Techniques for On-Site Stormwater Management in Cities

Like many cities in the Northeast United States, Philadelphia’s infrastructure is rapidly aging. As the city’s buildings, antiquated drainage systems, and roads age, the human health risks posed by natural disasters increase significantly. Flooding is of particular concern. According to this resource from Penn State’s PA Flood Risk tool, “flooding in Pennsylvania is the most frequent and damaging natural disaster that occurs throughout the Commonwealth.” Uncontrolled stormwater runoff strains the city’s aging infrastructure and erodes unstable land, threatens water quality, exacerbates flooding, and impacts ecosystems. In this post, we consider why poorly managed stormwater runoff is so dangerous for people, natural habitats, and the built environment. We also identify several techniques for sustainable on-site stormwater management and outline the many financial benefits for developers who choose to prioritize appropriate stormwater management in the projects they fund. Read on to learn more about managing runoff effectively through porous surfaces, green roofs, rain gardens, drainage ditches, detention ponds, and other techniques.

Why is Stormwater Runoff Dangerous for People, Natural Habitats, and the Built Environment?

Poorly managed stormwater runoff can result in horrific flash flooding events that devastate homes, damage vehicles, and injure residents. It can pollute our drinking water with heavy metals, fertilizers, and sewage overflow. Runoff also disrupts natural hydrological patterns, destroys habitats, and introduces invasive species. In some cases, poorly managed stormwater runoff can increase the water temperature of local lakes and rivers, exacerbating the heat island effect in nearby cities. In this section, we consider the potentially devastating impacts of unmitigated stormwater runoff in urban areas like Philadelphia. Let’s get into it.

Water Pollution from Fertilizers, Heavy Metals, and Sewage Overflow

When stormwater gathers sewage from cities, soil from agricultural areas, and debris from other areas, it can cause waterborne diseases, increase levels of toxic chemicals from litter, and alter soil nutrient profile by introducing too much of certain nutrients. In some cases, pollutants can also become aerosolized — resulting in air pollution too. Let’s take a closer look at each potential consequence of uncontrolled stormwater runoff.

First, let’s consider how stormwater runoff can increase levels of toxic chemicals and transmit waterborne diseases. Stormwater runoff can carry a range of chemical pollutants — including heavy metals, pesticides, and industrial chemicals. These contaminants not only pollute freshwater sources. They can also enter our food chain by consuming fish and other aquatic organisms from nearby rivers and lakes.

Long-term exposure to such contaminants can have adverse health effects on the population — including developmental issues, neurological problems, and cancer. In the short term, standing water can create breeding grounds for disease-carrying vectors like mosquitoes. This can increase the risk of contracting certain diseases.

Stormwater can also pollute freshwater sources with excessive nutrients. Excess nutrients like phosphorous and nitrogen, which are often present in stormwater runoff, can lead to the development of harmful algal blooms in water bodies. Some blooms produce toxins that pose health risks to humans if ingested or through skin contact with contaminated water.

Disruption of Natural Hydrological Patterns

Beyond the human health impacts of poorly managed stormwater runoff, there are also many consequences for the natural environment. In urban environments, construction, paving, and other human activity alter natural hydrological patterns. It effectively replaces natural permeable surfaces with impervious surfaces, which disrupts the balance of water absorption, groundwater recharge, and surface flow. This makes it far more difficult to control runoff — increasing the likelihood of flash floods, standing water, and more.

Habitat Destruction and Introduction of Invasive Species

Sadly, mismanaging runoff can also disrupt aquatic and terrestrial habitats that support natural biodiversity around cities like Philadelphia. Mismanagement can disrupt natural habitats by altering water flow patterns, causing erosion, and degrading marine ecosystems. At the same time that runoff displaces indigenous animals and plants, it also transports non-native plants, animals, and microorganisms. This may contribute to the spread of invasive species — further disrupting local ecosystems, introducing diseases, and more.

Aging Infrastructure & a Changing Climate: Threats Specific to Philadelphia

Flash Flooding Threatens Safety & Property

Flash flooding is one of the most devastating and unpredictable consequences of inadequate stormwater management practices. As noted in the introduction of this post, flash flooding is especially common in Philadelphia, where extreme rain events are growing more frequent and intense.

