Impervious surfaces are man-made or compacted surfaces that significantly reduce or prevent the infiltration of water into the underlying soil, causing precipitation to run off rather than soak into the ground. In stormwater management, impervious surfaces are a primary factor influencing runoff volume, peak flow rates, and water quality.
Common examples of impervious surfaces include asphalt and concrete pavement, rooftops, sidewalks, parking lots, and compacted gravel or soil that has been densified to the point where infiltration is minimal. These surfaces disrupt the natural hydrologic processes that would otherwise allow rainfall to infiltrate, recharge groundwater, and be taken up by vegetation.
As a result, areas with high percentages of impervious cover generate greater volumes of stormwater runoff and do so more rapidly. This leads to increased peak discharges, which can contribute to flooding, stream instability, and channel erosion. In addition, impervious surfaces accumulate pollutants such as oils, metals, nutrients, and sediments, which are then washed off during storm events and conveyed to receiving waters.
Impervious surfaces also reduce groundwater recharge by limiting infiltration, which can lower water tables and diminish baseflow in streams during dry periods. This alteration of the natural water balance is a key concern in urban and suburban watershed management.
To mitigate these impacts, stormwater management practices often aim to reduce effective imperviousness or manage runoff from these surfaces through techniques such as permeable pavements, green infrastructure, infiltration practices, and detention or retention systems.
So, impervious surfaces are a defining characteristic of developed landscapes and play a central role in shaping both the quantity and quality of stormwater runoff within a watershed.