Stormwater management (SWM) is the systematic process of controlling the runoff that results from precipitation. This control is necessary because urbanization fundamentally alters the natural path water takes after it falls to the earth. In a natural landscape, rain soaks into the soil and is filtered by vegetation, slowly recharging groundwater and feeding streams.
The construction of buildings, roads, and parking lots creates vast expanses of impervious surfaces that prevent water from soaking in. SWM addresses this change by implementing systems designed to either mimic the natural water cycle or safely divert the excess water. The methods used are designed to protect infrastructure, maintain water quality, and manage the volume and velocity of the water flowing across the landscape.
Why Unmanaged Runoff Requires Action
When natural ground cover is replaced by concrete and asphalt, the amount of runoff generated increases because the water cannot infiltrate the soil. This surge in water volume and velocity causes significant physical and environmental problems. Uncontrolled stormwater runoff is directly linked to urban flooding, as drainage systems are quickly overwhelmed by the rapid influx of water during a storm event.
The increased water flow leads to the erosion of stream banks and channels. This erosion destabilizes the banks and deposits fine sediment into the waterways, which can smother aquatic habitats and increase the turbidity, or cloudiness, of the water. High-velocity discharge from developed areas can increase annual runoff volumes by two to four times in suburban areas and up to fifteen times in highly urbanized environments.
Water quality is severely degraded as runoff travels across hard surfaces, picking up a wide range of contaminants, creating non-point source pollution. This water washes away pollutants such as motor oil, gasoline, and heavy metals that accumulate on roads and parking lots. It also collects fertilizers and pesticides from lawns, sediment from construction sites, and bacteria from pet waste. These pollutants are carried directly into local water bodies, often without any treatment, where they pose a threat to public health and aquatic life. Excess nutrients like nitrogen and phosphorus from fertilizers can trigger algal blooms, which deplete oxygen levels in the water and create “dead zones” that harm fish and other organisms. The introduction of these contaminants makes unmanaged stormwater one of the leading causes of water quality impairment in urban areas.
Conventional Stormwater Systems
The traditional approach to managing stormwater is often referred to as “gray infrastructure,” designed primarily for rapid conveyance and removal of water. This historical method views stormwater as a waste product that needs to be moved away from developed areas as quickly as possible to prevent flooding. These systems rely on a network of hard, non-porous structures to collect and transport runoff.
The system begins with elements like curb-and-gutter systems and catch basins, which channel surface runoff into a network of underground pipes. These pipes discharge the collected stormwater into a local receiving water body, such as a river, lake, or ocean.
In some cases, the gray infrastructure includes large, centralized detention or retention ponds. Detention basins hold water temporarily, slowly releasing it downstream after a storm to reduce peak flow rates. Retention ponds are designed to hold water permanently. This historical approach, while effective at flood control, typically discharges the collected stormwater with minimal or no treatment, transferring the pollution problem downstream.
Integrating Green Infrastructure Techniques
Modern stormwater management increasingly relies on Low-Impact Development (LID) or Green Infrastructure (GI), which contrasts with the conventional approach by managing water closer to where it falls. This strategy employs natural elements, such as soil and vegetation, to reduce runoff volume and filter pollutants before the water enters a waterway. This is accomplished by mimicking the natural processes of infiltration, filtration, and evapotranspiration that occur in undeveloped landscapes.
One common GI technique is the implementation of bioswales and rain gardens, which are shallow, vegetated channels or depressions designed to capture and temporarily hold runoff. Bioswales use specialized soil media and plants to slow the water’s velocity, allowing sediment to settle out and pollutants like heavy metals and hydrocarbons to be naturally filtered and absorbed by the soil and plant roots. These systems effectively reduce both the volume and the rate of stormwater discharge.
Permeable pavements, another widely used technique, are specially engineered surfaces for sidewalks, parking lots, and low-traffic roads that allow water to seep through the surface layer. This allows the stormwater to infiltrate into a stone sub-base layer underneath, reducing runoff and promoting groundwater recharge. This method prevents the water from becoming surface runoff and collecting pollutants.
Green roofs are utilized, where a layer of vegetation, soil, and drainage material is placed on top of a conventional roof structure. These systems capture rainfall directly, with the plants and soil absorbing a significant portion of the precipitation, which is then released back into the atmosphere through evapotranspiration. This process significantly reduces the amount of runoff from building tops.
Other techniques include planter boxes and urban tree canopies, which integrate vegetation and soil into the built environment to intercept and absorb rainfall. By integrating these decentralized, nature-based solutions, communities can manage the first inch of rainfall more effectively, reducing the strain on gray infrastructure and improving the quality of water that eventually reaches local ecosystems.