Hello everyone,
We are currently working on Option 1 to verify how much rainfall can be managed by our LID/GI design. However, the results of our calculation seem excessively high, and I would like to confirm whether this approach is correct.
Here are the project site details:
- Total site area (A): 49,308 m²
- Building footprint (B): 17,916.67 m²
- Paved area (C, all permeable pavement): 12,040.71 m²
- Other impervious area (D): 6,707.79 m²
- Landscape area (E): 14,642.83 m²
Using 30-year rainfall data in the calculator, we obtained the following percentiles:
- 80th: 33.94 mm
- 85th: 42.50 mm
- 90th: 57.58 mm
- 95th: 83.23 mm
Based on the example in the Reference Guide, we calculated the runoff as follows: Runoff = Rainfall − Depression Storage − Infiltration Loss
For the project area, landscape area (E) was classified as pervious, and the rest (B+C+D) was considered impervious.
According to the EPA SWM Guide, depression storage was set at 0.005 m for pervious surfaces and 0.003 m for impervious surfaces. Infiltration loss was applied only to pervious areas, with a default value of 0.002 m.
Since our goal is to achieve 1 point under LEED v4.1, we used the 80th percentile (33.94 mm):
- Pervious runoff = 12,642.83 × (0.03394 − 0.005 − 0.002) = 340.60 m³
- Impervious runoff = 36,665.17 × (0.03394 − 0.003) = 1134.42 m³
- Total runoff = 1,475.02 m³
Our LID/GI strategies include rainwater storage tank, green roof, permeable pavement, and landscape over underground structure(with at least 90cm of soil depth). Using local calculation methods (e.g., area × soil depth × porosity), the storage capacities are as follows:
- rainwater storage tank = 560 m³
- Green roof = 99.12 m³
- Landscape over underground structure = 1,513.06 m³
- Permeable pavement = 5,911.99 m³
When all of these are entered into the calculator, the result is 548%, which seems unusually high. It feels as though this might even qualify for an EP, but I suspect there may be a mistake in my methodology.
My questions are:
- Is this calculation method acceptable under LEED?
- Or am I misunderstanding or misapplying part of the calculation?
Any guidance would be greatly appreciated. Thank you!