Downspout disconnection into vegetated area (Source: PGDER)
In urban areas, downspouts are commonly connected to drain tiles that feed the sewer system, and the cumulative effect of thousands of connected downspouts can greatly increase the annual number, magnitude, and duration of CSO events. Downspout disconnection is the process of separating roof downspouts from the sewer system and redirecting roof runoff onto pervious surfaces, most commonly a lawn. This reduces the amount of directly connected impervious area in a drainage area.
Ideally, a downspout disconnection plan will work with the existing downspouts on a building. In some cases, however, downspouts can be relocated if the new position would drain to a more appropriate receiving area (e.g., a hedge). Re-pitching the gutters in order to direct the flow to another corner of the roof is another option. For buildings with internal drainage, disconnecting internal downspouts may be difficult or impractical. Other BMPs such as cisterns or vegetated roofs may be more appropriate in such a case.
For disconnection to be safe and effective, each downspout must discharge into a suitable receiving area. Runoff must not flow toward building foundations or onto adjacent property. Typical receiving areas for disconnected roof runoff include lawns, gardens, and other existing landscaping such as shrubs. Soil amendments can be used to increase soil permeability if necessary. However, site constraints such as small or non-existent lawns may dictate that runoff be directed into a rain garden or, most commonly, an infiltration practice.
Stormwater Management Objectives
Downspout disconnection into vegetated area (Source: LID Center)
Volume reductions occur through infiltration and evapotranspiration in the receiving area. The potential exists for disconnected roof runoff to be completely taken "out of the system" by spreading out and infiltrating over pervious surfaces and BMPs. Stormwater that eventually flows onto an impervious surface and then into the sewer will at least be initially detained by flowing over rough, pervious surfaces such as grass.
Downspout disconnection decreases the peak discharge by reducing the volume of roof runoff that enters the sewer and by increasing the discharge time over which it enters. Also, roofs are inherently distributed over a drainage area. Connected downspouts concentrate and centralize roof runoff, causing peak discharges from individual roofs to accumulate in a relatively small number of manmade conveyances. By contrast, downspout disconnection helps to keep separate the peak discharge from each individual roof.
Roof runoff contains deposited atmospheric pollutants, particles of roofing material, and nutrients and BOD loading from bird droppings. The concentrations of these pollutants will be reduced as the stormwater infiltrates and is taken up into plant roots. Also, receiving water quality will improve because CSOs will occur less frequently and with less magnitude as a result of the water quantity benefits of downspout disconnection.
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