THE STORMWATER CALCULATION PROCESS - Low Impact Development and Sustainable Stormwater Management - page 189

Step 6 . Design and incorporate structural BMPs that provide volume control for
the design storm volume increase indicated on Worksheet 4. Provide calcula-
tions and documentation to support the volume estimate provided by BMPs.
Indicate the permanent volume reduction provided by the BMPs proposed
(Worksheet 5). Also, indicate the area of non-volume-reducing BMPs included
in the design. BMPs that reduce volume include infiltration systems, bioreten-
tion, and pervious pavement and infiltration beds, among others (capture-reuse
is accounted for separately in Worksheets 9 and 10). Proceed to Flowchart B.
Step 7 . Determine applicable peak rate requirements and calculations. If applying
for an exemption for small sites and small impervious area sites, complete
Worksheet 6.
Step 8 . If additional peak rate calculations are required, demonstrate peak rate
mitigation through modeling. This modeling or routing should consider the
benefits of volume-control BMPs as outlined in Flowchart B and below. Pro-
vide additional detention capacity if needed.
8.1. List the applicable design criteria (local requirement, LID guidance, or
other) and what they requires on Worksheet 7.
8.2. Use one of the following methods to determine peak rate control for all
storms up to the 100-year storm or as required by local requirements.
8.3. List the pre- and postdevelopment peak rates for each design storm in
the space provided on Worksheet 7.
Other methods recommended for determining the effects of volume control
on peak rate mitigation are listed in the next section. Proceed to Flowchart C.
Step 9 . If the net increase in runoff volume associated with a 2-year, 24-hour
storm (pre- vs. postdevelopment) is being controlled adequately on-site (i.e.,
reduced by infiltration and/or evapotranspiration), there is no need to com-
plete Worksheet 8. However, if this net increase is not being controlled (i.e.,
permanently reduced), complete Worksheet 8. Select BMPs that will remove
the expected pollutants for the land use type. Often, multiple types of BMPs
used in series will be required to provide adequate treatment. As a guide, use
a series of BMPs that will achieve 80% removal of solids or better.
Step 10 . Determine the landscape irrigation demand (complete Worksheet 9).
The irrigation demand is evaluated for each landscape type on a particular site
by month. Worksheet 9 provides space for two types of landscape. If addi-
tional landscapes are proposed, the designer should use as many worksheets as
necessary. Once all the landscape irrigation demands have been determined,
the designer calculates the total landscape irrigation demand for the site by
summing the individual demands by month.
Step 11 . Determine the percentage of site area completely controlled (i.e., no
runoff in an average year) by a capture-reuse system (capture-reuse area)
that can be excluded from the stormwater management area (complete the
process outlined on Worksheet 10). Starting with a design storage volume
and contributing area, the designer can use Worksheet 10 to estimate the