In July of this year, five people were killed, and nearly a dozen vehicles were trapped during a flash flood event in Philadelphia’s suburban Washington Crossing neighborhood. As Anna Betts and Elise Young write in an article for The New York Times, “the water began to recede after 15 minutes” but several people had already died. Recent estimates revealed by NOAA have identified Philadelphia as “one of the top cities most at risk of extreme rains” as the climate continues to change. 

Extreme rain events are expected to increase, and the Delaware River is expected to rise. In fact, WHYY’s Zoe Reed writes that “sea level rise in Philadelphia will increase by 7 to 11 inches by the 2030s, 14 to 19 inches by the 2050s, and 24 to 38 inches by the 2080s.” The consequences of flash flooding events like the one that took five lives in July will only be worsened by poor stormwater management.

Poor Stormwater Management Exacerbates the Heat Island Effect

In this article for The Philadelphia Inquirer, Aubrey Whelan and Victoria Ke Li write that many Philadelphians do indeed suffer from the heat island effect. According to Li and Whelan, “a lack of tree cover and excessive hot surfaces, like asphalt, create ‘heat islands’ in Hunting Park, Strawberry Mansion, and Point Breeze.” Unmitigated stormwater management that reduces vegetation and heats nearby bodies of water could exacerbate this effect in Philly neighborhoods that already struggle against sweltering summers. 

Uncontrolled stormwater runoff contributes to the heat island effect in a few different ways. First, runoff can heat nearby bodies of water. Impervious surfaces like roads, parking lots, and buildings prevent rainwater from being absorbed into the ground. Instead, this water rapidly flows over these surfaces, picking up heat from the sun-warmed materials. As the heated runoff enters stormwater drainage systems, it can raise the temperature of water bodies such as rivers, lakes, and ponds. Elevated water temperatures can lead to warmer air temperatures in the vicinity, contributing to the overall heat island effect in urban areas.

Second, poorly managed stormwater runoff can destroy vegetation and green spaces. These spaces are crucial for natural cooling through processes like evapotranspiration. Without adequate vegetation to shade and cool surfaces, urban areas trap more heat. 

In contrast, well-designed stormwater management systems that prioritize permeable surfaces, green infrastructure, and natural drainage systems can mitigate the heat island effect by allowing rainwater to infiltrate the ground, supporting vegetation growth, and reducing the amount of heat-absorbing surfaces. By integrating effective stormwater management strategies, cities like Philadelphia can not only alleviate the heat island effect but also enhance overall urban livability and environmental sustainability.

Antiquated Sewers Overflow and Drain into the Delaware and Schuylkill Rivers

One of the most concerning consequences of poorly managed stormwater runoff in Philadelphia is sewage overflow and pollution of freshwater sources. In Philadelphia, the city’s stormwater and wastewater systems are connected. As this resource from the Philadelphia Water Department explains, “a single pipe carries both stormwater from streets, houses, and businesses as well as wastewater from houses and businesses to a water treatment plant” in certain neighborhoods. Storm drains do not connect to specific storm sewers but are instead combined.

Combined sewer systems like those described by the PWD are designed to collect both stormwater runoff and sewage in the same pipes. Under normal conditions, the combined flow is directed to plants, where it is treated before being released into the environment. However, during intense rainfall events or flash floods, the flow of stormwater exceeds the carrying capacity of these systems – leading to combined sewer overflows (CSOs). This manner of managing stormwater runoff means rivers can become polluted by sewage, but it also means that the city cannot reuse stormwater for other purposes.

Consequences of CSOs in Philadelphia

When a combined sewer system overflows, untreated sewage and stormwater runoff laden with toxic chemicals is discharged into nearby water bodies like the Delaware and Schuylkill Rivers. This can result in the contamination of water bodies with pollutants, pathogens, and other harmful substances present in the sewage. Unfortunately, the Delaware and Schuylkill Rivers are responsible for a significant portion of the city’s drinking water supply.

