Green Roof Runoff

Green roofs can significantly reduce the amount of rain water that would otherwise run off an impervious roof surface.

Green roof runoff.

Green roofs can reduce stormwater runoff via water wise gardening techniques. Due to the volume reduction actual nutrient loadings from green roofs are less than asphalt roofing runoff or otherwise manageable at the downspout. Green roof runoff does contain concentrations of some nutrients and other parameters but values are in line with other planted systems. Green roof runoff method depression storage.

Green roofs have a layer of plant material that absorbs water like a sponge. Their major drawback is the initial cost. The depth of water falling on the green roof that is permanently held within it. A study presented at the green roofs for healthy cities conference in june 2004 cited by the epa found water runoff was reduced by over 75 during rainstorms.

This is the amount of water that is lost to the environment due to evaporation and transpiration. Water is stored by the roof s substrate and. A green roof also helps control storm water runoff and retention. They capture water when it rains slowly releasing it through evaporation and plant use.

A green roof will typically intercept the first 5mm and more of rainfall providing interception storage. Research in the uk has shown the average depression storage value to be 5 of the soil substrate depth. Green roofs mitigate water runoff and sewer overflows. In the summer a green roof can typically retain between 70 80 of the run off.

Green roofs delay the time that runoff occurs reducing the number and volume of overflows in combined sewer systems csos green roofs slow the velocity of direct runoff reducing erosion of stream and river banks and the sedimentation and loss of habitat that result. Green roofs are very effective stormwater bmps. Green roofs play a significant role in retrofitting the low impact development lid practices in urban areas. Precipitation generally runs off the roof of a building into gutters and flows into a storm sewer.

A typical extensive green roof of 3 5 inches depth will reduce the volume of runoff by about 55 in central pennsylvania and attenuate the peak runoff rates to the level of the pre development land use. Vegetation and soil act as a sponge absorbing and filtering water that would normally plunge down gutters wash through polluted streets and over tax sewer systems.