As permeable grass is turned into concrete and steel, rain has to go somewhere.
If it’s not soaking into the grass, it needs to be accounted for. Rain that used to fall on green areas now falls on paved surfaces and roofs. Our streams and rivers are under pressure from the additional run-off, as is the infrastructure of the city.
Stormwater attenuation looks at returning the situation to neutrality as if the rain was still falling on green areas, even after we’ve built homes, patios and driveways. A Civil Engineer works out the flow of water that will come off these surfaces and how much storage is required to account for the difference.
This is called Stormwater Management Area Flow (SMAF). By reusing some of the water (retention) and releasing the excess slowly into the stormwater system (detention), you can reduce the load on infrastructure and protect sensitive areas.