Stormwater 101

What is stormwater? Stormwater is the flow of water that runs off land surfaces and impervious areas immediately following a rainfall or snowmelt event. Precipitation that is generated during a rainfall or snowmelt event can make its way back into the environment in several ways. Some of the precipitation infiltrates into the soil, some is taken up by plants, and some is evaporated into the atmosphere. The remaining portion runs off land surfaces and imperviousness. This run off is known as stormwater. For most developed areas, management of stormwater is shared by federal, state, and local government.

Example of excess rainfall that is prevented from infiltrating the ground due to impervious coverage (Cameron Street, Harrisburg).

What is the problem? Impervious areas are hardened “nonporous” surfaces such as rooftops, streets, or even tightly compacted soil. These areas prevent precipitation from naturally infiltrating, or soaking into the ground. This in turn prevents the groundwater from being replenished, or “recharged.” As stormwater runs over the land it accumulates debris, chemicals, and other pollutants and flows into a storm sewer system or directly into a waterbody. Anything that enters a storm sewer system is discharge untreated into the waterbodies we use for swimming, fishing, and providing drinking water.

When stormwater starts to flow at a high velocity, it carries with it a great amount of energy that can severely erode stream banks and cause excess sediment to wash into the water. Excessive sediment can destroy aquatic habitats and cloud the water, making it difficult for aquatic plants to grow.

Runoff may be carried through natural or manmade drainage ways, ditches, or swales. In most developing areas, stormwater is conveyed through a system of catch basins and pipes known as a storm sewer system. In some cases, these systems can be overwhelmed by excess precipitation and cause flooding and sewer overflows that cause contamination.

Diagram courtesy Arkansas Watershed Advisory Group