Stormwater Management: Back to the Future to Adapt to Climate Change

Growing awareness that climate change is already altering weather patterns has sparked interest in ratcheting up stormwater management requirements to account for larger amounts of rainfall. But newer methods aren’t always better, and a Moore administration proposal to double the amount of water that must be treated using the latest “environmental site design” techniques is likely to be less effective in accounting for these changes than alternative approaches derived from the latest climate change modeling results.

That’s not to say that stronger stormwater rules are a bad idea. The best approach, however, would be a combination of advanced practices designed to protect and improve the quality of water with more traditional techniques that are better suited to handle the increased quantity of runoff from bigger storms. To understand why climate change calls for a mix of the new and the old, it’s helpful to understand a little about the evolution of stormwater management practices.

A Brief History of Stormwater Regulation: From Flood Control to Environmental Health

In the beginning there was water – or to be more precise, there was unregulated runoff. Before the federal Clean Water Act ushered in the first generation of stormwater regulation in the early seventies, efforts to control runoff served mainly to help keep water from collecting around the foundations of buildings and the surfaces of roads to protect property and prevent flooding. This approach relied on ditches and pipes to divert water, washing pollutants into streams and rivers.

Images of the Willett Branch in Montgomery County show how rainfall was channeled into a series of pipes and concrete ditches when the Westbard neighborhood was developed in the early 1960s, diverting water from flooding basements and roads but sending untreated runoff into the Potomac. 

1972-82: A Watershed (Pun Intended) in Stormwater Management Regulation

The passage of the Clean Water Act in 1972 ushered in a new era of regulation aimed at finding better ways to manage stormwater than simply trying to prevent it from causing property damage. Maryland followed with the Stormwater Management Act of 1982, which established the first comprehensive stormwater management standards for new development projects.

Maryland’s law was driven in large part by a desire to clean up the Chesapeake Bay, which was suffering from upstream pollution that had damaged the state’s crab industry, but then-current stormwater management practices were suited mainly to capturing rainfall on site, which did little to treat polluted water or recharge local groundwater stores. Maryland’s rules resulted in the use of what are now considered relatively unsophisticated measures such as large ponds to collect water generated by only the largest and most infrequent storm events. While these policies improved water quality somewhat, smaller and more common storms that did not generate heavy rains continued to erode stream channels and carry pollutants into the Chesapeake Bay.

Lake Kittamaqundi, a man-made lake in Columbia, holds large quantities of runoff but does not help infiltration of rainwater to allow groundwater recharge

2nd Generation Regulation: “Best Management Practices” and “Unified Sizing Criteria”

By the 1990s the deficiencies of Maryland’s stormwater framework were increasingly apparent, as the health of the Bay had not dramatically improved, leading the state to adopt guidelines for “best management practices” (BMPs) to improve water quality as well as reducing the volume of runoff entering streams and rivers. These guidelines included “unified sizing criteria” specifying the minimum capacity of stormwater facilities in three categories: (1) water quality, (2) groundwater recharge and (3) stream channel protection. While the unified sizing criteria generally still could be – and were – met with the use of large, centralized ponds they were more successful in reducing the amount of pollutants carried into waterways than previous policies.

The introduction of “best management practices” helped to reduce the amount of sediment and other pollutants entering streams and rivers but continued to allow development projects to use large stormwater ponds to contain runoff, limiting infiltration and groundwater recharge.

3rd Generation Regulation: Environmental Site Design

Maryland raised the regulatory bar again with the Stormwater Management Act of 2007, which required the use of “environmental site design” (ESD) practices to the “maximum extent practicable” (MEP). This “ESD to the MEP” requirement forced a fundamental shift in the planning, engineering, and approval of stormwater management in real estate development. COMAR 26.17.02, the Code of Maryland Administrative Regulations, laid out specific guidelines and design criteria for implementation.

ESD practices call for infiltrating runoff across a wide area instead of collecting in a pond or ditch. By spreading the water out, ESD uses the soil to filter pollutants as runoff is soaked up into the ground. This process “recharges” groundwater and facilitates a more natural process that relies on trees and plants to soak up rainfall.

