, which is caused by a discrete number of sources, stormwater pollution is caused by the daily activities of people everywhere. It is often referred to as non-point source pollution because a particular source of contamination cannot be identified, but rather the pollution found in stormwater is from many diffuse sources.
In urban and suburban areas like those throughout southwestern Pennsylvania, much of the land surface is covered by impervious or non-penetrable surfaces like buildings and pavement, which do not allow rain and snowmelt to soak into the ground. As the rainwater and snowmelt run off streets, lawns, farms, and industrial sites, they carry fertilizers, dirt, pesticides, oil, grease, litter and many other pollutants to our rivers, lakes and streams.