Green Roof Runoff

Green roofs mitigate water runoff and sewer overflows.

Green roof runoff.

A green roof s plants remove air particulates produce oxygen and provide shade. Green roofs can reduce stormwater runoff via water wise gardening techniques. Research in the uk has shown the average depression storage value to be 5 of the soil substrate depth. 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. The increased urbanization of towns and cities has resulted in less green space and more impervious surfaces. Their major drawback is the initial cost. Green roofs are very effective stormwater bmps.

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. In the summer a green roof can typically retain between 70 80 of the run off. Green roofs play a significant role in retrofitting the low impact development lid practices in urban areas. Green roof runoff does contain concentrations of some nutrients and other parameters but values are in line with other planted systems.

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. Precipitation generally runs off the roof of a building into gutters and flows into a storm sewer. The depth of water falling on the green roof that is permanently held within it. 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.

A green roof will typically intercept the first 5mm and more of rainfall providing interception storage. They capture water when it rains slowly releasing it through evaporation and plant use. Green roofs can significantly reduce the amount of rain water that would otherwise run off an impervious roof surface. The amount will be dependent on the depth and type of substrate in the green roof system.

A green roof also helps control storm water runoff and retention. Due to the volume reduction actual nutrient loadings from green roofs are less than asphalt roofing runoff or otherwise manageable at the downspout.