THE STORMWATER CALCULATION PROCESS - Low Impact Development and Sustainable Stormwater Management - page 188

APPENDIX A
THE STORMWATER
CALCULATION PROCESS
Flowchart A is provided to guide the user through the first steps of the stormwater
calculation process and can be thought of as a series of steps executed through
a series of worksheets.
Step 1 . Provide general site information (Worksheet 1).
Step 2 . Identify sensitive natural resources, and if applicable, identify which
areas will be protected (Worksheet 2).
Step 3 . Identify the area of the site contributing to a capture-reuse system (Work-
sheet 3). Identify the area of the site for which stormwater runoff must be
managed, or the stormwater management area (Worksheet 3). ( Note: Steps 10
and 11 discuss the procedure for taking stormwater credit for capture-reuse
strategies.)
Step 4 . Incorporate nonstructural best management practices (BMPs) into the
stormwater design. Quantify the benefits of nonstructural BMPs (Worksheet 3).
For nonstructural BMPs being proposed, provide nonstructural BMP checklists
to demonstrate that these BMPs are appropriate.
Step 5 . Estimate the target volume of runoff associated with the design storm
event (e.g., the 85th percentile storm runoff volume, the net increase in a
2-year storm), using the runoff curve number or small storm hydrology
method. The use of weighted curve numbers for pervious and impervious
areas is not acceptable. Runoff volume should be calculated based on major
land use types and soil types (Worksheet 4).