Environmental Engineering Reference
In-Depth Information
of stormwater management. This peak-rate criterion is generally based on larger
storm events of limited frequency (such as the 100-year storm event). By con-
trast, the additional elements of stormwater management—volume, groundwater
recharge, water quality, and stream channel protection—are based largely on the
smaller, more frequent storm events (e.g., 2-year storms and smaller).
Engineers and regulatory officials are generally familiar with the engineering
methods and models used to evaluate the rate of runoff for large storm events.
There is some consistency in the calculation methodologies used, with the rational
and modified rational methods probably being the two most common methodolo-
gies applied to estimate rate of runoff. However, local county and city regulations
are sometimes based on unreferenced equations. Furthermore, most regulations
have been based primarily on peak-rate (release-rate) calculations, which may
not be easily integrated with comprehensive stormwater management or LID.
To manage stormwater for volume, groundwater recharge, quality, channel pro-
tection, and hydromodification and to integrate both nonstructural and structural
BMPs and take reasonable credit for their use, additional or expanded analyti-
cal methods are needed. In Appendix A we provide guidance on recommended
procedures and methodologies to improve stormwater management, and include
worksheets and flowcharts intended to assist in this process.
As stated in the site design process, applicants are urged to meet with the
reviewing engineer(s) at the initiation of a project proposal, at which time BMPs
to be integrated into the site plan should be discussed. Methodologies to be used
in this process should be reviewed and agreed upon. Although not mandated, the
following methodological procedure has been developed to provide additional
guidance to applicants. These recommended procedures are intended to reduce the
extent of judgment that will be required as part of the site design and stormwater
management engineering process.
Methods Involving No Routing
Simple Volume Diversion (Off-Line Storage)
This is a very simple way
to account partially for the effect of volume control BMPs on peak runoff rates.
Many computer models have components that allow a “diversion” or “abstrac-
tion.” The total volume reduction provided by the applicable structural and
nonstructural BMPs can be diverted or abstracted from the modeled runoff before
it is routed to the detention system (if detention is needed). This approach is very
conservative because it does not give any credit to the increased time of travel,
fully account for ongoing infiltration, and so on, associated with the BMPs. Even
this conservative approach can reduce the detention storage requirements sig-
nificantly. This method can and should be used in conjunction with the travel
time/time of concentration adjustment explained below and does not entail any
routing through the volume-reducing BMP(s).
Travel Time/Time of Concentration Adjustment
In LID design, time of
concentration is the time it takes a drop of water to move from the farthest point in
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