The Delaware River – which forms the eastern border of Philadelphia – serves as a major source of drinking water. The PWD operates water treatment plants that draw water from the Delaware River, treat it to meet drinking water standards, and distribute it throughout the city. The Schuylkill River – which runs through the western part of Philadelphia – is another vital source of drinking water. The Fairmount Water Works, located on the banks of the Schuylkill River, historically played a role in treating water for the city. Today, the PWD also draws water from the Schuylkill River and treats it at various water treatment plants before distribution.

While the water from these rivers goes through extensive treatment to ensure that it meets strict water quality standards, contamination from stormwater runoff can overwhelm treatment plants and slow these necessary processes. If both the Delaware and Schuylkill Rivers were heavily polluted, it would have severe consequences for Philadelphia’s fresh water supply, the overall well-being of city residents, and the natural environment. It would not only strain Philadelphia’s aging degrading infrastructure, threaten the health of residents, and inconvenience businesses. 

If these rivers were seriously contaminated by sewage runoff and stormwater pollution, Philadelphia might need to seek fresh water from other sources in the region – which would place financial stress on the city. Severe pollution of these rivers could also harm aquatic ecosystems.

Techniques for On-Site Stormwater Management

Thankfully, there are many ways architects, engineers, and developers can integrate on-site stormwater management to reduce the intensity and frequency of runoff during extreme rainfall events. These include natural land masses like bioswales and riparian buffers, which are created by people to protect habitats and urban areas from runoff. Techniques also include the implementation of rain gardens, rain chains, previous pavers, green roofs, and sustainable construction techniques.

Bioswales and Riparian Buffers

We begin with landscape features like bioswales and riparian buffers — both of which are natural formations recreated or reinforced by people to slow the flow of runoff, improve filtration, and prevent erosion. They also protect aquatic environments, which often suffer from pollution when stormwater is poorly managed.

Bioswales are gently sloping channels that direct runoff that flows off of impervious surfaces like paved roads, asphalt parking lots, and concrete sidewalks. They are planted with a variety of native grasses, shrubs, and other vegetation that have strong root systems selected to help stabilize soil. This green infrastructure solution to stormwater mismanagement is easy to maintain and often aesthetically pleasing.

While they serve a similar purpose, riparian buffers are slightly different than bioswales. Riparian buffers are areas of natural vegetation located along the banks of rivers and other water bodies intended to create a transition between aquatic environments and upland areas from which stormwater flows. Riparian buffers are typically broader and encompass a wider area along the water body’s edge, whereas bioswales are skinnier and depressed like thin streams.

Riparian buffers protect natural water bodies and surrounding habitats, while bioswales are designed to manage stormwater runoff in developed areas. Both contribute to the overall goal of sustainable water management and environmental preservation. They also help restore natural hydrologies that were disrupted by poorly managed runoff.

Rain Gardens

Rain gardens are man-made landscape features that help manage stormwater runoff by collecting and treating rainwater from impervious surfaces like roofs, driveways, and sidewalks. Unlike those impervious surfaces, rain gardens have well-draining, highly permeable soil that allows rainwater to infiltrate the ground slowly. This process reduces the volume of runoff that would otherwise flow directly into storm drains, sewer systems, or water bodies.

A rain garden is planted with a variety of native vegetation that is usually selected for its ability to both thrive in wet conditions and tolerate periods of drought. Rain gardens are strategically located within urban environments to capture and manage rainwater while providing green spaces for residents. Many — like those in our Taupe Towns project — are built into sidewalks. Each Taupe Towns townhome includes a solar pergola, rooftop garden, rain garden, and other sustainable features that make the project truly special.

They provide multiple benefits for stormwater management, environmental health, and human health. In addition to interrupting the flow of runoff, rain gardens also increase local biodiversity by encouraging insects and animals to spread the seeds of those plants. They can aid in reducing the heat island effect in urban areas too. Rain gardens release water vapor through a process called evapotranspiration, which contributes to local cooling.

Rainwater Harvesting and Rain Chains

Some architects, developers, and property owners also choose to collect rainwater instead of diverting it directly into the ground. Rain chains are commonly used as one part of a rainwater harvesting system. These chains are decorative alternatives to traditional downspouts on the sides of buildings.

