Best Management Practices
In short a stormwater BMP (Best Management Practice) is a treatment for stormwater. Different types of BMP's have varying levels of treatment for water quality and quantity.
Many new developments are required to install stormwater BMP's to reduce nutrients and the volume of the runoff. The property owners or association is then responsible for regular inspection and maintenance to keep the BMP functioning properly. The responsible party must inspect the BMP at a minimum of monthly and after each significant rainfall.
The Stormwater Division also inspects each stormwater BMP annually to ensure that proper inspection and maintenance is being completed. If the annual inspection reveals a need for maintenance or repair, the responsible party will be given 30 days to bring the BMP into compliance. After this time the Stormwater Division will take steps to bid out the work to be completed and then charge the BMP owner for the cost plus a 25% surcharge.
Types of Stormwater BMP's
Wet Pond - The wet pond is the most recognizable stormwater BMP. These ponds typically range from 2 to 5 foot deep and provide stormwater treatment by reducing nutrients such as Nitrogen and Phosphorus, collecting trash and sediment, and by holding large rainfalls to be released slowly over time.
Dry Pond - Dry ponds are very similar to wet ponds except that it has no permanent pool of water between rainfall events. Dry ponds do provide some nutrient removal but are mainly used as peak flow control by catching and holding rainwater and slowly releasing it over a period of 2-5 days.
Constructed Wetland - Wetlands are one of the most effective means of pollutant removal. A wetland acts as a sponge and the plants use up the excess nutrients leaving behind cleaner water. Wetlands require special attention and maintenance to maintain the correct water level so that all the plants can survive.
Bioretention - Bioretention or commonly known as "rain gardens" are also very effective at pollutant removal. These cells also increase infiltration of stormwater into the soil where it is treated and can recharge the ground water. Because bioretention cells can be small, they are often used in places where land is a premium and when properly landscaped and maintained will blend seamlessly into the other landscaping on the property.
Level Spreader - A level spreader is basically a long pool of water with a level lip on one side that allows water to spread out and spill evenly over the edge. By spreading the water out over a large area, the water will not erode the land as much and will come in contact with more plants and grass that provide treatment.
Grassed Swale - Most people may think that a grassed swale is nothing more than a ditch but there is more to it. A true grassed swale bmp is shallow with a wide flat bottom that allows stormwater to spread out, slow down and soak into the ground as much as possible. This provides treatment as the plants and soil filter out pollutants and nutrients.
Permeable Pavement - There are various methods of installing pavement that allows stormwater to infiltrate while still providing the structural benefits of pavement. Pervious concrete, pervious asphalt, stone pavers, and grid pavers, are all means of infiltration and stormwater treatment.
Propietary Devices and Other BMP's - There are many different creative methods of treating stormwater. Many of these include some type of filter or baffle that must be regularly maintained. These devices are often expensive and are approved on an individual basis if appropriate treatment can be shown. Most, however, take up far less space and may work well in urban situations.
Because there are so many varieties of these devices, no specific guidance is given for design or inspection and maintenance. Follow the manufactures instructions.
Workshops and Certifications
The Stormwater Division also hosts several workshops about BMP maintenance. The division hopes to have a short free seminar twice a year in which all BMP owners and maintenance professionals may come discuss methods that are and are not successful.
The North Carolina State University Biological and Agricultural Engineering Department along with the NC Extension Service also provides many workshops and seminars on stormwater BMP's. They also offer a certification for BMP inspection and maintenance that is highly suggested for any one who owns, inspects, or maintains a stormwater BMP.