Stormwater best management practices, called BMPs, are useful for reducing the spread of pollutants caused by stormwater runoff. This runoff poses a threat to the environment because it can collect pollutants from impervious surfaces, such as city streets and parking lots, and flow into nearby waterways.
Stormwater control measures help protect surrounding lands and other natural resources that could be compromised by pollutants such as fertilizers, sediment, pesticides, and other contaminants, both chemical and natural.
Table of Contents
- Why is Stormwater Runoff a Problem?
- Examples of Structural Stormwater BMPs
- Examples of Non-Structural Stormwater BMPs
- Benefits of Using BMPs for Stormwater Management
- What Stormwater BMPs Should Your Construction Site Use?
- Stormwater Management Resources and Guides
Why is Stormwater Runoff a Problem?
Runoff picks up oil, fertilizer, and other pollutants and carries them through ditches and storm drains. Ultimately, it deposits these contaminants in natural waterways, lakes, and oceans, creating health hazards for homeowners, farmers, and the overall ecosystem. Left unmanaged, runoff can degrade stormwater quality, compromising access to clean water and drinking water for people in nearby cities and towns, which is why implementing stormwater BMPs on your construction site is important.
Examples of Structural Stormwater BMPs
Structural BMPs employ physical structures to reduce runoff and mitigate against the spread of pollutants through water resources. They can employ natural elements such as soil or vegetation, or they can involve the construction or placement of manmade items such as rain barrels and cisterns.
1. Point BMPs
Point BMPs capture drainage at a specific location upstream.
- Constructed wetlands: These shallow aquatic wetlands reduce runoff and peak flow rates. Also called stormwater wetlands, they are sometimes created by retrofitting temporary water detention basins.
- Infiltration basins: Infiltration basins are designed to allow water to drain (or infiltrate) into the ground. They provide a mechanism for groundwater recharge, where water moves downward from surface water into groundwater.
- Sand filters (surface): This mechanism consists of a sand bed that filters out pollutants, which are then treated through settling and absorption into the sand. This method results in an 80% removal rate of total suspended solids.
- Rain barrels: These devices come in all sizes and can be found for sale or made by hand. They work by collecting rainwater from downspouts, preventing it from carrying contaminants into the ecosystem; the water is then reused for things like washing or landscaping.
- Rain gardens: These gardens won’t make it rain, but they will capture rain when it falls. They use wildflowers and other native vegetation to help the soil soak up rainwater.
- Cisterns: Like rain barrels, cisterns capture water runoff from roofs for reuse in irrigation or for other non-potable uses such as toilet flushing. They can be made of fiberglass, brick, plastic, concrete, or other materials.
- Wet ponds: Also known as retention basins, wet ponds contain a certain level of water permanently, with the capacity to add more during storms. In addition to handling peak storm levels of water, they can serve as way stations for migrating birds.
- Detention basins: Detention basins are earthen structures created through the use of a natural depression or via excavation. Like retention basins, they are used to control peak rate runoff.
2. Linear BMPs
Linear BMPs, as the name suggests, are laid out in a line. They’re narrow structures next to stream channels, such as swales and trenches, that filter runoff.
- Grassed swales: Swales are shallow channels with gentle slopes on both sides that guide runoff toward treatment areas and promote infiltration.
- Infiltration trenches: Infiltration trenches serve the same function as infiltration basins, but with a linear design. Their highly permeable soils promote the drainage of water directly into the ground, rather than it continuing as runoff.
- Vegetated filter strips: Areas planted with grasses, trees, and shrubs, these “buffer strips” (as they’re also known) are placed between pollution sources and the body of water. They filter pollutants out of the runoff before it reaches a lake, pond, or ocean.
- Sand filters (non-surface): These operate similarly to surface sand filters. In this case, however, they are underground, usually contained in a concrete shell consisting of three chambers.
3. Area BMPs
Areas BMPs cover a specific area, providing a more eco-friendly alternative to impervious areas. They’re used in place of standard impervious surfaces such as concrete, asphalt, and hard tiles.
- Green roofs: A green roof is a flat or slightly sloped roof planted with a layer of vegetation in soil. The soil can be relatively shallow (less than 6 inches) or deep, depending on whether it’s an extensive or intensive green roof. In this application of green building, the soil minimizes runoff as the plants soak up the water, keeping it from draining off the roof via a downspout.
- Porous pavement: Porous or permeable pavement can reduce runoff by infiltrating rainwater as well as melting snow. It also filters pollutants as they pass through it. It can take the form of pervious asphalt or concrete, plastic grid pavers, or interlocking pavers.
Examples of Non-Structural Stormwater BMPs
Whereas structural BMPs filter water runoff after it’s generated, non-structural BMPs act to keep stormwater runoff from being generated. Instead of addressing the problem after the fact, they work to prevent it from occurring. As a result, they’re both more efficient and more cost-effective than structural BMPs.
1. Protection of Sensitive Areas
This involves protecting sensitive areas from pollutants by designating them as conservation areas. Minimizing the sources of water pollution can help protect not only these areas but those in valleys downstream.
2. Minimizing Disturbance and Maintenance
The idea here is to prevent the generation of stormwater runoff by limiting the need for soil disturbance, grading, and vegetation removal. Preserving natural topography also reduces or eliminates the need for landscaping and maintenance.
3. Watershed Planning
Watershed planning takes a macro approach to mitigating runoff issues by looking at the entire watershed: from the highest elevation, where water is collected, to low points where it is deposited into bodies of water. Rather than seeking to solve problems at each point in the process, watershed planning forms partnerships among stakeholders within the watershed and takes into account the unique character of the watershed as a whole.
