Stormwater Basics

The term stormwater generally refers to the accumulation of water that results from precipitation (rain or snow). The amount of stormwater that runs off the surfaces over which the precipitation occurs depends on the type of surface—vegetated surfaces are commonly able to minimize stormwater runoff by soaking up the water in the soil (infiltration) and the plants (root uptake and evapotranspiration) while urban surfaces like asphalt and concrete prevent infiltration due to their impervious characteristics. Precipitation over urban areas not only results in increased stormwater runoff volumes (high risk of flooding), but increased transportation of pollution as well. Pollutants like sediment, oil, trash, and nutrients ends up on urban surfaces, and is washed into streams, rivers, and lakes if there is no intervention.

 The National Pollutant Discharge Elimination System (NPDES) regulates stormwater discharges from three potential sources: construction activities, industrial sources and municipal separate storm sewer systems (MS4s). Most stormwater discharges are considered point sources, and operators of these sources may need to have an NPDES permit. The purpose of the permitting mechanism is to prevent stormwater runoff from washing harmful pollutants into local surface waters.

Why is Stormwater an Issue?

Managing stormwater has two primary concerns: runoff volumes and velocities that can cause flooding and stream bank erosion, and the pollution carried by runoff.

Stormwater can pick up and convey sediment, trash, debris, chemicals, nutrients, metals and other pollutants that flow into a storm sewer system or directly to a lake, stream, river, wetland or coastal water. Stormwater that is not properly treated discharges into the water bodies used for swimming, fishing and drinking water. Some of the most common pollutants of concern conveyed in untreated stormwater runoff are:

• Sediment - Sediments can cloud the water bodies and make it difficult or impossible for aquatic plants to grow. Sediment also can destroy aquatic habitats.
• Excess nutrients - Nutrients can cause algal blooms. When algae die, they sink to the bottom and decompose in a process that removes oxygen from the water. Fish and other aquatic organisms can't exist in water with low dissolved oxygen levels.
• Bacteria and other pathogens - Bacteria can wash into swimming areas and create health hazards, often making beach closures necessary.
• Heavy metals - Metals that originate from vehicle tires and metal roofs, such as copper and zinc, are conveyed in stormwater runoff to rivers and streams, where they can eventually become concentrated enough to disrupt aquatic ecosystems.
• Trash & debris - Trash and debris such as plastic bags, six-pack rings, bottles and cigarette butts washed into water bodies can choke suffocate, or disable aquatic life like ducks, fish, turtles and birds.
• Household hazardous wastes - Insecticides, pesticides, paint, solvents, used motor oil and other auto fluids can poison aquatic life. Land animals and people can become sick from eating diseased fish and shellfish or ingesting polluted water.

What does Stormwater Affect?

Stormwater literally affects every aspect of life on the planet, from the rivers and streams that receive runoff, to the wildlife and indigenous plants that occupy our watersheds and call those water bodies home. Everyday activities such as driving, maintaining vehicles and lawns, disposing of waste and even walking pets often cover impervious surfaces with a coating of various harmful materials. Eventually every drop of oil, sediment particle, chemical, or trash and debris that comes to land on impervious surfaces across the world will be relocated by stormwater. If we can’t manage the pollutants before they end up on impervious surfaces, we need to intercept them before they ultimately come to reside in watersheds, the home of so many different plants and animals. If concern for ecological health is not compelling enough, then let’s consider the impact on each taxpayer when required to pay increased rates to operate the water or sewer treatment plants that remove these pollutants prior to the water either being distributed to citizen's homes or put back into the system.

Drinking Water - One of the most important resources is the watershed and the need for drinking water. In the United States, citizens take for granted the access to clean drinking water. As watersheds become contaminated, the cost people will need to pay for drinking water will increase. Other countries have had to deal with this problem for years. In China, large tank systems boil the water to purify it before consumption. In some of the top hotels, guests are given bottled water for drinking and brushing teeth. Drinking the tap water is not advised, as much of it is not potable.

Natural Areas - The degradation caused by urban stormwater pollution is serious. Changes in land use that increase impervious cover lead to flooding, erosion, habitat degradation and water quality impairment. Construction sites, power plants, failed septic systems, illegal discharges and improper sewer connections all contribute substantial amounts of pollution in runoff. This pollution in turn impacts important natural resources as well as other equally important activities such as commercial and recreational fishing, swimming and boating. Urban runoff can harm aquatic life in many ways due to changes in water chemistry and habitat loss. For example, when metals and organics enter stormwater and are carried to the watershed, they are toxic to fish and other forms of aquatic life. Urban stormwater runoff is not alone in causing these impacts; industrial and agricultural runoff are major contributors to water quality impairment when discharges are uncontrolled.

Flood Control - Pollution is visible to almost everyone; however, there is another aspect of stormwater that needs consideration. As the amount of impervious surfaces has increased, there has been an increase on nature’s systems. Natural waterways have formed over time to accommodate runoff. As more impervious surfaces are built, the volume of stormwater runoff contributing to the natural waterways also increases. Water that would otherwise have percolated into the ground and eventually reached the water table is instead directed to rivers, lakes and streams that are not designed to handle this increase. The system overload leads to higher water levels and eventually to natural scour and erosion of the lake, stream and riverbanks. In many cases, the system cannot adjust, or is not designed to accommodate the extra volume, and that can result in flooded parking lots or riverbanks spilling over, costing communities thousands, if not millions, in property damage.

BMP vs SCM

Best management practice (BMP) is the overarching term for a permanent structural or non-structural method or device used to treat or manage stormwater runoff. A stormwater control measure (SCM) is a type of BMP—a permanent structural device used to manage stormwater runoff volume, flow rate, and/or pollutant load. The distinction between the two terms is primarily the structural nature of a SCM. Often, the terms are used interchangeably; however, it is useful to define the difference between them because SCM is used more commonly in technical language pertaining to the testing and evaluation of structural management solutions. SWEMA’s manufacturer members may generally refer to their products as SCMs, and so it is the favored term in much of our content.