Page 91 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

72 Cha pte r T w o

To determine the percent imperviousness, the following steps should be taken:

• Identify the different surface types on the site.
• Calculate the total area for each of these surface types A, using the site drawing.
i
• Use Table 2.6.1 or other sources of the rational runoff coefficient such as textbooks
or manufacturers’ information to assign the individual runoff coefficient C for
i
each type of surface.
• Finally use Eqs. (2.6.2) and (2.6.3) to determine the percent imperviousness.

The values in Table 2.6.1 have been taken from LEED-NC 2.1 with the pervious
pavement grids approximated from values given there, and the pervious concrete value
as determined in a laboratory study at the University of South Carolina (Valavala et al.,
2006). Any surface that totally captures the rainfall and prevents it from runoff such as
a retention basin or area that has the rainfall directed into a drywell or other infiltration
system also has a runoff coefficient of zero unless the volume of the structure is
exceeded.

Surface Type Typical Runoff Coefficient

Pavement, asphalt 0.95
Pavement, concrete 0.95
Pavement, brick 0.85
Pervious pavement, plastic grid with grass 0.20
Pervious pavement, concrete grid with grass 0.60
Pervious pavement, concrete paver grid with gravel 0.85
Pervious concrete (0–1% slope) ~0
Turf, flat (0–1% slope) 0.25
Turf, average (1–3% slope) 0.35
Turf, hilly (3–10% slope) 0.40
Turf, steep (>10% slope) 0.45
Roofs, conventional 0.95
Roof, garden roof (<4-in substrate) 0.50
Roof, garden roof (4- to 8-in substrate) 0.30
Roof, garden roof (9- to 20-in substrate) 0.20
Roof, garden roof (>20-in substrate) 0.10
Vegetation, flat (0–1% slope) 0.10
Vegetation, average (1–3% slope) 0.20
Vegetation, hilly (3–10% slope) 0.25
Vegetation, steep (>10% slope) 0.30
TABLE 2.6.1 Typical Runoff Coefficients (Two-Year Storms)