Page 427 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

Low-Impact Development and Stormwater Issues 387

Equations (10.2.7) and (10.2.8) can thus be modified for the pervious concrete system
model as follows:

t = (S + S )/[C IA + (I – F)(A )] time to fi ll (10.2.13)
fi ll Gravel Pervious up up BMP-PC
Q = C IA + (I – F)(A ) steady-state fl ow out (10.2.14)
out up up BMP-PC
Many other initial conditions and configurations can be modeled for pervious concrete
systems. In like manner, other BMP systems can be modeled using the basic concepts of
a box model and mass balances. First determine the stormwater flows in, then how
and where stormwater might flow out, and internally determine if there are separate
layers or cells within the BMP. For instance, rain gardens collect water that falls on
them to decrease runoff from these areas and may also collect and infiltrate stormwater
from neighboring areas. Figures 10.2.3 and 10.2.4 show how rainwater that falls on a
roof may be directly collected into a roof top garden (green roof) or diverted to a rain
garden on the site. Note how the green roof has low-water needs-succulent plant
species.) Figure 10.2.5 is a photograph of a storage facility in Oregon where the
pervious concrete collects rainwater that falls directly on it and also stormwater from
the roof. Then perform water and pollutant mass balances around each layer, and for
the overall LID practice.

FIGURE 10.2.3 Roof top garden at Rachel Carson Elementary School in Sammamish, WA.
(Photograph courtesy Coughlin Porter Lundeen taken September 2008.)