Water-quality-and-CN-calcs.xls

For the Civil Engineering Stormwater modeling field, this calculates Water Quality volume for the Florida area (based on SJRWMD requirements) and calculates the CN (based on TR-55 methodology). Please note that this spreadsheet is password protected and cell formulae cannot be seen.

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Calculation Reference
Hydraulics

The St. Johns River Water Management District (SJRWMD) in Florida has specific requirements for stormwater management, including the calculation of the Water Quality Volume (WQv) to ensure proper treatment of pollutants in stormwater runoff.

1. Calculate the Water Quality Volume (WQv):

To calculate the WQv, follow the SJRWMD guidelines. The WQv is typically calculated based on the runoff volume from the site for a specific design storm event (often the 2.5 inches, 24-hour rainfall event). The formula for WQv is:

WQv = Vr * A * 0.5

where: WQv = Water Quality Volume (cubic feet) Vr = Runoff coefficient (dimensionless) A = Drainage area (acres)

2. Calculate the Curve Number (CN) based on the TR-55 methodology:

The Curve Number (CN) is a dimensionless parameter used to estimate the amount of stormwater runoff generated by a particular land use or surface type. The TR-55 methodology provides guidance for determining the CN based on soil types (grouped as A, B, C, or D) and land use. Here's how to calculate CN:

a. Identify the land use and soil type for your site.

b. Use the TR-55 table to find the CN value for each land use and soil type combination. Some example CN values:

• Urban Districts (business, commercial, industrial):

  • Type A: 77
  • Type B: 85
  • Type C: 92
  • Type D: 95

• Residential (1/4-acre lots):

  • Type A: 68
  • Type B: 77
  • Type C: 86
  • Type D: 89

• Open spaces (lawns, parks, golf courses):

  • Type A: 39
  • Type B: 61
  • Type C: 74
  • Type D: 80

c. Calculate the weighted CN for the entire site by considering the proportion of each land use and soil type combination:

CN_weighted = (CN1 * A1 + CN2 * A2 + ... + CNn * An) / (A1 + A2 + ... + An)

where: CN1, CN2, ..., CNn = CN values for different land use and soil type combinations A1, A2, ..., An = areas corresponding to each land use and soil type combination

3. Use the CN to calculate runoff coefficients:

The runoff coefficient (Vr) can be calculated using the CN value:

Vr = (CN - 10) / 100

Use the Vr value to calculate the Water Quality Volume (WQv) following the SJRWMD requirements. Always consult local stormwater management guidelines or a professional engineer to ensure that your calculations are accurate and appropriate for your project.