They are used to guide rainwater from a roof gutter to the ground or a collection system for rainwater harvesting. Rain chains typically consist of a series of linked metal cups, chains, or other decorative elements that hang vertically from the gutter down to the ground or a container. Rain chains play a small part in stormwater management.

In combination with rain chains — which are largely decorative — many property owners also use rain barrels, modified gutters, specific roofing materials, and pumps for rainwater harvesting.

Pervious Pavers, Grass, and Mulch Instead of Asphalt and Concrete

Grass, mulch, and pervious pavers are also commonly used in stormwater management systems because — like rain gardens — they absorb and filter runoff rather than allowing it to flow over them like asphalt and concrete do. Instead of one giant slab of concrete, permeable or pervious pavers are designed to allow water to pass through small gaps and infiltrate into the ground below. Unlike traditional impervious surfaces like concrete or asphalt, pervious pavers facilitate stormwater management by promoting water infiltration, reducing runoff, and supporting natural groundwater recharge.

Pervious pavers have the ability to dissipate heat more effectively than traditional impervious surfaces. This can help mitigate the urban heat island effect, where urban areas experience higher temperatures due to the heat-absorbing nature of conventional pavements. They are commonly used in rear yards and driveways to prevent runoff while also adding to the aesthetic of outdoor spaces and cooling those areas. For example, we added pervious pavers to the rear driveway of our Hemlock Towns project to not only limit runoff but also to increase the amount of green space in Philadelphia. Grass and mulch perform similar functions.

Notably, the use of permeable pavement, rain gardens, mulch, and grass also stabilizes and ultimately replenishes aquifers. Groundwater aquifers are naturally occurring layers of permeable rock, sediment, or soil that form underground. These formations control the flow of groundwater — supplying water to wells and springs. Accessibility of said water changes as the water table rises and falls. Proper stormwater management practices can help recharge groundwater aquifers — enhancing their water storage capacity and contributing to sustainable water resources.

Green Roofs

In recent years, green roofs have increased in popularity due to their ability to collect and redirect rainwater while limiting the heat island effect and enhancing the aesthetics of urban environments. They are designed with layers of vegetation and a growing medium atop a waterproof membrane. In combination with the waterproof membrane, these layers of vegetation act as a natural sponge that absorbs, retains, and gradually releases water.

These elements also act as a natural water filter by capturing pollutants and contaminants from rainwater as it passes through. This filtering process improves the quality of stormwater runoff that eventually reaches the ground and surrounding water bodies. Plus, green roofs provide functional outdoor green spaces for residents living in urban environments.

Our mixed-use Ivy Commons development is one such haven for Philadelphians. It boasts a shared intensive green roof with rain gardens and a solar pergola. The green roof not only adds to the building’s aesthetics but also absorbs CO2, produces oxygen, improves air quality, reduces the heat island effect, and naturally absorbs rainwater, preventing runoff. It protects the surrounding environment from runoff while enhancing the mental and physical health of residents

Appropriate Site Grading

Last but certainly not least, appropriate site grading is needed to properly direct runoff away from buildings without rapidly sending it throughout the larger urban environment. Grading involves shaping the land’s surface to direct the flow of water toward appropriate drainage systems such as swales, ditches, and stormwater collection points. This prevents water from pooling in undesirable locations and reduces the risk of flooding.

Financial Benefits of Investing in Stormwater Management Measures

Investing in appropriate stormwater management when designing and building new developments extends the lifetime of those developments while limiting the amount of money needed to repair structures after a disaster occurs. It reduces your liability by protecting residents and the surrounding environment during extreme weather events, cuts insurance costs, and reduces maintenance costs.

Investing in stormwater management also reduces the need for future retrofits or renovations, as that property is “ahead of the curve” when it comes to evolving regulatory requirements. Furthermore, effective stormwater management systems set your property apart from others should you choose to sell. Lower maintenance, disaster response, and insurance costs can be a major selling point for buyers and investors.

Last but not least, many rebates, grants, and other forms of financial assistance exist for projects that contribute to local watershed health and stormwater management. Some conservation districts provide financial aid and/or technical support for these projects, and many water utilities offer incentives to developers who implement such systems to limit strain on local infrastructure.




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