Storm drains are often unable to manage the quantity of rain that falls during heavy rains and/or storms. When storm drains are inundated, basement and street flooding can occur. Unlike catastrophic flooding events, this type of urban flooding occurs in built-up areas where human-made drainage systems are prevalent. Urban flooding is the primary cause of sewer backups and basement flooding, which can affect properties repeatedly.
Clogged drains also contribute to flooding by the obstruction of storm drains. Communities or cities can help reduce this by cleaning leaves from the storm drains to stop ponding or flooding into yards. Snow in the winter can also clog drains when there is an unusual amount of rain in the winter and snow is plowed atop storm drains.
Runoff into storm sewers can be minimized by including sustainable urban drainage systems (UK term) or low impact development or green infrastructure practices (US terms) into municipal plans. To reduce stormwater from rooftops, flows from eaves troughs (rain gutters and downspouts) may be infiltrated into adjacent soil, rather than discharged into the storm sewer system. Storm water runoff from paved surfaces can be directed to unlined ditches (sometimes called swales or bioswales) before flowing into the storm sewers, again to allow the runoff to soak into the ground. Permeable paving materials can be used in building sidewalks, driveways and in some cases, parking lots, to infiltrate a portion of the stormwater volume.
Many areas require that properties have detention tanks that temporarily hold rainwater runoff, and restrict the outlet flow to the public sewer. This lessens the risk of overburdening the public sewer during heavy rain. An overflow outlet may also connect higher on the outlet side of the detention tank. This overflow prevents the detention tank from completely filling. Restricting water flow and temporarily holding the water in a detention tank public this way makes it far less likely for rain to overwhelm the sewers.