Stormwater runoff occurs when precipitation flows over impervious surfaces like roads, parking lots, and rooftops instead of soaking into the ground. This process prevents groundwater recharge and collects pollutants, which are then channeled into local waterways. The transformation of natural landscapes into urban environments significantly increases the speed and volume of runoff, leading to erosion and flooding. Stormwater Best Management Practices (BMPs) are techniques developed to manage the quality and quantity of this accelerated runoff.
Defining Stormwater Best Management Practices
Stormwater BMPs manage the adverse effects of rainfall in developed areas by replicating the natural hydrologic cycle, which is disrupted by impervious surfaces. They are implemented to control the volume of water flow and treat contaminants. This control is necessary due to non-point source pollution, which originates from many diffuse sources rather than a single pipe.
Runoff picks up pollutants, including sediment, heavy metals, oil, grease, pesticides, and excessive nutrients like nitrogen and phosphorus. These substances degrade the health of streams and rivers, threatening aquatic ecosystems and human water supplies. BMPs mitigate this threat using physical, chemical, and biological processes to remove or neutralize pollutants before the water is released downstream. Although managing water quantity to control flash flooding is a function, the primary necessity of a BMP is improving water quality.
Classifying BMPs: Structural and Non-Structural Solutions
Stormwater management classifies BMPs into two categories based on function and form. Structural BMPs involve constructing permanent, engineered facilities built into the landscape to manage runoff physically. These solutions capture, detain, filter, or infiltrate stormwater once it becomes runoff. Examples include retention ponds, constructed wetlands, and specialized filtration devices.
Non-structural BMPs focus on policies and planning efforts to prevent runoff and pollution at the source. These community-wide or site-level practices do not rely on fixed physical structures. Examples include public education campaigns, urban fertilizer ordinances, street sweeping programs, and land-use planning that minimizes soil disturbance. These methods promote behavioral change and sustainable development patterns to reduce the problem before an engineered solution is required.
Common Examples of Structural BMPs
Structural BMPs manage stormwater through three primary mechanisms: infiltration, filtration, and detention. Bioretention areas, or rain gardens, exemplify infiltration and filtration. These shallow, landscaped depressions utilize an engineered soil mix and dense vegetation to filter runoff. As water passes through the filter media, pollutants like suspended solids, heavy metals, and nutrients are physically trapped, adsorbed, or taken up by plant roots.
A bioretention area can remove up to 90% of nutrients and chemicals and 80% of sediments from the runoff volume they treat. The system is typically layered, often featuring an underdrain at the base to ensure filtered water can be slowly discharged, preventing excessive standing water.
Permeable pavements replace traditional impervious asphalt with porous materials like specialized concrete, asphalt, or interlocking pavers. These surfaces allow rainwater to percolate directly through the pavement into a stone base layer below. The water is temporarily stored there and slowly infiltrated into the native soil or released.
This infiltration mechanism reduces runoff volume and recharges groundwater supplies, while the stone base filters suspended solids. Detention and retention ponds are larger-scale structural solutions located at the end of a drainage system. Detention ponds temporarily hold excess stormwater and slowly release it to control peak flow rates and prevent downstream flooding. Retention ponds maintain a permanent pool of water, allowing for continuous settling of pollutants and providing biological water quality treatment.
Ensuring Long-Term Effectiveness Through Maintenance
The long-term success of any stormwater BMP hinges on consistent maintenance. Structural BMPs are not “set-it-and-forget-it” systems; their lifespan requires proper care. Neglecting maintenance can lead to system failure, transforming the BMP from a functional asset into a pollution source or a flood hazard.
For infiltration-based systems, sediment build-up is a primary failure point, as fine particles clog the filter media or the porous surface of permeable pavement, reducing water absorption. Routine tasks, including removing accumulated sediment from forebays, clearing debris from inlets and outlets, and managing vegetation growth, are necessary to preserve capacity. Without regular attention, a BMP can quickly fail to meet its water quality and quantity objectives, necessitating costly rehabilitation or replacement.