A Green Future for Stormwater Management

Across the globe we are seeing extreme rain events that are wreaking havoc on infrastructure and the environment due to climate change:

• In August of 2022 Death Valley, California received nearly a year’s worth of rainfall in three hours resulting in the loss of critical water sources and major damage to property and stormwater management practices in place.
• Last fall in Liguria and Piedmont Regions of Italy received over 35 inches of rainfall in just 24 hours, which is about how much rain New York City typically gets in one year.

Not every rain event is as extreme as these, but every rain event has the potential to cause damage and spread contaminants if not managed properly or sufficiently.

To combat these issues, engineers use green infrastructure to meet pollutant removal goals, reduce channel erosion, prevent overbank flooding and to help control extreme floods. When designing and implementing green infrastructure techniques the Water Quality Volume (WQv) is used to determine how much stormwater these systems will be able to treat for pollutants. The WQv is a calculated value used to determine how much stormwater a Stormwater Management Practice (SMP) needs to treat to meet quality standards. The WQv is used to improve water quality by capturing and treating runoff from small, frequent storm events that tend to contain higher pollutant levels. The quantity of water that must be treated is directly related to the area of impervious cover being added to a site and the 90^th percentile rain event. Because of this direct relationship, Runoff Reduction techniques also reduce the WQv. Green infrastructure achieves Runoff Reduction through the following practices:

• Green roofs, stormwater planters and rain gardens used to reduce the contributing stormwater volume.
• Techniques for reducing contributing area such as the preservation/restoration of conservation areas and vegetated channels for conveying stormwater.
• Stormwater Management Practices (SMPs) with runoff reduction capacity such as Infiltration Practices, Bioretention Basins and Dry Swales.

TRC engineers have been supporting clients across the country by designing and implementing green infrastructure to combat climate change. Recently, the New Providence, NJ and New York City, NY offices have been working on a Stormwater Pollution Prevention Plan (SWPPP) and hydrologic calculations in Yorktown, New York for a proposed solar field. Due to the location of the site, infiltration practices are not permitted, therefore the team is utilizing a combination of Dry Swales and Bioretention Basins to meet the pollutant removal goals and to replicate pre-development hydrology in accordance with the New York State Department of Environmental Conservation (NYSDEC) stormwater regulations. These SMPs were designed to meet both the WQv and Runoff Reduction criteria, as well as to replicate pre-development hydrology.

Dry Swales act as regular swales but filter water through layers of permeable soil and gravel to an underdrain.

Bioretention Basins act as regular basins but filter water through layers of mulch, planting soil, and gravel to an underdrain. These underdrains either lead to another SMP or convey treated water offsite.

It is critical to analyze stormwater management practices through the lens of environmental, social and governance (ESG) to combat climate change and the extreme weather events that are becoming more common. Stormwater has the potential to spread contaminants and can result in the loss of critical water sources if not managed properly. Many techniques and SMPs sometimes require large, open areas to mitigate the environmental effects of stormwater. In city landscapes these areas are simply not available. When compounded with the large amount of impervious area and many sources of contaminants, this is a major issue. To combat these issues state agencies have begun to update their stormwater regulations, making more robust guidance and procedure documents, taking into consideration the new challenges we face.

TRC Experts Can Help

Overlooking the stream that usually runs calmly past your business is normally an enjoyable break in your day, but not today! It’s raining cats and dogs and that stream is now rapidly rising. You look back at your stormwater outfall pipe, which usually would be dumping water from your office park right into the stream and you notice that the flow isn’t coming. Then you remember that TRC just finished designing and installing your new stormwater infrastructure, storing your runoff on this rainy day. Extreme rainfall events are becoming more and more frequent. More than 8 inches of rain fell in a portion of New Jersey in just a few hours during Tropical Storm Ida in 2020, and Houston, Texas endured over 30 inches of rain during Hurricane Harvey in 2017. These extreme rain events are predicted to become even more frequent in the future due to the impacts of climate change. How will your stormwater infrastructure handle the increased loads? TRC has the expertise and vision to assess your existing vulnerabilities and infrastructure through modeling current and anticipated precipitation events, provide updated plans, and find the right solution for your facility. Give TRC a call so you can relax on those rainy days and stay focused on your business.