4. Community Education
Community education seeks to promote BMPs by providing information about the benefits of preventing runoff and how to utilize methods such as cisterns, rain barrels, and rain gardens.
5. Procedural and Institutional Ordinances
Local government ordinances can be used to mandate best practices, such as waste control on construction sites, which can minimize the level of runoff contamination. This can include public notice of procedures and a review of site plans. Such ordinances can reduce the source of contamination but do not address the runoff itself.
Benefits of Using BMPs for Stormwater Management
The benefits of managing stormwater runoff don’t end with reducing pollutants. Stormwater BMPs can reduce the threat of flooding, create whole new ecosystems, and provide economic benefits.
- Economic benefits: The economic benefits of stormwater management include increased economic development, more green jobs, and increased land and property values.
- Improved water quality: Removing pollutants from water improves water quality, both for humans and for the ecosystem as a whole.
- Better flood control: Flood risks are mitigated as a large amount of runoff is diverted to BMPs. It can be held there temporarily and released gradually to keep water levels manageable.
- Creates environmental amenities: Ponds, basins, rain gardens, and marshes can create stopover sites for migrating waterfowl like herons and egrets. They can also provide permanent habitat for other wildlife, creating new ecosystems. For humans, they can provide spots for hiking, fishing, swimming, and other recreational activities.
What Stormwater BMPs Should Your Construction Site Use?
In evaluating what BMPs to use on a construction site, a property owner will need to collect information about the site. Measure the site area and identify drainage areas. Then develop a site design plan and prepare a site map. The BMPs you decide to use will vary depending on the character of your site and the region where it’s located.
One step you can take is to be responsible with land use: protect natural features, such as existing vegetation, keeping exposed soil and clearing to a minimum. Limit grading, and take measures to control sediment. Cover or seed any dirt stockpiles. And for those areas that do need to be cleared, plant vegetation or take other steps to stabilize those areas once the job is complete.
Terracing your slopes and installing sediment barriers can help divert water away from slopes, bolstering erosion control and reducing the amount of material picked up in runoff. Silt fencing can help, too, but it’s important to make sure the bottom of fences are buried in the ground and that the fencing material is attached to the stakes securely. Fences shouldn’t be put up in a waterway.
Remove potential runoff debris by clearing mud and dirt away from entrances used by heavy equipment or other vehicles, and ensure the entrances don’t become buried beneath soil.
How To Implement Stormwater Management Plans
Once you’ve decided on a plan, you’ll need to implement it, maintain it, and monitor it to ensure it’s effective.
The contractors who install the BMPs should be trained to do so, and be sure to adhere to BMP maintenance requirements. BMPs should be inspected regularly to ensure they’re operating effectively, and routine maintenance should be provided.
Since many BMPs are only designed to handle a certain amount of sediment, leaving them too long without maintaining them can render them ineffective when it comes to sediment control and lead to more pollution.
As part of construction management, monitor progress and keep records so you know whether you need to modify existing BMPs or choose more appropriate ones. If you decide to add new BMPs, update your plan.
How Effective Are Stormwater BMPs?
According to the EPA, the best way to measure stormwater BMP effectiveness is to estimate the total load of the pollutant that could be removed by the BMP. You can calculate the total load by multiplying the volume of water discharged from the BMP by the average pollutant concentration.
This can be difficult to measure unless multiple samples are taken for many rain events over time. However, it is important to consider which BMPs are most effective for filtering out pollutants.
Stormwater Management Resources and Guides
If you’re a construction manager or property owner looking for ideas and information about Stormwater BMPs, you can refer to projects at EPA demonstration sites and check out the agency’s Siting Tool.
Environmental Protection Agency (EPA) BMP Siting Tool
The EPA’s BMP Siting Tool can be used to identify good locations to implement different kinds of BMPs. It takes into account factors such as the depth of the groundwater table, slope, drainage area, soil type, and road, stream, and building buffers.
EPA Stormwater Management BMPs
The BMPs mentioned above are more than just ideas. They’re sustainable construction projects that have been implemented in practice by the EPA in various low-impact development (LID) projects.
National Pollution Discharge Elimination System (NPDES) Program
Created under the Clean Water Act in 1972, the NPDES program is designed to regulate point sources where pollution is discharged. NPDES controls pollution at two levels, by establishing technology-based and water-quality-based limits.
Permits, tailored to each individual facility, are issued either by the EPA or the state in which projects are undertaken. They last for a specific period of time, up to five years, before which the facility must reapply.
A construction stormwater general permit is required for discharge from construction activity that ultimately disturbs one acre or more, as well as smaller sites included in a larger common plan of development. A number of industrial activities are covered, including coal and mineral mining; salvage and scrap yards; landfills; and heavy manufacturing sites such as chemical plants, paper mills, steel mills, and petroleum refineries.
Under a Construction General Permit, projects must develop a stormwater pollution prevention plan. This plan must be kept up to date so it reflects current site conditions. In addition, landowners/project directors must install and maintain sediment and erosion controls for the duration of construction to control stormwater discharges.
Pollution prevention controls must be in place to minimize the discharge of pollutants from stormwater as well as from leaked or spilled material. Inspections must be conducted either weekly or biweekly and within 24 hours of any storm event involving more than a quarter-inch of rainfall.
A wide variety of stormwater BMPs are available to mitigate pollutants in runoff, which can affect surface waters and contaminate storm sewer systems. Identifying which BMPs are appropriate for your area to best mitigate runoff issues can help preserve human health and the environment, while sometimes enhancing aesthetics and even creating new ecosystems.