Examples of decentralized stormwater management practices

Total Maximum Daily Load and Implementation of ESD

In 2010 the EPA established the Chesapeake Bay “total maximum daily load,” or TMDL, for several types of pollutants. The TMDL essentially put the bay on a “pollution diet” by setting goals for how much sediment, nitrogen, and phosphorus can be released into the Bay by various land uses and requiring states with waterways draining into the Bay to adopt rules to achieve the TMDL goals. Maryland developed stringent nutrient and sediment load reduction targets as part of a broader Chesapeake Bay Watershed Agreement.

The Chesapeake Bay TMDL influenced the implementation of ESD to the MEP, putting nearly all the focus of stormwater management regulation on improving water quality rather than capturing large volumes of rainfall. Instead of using large, centralized facilities to capture water and hold it in one location, ESD relies on small stormwater management facilities spread across a site.  Each facility has a small drainage area where it collects stormwater and then allows it to move safely into the environment.

The ESD approach works best for the most common storms, which may be of extended duration but are low in intensity. When a large amount of rain falls in a short period of time, on the other hand, ESD practices are often overwhelmed as volumes exceed their absorption capacity.

Example of shift from 2nd to 3rd generation stormwater management regulations

Senate Bill 227: Will the 4th Time Be a Charm?

In 2021, after severe storms caused serious flooding and property damage in several communities, the General Assembly passed Senate Bill 227, which mandated a comprehensive update to Maryland’s existing stormwater management regulations. It also required the Maryland Department of the Environment (MDE) to update its assumptions about the frequency and intensity of storm events based on recent precipitation data and revise its regulations accordingly.

MDE is now working to implement SB 227, and it has solicited input from industry and environmental leaders in the AstoRM Stakeholder Advisory Committee. Rodgers Consulting has been an active participant, providing extensive comments during the review process.

A draft of MDE’s proposed framework includes important changes to reflect the reality of climate change, including integration of the latest rainfall precipitation data to ensure that stormwater management standards remain calibrated to match real-world conditions.

In crucial respects, however, MDE’s proposal is flawed – particularly in its continued emphasis on the use of ESD practices to handle larger volumes of water. Even without climate change, ESD practices are often unable to handle the rainfall associated with anything but the most frequent and smallest storms. Current climate modeling predicts that the most infrequent and biggest storms are going to bring more precipitation while the most frequent and smallest storms are unlikely to change in the future. ESD is poorly suited to dealing with these short duration, high-volume weather events and should limited to those storm events that ESD can effectively handle and are not predicted to change in the future.

The First Flush: Why a Combination of Facilities is Best to Address Volume and Quality

The changes needed to make MDE’s proposal workable are not drastic – but they are a little counterintuitive. Just because ESD is better than earlier practices in addressing the need to remove pollutants does not mean that twice as much ESD is the best approach to dealing with the need to manage larger volumes of rainfall while continuing to maintain and improve environmental health. Instead of simply increasing the required capacity of ESD-compliant facilities, the rules should call for a combination of ESD with previously accepted practices that are better able to handle large quantities of water that fall over a short period of time.

ESD alone may not provide the attenuation necessary for high intensity, short duration storms, leading to excessive velocities and stream erosion

Ponds and other stormwater facilities designed to capture larger volumes of water are not as effective as ESD practices in removing pollutants from runoff, but they don’t need to be. That’s because the first 30 to 35 percent of the rain that falls during a storm – the so-called “first flush” – washes about 80 percent of the pollutants off of the soil and impervious surfaces such as driveways and parking lots. ESD practices filter the pollutants caught in the first flush, while ponds and other high-volume facilities collect the cleaner excess runoff that ESD cannot handle. 

Right-Sizing ESD for Sound Environmental Management and Land Use Policy

Land planners, civil engineers, and environmental consultants working with real estate developers continue to focus on methodologies associated with ‘right-sizing’ the ESD framework. It is important to get this right to avoid making inefficient use of land and building stormwater facilities that are ineffective in providing the intended results.   

A more nuanced approach to stormwater management that draws the best from new and traditional practices can help Maryland fortify its resilience against climate change, make growth more sustainable, and improve water quality for the benefit of humans and the natural environment.

Combination of ESD and Standard BMP’s provide attenuation for types of storms expected to increase in frequency and redundant protection to downstream waters

William “K.C.” Reed is a civil engineer with Rodgers Consulting who serves as an industry representative on the Maryland Department of the Environment’s Advancing Stormwater Resiliency in Maryland (A-StoRM) Advisory Committee