Impervious surfaces are constructed areas that prevent water from soaking into the underlying soil. These surfaces, including asphalt roads, concrete sidewalks, building rooftops, and compacted soils, force precipitation to become surface runoff. Managing this rapid runoff is a primary goal in maintaining healthy urban and suburban environments, influencing local water cycles and infrastructure. The reduction of these surfaces and the adoption of alternative materials are practical methods for better water management.
Defining the Environmental Costs of Impervious Surfaces
The inability of water to infiltrate surfaces increases the volume and velocity of stormwater runoff. This surge contributes to higher peak flow rates, which can quickly overwhelm conventional storm drainage systems and lead to localized flooding. The faster movement of water also increases its erosive power, causing scour and instability in natural stream banks.
This rapid surface flow prevents the natural replenishment of underground water sources, known as groundwater recharge. When less water seeps into the earth, base flow in streams and rivers declines during dry periods, affecting ecosystems that depend on consistent water levels. Furthermore, as water rushes over pavement, it collects non-point source pollutants, including oils, heavy metals, pesticides, and excess nutrients.
The resulting polluted runoff is discharged directly into lakes and streams, degrading water quality and harming aquatic organisms. Dark-colored impervious surfaces like asphalt absorb solar energy, raising ambient air and surface temperatures and creating the urban heat island effect.
The runoff itself can also become heated as it moves across warm pavement, increasing stream temperatures. This warm water transfer creates thermal shock for aquatic life, stressing the biological health of receiving waterways. Addressing these issues requires removing existing hardscape and replacing it with materials that restore natural infiltration.
Physical Reduction and Landscape Conversion Techniques
One direct approach to managing runoff involves the physical removal of unnecessary hardscape, a process often called de-paving. Patios, unused driveways, or sections of parking lots can be broken up and converted back into pervious areas. Following removal, the underlying soil must be addressed, as it is severely compacted from years of being covered.
Compacted soils require tilling and amendment with topsoil or organic matter to loosen them and allow vegetation to establish. Once prepared, the area can be converted into gardens or lawn space, utilizing the deep root systems of shrubs and trees. These deeper roots absorb water and create natural pathways for increased infiltration, which is more effective than compacted turf grass.
The installation of rain gardens, also known as bioretention areas, is another effective technique that works in conjunction with existing hard surfaces. These are shallow, engineered depressions typically located downslope from a roof downspout or driveway. Rain gardens are planted with water-tolerant native vegetation chosen to withstand both wet and dry conditions.
The garden’s engineered soil mix and plants filter runoff, allowing water to slowly percolate into the ground while trapping sediments and pollutants. Beyond dedicated gardens, redirecting water from existing impervious surfaces can yield substantial benefits. This involves disconnecting roof downspouts from the storm sewer system and directing the flow to a vegetated area.
This allows the runoff to spread out over a lawn or a filter strip, slowing the flow and giving the water a chance to soak into the ground. Directing pavement runoff to a pervious area increases the time the water takes to reach a drain, providing an opportunity for natural filtration. This method treats a portion of the runoff without requiring full removal of the impervious surface.
Implementing Permeable Paving Alternatives
When a paved surface is necessary for access or parking, permeable paving materials offer a functional alternative to traditional concrete and asphalt. Unlike conventional surfaces designed to be watertight, permeable systems allow precipitation to pass through the surface into an underlying stone base layer. This reservoir temporarily stores the water before it gradually infiltrates the native soil below, mimicking the natural hydrologic cycle.
Permeable interlocking concrete pavers (PICP) consist of nonporous, brick-like units separated by designed joints. These gaps are filled with crushed stone or aggregate, which serves as the pathway for water infiltration. The paver units provide the structural load-bearing capacity, while the open joints manage the stormwater runoff.
Porous concrete and porous asphalt are distinct from their traditional counterparts due to specialized mix designs. These materials intentionally reduce the fine aggregate content, creating interconnected voids that allow water to flow directly through the pavement layer. They are often used in high-traffic areas like parking lots where a continuous, durable surface is required.
Grid systems provide a durable, vegetated alternative for lower-traffic areas such as residential driveways and overflow parking. These systems use plastic or concrete grids to form a stable surface structure. The cells are filled with gravel or soil and grass, protecting the infill from compaction. The grid maintains structural integrity while the infill allows precipitation to soak directly into the ground, reducing runoff volume.