A storm sewer system is a network of pipes, catch basins, and outfalls designed to collect and transport rainwater runoff from streets, parking lots, and other surfaces to discharge points, typically waterways or retention facilities. Storm sewers are separate from sanitary sewers carrying household wastewater. Storm sewer systems prevent flooding by moving water away from developed areas quickly. However, storm sewers also transport pollutants picked up from surfaces—oil, sediment, debris, and chemicals—directly to receiving waters, creating water quality concerns. Municipalities are increasingly implementing stormwater management practices like retention ponds, green infrastructure, and treatment systems to reduce pollution from storm sewer discharges. Storm sewer capacity is a key consideration for development approvals.