Page 422 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

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monitoring for maintenance needs. The ASTM C09/49 pervious concrete technical
committee has recently released two standard testing methods specific for pervious
concrete. It is also developing additional standards with respect to durability, hardened
density, and strength. The two ASTM standards already available are ASTM C1688/
C1688M-08 Standard Test Method for Density and Void Content of Freshly Mixed
Pervious Concrete and ASTM C1701/C1701M-09 Standard Test Method for Infiltration
Rate of In Place Pervious Concrete. The American Concrete Institute technical committee
on pervious concrete (ACI 522) has also released a specification for pervious concrete:
ACI 522.1-08: Specification for Pervious Concrete Pavement.

10.2 Modeling LID BMPs
There are many commercial stormwater models that are commonly used for design and
assessment. Many of these have opportunities for including some of the LID scenarios
for stormwater management into the assessment. In the future, more individual LID
practices and technologies will be included in them. However, given the many options
for LID and the site-specific needs and variations for each, it is helpful to have some
simple methods to perform preliminary design and performance estimates of the
practices. The following section gives some simple mass balance methods for preliminary
design and assessment of many LID technologies.

Overall Mass Balances
In stormwater management there are two types of mass balances that are of interest: a
mass balance on the water and a mass balance on the pollutant in question. Regardless of
the type of substance for which its mass is being balanced, the balances can be summarized
with Eqs. (10.2.1) and (10.2.2). Given the “system” or unit operation that is being evaluated,
Eq. (10.2.1) represents a dynamic situation where the mass of the substance in question
within the system changes with time. Equation (10.2.2) represents a steady-state situation
where the mass of the substance in question within the system is constant.

Rate of accumulation =Σ Rates in − Σ Rates out ± Σ Internal reactions/processes
dynamic mass balance (10.2.1)
0 =Σ Rates in − Σ Rates out ± Σ Internal reactions/processes
steady-state mass balance (10.2.2)
Stormwater Mass Balances
The first mass balance explained here is the total stormwater mass balance around a
system such as a BMP or other LID practice. Let the BMP be represented by a box, and
let the typical flows in and out be as shown in Fig. 10.2.1.

Q misc I E
Q up BMP
Q out
S BMP
F

FIGURE 10.2.1 Simple box model of a stormwater mass balance on a BMP.