Rainfall hitting impervious surfaces (paved roads, rooftops, parking lots) cannot soak into the ground and becomes stormwater runoff. This water flows across the land, picking up pollutants and debris before entering storm drains or waterways. Best Management Practices (BMPs) are tools developed to manage this runoff effectively. BMPs mitigate the environmental impact of development by controlling the water’s path and contents. The goal is to protect local streams, rivers, and lakes from the negative effects of this rapid, contaminated flow.
The Purpose and Function of Stormwater BMPs
Stormwater Best Management Practices are implemented to achieve two primary objectives: flow control and quality control. Flow control focuses on reducing the volume and velocity of water leaving a developed site during a rain event. By slowing the runoff, these practices help prevent downstream erosion, streambank instability, and flash flooding in urban areas. This management of water quantity ensures the natural receiving environment is not overwhelmed by rapid surges of water.
Quality control involves removing various pollutants the runoff has collected. Common contaminants include sediment, excess nutrients like nitrogen and phosphorus from fertilizers, and pathogens. BMPs employ physical, chemical, and biological processes to filter out these substances before the water is released. These practices are often mandated by federal and local environmental regulations to protect aquatic life and safeguard public health.
Categorizing BMPs: Structural and Non-Structural Approaches
Stormwater management practices are divided into two broad categories based on their nature and application. Structural BMPs include physical, engineered systems that are permanently constructed into the landscape. These facilities actively capture, treat, or store runoff and require a physical alteration of the site’s topography or drainage system. They are designed to manage water after it has become runoff, acting as a fixed part of the infrastructure.
Non-structural BMPs focus on preventing pollution and reducing runoff at the source through planning and procedural changes. These are operational, behavioral, or educational measures, not physical structures. Examples include land use planning policies, conservation of natural areas, and public outreach programs. Non-structural approaches aim to minimize the problem before it requires an engineered solution, often resulting in more cost-effective outcomes.
Specific Examples of Structural BMP Systems
Structural BMPs are designed with specific mechanisms to manage stormwater, typically categorized by how they interact with the water.
Infiltration Systems
One category is infiltration systems, which are engineered to encourage runoff to soak into the ground, thereby recharging groundwater supplies. Permeable pavements, for example, use specialized porous materials that allow rainwater to pass through the surface into an underlying stone reservoir. Infiltration trenches are shallow, gravel-filled ditches that collect runoff and allow it to slowly filter into the native soil.
Filtration Systems
Filtration systems function by passing stormwater through a specific medium to remove fine particles and associated pollutants. Bioretention areas, often called rain gardens, are landscaped depressions that use a mixture of engineered soil, mulch, and vegetation to filter runoff. These systems rely on the soil matrix and plant roots to absorb and break down contaminants. Sand filters operate by directing runoff through a layer of sand, which acts as a physical sieve to trap sediment and debris before the cleaner water drains away.
Detention and Retention Systems
The third category includes detention and retention systems, which focus on temporarily holding water. Dry detention basins are engineered depressions that collect stormwater during a storm and slowly release it over several hours through a controlled outlet structure. These are primarily used for flow control, as they are typically dry between rain events. Wet retention ponds, in contrast, maintain a permanent pool of water, allowing pollutants to settle out through gravity and biological action before the water is discharged.
The Role of Maintenance and Operational BMPs
The success of any stormwater management strategy relies on consistent maintenance and effective operational practices. Structural BMPs, such as filtration beds and ponds, require routine upkeep to ensure they function as intended. Maintenance activities include periodic dredging of accumulated sediment from the bottom of retention ponds or replacing clogged filter media in bioretention areas. If these systems are neglected, their capacity to control flow and remove pollutants significantly diminishes.
Operational BMPs are a form of non-structural practice focused on good housekeeping and spill prevention to stop pollution at the source. This includes regularly sweeping streets and parking lots to remove sediment, trash, and debris before a storm washes them into the drainage system. Proper material storage protocols require that chemicals, fertilizers, and construction materials be kept in covered areas away from rain. Implementing spill prevention and response plans limits the environmental damage from accidental releases of toxic substances.