Storm water runoff calculations.

Calculation Reference
Strorm Water Design

To calculate stormwater runoff and appropriately size a drain, you can use the Rational Method. This method is based on the assumption that the rate of runoff (Q) is proportional to rainfall intensity (i), drainage area (A), and a runoff coefficient (C) that represents the portion of rainfall that becomes runoff. Here's the formula:

Q = CiA

where: Q = Runoff rate (cubic feet per second, or CFS) C = Runoff coefficient (dimensionless) i = Rainfall intensity (inches per hour, or in/hr) A = Drainage area (acres)

To use the Rational Method, follow these steps:

  1. Determine the drainage area (A): Measure or estimate the area that contributes runoff to the drain. This can be done using site plans, aerial imagery, or topographic maps.

  2. Select the appropriate runoff coefficient (C): The runoff coefficient represents the fraction of rainfall that becomes runoff. It depends on factors such as surface type, soil type, and slope. Here are some typical values:

  • Pavement or concrete: 0.9 - 0.95
  • Asphalt: 0.85 - 0.9
  • Gravel: 0.7 - 0.8
  • Lawns (flat): 0.1 - 0.3
  • Lawns (moderate slope): 0.2 - 0.4
  • Lawns (steep slope): 0.3 - 0.6
  1. Determine the rainfall intensity (i): You'll need to refer to the local rainfall data and choose an appropriate design storm for your area. This is usually represented as a return period (e.g., 10-year, 25-year, or 100-year storm) and can be found in the local stormwater management guidelines or on the websites of national weather agencies.

  2. Calculate the runoff rate (Q): Plug your values for C, i, and A into the Rational Method formula to calculate the runoff rate (Q).

  3. Size the drain: The drain should be sized to handle the calculated runoff rate (Q). You can consult manufacturer's specifications for drain capacity or consult with a professional engineer to ensure that the selected drain is adequate for the intended application.

Keep in mind that local regulations and guidelines may require more complex calculations or different methods. Consult with a professional engineer or stormwater management expert to ensure that your design complies with all relevant regulations.

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12 Oct 2007
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27 Apr 2023
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