Especially in urban areas, over
75% of a rain event can become surface runoff
depositing pollutants into waterways. Many
older cities have combined stormwater and sanitary
sewers where, in the event of a significant
rainfall, the sewage treatment plant becomes
unable to treat all of the excess stormwater
flowing into it. Bypass valves are opened, and
raw sewage can be directed to the receiving
waters. Green roofs help alleviate this problem
through retention of rainfall and detention
of roof water runoff. Testing shows:
Green roofs can greatly reduce the risk of flooding,
sewer overflows, and subsequent discharges. Once
the green roof is saturated, the water slowly
percolates through the GreenGrid® growth
media, becoming roof runoff. This occurs several
hours after peak flows from impervious surfaces
have subsided and provides additional time for
municipal sewer systems to handle other uncontrolled
runoff. For the building owner, a GreenGrid® green
roof can often mean that costly stormwater detention
and retention systems can be substantially reduced
in size or even rendered unnecessary. |
