Page 424 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

384 Cha pte r T e n

flow rate we are interested in determining. The direct rainfall flow into the BMP has
been estimated in Eqs. (10.2.3) and (10.2.4) as the rainfall rate times the surface area
of the BMP, and both the soil infiltration flow out (exfiltration) and the evaporation
loss have been estimated by some sort of areal soil/subsurface infiltration rate and
some sort of areal evaporation rate (see also Chap. 3 for evapotranspiration rates).
The flow from the upslope areas can be estimated by Eq. (10.2.5), using the following
variables:

A Land area upslope of BMP that drains into it (length squared, usually acres)
up
C Average stormwater runoff coefficient of land surface upslope of BMP
up
This results in Eq. (10.2.6).

Q = C IA upslope rational method runoff estimate contribution (10.2.6)
up up up
By using these equations, many different situations can be modeled. For the first scenario,
assume that it is raining at a constant rainfall rate I, that the only flows in are from upslope
runoff and the rain, that there is negligible evaporation, and that there was no water initially
stored in the BMP. The equations can be used to estimate the time it might take to start runoff
out of the BMP (which is essentially the time to fill the storage volume during which the
runoff out of the BMP is zero) if the flows in are greater than the infiltration rate.

t = S /[C IA + I(A ) − F(A )] time to fill in fi rst scenario (10.2.7)
fi ll BMP up up BMP BMP
Now, assume that the storage volume S has filled and the rain continues. The
BMP
equations can also be used to estimate the steady-state runoff out of this BMP. In this
case, the outlet for the flow out is above the storage volume height, a common condition
in many cases. If there are multiple outlets, then the equations will need to be solved for
each stage of storage within the BMP.

Q = C IA + I(A ) − F(A ) steady-state flow out in second scenario (10.2.8)

out up up BMP BMP
The main way to model the stormwater mass balance around any BMP is to first
make a simplified sketch of the system, labeling potential flows in and out and internal
items such as storage as in Fig. 10.2.1. Then the initial conditions are used along with
Eqs. (10.2.3) and (10.2.4) to model the system. For solutions, the equations are further
simplified by allowing negligible flows to be neglected for the model and by using
accepted models and parameters such as the rational method or soil infiltration rates to
substitute for many of the variables.

Pollutant Mass Balances
The mass balance of pollutants around a BMP can be modeled by using the box model
in the stormwater mass balance model and including the concentration of the pollutant
in each flow, or within the BMP, using the following definitions:
C Average concentration of a pollutant in a BMP
PBMP
C Average concentration of a pollutant infiltrating into ground
PF