Runoff reduction is a key component of effective stormwater management. Find out why, and what you can do about it. 

What is stormwater runoff?

Stormwater runoff is the surface water that flows over impermeable surfaces such as roads, sidewalks, parking lots and roofs following rainfall.

While rainfall is most commonly the source of surface water, in certain climates melting snow and ice is also a significant seasonal contributor to stormwater runoff.

Increasing urban development and the growth of the built environment has created more and more surfaces that water cannot pass through—increasing the volume of runoff that has to find its way back into the water table.

Why is stormwater runoff a problem?

When rainwater travels across an impermeable surface it picks up whatever materials are present on that surface, carrying those materials into the first body of water that it encounters.

In urban and developed areas it is common for stormwater runoff to pick up trash, solids such as silt, sand and grit, nutrients, organic materials, hydrocarbons, metals and many other potential pollutants. Many of these are sediment-bound, meaning that catching solids is a critical component of reducing overall pollution.

If these materials reach our rivers, lakes or oceans they present a significant risk of pollution, damaging ecosystems and endangering aquatic life.

The dangers of pollutants such as oil, metals and pesticides are relatively obvious, but even something as seemingly innocuous as dirt can accumulate in waterways, increasing turbidity, reducing the ability of light to reach deeper levels within the water column, and harming the aquatic plants on which the ecosystem depends.

What’s the solution to stormwater runoff?

There are three main approaches to addressing the problem of pollution in stormwater runoff:

1. Reduce pollution
2. Reduce the amount of stormwater runoff
3. Capture the pollution carried in stormwater runoff

Reducing pollution

The first and most obvious way to tackle the issue is to reduce the amount of pollution that finds its way onto our surfaces.

This is deceptively complex, however, as the sources of stormwater pollution are varied and are fundamental parts of everyday life: car tires deposit rubber particles, while car brakes deposit tiny flakes of metal; household fertilizers, weed killers and other products leave a range of chemicals on paved surfaces; animal waste contains nutrients that can trigger harmful algal blooms; and so on.

It is impossible to prevent some degree of surface pollution without drastically altering the way we live and work, so realistically speaking we can only hope to reduce—rather than eliminate—these sources of pollution.

Reducing stormwater runoff

In terms of stormwater management, the ultimate objective of any built environment is to perfectly mimic the drainage that existed prior to the development. In other words, if 95% of rainfall in one square mile infiltrated into the soil prior to development, then 95% of rainfall should infiltrate into the soil in that same square mile after development.

This is usually difficult to achieve, however, as the built environment behaves differently than the natural environment, but civil engineers and designers have tools and techniques (often referred to as Best Management Practices, or BMPs) that can help them to mimic the pre-development natural environment.

The key words are “slow” and “hold”. The slower the water moves, the less pollutants it picks up and transports, while if the water is held in place, pollutants are more likely to settle out, and the water itself has the opportunity to infiltrate into the soil.

Features such as swales, detention and retention ponds, green roofs and other engineered features aim to achieve just this, helping to detain water and thereby preventing it from flowing freely over impermeable surfaces, or allowing it to infiltrate in place.

While these features aren’t natural, they help a built environment to behave more naturally, and in many cases also provide amenity for residents.

Capturing stormwater pollution

It is unlikely that any developed environment will be able to exactly mimic its pre-development stormwater drainage patterns, so the third approach is to put in place “safety nets” to catch any surface water pollution that is able to pass through and make its way toward a body of water.

Examples include screens, hydrodynamic separators, filters and other manufactured treatment devices. These are typically placed upstream of a body of water or other drainage system, where they simply capture pollutants, removing them from the stormwater runoff and ensuring that treated surface water passes into the natural environment.

Some runoff reduction BMPs incorporate a stormwater treatment element as well—retention ponds, for example, are used to hold water in place but may also allow pollutants to settle out (sometimes using flocculants to increase settling) and either be digested by microorganisms, or dredged up manually at regular intervals and disposed of.

While many of these techniques are effective, they are often large and require significant maintenance to ensure that they continue to work as designed. Some smaller technologies also combine infiltration and treatment, however, providing a combination of runoff reduction and pollution capture in a small footprint. These systems are particularly beneficial in urban, city-center environments where space is limited and many of the surfaces are impermeable.

The NPDES system

In response to the environmental risks posed by stormwater runoff, under the Clean Water Act the National Pollutant Discharge Elimination System (NPDES) puts in place a system of permits that regulate businesses that discharge from a point source into a body of water in the US.

The EPA manages the program, though administration is at the state level. Almost all US states have full or partial NPDES authorizations.

Different state approaches to stormwater runoff

Some states have adopted particularly robust, comprehensive measures, and two in particular are leading the way on reducing and treating surface water runoff.

Washington

Washington passed the Washington State Water Pollution Control Act, which acts as the primary law governing water quality in the state. Its objective is to “maintain the highest possible standards to insure the purity of all waters of the state”, and requires that the state will “require the use of all known available and reasonable methods by industries and others to prevent and control the pollution of the waters of the state”.

Washington has long been a leading proponent of water quality in the US, and in 2020 was the first to publish Water Quality Standard variances for polychlorinated biphenyls (PCBs) that require facilities to incorporate additional pollution controls.

Related:

• Up-Flo® Filter TAPE Certified for GULD
• Hydro StormScape® TAPE Certified for PULD

New Jersey

New Jersey has adopted a “Green Infrastructure” philosophy that is designed to manage stormwater runoff on site. Its intention is to put in place technologies and infrastructure elements that reduce, store and infiltrate surface water in place.

While the term “Green Infrastructure” might imply the use of natural features, this is not necessarily the case—a range of engineered technologies, both above and below ground, may be used in conjunction to achieve the objective of mimicking pre-development drainage conditions. Indeed, “green” features on their own are unlikely to be able to adequately manage runoff deriving from a built and largely impermeable environment.

Related:

• NJDEP Authorizes First Defense® High Capacity with highest stormwater treatment loading rate in industry
• Hydro StormScape® NJDEP certified for 80% TSS removal