Quick Summary: Rainwater does not simply disappear once a storm passes. Some soaks into the ground, some evaporates, and some becomes runoff that travels through drains, pipes, basins, and local waterways. Stormwater management systems help control that flow, reduce flood risk, and protect water quality.
Following the Water Beyond the Storm Drain
During a heavy rainstorm, it is easy to spot water running along a curb and disappearing into a storm drain. Once the rain stops and the pavement dries, most of us do not give it another thought.
But that drain is only one stop along the way.
Where rainwater goes next depends on the surrounding landscape and the infrastructure beneath it. Understanding that journey helps explain why stormwater management is an important part of planning communities, protecting property, and maintaining healthy waterways.
How Rain Becomes Stormwater Runoff
Before an area is developed, rainfall has more opportunities to soak into the soil. Trees and other vegetation intercept some of the rain, while water that reaches the ground may infiltrate, evaporate, or flow toward a stream.
Buildings and pavement change that process. Roofs, roads, sidewalks, and parking lots are impervious surfaces, which prevent water from readily soaking into the ground. Instead, rain flows across these surfaces as stormwater runoff. Even lawns on graded, compacted soil absorb far less water than undisturbed ground.
Think about the difference between rain falling on a wooded property and rain falling on a shopping center parking lot. The wooded area offers soil and vegetation that can absorb and slow water. On the parking lot, much of that same rainfall moves toward the nearest drainage inlet.
Development can therefore increase both the amount of runoff and how quickly it reaches the drainage system. Stormwater management systems help address those changes.
Where Does Water Go After It Enters a Storm Drain?
Once runoff enters a storm drain, it typically moves through a network of pipes, ditches, or channels. From there, it may flow into a stormwater management facility or discharge into a nearby stream, river, or other waterway.
One common misconception is that anything entering a storm drain goes to a wastewater treatment plant. In communities with separate storm sewer systems, that is generally not the case.
Sanitary sewers carry wastewater from toilets, sinks, and showers to treatment facilities. Storm sewers collect rainfall runoff through a separate network, typically discharging it to local waterways, sometimes after passing through a stormwater facility, but without the treatment a wastewater plant provided.
Some older communities use combined sewer systems, where stormwater and wastewater share the same pipes. These systems normally convey water to a treatment plant, but heavy rainfall can exceed their capacity and cause combined sewer overflows, which can release a mix of untreated stormwater and sewage into waterways. Philadelphia, Pittsburgh and Harrisburg are among the many cities with combined systems.
This connection between streets and streams is why stormwater management involves more than keeping roads dry. What washes into a drain can also affect the water downstream.
Slowing the Flow: How Stormwater Systems Work
If runoff from an entire neighborhood or commercial property reached a stream at once, that sudden surge could contribute to flooding and erosion. Stormwater facilities help manage the timing of that flow, and some also reduce runoff volume or remove pollutants.
A familiar example is the basin you might see beside a residential development or office park. Although it may look like an open grassy depression, a dry detention basin is designed to temporarily collect water during a storm and release it at a controlled rate. It generally drains within a set period after each storm so it has room for the next rainfall.
A wet pond, often called a retention pond, works differently. It maintains a permanent pool of water, with additional space above that pool to store runoff. Along with controlling discharge rates, wet ponds can improve water quality by allowing sediment to settle and supporting other pollutant-removal processes, such as nutrient uptake by plants.
That means water remaining in a pond is not automatically a sign of a drainage problem. The opposite can be true for a dry retention basin. Water that lingers several days after a storm may indicate a clogged outlet or poorly draining soils. The important question is whether the facility is functioning as designed.
Not every stormwater facility is visible. Some properties use underground chambers, tanks, or other storage structures beneath parking lots or landscaped areas. These systems temporarily store runoff, then either release it through controlled outlets or allow it to infiltrate into the soil, helping manage stormwater in areas with limited surface space. Depending on the design, additional features may be needed to provide filtration, and pretreatment is often needed to keep sediment from clogging the system.
Giving Rainwater a Place to Soak In
Other stormwater approaches focus on managing rainfall closer to where it lands.
Green infrastructure uses vegetation, soil, and permeable materials to capture, slow, and filter runoff. A planted area beside a parking lot, for example, may be a rain garden or bioretention facility designed to receive water from the surrounding pavement.
Vegetated swales can guide runoff through shallow channels while slowing and filtering it. Permeable pavement allows water to pass into a stone storage layer beneath the surface, while infiltration systems give collected runoff an opportunity to soak into suitable soils.
These features can work alongside conventional drains, pipes, and storage facilities. Their performance depends on site conditions, including soil infiltration rates, depth to bedrock or groundwater, and the presence of carbonate geology, which can increase sinkhole risk and limit infiltration in parts of Southeastern Pennsylvania.
Why the Water’s Journey Matters
Rainwater may start relatively clean, but its journey across developed land can change that. Along the way, runoff can pick up oil, fertilizer, sediment, litter, pet waste, road salt, and metals.
When that runoff reaches a waterway, those pollutants can travel with it. Fast-moving water can also erode streambanks and carry additional sediment downstream.
Effective stormwater management considers both the flow of water and its quality. Slowing runoff, reducing the amount leaving a site, and removing pollutants where feasible help protect the places where that water eventually ends up.
Good Design Needs Ongoing Care
Even a well-designed stormwater system needs maintenance.
Leaves and litter can block inlets, sediment can reduce available storage, and clogged outlets can interfere with drainage. Vegetation also needs care to continue performing its intended role. This includes routine mowing and keeping trees and woods growth off basin embankments, where roots can weaken the berm.
Routine inspections and maintenance help address these issues. Cleaning drainage structures, removing accumulated sediment, and maintaining planted areas are all part of keeping a system functioning.
A clear maintenance plan establishes who is responsible for that work and when it should happen, helping protect the investment long after construction is complete. In Pennsylvania, responsibility for long-term operation and maintenance of post-construction stormwater facilities is typically documented in an O&M Plan, often with a recording agreement with the municipality.
Planning for the Next Storm
Effective stormwater management begins with understanding the site. Engineers evaluate rainfall, slopes, drainage areas, soil conditions, impervious surfaces, land cover, and existing draining patterns to estimate how much runoff a project will generate and where it will travel.
That information helps determine the appropriate combination of drainage, storage, infiltration, and treatment features. The design must also account for the surrounding drainage network and applicable local and state requirements, such as municipal stormwater ordinances, PA DEP Chapter 102 permitting, and NJ DEP Stormwater Management Rules.
For municipalities, developers, and property owners, addressing stormwater early can reduce costs and avoid redesign, since stormwater facilities often affect site layout, grading, and usable area.
Carroll Engineering Corporation provides stormwater management services throughout Pennsylvania and New Jersey, including design of detention and infiltration systems, grading, erosion and sediment control, MS4 program support, and permitting support.
Connect with Carroll Engineering at info@carrollengineering.com or 215-343-5700 to discuss your project’s stormwater and drainage needs.