stormwater

Stormwater refers to precipitation, such as rain or melted snow, that does not infiltrate the ground and instead becomes surface runoff. In urban and developed areas this runoff is collected by municipal drainage systems and discharged into nearby waterways, lakes, or oceans, often with little or no treatment.

As runoff moves across surfaces like roofs, roads, and fields, it can pick up pollutants including sediment, nutrients, metals, hydrocarbons, pesticides, and bacteria. The specific mix depends on land use and activity, and stormwater quality can influence the health of receiving waters.

Stormwater can contribute to flooding, erosion, and habitat degradation. It alters the timing and magnitude of freshwater flows, transports pollutants, and can lead to increased sediment deposition in rivers and reservoirs. Where possible, natural storage and infiltration of runoff help mitigate these effects.

Infrastructure for stormwater typically includes networks of drains, pipes, culverts, and outfalls. In many regions stormwater is managed separately from sanitary sewage, though older cities may have combined systems that can overflow during heavy rain events.

Management combines structural controls with non-structural approaches. Structural measures include detention and retention basins, wetlands, infiltration trenches, green roofs, and permeable pavements. Non-structural strategies emphasize protecting source areas, land-use planning, and the use of green infrastructure such as bioswales and rain gardens. Frameworks like Low Impact Development and Sustainable Urban Drainage Systems are used to guide planning and design.

Regulation varies by country but typically includes standards and permits for discharges. In the United States, stormwater management is commonly addressed under the Clean Water Act with municipal separate storm sewer system permits and related controls, supported by maintenance and monitoring programs.