Page 423 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

Low-Impact Development and Stormwater Issues 383

In this simple box model of a stormwater mass balance around a BMP, the following
variables are used:
2
A Surface area of BMP (length squared, usually acres or ft )
BMP
E Evaporation rate (length/time, usually in/h)
F Infiltration rate (length/time, usually in/h)
I Rainfall rate (length/time, usually in/h)
3
Q Miscellaneous flows into a BMP (length /time, usually ft /s or cfs)
3
misc
3
3
Q Total runoff out of a BMP (length /time, usually ft /s)
out
Q Runoff from upslope areas into a BMP (length /time, usually ft /s)
3
3
up
3
3
S Storage volume in a BMP (length , usually ft )
BMP
t Time to fill
fill
Assuming that there are no internal reactions within the BMP that remove or add water,
these variables can be substituted into Eqs. (10.2.1) and (10.2.2) and the following water
mass balance equations result:
Accumulation rate = Q + Q + I(A ) − E(A ) − F(A ) − Q
misc up BMP BMP BMP out
stormwater dynamic mass balance (10.2.3)
0 = Q + Q + I(A ) − E(A ) − F(A ) − Q
misc up BMP BMP BMP out
stormwater steady-state mass balance (10.2.4)
In addition to these equations, the initial state of the BMP with respect to the mass of
water within it must be known. For the initial condition where the storage volume within
the BMP is not full, Eq. (10.2.3) represents the condition when the storage volume within
the BMP is filling with water if the flows in are greater than the flows out; or if there is any
water within the BMP, then losing water when the flows out are greater than the flows in.
Equation (10.2.4) represents the condition when the amount of stormwater that comes in
equals what goes out. This usually means the storage volume is full, or that the storage
volume is not “filling” since the potential for flow out is greater than what comes in.
What is needed now is some way to estimate the flows. The simplest model to use
for many of the flows in the stormwater mass balance is based on the rational runoff
method, and this is presented here. (However, flows from other models can be substituted
into the overall mass balances if they are used.) In the rational runoff method, the governing
generic equation relates the total runoff Q from a site to the average rational runoff
coefficient C, the rainfall rate I, and the land area A in the following manner:
T
Q = CIA generic rational method equation (10.2.5)
T
[If the units used are runoff in cubic feet per second, rainfall rate in inches per hour, and
area in acres, then it just so happens that the conversion factor from acre-inches per hour
(acre·in/h) to cubic feet per second is approximately 1, and no unit conversions are
needed. See Sec. 2.6 for additional information about the rational method and C.]
In the case of most BMPs, we may have upslope overland flow into the system,
rainfall directly on the top surface of the BMP, some additional water sources (such as
pipe flows from elsewhere), evaporation from the top of the BMP, infiltration into the
ground/subsurface underneath, and then the flow out of the BMP, which is the main