June 14, 2018

Many older cities across the United States still convey wastewater and stormwater in the same pipes or systems. The older gray infrastructure transports the wastewater it collects to a treatment plant where it’s treated and ultimately discharged into waterways. During heavy rainfall events, however, the volume of wastewater often exceeds the capacity as it combines with stormwater runoff that is conveyed in the same sewer system causing it to overflow. When this occurs, untreated wastewater makes its way to nearby rivers, lakes and streams.

With these combined sewer overflow (CSO) events posing an increasing risk to human health and natural resources, the U.S. Environmental Protection Agency (EPA) is enforcing regulatory Consent Decrees in older cities, mandating they separate their stormwater from their wastewater to help mitigate CSOs. In response to the EPA’s call to action, many of these communities are undertaking implementation of cost-effective, resilient and sustainable solutions to begin managing severe wet-weather impacts. In today’s post, I explore some best practices as they apply to sustainable stormwater management and stormwater resiliency.


Bridging the Gap

To separate stormwater from wastewater, older cities under these regulatory Consent Decrees are committing to a range of solutions. These include building deep tunnel systems for storage and implementing green infrastructure. And in some cases, they are doing both.

The definition of stormwater management has changed over time, and I am often asked to break down the meaning of “sustainable stormwater management” in terms of best practices that can be implemented given today’s extreme weather events and aging infrastructure, as well as the need to be more resilient going forward. At present, I believe that sustainable stormwater management should include:

  • Mitigation of both quantity and quality.
  • Separation of stormwater runoff from wastewater systems.
  • Incorporation of green infrastructure combined with maintenance and operation.
  • Mitigation of extreme weather flood flows.
  • Consideration of the age, capacity and integrity of the storm sewer infrastructure.
  • Partnering with adjacent entities to maximize economic, quantity and quality benefits.
  • Assessing what a predictive design approach might include, and documenting the potential benefits of employing a forward-looking approach.
  • An understanding of the dynamics between a flood event and stormwater runoff (this nexus continues to be blurred as extreme weather events increase).
  • Understanding the concepts of Consequence of Failure (COF) and Probability of Failure (POF) associated with your drainage or stormwater infrastructure.

Scale: L-Low; M-Moderate; S-Significant and H-High (source: Adopted from US DOT- FHWA – Risk-Based Transportation Asset Management: Evaluating Threats, Capitalizing on Opportunities 2012).


Digging Deeper

Over recent years, the U.S. has seen an increase in extreme weather events including intense rain and snow storms due to the possible effects associated with climate change. Along with the continued development of the built environment, this is placing additional stressors on an already over-taxed drainage/storm sewer infrastructure system that has either reached or exceeded its expected life cycle.

Now more than ever, we need to develop an approach that can address the nexus between water, wastewater and stormwater utilities, aka the utility triangle. Along with offering more cost-effective and sustainable solutions for older cities, green infrastructure helps them to become more resilient to severe weather events by reducing flooding and the heat island effect, as well as improving overall water quality for the community.



According to Federal Emergency Management Agency (FEMA) administrator William B. “Brock” Long, local government agencies are far better equipped to prepare for natural disasters because of their ability to affect building codes, land-use planning and system designs—all at a local level. For this reason, it is important to empower local governments by continuing to provide funding for natural disaster preparedness.

We also need to continue to dig deeper when it comes to understanding what makes something resilient in respect to stormwater runoff associated with extreme-weather events. There are several key points worth exploring in terms of potential benefits as they relate to stormwater resiliency. These include:

  • Storm sewer system redundancy and modularization. This allows for risk to be spread across multiple elements that provide similar functions. This overlapping function helps mitigate system failure. Without it, if a portion of the system goes down, the entire system is vulnerable to flooding.
  • Performing a resiliency assessment associated with drainage and stormwater assets. These assessments help identify system needs and the current system condition, while serving as a catalyst for the development of strategies in relation to operational and recovery post-extreme weather events.
  • Distinguishing significant stormwater runoff from flooding during an extreme weather event. This needs to be better understood and must be assessed in respect to overall design goals.


The Times They Are a-Changin’

A decade ago, stormwater management was relatively straightforward—if you managed the quantity, you managed the stormwater. Today, ancillary stressors and elements such as extreme weather meteorology and forecasting, land planning, local government policy, flood control and management, green infrastructure, GIS, civil engineering, disaster planning and recovery, hydraulics, soil science and more should all form a part of your evaluation criteria in determining if your stormwater management program or system should be resilient or sustainable.

Simply put, stormwater management is not what it used to be. As professionals, we need to be knowledgeable of these elements while incorporating them within limited budgets or at the expense of existing budget services. And therein lies the challenge.


This post was authored by Mark McCabe when he served as a senior environmental scientist in Gresham Smith’s Water + Environment market.